essay on ocean currents

ENCYCLOPEDIC ENTRY

Ocean currents.

Ocean currents are like vast rivers, flowing along predictable paths.

Biology, Ecology, Earth Science, Oceanography

Rubber Duck on the Beach

A rubber duck washed up on the beach after being carried out in the ocean.

Creacart/Getty

Ocean water is constantly moving, and not only in the form of waves and tides . Ocean currents flow like vast rivers, sweeping along predictable paths. Some ocean currents flow at the surface; others flow deep within water. Some currents flow for short distances; others cross entire ocean basins and even circle the globe.

By moving heat from the equator toward the poles, ocean currents play an important role in controlling the climate. Ocean currents are also critically important to sea life. They carry nutrients and food to organisms that live permanently attached in one place, and carry reproductive cells and ocean life to new places.

Rivers flow because of gravity . What makes ocean currents flow?

Tides contribute to coastal currents that travel short distances. Major surface ocean currents in the open ocean, however, are set in motion by the wind, which drags on the surface of the water as it blows. The water starts flowing in the same direction as the wind.

But currents do not simply track the wind. Other things, including the shape of the coastline and the seafloor, and most importantly the rotation of the Earth, influence the path of surface currents .

In the Northern Hemisphere, for example, predictable winds called trade winds blow from east to west just above the equator. The winds pull surface water with them, creating currents . As these currents flow westward, the Coriolis effect —a force that results from the rotation of the Earth—deflects them. The currents then bend to the right, heading north. At about 30 degrees north latitude, a different set of winds, the westerlies, push the currents back to the east, producing a closed clockwise loop.

The same thing happens below the equator, in the Southern Hemisphere, except that here the Coriolis effect bends surface currents to the left, producing a counter-clockwise loop.

Large rotating currents that start near the equator are called subtropical gyres. There are five main gyres: the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres, and the Indian Ocean Subtropical Gyre.

These surface currents play an important role in moderating climate by transferring heat from the equator towards the poles. Subtropical gyres are also responsible for concentrating plastic trash in certain areas of the ocean.

In contrast to wind-driven surface currents , deep- ocean currents are caused by differences in water density. The process that creates deep currents is called thermohaline circulation —“thermo” referring to temperature and “haline” to saltiness.

It all starts with surface currents carrying warm water north from the equator. The water cools as it moves into higher northern latitudes, and the more it cools, the denser it becomes.

In the North Atlantic Ocean, near Iceland, the water becomes so cold that sea ice starts to form. The salt naturally present in seawater does not become part of the ice, however. It is left behind in the ocean water that lies just under the ice, making that water extra salty and dense. The denser water sinks, and as it does, more ocean water moves in to fill the space it once occupied. This water also cools and sinks, keeping a deep current in motion.

This is the start of what scientists call the “global conveyor belt,” a system of connected deep and surface currents that moves water around the globe. These currents circulate around the globe in a thousand-year cycle.

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Smithsonian Ocean

Currents, Waves, and Tides

Introduction, additional resources.

Looking toward the sea from land, it may appear that the ocean is a stagnant place. But this is far from the truth—the ocean is constantly in motion. Water is propelled around the globe in sweeping currents, waves transfer energy across entire ocean basins, and tides reliably flood and ebb every single day. But why does this occur?

Ocean movement is created by the governing principles of physics and chemistry. Friction, drag, and density all come into play when describing the nature of a wave, the movement of a current, or the ebb of a tide. Ocean motion is influenced by occurrences here on Earth that are familiar, like heat changes and wind. It also requires a shift in perspective to encompass the movement of planets, the Moon, and the Sun. Though it appears we live on a stable and stationary planet, we are, in fact, whipping through space around the Sun in an orbit and spinning on an axis. This planetary movement has a strong effect on how oceans move.  

While the ocean as we know it has been in existence since the beginning of humanity, the familiar currents that help stabilize our climate may now be threatened.  Climate change is altering the processes that propel water across the globe, and should this alter ocean currents, it would likely lead to a cascade of even more change.

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A large movement of water in one general direction is a current. Currents can be temporary or long-lasting. They can be near the surface or in the deep ocean. The strongest currents shape Earth’s global climate patterns (and even local weather conditions) by moving heat around the world.

Surface Currents

At the surface, currents are mainly driven by four factors—wind, the Sun’s radiation, gravity, and Earth’s rotation. All of these factors are interconnected. The Sun’s radiation creates prevailing wind patterns, which push ocean water to bunch in hills and valleys. Gravity pulls the water away from hills and toward valleys and Earth’s rotation steers the moving water.

Sun and Wind

Wind is a major force in propelling water across the globe in surface currents. When air moves across the ocean’s surface, it pulls the top layers of water with it through friction, the force of resistance between two touching materials moving over one another. Surface ocean currents are driven by consistent wind patterns that persist throughout time over the entire globe, such as the jet stream. These wind patterns (convection cells) are created by radiation from the Sun beating down on Earth  and generating heat.

The Sun’s radiation is strongest at the equator and dissipates the closer you get to the poles. This uneven distribution of heat causes air to move. The hot air over the equator rises and moves away from the equator. Likewise, cold air from the poles sinks and moves towards the equator. The clashing of hot air originating at the equator and cold air originating at the poles creates regions of high atmospheric pressure and low atmospheric pressure along specific latitude lines. It would make intuitive sense that the hot air and cool air would meet in the middle of the equator and the North or South pole, however, in reality it is much more complicated . A combination of Earth’s rotation, the fact that Earth is tilted on an axis, and the placement of most continents in the Northern Hemisphere, create pressure systems that divide each hemisphere into three distinct wind patterns or circulation cells.

In the Northern Hemisphere, the most northern system, the polar cell, blows air in a consistent southwestern direction toward a pocket of low pressure along the 60-degree latitude line. The middle system, the Ferrel cell, blows in a consistent northeastern direction toward the same 60-degree low. And the most southern system, the Hadley cell, blows air in a consistent southwestern direction toward a region of low pressure along the equator. The result is a global pattern of prevailing wind, and it is this consistent wind that impacts the ocean.

While it may appear that the ocean is a flat surface, the reality is that it is a series of hills and valleys in the water. At the places where the wind generated currents converge into each other, the ocean water is pushed to build a slight hill. Likewise, where the winds diverge, the ocean water dips in a slight depression.

Gravity and Earth's Rotation

Wind pushes water into hills of high pressure which leave behind valleys of low pressure. Since water is a liquid that prefers to stay at a level height, this creates an unstable situation. Following the pull of gravity, ocean water moves from the built-up areas of high pressure down to the valleys of low pressure.

But as the water moves from hills to valleys, it does so in a curved trajectory, not a straight line. This curving is a result of Earth’s spin on its axis.

On Earth, movement in a straight line over long distances is harder than it may seem. That’s because Earth is constantly rotating, meaning every object on its surface is moving at the speed at which the Earth is spinning on its axis. From our perspective, stationary objects are just that, unmoving. In reality, they are whipping around at a speed of roughly 1,000 miles per hour (1600 km/hr) at Earth’s equator. It is that whipping, rotating motion that influences the movement of any object not in direct contact with the planet’s surface, making straight appearing trajectories actually bend. It also influences the movement of ocean currents. Scientists refer to this bending as the Coriolis Effect .

It is easiest to understand this phenomenon when thinking about travel in a northern or southern direction. Since Earth is essentially a sphere and it spins around an axis, anything near Earth’s equator will travel the fastest—since Earth is rotating at a constant rate and the equator runs along the widest part of the sphere, any object there must travel the entirety of Earth’s circumference in one rotation. As you get closer and closer to the poles, the distance traveled in one rotation gradually shrinks until it reaches zero at either pole. Therefore, an object on the surface will gradually spin slower the closer it gets to a pole. 

But leave the surface of the planet, and the anchor keeping you in sync with the land beneath you disappears. Any moving object (plane, boat, hot air balloon, water) will begin its travels at the rotating speed of the location where it took off from. If it should travel north or south, the ground beneath it will be traveling at a different speed. Travel North from the Equator, and the ground will gradually spin slower beneath you. This causes an object attempting to travel in a straight line to veer to the right in the Northern Hemisphere and veer to the left in the Southern Hemisphere relative to the direction traveling. 

Understanding how the rotating Earth affects movement to the west or east is a bit trickier. Envision an elastic string attached to a ball on one end and an anchored point at the other. The faster the ball is spun around the anchor, the more the elastic stretches and the farther the ball travels from the center point. An object traveling on Earth behaves the same way. If the object moves east, in the direction that Earth is spinning, it is now traveling around the axis of Earth faster than it was when it was anchored—and so, the object wants to move out and away from the axis. Still tethered by gravity, the object does so by moving toward the equator, the place on Earth that is the greatest distance from the axis. Travel west, the opposite direction that Earth is spinning, and now the object is spinning slower than Earth’s surface and so it wants to move toward the axis. It does so by moving toward the pole. This again appears as a bend to the right in the Northern hemisphere and to the left in the Southern hemisphere.

Water moving along Earth’s surface is also subject to the Coriolis effect which causes moving water to curve in the same directions described above. In the Northern Hemisphere, surface water curves to the right and in the Southern Hemisphere it curves to the left of the direction it is forced to move.

Swirling Gyres

Earth’s rotation is also responsible for the circular motion of ocean currents. There are 5 major gyres—expansive currents that span entire oceans—on Earth. There are gyres in the Northern Atlantic, the Southern Atlantic, the Northern Pacific, the Southern Pacific, and the Indian Ocean. Similar to surface waters, Northern gyres spin clockwise (to the right) while gyres in the south spin counterclockwise (to the left). 

The center of the gyres are relatively calm areas of the ocean. The Sargasso Sea, known for its vast expanses of floating Sargassum seaweed, exists in the North Atlantic gyre and is the only sea without land boundaries. Today, gyres are also areas where marine plastic and debris congregate. The most famous one is known as the Great Pacific Garbage Patch , but all five gyres are centers of plastic accumulation.

Ekman Transport

Wind moving across the ocean moves the water beneath it, but not in the way you might expect. The Coriolis Effect, the apparent force created by the spinning of Earth on its axis, affects water movement, including movement instigated by wind. Recall that Coriolis causes the trajectory of a moving object to veer to the right or the left depending upon the hemisphere it is located in. But in this case, the three-dimensional nature of the ocean plays into the direction of the water’s overall movement. Wind blowing over water will move the ocean water underneath it in an average direction perpendicular to the direction the wind is traveling.

As wind blows over the surface layer of water, friction between the two pulls the water forward. As we know, when water (and other objects) moves across Earth’s surface it bends due to the Coriolis Effect. The top most layer of water will bend away from the direction of the wind at about 45 degrees. For simplicity, we will assume that this scenario is in the Northern Hemisphere and all movement bends to the right. As the top layer of water begins to travel, it in turn pulls on the water layer beneath it, just as the wind had. Now this second water layer begins to move, and it travels in a direction slightly to the right of the layer above it. This effect continues layer by layer as you move down from the surface, creating a spiral effect in the moving water.

In addition to a change in direction, each sequential layer down loses energy and moves at a slower speed. Friction causes the water to move, but drag resists that movement, so as we travel from the top layer to the next, some of the energy is lost. When all the layers down the spiral are accounted for, the net direction of the water is perpendicular to the direction of the wind.

Deep Currents

The ocean is connected by a massive circulatory current deep underwater. This planetary current pattern, called the global conveyor belt , slowly moves water around the world—taking 1,000 years to make a complete circuit. It is driven by changes in water temperature and salinity, a characteristic that has scientists refer to the current as an example of thermohaline circulation.

  Both heat and salt contribute to the ocean water’s density. Saltier and colder water is heavier and denser than less salty (or fresher), warmer water. Around the globe there are areas where the heat and saltiness of ocean water (and therefore, its density) change. The most important of these areas is in the North Atlantic.

As warm Atlantic water from the Equator reaches the cold polar region in the North via the Gulf Stream, it rapidly cools. This region is also cold enough that the ocean water freezes, but only the water turns to ice. As the water freezes it leaves the salt behind, causing the surrounding water to become saltier and saltier. The cold, salty water then sinks in a mass movement to the deep ocean. It is this sinking that is a main driver for the entire deep-water circulation system that moves massive quantities of water around the globe. Cooling also occurs near Antarctica, but not to the extremes that happen in the Northern Hemisphere.

Another area of the ocean where massive amounts of water move to the ocean’s depths is in the Mediterranean. In this area, evaporation is the main driver that changes the salinity of the ocean water. As water in the Mediterranean evaporates, it leaves the salt behind. This super salty ocean water then bleeds into the Atlantic via the thin mouth of the Mediterranean, also known as the Strait of Gibraltar.

When cold, salty water circulates the globe and gradually becomes warmer, it begins to rise. The “old” deep water is full of nutrients that have accumulated from the sinking of waste from the productive surface waters up above. Locations where the “old” water rises are highly productive areas because they contain ample nutrients and have access to sunlight—the perfect combination for photosynthesis.

Currents and Change

Because ocean circulation is driven by temperature change, any variation to the planet’s climate could significantly alter the system. Scientists worry that the melting ice caused by global warming may weaken the global conveyer belt by adding extra fresh water in the Arctic. A 2018 study found that the massive ocean current that courses around the Atlantic Ocean, called the Atlantic Meridional Overturning Circulation, has decreased in strength by about 15 percent since 400 AD and is now the weakest it has been in 1,600 years. Ironically, despite an overall increase in global temperatures, many places in North America and Europe may get colder as a result.

Rip Currents

Not all currents occur at such a large scale. Individual beaches may have rip currents that are dangerous to swimmers.  Rip currents are strong, narrow, seaward flows of water that extend from close to the shoreline to outside of the surf zone. They are found on almost any beach with breaking waves and act as “rivers of the sea,” moving sand, marine organisms, and other material offshore.  Rip currents are formed when there are alongshore variations in wave breaking. In particular, rip currents tend to form in regions with less wave breaking sandwiched between regions of greater wave breaking.  This can occur when there are gaps in sand bars nearshore, from structures like piers or jetties, or from natural variations in how waves are breaking.

Rip currents can move faster than an Olympic swimmer can swim, at speeds as fast as eight feet (2.4 meters) per second. At these speeds, a rip current can easily overpower a swimmer trying to return to shore. Instead of attempting to swim against the current, experts suggest not to fight it and to swim parallel to shore. For more safety tips visit NOAA’s guide to rip current safety .

Currents and Nature

Unseen by the human eye, thousands of microscopic animals hitch rides across oceans on an oceanic highway. These animals, called zooplankton, move at the whim of ocean currents . Off the Eastern Shore of the United States, one of the most powerful ocean currents— the Gulf Stream —is transporting zooplankton from the Gulf of Mexico, around the tip of Florida, up to Cape Cod in Massachusetts and then across the North Atlantic Ocean towards Europe. The currents enable the young creatures to find their way to hospitable places where they grow into adults.

Other ocean creatures hitch rides on currents using floating debris, like mats of seaweed, tree trunks, and even plastic. They use these havens to survive the otherwise perilous open ocean. After the 2011 tsunami that prompted the Fukushima Daiichi power plant meltdown in Japan, debris from the Japanese coast began washing ashore on the West coast of North America, bringing with it over 280 Japanese species. The movement of species across ocean basins helps maintain populations across the entirety of a species’ range. It also ensures the diversity of genetics within a population, an important factor for keeping species resistant and resilient to hardships like disease and environmental disasters.   

Currents also influence where large adult species can and want to go. Turtles and whales migrate annually to the plentiful waters of Georges Bank off the coast of New England, a place that is productive because of the warm waters brought north from the equator.

Sculpting seawater into crested shapes, waves move energy from one area to another. Waves located on the ocean’s surface are commonly caused by wind transferring its energy to the water, and big waves, or swells, can travel over long distances. 

When waves crash onshore they can make a significant impact to the landscape by shifting entire islands of sand and carving out rocky coastlines. Storm waves can even move boulders the size of cars above the high tide line, leaving a massive boulder hundreds of feet inland. Until recently, scientists attributed the placement of these rogue boulders to past tsunami damage, however, a 2018 study upended this notion by carefully recording the movement of boulders along a swath of rocky coastline in Ireland over a time period in which no tsunamis occurred. In addition to over 1,000 mid-sized boulders, many reaching over 100 tons in weight, scientists recorded the movement of a 620-ton boulder (the same weight as 90 full-sized African elephants ), showing that storm waves moved it over 8 feet (2.5 meters) in just one winter.

Anatomy of a Wave

A wave forms in a series of crests and troughs. The crests are the peak heights of the wave and the troughs are the lowest valleys. A wave is described by its wavelength (or the distance between two sequential crests or two sequential troughs), the wave period (or the time it takes a wave to travel the wavelength), and the wave frequency (the number of wave crests that pass by a fixed location in a given amount of time). When a wave travels, it is passing through the water, but the water barely travels, rather it moves in a circular motion.

Wave Formation

Surface waves.

Waves on the ocean surface are usually formed by wind. When wind blows, it transfers the energy through friction. The faster the wind, the longer it blows, or the farther it can blow uninterrupted, the bigger the waves. Therefore, a wave's size depends on wind speed, wind duration, and the area over which the wind is blowing (the fetch). This variability leads to waves of all shapes and sizes. The smallest categories of waves are ripples, growing less than one foot (.3 m) high. The largest waves occur where there are big expanses of open water that wind can affect. Places famous for big waves include Waimea Bay in Hawaii, Jaws in Maui, Mavericks in California, Mullaghmore Head in Ireland, and Teahupoo in Tahiti. These large wave sites attract surfers, although occasionally, waves get just too big to surf. Some of the biggest waves are generated by storms like hurricanes. In 2004, Hurricane Ivan created waves that averaged around 60 feet (18 meters) high and the largest were almost 100 feet (30.5 meters) high. In 2019, hurricane Dorian also created a wave over 100 feet high in the northern Atlantic.

Giant waves don’t just occur near land. ‘Rogue waves, ' which can form during storms, are especially big—there are reports of 112 foot (34 m) and 70 foot (21 m) rogue waves —and can be extremely unpredictable. To sailors, they look like walls of water. No one knows for sure what causes a rogue wave to appear, but some scientists think that they tend to form when different ocean swells reinforce one another. Many of the largest rogue waves recorded have been in the North Sea in the North Atlantic Ocean. One was recorded by a buoy in 2013 and measured 62.3 feet (19 m) and another nicknamed the Draupner wave was a massive wall of water 84 feet (25.6 m) high that crossed a natural gas platform on New Year's Eve, 1995.

Tsunami Waves

A classic tsunami wave occurs when the tectonic plates beneath the ocean slip during an earthquake. The physical shift of the plates force water up and above the average sea level by a few meters. This then gets transferred into horizontal energy across the ocean’s surface. From a single tectonic plate slip, waves radiate outwards in all directions moving away from the earthquake.  

When a tsunami reaches shore, it begins to slow dramatically from contact with the bottom of the seafloor. As the leading part of the wave begins to slow, the remaining wave piles up behind it, causing the height of the wave to increase. Though tsunami waves are only a few feet to several meters high as they travel over the deep ocean, it is their speed and long wavelength that cause the change to dramatic heights when they are forced to slow at the shore.

Tsunami waves are capable of destroying seaside communities with wave heights that sometimes surpass around 66ft (20 m) . Tsunamis have caused over 420,000 deaths since 1850—over 230,000 people were killed by the giant earthquake off Indonesia in 2004, and the damage caused to the Fukushima nuclear reactor in Japan by a tsunami in 2011 continues to wreak havoc. Although tsunamis cannot be predicted in advance when an earthquake occurs, tsunami warnings are broadcast and any waves can be tracked by a global network of buoys – this early warning system is essential because tsunamis can travel at over 400 miles per hour (644 km/hr). The highest tsunami wave reached about 1,720 ft (524 m), a product of a massive earthquake and rockslide. When the wave hit shore, it was said to destroy everything.

There are also other, usually less destructive tsunami waves caused by weather systems called meteotsunamis.  These tsunami waves have similar characteristics to the classical earthquake driven tsunamis described above, however they are typically much smaller and focused along smaller regions of the oceans or even Great Lakes.  Meteotsunamis are often caused by fast moving storm systems and have been measured in several cases at over 6 feet (2 meters) high.  A 2019 study found that smaller meteotsunami waves strike the east coast of the U.S. more than twenty times a year!

Tides are actually waves, the biggest waves on the planet, and they cause the sea to rise and fall along the shore around the world. Tides exist thanks to the gravitational pull of the Moon and the Sun, but vary depending on where the Moon and Sun are in relation to the ocean as Earth rotates on its axis. The Moon, being so much closer to Earth, has more power to pull the tides than the Sun and therefore is the primary force creating the tides.

What Causes the Tides?

The Moon’s gravitational pull causes water to bulge on both the side of Earth closest to the Moon and on the opposite side of the planet. The Moon’s gravity has a stronger pull on the side of Earth that is closest to it, which makes the ocean bulge on that side, while on the opposite side of the planet the centrifugal force created by the Moon and Earth orbiting around one another pulls the ocean water out. Centrifugal force is the same force that smooshes riders to the outside walls of spinning carnival rides.

Meanwhile, Earth continues to spin. As Earth rotates, the water bulges stay in line with the Moon while the planet’s surface moves underneath it. A specific point on the planet will pass through both of the bulges and both of the valleys. When a specific place is in the location of a bulge it experiences a high tide. When a specific place is in the location of a valley it experiences a low tide. During one planetary rotation (or one day) a specific location will pass through both bulges and both valleys, and this is why we have two high tides and two low tides in a day. But, while Earth takes 24 hours to complete one rotation, it must then rotate an additional and 50 minutes to catch up with the orbiting Moon. This is why the time of high tide and the time of low tide change slightly every day.

The Sun also has a part to play in causing the tides, and its location in relation to the Moon alters the strength of the pull on the ocean. When the Sun and Moon are in line with one another they reinforce each other’s gravitational pulls and create larger-than-normal tides called spring tides. This happens when the Moon is either on the same side of Earth as the Sun or directly on the opposite side of Earth. Smaller-than-usual tidal ranges, called neap tides,  occur when the gravitational force of the Sun is at a right angle to the pull from the Moon. The two forces of the Sun and Moon cancel each other out and create a neap tide.

Continental Interference

If Earth were a sphere covered by water, only the water would be able to move freely over the planet’s surface and the two tides in a day at each location would be more or less the same. But continents obstruct the flow of water, causing this seemingly simple daily cycle to be a bit more complicated. Because of continental obstruction, some locations experience two tides a day that are more or less the same height (known as semidiurnal tides), some locations experience one tide at one height and the second at a different height (mixed semidiurnal tides), and some locations have so much interference from land that they only experience one high tide and one low tide per day (diurnal tides). 

The local geography can also affect the way the tides behave in a location. Shores around coastal islands and inlets may experience delayed tides compared to smoother surrounding coasts since the water must funnel in through constrained waterways.

Tides and Nature

The intertidal zone , the coastal area tides submerge for part of the day, is home to many ocean creatures. It takes a special set of adaptations to live a life half the time scorched by the Sun and the other submerged underwater. Moreover, the incoming tide promises a constant pounding by ocean waves. Despite this, it’s a place where species thrive. Shelled mollusks like periwinkles, muscles, and barnacles cling to rocks, sea stars wedge themselves in crevices, and crabs hide in fronds of algae.

A red tide is not a true tide at all but rather a term used to describe the red color of an algal bloom. Algae are integral to ocean systems, but when they are supplied with excessive amounts of nutrients they can explode in number and smother other organisms. The algae may produce toxins or they can die, decay, and the bacteria decomposing them take up all the oxygen. This massive growth of algae can become harmful to both the environment and humans, which is why scientists often refer to them as harmful algal blooms or HABs.

Monitoring Tides

Tidal movements are tracked using networks of nearshore water level gauges, and many countries provide real-time information with tidal listings and tidal charts. Tides can be tracked at specific locations in order to predict the height of a tide, i.e. when low and high tide will occur in the future. The Bay of Fundy in Nova Scotia, Canada has the highest tidal range of any place on the planet. The tides there range from 11 feet (3.5 m) to 53 feet (16 m) and cause erosion, creating massive cliffs. This erosion also releases nutrients into the water that help support marine life. The currents associated with the tides are called flood currents (incoming tide) and ebb currents (outgoing tide). Having reliable knowledge about the tides and tidal currents is important for navigating ships safely, and for engineering projects such as tidal and wave energy , as well as for planning trips to the seashore.

Websites: NOAA Tides and Currents USGS Life of a Tsunami UCAR Center for Science Education Thermohaline Circulation

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8.4: Ocean Currents

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Ocean water is constantly in motion (Figure 14.7). From north to south, east to west, and up and down the shore, ocean water moves all over the place. These movements can be explained as the result of many separate forces, including local conditions of wind, water, the position of the moon and Sun, the rotation of the Earth, and the position of land formations.

Figure 14.7 : Ocean waves transfer energy through the water over great distances.

Lesson Objectives

• Describe how surface currents form and how they affect the world’s climate.
• Describe the causes of deep currents.
• Relate upwelling areas to their impact on the food chain.

Surface Currents

Wind that blows over the ocean water creates waves. It also creates surface currents , which are horizontal streams of water that can flow for thousands of kilometers and can reach depths of hundreds of meters. Surface currents are an important factor in the ocean because they are a major factor in determining climate around the globe.

Causes of Surface Currents

Currents on the surface are determined by three major factors: the major overall global wind patterns, the rotation of the Earth, and the shape of ocean basins.

When you blow across a cup of hot chocolate, you create tiny ripples on its surface that continue to move after you’ve stopped blowing. The ripples in the cup are tiny waves, just like the waves that wind forms on the ocean surface. The movement of hot chocolate throughout the cup forms a stream or current, just as oceanic water moves when wind blows across it.

But what makes the wind start to blow? When sunshine heats up air, the air expands, which means the density of the air decreases and it becomes lighter. Like a balloon, the light warm air floats upward, leaving a slight vacuum below, which pulls in cooler, denser air from the sides. The cooler air coming into the space left by the warm air is wind.

Because the Earth’s equator is warmed by the most direct rays of the Sun, air at the equator is hotter than air further north or south. This hotter air rises up at the equator and as colder air moves in to take its place, winds begin to blow and push the ocean into waves and currents.

Wind is not the only factor that affects ocean currents. The ‘Coriolis Effect’ describes how Earth’s rotation steers winds and surface currents (Figure 14.14). The Earth is a sphere that spins on its axis in a counterclockwise direction when seen from the North Pole. The further towards one of the poles you move from the equator, the shorter the distance around the Earth. This means that objects on the equator move faster than objects further from the equator. While wind or an ocean current moves, the Earth is spinning underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line. Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west. The Coriolis Effect bends the direction of surface currents.

The third major factor that determines the direction of surface currents is the shape of ocean basins (Figure 14.15). When a surface current collides with land, it changes the direction of the currents. Imagine pushing the water in a bathtub towards the end of the tub. When the water reaches the edge, it has to change direction.

Figure 14.15 : This map shows the major surface currents at sea. Currents are created by wind, and their directions are determined by the Coriolis effect and the shape of ocean basins.

Effect on Global Climate

Surface currents play a large role in determining climate. These currents bring warm water from the equator to cooler parts of the ocean; they transfer heat energy. Let’s take the Gulf Stream as an example; you can find the Gulf Stream in the North Atlantic Ocean in Figure 14.15. The Gulf Stream is an ocean current that transports warm water from the equator past the east coast of North America and across the Atlantic to Europe. The volume of water it transports is more than 25 times that of all of the rivers in the world combined, and the energy it transfers is more than 100 times the world’s energy demand. It is about 160 kilometers wide and about a kilometer deep. The Gulf Stream’s warm waters give Europe a much warmer climate than other places at the same latitude. If the Gulf Stream were severely disrupted, temperatures would plunge in Europe.

Deep Currents

Surface currents occur close to the surface of the ocean and mostly affect the photic zone. Deep within the ocean, equally important currents exist that are called deep currents . These currents are not created by wind, but instead by differences in density of masses of water. Density is the amount of mass in a given volume. For example, if you take two full one liter bottles of liquid, one might weigh more, that is it would have greater mass than the other. Because the bottles are both of equal volume, the liquid in the heavier bottle is denser. If you put the two liquids together, the one with greater density would sink and the one with lower density would rise.

Two major factors determine the density of ocean water: salinity (the amount of salt dissolved in the water) and temperature (Figure 14.16). The more salt that is dissolved in the water, the greater its density will be. Temperature also affects density: the colder the temperature, the greater the density. This is because temperature affects volume but not mass. Colder water takes up less space than warmer water (except when it freezes). So, cold water has greater density than warm water.

Figure 14.16 : Thermohaline currents are created by differences in density due to temperature (thermo) and salinity (haline). The blue arrows are deep currents and the red ones are surface currents.

Figure 14.17 : Surface and deep currents together form convection currents that circulate water from one place to another and back again. A water particle in the convection cycle can take 1600 years to complete the cycle.

More dense water masses will sink towards the ocean floor. Just like convection in air, when denser water sinks, its space is filled by less dense water moving in. This creates convection currents that move enormous amounts of water in the depths of the ocean. Why is the water temperature cooler in some places? Water cools as it moves from the equator to the poles via surface currents. Cooler water is more dense so it begins to sink. As a result, the surface currents and the deep currents are linked. Wind causes surface currents to transport water around the oceans, while density differences cause deep currents to return that water back around the globe (Figure 14.17).

As you have seen, water that has greater density usually sinks to the bottom. However, in the right conditions, this process can be reversed. Denser water from the deep ocean can come up to the surface in an upwelling (Figure 14.18). Generally, an upwelling occurs along the coast when wind blows water strongly away from the shore. As the surface water is blown away from the shore, colder water from below comes up to take its place. This is an important process in places like California, South America, South Africa, and the Arabian Sea because the nutrients brought up from the deep ocean water support the growth of plankton which, in turn, supports other members in the ecosystem. Upwelling also takes place along the equator between the North and South Equatorial Currents.

Figure 14.18 : An upwelling forces denser water from below to take the place of less dense water at the surface that is pushed away by the wind.

Lesson Summary

• Ocean waves are energy traveling through the water.
• The highest portion of a wave is the crest and the lowest is the trough.
• The horizontal distance between two wave crests is the wave’s length.
• Most waves in the ocean are wind generated waves.
• Ocean surface currents are produced by major overall patterns of atmospheric circulation, the Coriolis Effect and the shape of each ocean basin.
• Ocean surface circulation brings warm equatorial waters towards the poles and cooler polar water towards the equator.
• Deep ocean circulation is density driven circulation produced by differences in salinity and temperature of water masses.
• Upwelling areas are biologically important areas that form as ocean surface waters are blown away from a shore, causing cold, nutrient rich waters to rise to the surface.

Review Questions

• What factors of wind determine the size of a wave?
• Define the crest and trough of a wave.
• What is the most significant cause of the surface currents in the ocean?
• How do ocean surface currents affect climate?
• What is the Coriolis Effect?
• Some scientists have hypothesized that if enough ice in Greenland melts, the Gulf Stream might be shut down. Without the Gulf Stream to bring warm water northward, Europe would become much colder. Explain why melting ice in Greenland might affect the Gulf Stream.
• What process can make denser water rise to the top?
• Why are upwelling areas important to marine life?
• Provided by : Wikibooks. Located at : http://en.wikibooks.org/wiki/High_School_Earth_Science/Ocean_Movements . License : CC BY-SA: Attribution-ShareAlike

Understanding Global Change

Discover why the climate and environment changes, your place in the Earth system, and paths to a resilient future.

Ocean circulation

The ocean covers 71% of Earth’s surface and is constantly in motion. Large masses of water that move together, called ocean currents, transport heat , marine organisms, nutrients , dissolved gasses such as carbon dioxide and oxygen , and pollutants all over the world.  Climate and ecosystems everywhere on Earth, even those far from the ocean, are affected by the ocean circulation.

On this page:

What is ocean circulation, earth system models about ocean circulation, how human activities influence ocean circulation, explore the earth system, investigate, links to learn more.

For the classroom:

• Teaching Resources

Global Change Infographic

Ocean circulation is an essential part of How the Earth System Works.  Click the image on the left to open the Understanding Global Change Infographic . Locate the ocean circulation icon and identify other Earth system processes and phenomena that cause changes to, or are affected by, ocean circulation.

Graphic courtesy of SAGE

Ocean circulation patterns, the movement of large masses of water both at and below the surface, are determined by atmospheric circulation patterns, variation in the amount of sunlight absorbed with latitude, and the water cycle . Surface currents, also called horizontal currents, are primarily the result of wind pushing on the surface of the water, and the direction and extent of their movement is determined by the distribution of continents . Currents, like winds in the atmosphere, do not move in straight lines because of the spin of the Earth, which causes the Coriolis effect.

Currents that move up and down in the water column, also called vertical currents, are created by differences in the density of water masses, where heavier waters sink and lighter waters rise.  This type of ocean circulation is called thermohaline circulation (therme=heat, halos=salt) because the vertical movement is caused by differences in temperature and salinity (the amount of salt in water).  Adding heat decreases the density of water, while adding salt increases the density of water. Thermohaline circulation occurs because winds move warm surface waters from the equator towards the poles, where the water cools and increases in density. Some of this water gets so cold that it freezes, leaving its salt behind in the remaining water, further increasing the density of this water.  This cold, salty water near the poles (primarily in the North Atlantic and near Antarctica) sinks and spreads along the bottom and eventually rises back towards the surface of the ocean. It takes about 1000 years for water to circulate around what is called the global conveyor belt that moves water three dimensionally throughout the world’s ocean basins.

Graphic courtesy of NASA/JPL

This model shows some of the cause and effect relationships among components of the Earth system related to ocean circulation. While this model does not depict the ocean circulation patterns that results from atmospheric wind and density differences in water masses, it summarizes the key concepts involved in explaining this process. Hover over the icons for brief explanations; click on the icons to learn more about each topic. Download the Earth system models on this page.

The model below shows some of the additional phenomena that ocean circulation patterns affect. Ocean circulation is such an important process in the Earth system because currents transport heat , oxygen , nutrients , and living organisms . Most of the sunlight absorbed by water on Earth’s surface gets stored in our oceans as heat, and heat from the atmosphere is also absorbed by the ocean, which increases the ocean’s temperature. This heat is then transported by currents and re-radiated, influencing regional air temperatures and climates all over the globe. For example, the Gulf Stream in the Atlantic Ocean brings heat from near the equator to Europe, making it much warmer than other areas at similar latitudes. Hover over the icons for brief explanations; click on the icons to learn more about each topic.

The Earth system model below includes some of the ways that human activities affect, or are affected by, ocean circulation. As the world warms due to increased levels of greenhouse gases in the atmosphere from human activities, changes in ocean and atmospheric circulation patterns will alter regional climate and ecosystems around the globe. Hover over or click on the icons to learn more about these human causes of change and how they influence, or are influenced by, ocean circulation.

Click the icons and bolded terms (e.g. nutrient levels, atmospheric circulation, etc.) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.

To learn more about teaching ocean circulation, visit the Teaching Resources page.

Learn more in these real-world examples, and challenge yourself to construct a model that explains the Earth system relationships.

• Global change drove the evolution of giants
• NOAA: Ocean currents
• NOAA: How does the ocean affect climate and weather on land?
• UCAR: Ocean on the move: Thermohaline circulation
• NASA: Thermohaline Circulation
• NOAA: What is upwelling?

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Keep exploring

Find even more resources on ocean currents  in our searchable resource database.

Ocean water is on the move, affecting your climate, your local ecosystem, and the seafood that you eat. Ocean currents , abiotic features of the environment, are continuous and directed movements of ocean water. These currents are on the ocean’s surface and in its depths, flowing both locally and globally.

Map of temperature of the North Atlantic Ocean shows the warm Gulf Stream current along the East Coast of the United States transporting heat northward towards the cooler high latitudes. (Image credit: NOAA)

Winds, water density, and tides all drive ocean currents. Coastal and sea floor features influence their location, direction, and speed. Earth’s rotation results in the Coriolis effect which also influences ocean currents. Similar to a person trying to walk in a straight line across a spinning merry-go-round, winds and ocean waters get deflected from a straight line path as they travel across the rotating Earth. This phenomenon causes ocean currents in the Northern Hemisphere to veer to the right and in the Southern Hemisphere to the left.

Drifters, buoys, Argo floats and more help scientists monitor the global ocean, including areas that are difficult to travel to via research ship.

Surface currents

Differences in water density, resulting from the variability of water temperature ( thermo ) and salinity ( haline ), also cause ocean currents. This process is known as thermohaline circulation. In cold regions, such as the North Atlantic Ocean, ocean water loses heat to the atmosphere and becomes cold and dense. When ocean water freezes, forming sea ice, salt is left behind causing surrounding seawater to become saltier and denser. Dense-cold-salty water sinks to the ocean bottom. Surface water flows in to replace the sinking water, which in turn becomes cold and salty enough to sink. This "starts" the  global conveyer belt , a connected system of deep and surface currents that circulate around the globe on a 1000 year time span. This global set of ocean currents is a critical part of Earth’s climate system as well as the ocean nutrient and carbon dioxide cycles.

Biological influence

Ocean currents are an important abiotic factor that significantly influences food webs and reproduction of marine organisms and the marine ecosystems that they inhabit. Many species with limited mobility are dependent on this "liquid wind" to bring food and nutrients to them and to distribute larvae and reproductive cells. Even fish and mammals living in the ocean may have their destinations and food supply affected by currents.

Upwelling currents bring cold nutrient-rich waters from the ocean bottom to the surface, supporting many of the most important fisheries and ecosystems in the world. These currents support the growth of phytoplankton and seaweed which provide the energy base for consumers higher in the food chain, including fish, marine mammals, and humans.

EDUCATION CONNECTION

Educators can use ocean currents to help students learn and appreciate the interaction of Earth's systems and how scientists study these processes with drifting buoys,  sound monitors , and other methods. The lesson plans, labs, and other resources in this collection can help students understand how distant abiotic factors, such as water density, Earth’s rotation, and ocean currents can impact local climate and biomes, the beaches we visit, and the seafood that we eat.

Ocean Currents Map

Why are ocean currents important, types of ocean currents.

Ocean currents are driven by a variety of factors, including tides, winds, and changes in water density. These factors work together to create a complex system that has a significant impact on our weather, marine travel, and oceanic ecosystems. Tides, which are caused by the gravitational pull of the moon and the sun, play a role in driving ocean currents. The rising and falling of tides create a rhythmic movement of water, contributing to the flow of currents. Winds also have a strong influence on ocean currents. Global wind systems, driven by the uneven heating of the Earth's surface, transfer heat from the tropics to the polar regions. This heat transfer creates pressure differences in the atmosphere, which in turn generate winds. These winds, known as surface winds, push the surface waters of the ocean, creating surface currents. In addition to tides and winds, changes in water density contribute to the formation of ocean currents. Variations in temperature and salinity, both of which affect water density, play a crucial role. This process, known as thermohaline circulation, drives deep ocean currents. In cold regions like the North Atlantic Ocean, differences in water density caused by variations in temperature and salinity are particularly important. It is important to note that ocean currents are not solely influenced by abiotic factors. Biological factors also come into play. The distribution of food and nutrients in the ocean can be influenced by ocean currents, which in turn affects marine ecosystems. In conclusion, the driving forces behind ocean currents are diverse and interconnected. Tides, winds, and changes in water density all contribute to the complex system of currents that shape our planet's climate system and support marine ecosystems.

Surface Currents

Deep ocean currents, tidal currents.

Ocean Currents Map PDF

The Great Pacific Garbage Patch

This collection of litter (composed mostly of tiny pieces of plastic) is located in the north pacific. the trash is collecting in the calm center of the north pacific subtropical gyre. a gyre is a large system of swirling ocean currents. the north pacific subtropical gyre is made of four separate currents: the california current, the north equatorial current, the kuroshio current, and the north pacific current. these four currents are moving large amounts of trash towards the great pacific garbage patch — helping it grow ever larger., to understand ocean currents, it's best to start with understanding the waves. and how it plays a large role in creating energy..

The five major oceans wide gyres are the North Atlantic, South Atlantic North Pacific South Pacific, Indian Ocean, Ocean gyres and world map pacific of plastic pollution. The currents we see at the beach are called coastal currents that can affect land and wave formations. Currents travel around 5.6 miles per hour in warmer waters of the northern hemisphere and in the North Pacific moves much slower in cold water at 0.03 to 0.06 miles per hour.

Connected One World Ocean

Currents & marine organisms.

Ocean currents exist both on and below the surface. Some currents are local to specific areas, while others are global. And they move a lot of water. The largest current in the world, the Antarctic Circumpolar Current, is estimated to be 100 times larger than all the water flowing in all the world’s rivers! All of this moving water helps more stationary species get the food and nutrients they need. Instead of going looking for food, these creatures wait for the currents to bring a fresh supply to them. Currents also play a major role in reproduction. The currents spread larvae and other reproductive cells. Without currents many of the ocean’s ecosystems would collapse.

What Can You Do?

Cleaning up the Great Pacific Garbage Patch is a challenge. It is not close to any coastline, which means no one country or organization has stepped up to take responsibility for its cleanup. However, many ocean conservation organizations, such as Ocean Blue Project, one of the best Ocean cleanup organizations removing 1 million pounds of plastic by 2025. Help save our blue economy by making a one time donation to help remove plastic pollution from a beach near you.  

The best way to support this effort — reduce your use of single-use plastics. If less plastic is being used, then less of it will end up in our oceans.

What are the Five Oceans of the World?

"the five bodies of water and the global ocean produces more then half oxygen humans breath.", historically the ocean was thought of having 4 oceans the pacific, atlantic, indian, and arctic. today we have five bodies of water and our one world ocean or five oceans aka ocean 5, and two seas covering over 71 percent of the earths surface and over 97 percent of the earth’s water. only 1% of earths water is freshwater and percent or two is part of our ice glaciers. with sea level rise just think of our ice melting and how a percent of earth would so be under water. the oceans of the world host over 230,000 marine animals species and more could be discovered as humans learn ways to explore the deepest sections of the ocean. we all share the same ocean our one world ocean, learn more about how we can protect the microplastics that are harming fish and how we can support the ocean cleanup..

The Southern Ocean also known as the antarctic area.

The Antarctic ocean is the smallest of our oceans and the fourth largest and is full of wildlife and mountains of ice lastly throughout the year. Although this area is so cold humans have managed to live here. One of the largest setbacks is with global warming most of the ice mountains is expected to melt by 2040. The depth of The Antarctic Ocean is 23,740′ in depth. The Southern Ocean also known as the Antarctic Area: 7.849 million mi².  How many people live in the Antarctic? No humans  live in Antarctica  permanently, but around 1,000 to 5,000  people live  through the year at the science stations in  Antarctica . The only plants and animals that can  live  in cold  live  there. The animals include penguins, seals, nematodes, tardigrades and mites.

Fun facts: Between Africa and Austral

Indian Ocean is located between Africa and Austral-Asia and the Southern Ocean. is the third largest of our oceans and covers a fifth ( 20%) of our earths surface. Until the mid 1800s the Indian Ocean was called the Eastern Oceans. The Indian Ocean is around 5.5 times the size of United States and is a warm body of water depending on the Ocean Currents of the Equator to help stabilize the temperatures. 

Atlantic Ocean ​boards North America, Africa, South America, and Europe. This Ocean is the second largest of our five oceans and home of the largest islands in the world. The Atlantic Ocean covers 1/5 of the earths surface and 29% of the waters surface area.

The  Atlantic Ocean  ranks the second for the most  dangerous ocean  waters in the world. This  ocean  water is usually affected by coastal winds, temperature of the water surface currents maps. 

6 Types of Plants That Live in the Atlantic Ocean

• Kelp. Kelp grows in cold coastal waters. …
• Seagrass. …
• Red Algae. …
• Coral and Algae. …
• Coralline Algae.

Pacific Ocean Temperatures or conditions are split:  cold  in east, and warmer in west. In Oregon the body of water is average 54 degrees. Winter has huge Oregon King Tides leaving the norther waters super rough seas. 

Fun Facts For Youth: Atolls are in the warmer conditions of the Pacific Ocean and are the Coral Sea Islands West of the Barrier Reef in Australia. Atolls are only found in the warm ocean waters, located in the southern water bodies of our ocean. 

Ocean Plastic The Pacific Ocean is also the home for the most  micro plastics  floating in our oceans. The plastic are caused by humans littering by accident or just littering. Plastic pollution makes its way to the ocean in many directions by getting into street drains, rivers, blowing in the wind, or from fishing boats. learn about how some animals help lower plastic pollution.

6 Types of Plants That Live in the Pacific Ocean

• Kelp. Kelp grows in cold coastal water bodies.
• Coral and Algae
• Coralline Algae

How do Ocean Currents affect Climate

Ocean currents move warm and cold water, to polar regions and tropical regions influencing both weather and climate and changing the regions temperatures. Learn more about Ocean Blue nonprofit working to remove plastic from our Ocean. 

Ocean currents, also known as continuous and directed movements of ocean water, play a crucial role in shaping our climate, local ecosystems, and even the seafood we enjoy.

These currents are a result of various factors, including tides, winds, and changes in the water’s density. They can be categorized into two types: surface currents and deep ocean currents, which together create a complex system with far-reaching effects on our environment. Surface currents, influenced by tides and winds, occur on the ocean’s surface and have a significant impact on weather patterns and marine travel.

Prevailing Winds – Wind Currents of The World

They can create favorable conditions for sailing or hinder maritime transportation, influencing trade routes and travel times.

These currents also have a direct influence on coastal ecosystems, affecting the distribution of nutrients and the migration patterns of marine species.

What Causes Deep Ocean Currents

Deep ocean currents, on the other hand, are driven by changes in water density, caused by variations in temperature and salinity. These currents flow in the depths of the ocean, and their slow but steady movement plays a critical role in regulating Earth’s climate.

They help distribute heat around the globe, influencing regional and global temperature patterns. Deep ocean currents also play a crucial role in the transport of nutrients and oxygen to deep-sea ecosystems, supporting a diverse array of marine life. It is important to note that ocean currents are not solely influenced by natural factors.

Coastal and sea floor features, such as underwater mountains or canyons, can alter the direction, speed, and location of these currents.

Additionally, the Coriolis effect, a result of Earth’s rotation, also contributes to the complex movement of ocean currents. In summary, ocean currents are dynamic and intricate systems that are driven by tides, winds, water density, and influenced by coastal and sea floor features.

How Does The Ocean Affect Climate and Weather on Land

Their impact extends beyond the surface of the ocean, affecting weather patterns, marine travel, and the delicate balance of marine ecosystems. Understanding these currents is crucial for comprehending the interconnectedness of our planet’s climate and ecosystems.

Why Is The Ocean Blue

Clean water is blue because water absorbs and reflects the blue sky as light bounces red light, red orange yellow, light spectrum of reflections of light as a significant to lowering sediments as for taking care of our wild rivers protective sediments runoff destroying our ocean., clean water is blue because water absorbs and reflects the blue sky as light bounces red light, red orange yellow, light spectrum of reflections of light as a significant to lowering sediments as for taking care of our wild rivers protective sediments runoff destroying our ocean. ocean blue feels beach cleanups conjointly facilitate the long wavelength of the blue color by lowering floating ocean plastics have to be compelled to facilitate keep our ocean blue by protecting clean water. because the ocean absorbs the red yellowness wavelength of light as the aspect of the white lightweight you’ll usually see a glimpse of reminder red etc once viewing the blue ocean reflections we tend to see most frequently. the blue color lower floating sediments that may lower the short wavelengths of lightweight of sunshine spectrum that permits our ocean blue wavelengths reflections of sunshine to be the blue light color. therefore removing plastic floating in our ocean helps permit blue ocean water and our water molecules of safe of blue water., ways to contribute to the ocean.

Ocean Activities for Kindergarten to 2nd Grade

Save the Whales Graphic Hoody

Save the Whale Graphic T-Shirt

Save The Ocean Shirt Microplastics Bird T-shirt (Unisex)

Eco Friendly Reusable Water Bottle

Ocean Themed Clothing Save The Ocean Birds Shirt (Unisex)

Seastar Save The Ocean T-Shirt (Unisex)

Sea the Change Microplastic Turtle Graphic T-Shirt (Unisex)

Marine Biology for Kids Science Kit — Marine Science Ocean Debris STEM Kit

Pride for the Ocean Trucker Hat

Ocean Blue Logo Trucker Hat (Adult)

STEM Activities for Preschoolers

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The Role of Ocean Currents in Climate

ThinkTV, Teachers' Domain

This video segment uses data-based visual NOAA representations to trace the path of surface ocean currents around the globe and explore their role in creating climate zones. Ocean surface currents have a major impact on regional climate around the world, bringing coastal fog to San Francisco and comfortable temperatures to the British Isles.

Notes from our reviewers

The CLEAN collection is hand-picked and rigorously reviewed for scientific accuracy and classroom effectiveness. Read what our review team had to say about this resource below or learn more about how CLEAN reviews teaching materials .

• Teaching Tips This video provides some useful visuals for a unit on global ocean currents. There is a very nice research project in the teaching tips section asking students to research a coastal city to determine how ocean currents determine its climate.
• About the Science Video focuses on role of surface ocean currents in global climate. Comment from expert scientist: Asks students to think on both global and local scales. Includes (though necessarily briefly) both major processes that move ocean water: wind forcing and density. Relates the oceans to climate processes on large scales (e.g. heat uptake by the ocean) and smaller scales (e.g. El Niño, local coastal climates).
• About the Pedagogy This video includes a background essay and teaching tips. Links to other videos and resources are included, making it easy to build a more comprehensive unit on ocean currents.
• Technical Details/Ease of Use The quality of the streaming video is likely not suitable for classroom projection. The download versions are of higher quality.

Ocean Currents: The Driving Forces and Implications for Earth’s Climate | Sociology UPSC | Triumph IAS

Table of Contents

Ocean Currents

(relevant for geography section of general   studies paper prelims/mains).

Ocean currents represent the perpetual flow of water within the ocean, akin to aquatic counterparts of rivers. These currents adhere to established trajectories, mirroring the presence of rivers within the oceanic expanse. Oceanic currents are classified into two well-defined systems—the surface circulation, which agitates the upper stratum of the ocean, and the deep circulation, which courses through the depths of the sea floor.

Ocean Currents and Their Causes:

Ocean currents are instigated by a confluence of factors, including wind patterns, disparities in water density due to temperature and salinity discrepancies, gravitational forces, and occurrences like earthquakes or storms.

• Influence of Gravity: Surface currents within the ocean are primarily driven by global wind systems powered by solar energy. Additionally, Coriolis forces resulting from the Earth’s rotation intersect with these currents. Role of
• Planetary Winds: The arrangement of surface currents is dictated by the prevailing wind directions. The interaction of surface wind-driven currents with geographic features also produces upwelling currents, which then give rise to deepwater currents.
• Density Variation: Another determinant of ocean currents is the variance in water density due to temperature and salinity differences, leading to thermohaline circulation. This mechanism propels water masses through the ocean’s depths, carrying essential elements like nutrients, oxygen, and heat.
• Unforeseen Events: Geophysical incidents such as powerful storms and underwater earthquakes possess the capacity to initiate significant ocean currents , causing the movement of water masses toward shorelines and shallower areas. Earthquakes may also trigger rapid downslope shifts of water-soaked sediments, creating potent turbidity currents.
• Influential Topography: When a wide-ranging current is funnelled into a confined space, its strength can intensify markedly. On the ocean floor, water masses channelled through narrow gaps in ridge systems or around seamounts can give rise to currents of greater magnitude than those found in the surrounding waters. This phenomenon has implications for the distribution and abundance of marine organisms and for the endeavours of scientists studying them, along with their equipment.

The ocean currents act as the global conveyor belt and thus play a dominant role in determining the climate of many of Earth’s regions.

Sample Question for UPSC Sociology Optional Paper:

Q 1: “How do ocean currents impact the social and economic activities of coastal communities?” Answer: Ocean currents influence fish migration patterns, which directly affect the livelihoods of fishermen. They also play a role in the climate of coastal regions, thereby affecting agriculture and tourism.

Q 2: “What role do ocean currents play in global inequality?” Answer: Ocean currents can have varying effects on different regions, influencing climate and therefore agricultural productivity, which can contribute to economic disparities between nations.

Q 3: “Discuss the sociological implications of thermohaline circulation.” Answer: Thermohaline circulation affects global climate, which in turn impacts food security, migration patterns, and economic stability, thereby having broader sociological implications.

Related Blogs …

To master these intricacies and fare well in the Sociology Optional Syllabus , aspiring sociologists might benefit from guidance by the Best Sociology Optional Teacher and participation in the Best Sociology Optional Coaching . These avenues provide comprehensive assistance, ensuring a solid understanding of sociology’s diverse methodologies and techniques.

Ocean currents, surface currents, deep circulation, thermohaline circulation, wind patterns, water density, temperature, salinity, gravitational forces, climate change, marine ecology, Coriolis forces, Ocean currrents

Choose T he Best Sociology Optional Teacher for IAS Preparation?

At the beginning of the journey for Civil Services Examination preparation, many students face a pivotal decision – selecting their optional subject. Questions such as “ which optional subject is the best? ” and “ which optional subject is the most scoring? ” frequently come to mind. Choosing the right optional subject, like choosing the best sociology optional teacher , is a subjective yet vital step that requires a thoughtful decision based on facts. A misstep in this crucial decision can indeed prove disastrous.

Ever since the exam pattern was revamped in 2013, the UPSC has eliminated the need for a second optional subject. Now, candidates have to choose only one optional subject for the UPSC Mains , which has two papers of 250 marks each. One of the compelling choices for many has been the sociology optional. However, it’s strongly advised to decide on your optional subject for mains well ahead of time to get sufficient time to complete the syllabus. After all, most students score similarly in General Studies Papers; it’s the score in the optional subject & essay that contributes significantly to the final selection.

“ A sound strategy does not rely solely on the popular Opinion of toppers or famous YouTubers cum teachers. ”

It requires understanding one’s ability, interest, and the relevance of the subject, not just for the exam but also for life in general. Hence, when selecting the best sociology teacher, one must consider the usefulness of sociology optional coaching in General Studies, Essay, and Personality Test.

The choice of the optional subject should be based on objective criteria, such as the nature, scope, and size of the syllabus, uniformity and stability in the question pattern, relevance of the syllabic content in daily life in society, and the availability of study material and guidance. For example, choosing the best sociology optional coaching can ensure access to top-quality study materials and experienced teachers. Always remember, the approach of the UPSC optional subject differs from your academic studies of subjects. Therefore, before settling for sociology optional , you need to analyze the syllabus, previous years’ pattern, subject requirements (be it ideal, visionary, numerical, conceptual theoretical), and your comfort level with the subject.

This decision marks a critical point in your UPSC – CSE journey , potentially determining your success in a career in IAS/Civil Services. Therefore, it’s crucial to choose wisely, whether it’s the optional subject or the best sociology optional teacher . Always base your decision on accurate facts, and never let your emotional biases guide your choices. After all, the search for the best sociology optional coaching is about finding the perfect fit for your unique academic needs and aspirations.

To master these intricacies and fare well in the Sociology Optional Syllabus , aspiring sociologists might benefit from guidance by the Best Sociology Optional Teacher and participation in the Best Sociology Optional Coaching . These avenues provide comprehensive assistance, ensuring a solid understanding of sociology’s diverse methodologies and techniques. Sociology, Social theory, Best Sociology Optional Teacher, Best Sociology Optional Coaching, Sociology Optional Syllabus. Best Sociology Optional Teacher, Sociology Syllabus, Sociology Optional, Sociology Optional Coaching, Best Sociology Optional Coaching, Best Sociology Teacher, Sociology Course, Sociology Teacher, Sociology Foundation, Sociology Foundation Course, Sociology Optional UPSC, Sociology for IAS,

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Ocean Currents

What is Ocean Current? It is a horizontal movement of seawater that is produced by gravity, wind, and water density. Ocean currents play an important role in the determination of climates of coastal regions.

As an important topic of Geography (Oceanography), questions could be framed either in Prelims or Mains (GS 1) papers of the IAS Exam .

Candidates can check previous years’ Geography Questions in UPSC Prelims in the linked article.

Understand the basics of ocean currents in this article.

Ocean Water and Ocean Currents

The movement of ocean water is continuous. This movement of ocean water is broadly categorized into three types:

The streams of water that flow constantly on the ocean surface in definite directions are called ocean currents.

Ocean currents are one of the factors that affect the temperature of ocean water.

• Warm ocean currents raise the temperature in cold areas
• Cold ocean currents decrease the temperature in warmer areas.

To understand the ocean waves and related concepts, check the links below:

Relevant Facts about Ocean Currents for UPSC

• The magnitude of the ocean currents ranges from a few centimetres per second to as much as 4 metres (about 13 feet) per second.
• The intensity of the ocean currents generally decreases with increasing depth.
• The speed of ocean currents is more than that of upwelling or downwelling which are the vertical movements of ocean water.
• Warm Ocean Currents
• Cold Ocean Currents

What causes ocean currents?

Horizontal pressure-gradient forces, Coriolis forces, and frictional forces are important forces that cause and affect ocean currents. NCERT Notes on Factors Affecting Wind mention Coriolis Force that one can read in the linked article.

Rise and fall of the tide

Tides give rise to tidal currents. Near the shore, tidal currents are the strongest. The change in tidal currents is periodical in nature and can be predicted for the near future. The speed of tidal currents at some places can be around 8 knots or more.

The ocean currents at or near the ocean surface are driven by wind forces.

Thermohaline Circulation

‘Thermo’ stands for temperature and ‘Haline’ stands for salinity. The variations in temperature and salinity at different parts of the oceans create density differences which in turn affect the ocean currents.

What is a Frictional Force?

The movement of water through the oceans is slowed by friction, with surrounding fluid moving at a different velocity. A faster-moving layer of water and a slower-moving layer of water would impact each other. This causes momentum transfer between both layers producing frictional forces.

What are geostrophic currents?

When the pressure gradient force on the ocean current is balanced by the Coriolis forces, it results in the geostrophic currents.

• The direction of geostrophic flow is parallel to an isobar.
• The high pressure is to the right of the flow in the Northern Hemisphere, and the high pressure to the left is found in the Southern Hemisphere.

North and South Equatorial Currents

North Equatorial Current

• North Equatorial Current flows from east to west in the Pacific and the Atlantic Ocean.
• North Equatorial Current flows between the latitudes of 10 degrees and 20 degrees north.
• It is not connected to the equator.
• Equatorial circulation separates this current between the Pacific and Atlantic oceans.

South Equatorial Current

• It flows in the Pacific, Atlantic, and Indian oceans.
• The direction of the south equatorial current is east to west.
• The latitudes in which the current flows are between the equator and 20 degrees south.
• It flows across the equator to 5 degrees north latitudes in the Pacific and Atlantic Oceans.

What is the Equatorial Counter Current?

It is found in the following three oceans:

• Indian Ocean
• Atlantic Ocean
• Pacific Ocean

It is found between north and south equatorial currents at about 3-10 degrees north latitude.

What is Antarctic Circumpolar Current?

The ocean current that flows clockwise around the Antarctic is called the Antarctic Circumpolar Current. It is also called West Wind Drift. It is a feature of ocean circulation of the Southern Ocean.

• It does not have a well-defined axis
• It consists of a series of individual currents which are separated by frontal zones.

What is a Global Conveyor Belt?

A system of ocean currents that helps in the transportation of water around the world is called a global conveyor belt. As per National Geographic, “Along this conveyor belt, heat and nutrients are moved around the world in a leisurely 1000-year cycle.”

Distribution of Ocean Currents

The ocean currents are distributed across five oceans. The list of important ocean-wise currents is given below:

Download the  UPSC syllabus from the linked article as it will help candidates to remain on track while they prepare for any topic.

Frequently Asked Questions on Ocean Currents

Q 1. what is meant by ocean current, q 2. what are tidal currents.

For Geography preparation, check the links below:

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How the Ocean Current Affect Animals’ Life in the Sea Essay

Introduction, ocean currents and marine turtles, nekton’s (fishes), sea urchins, works cited.

An ocean current refers to a continuous water flow in the ocean following a defined path. This occurs either at the surface of the ocean or below the surface, it also may be parallel or vertical to the surface. These currents are either caused by wind or changes in density (Thermohaline currents). Ocean currents affect the climate, temperatures, and biotic systems especially the fisheries but also those plants and animals on the seashore (Gray et al. 1).

Ocean currents affect marine life in different ways some of these include; as water flows along a given path, there are many sea animals along the same path. Depending on the strength of the ocean current, sea animals along the path are flown along with the water, and the animals are moved to new regions that are sometimes thousands of kilometers away causing redistribution of marine life. During the flow, nutrients are also moved from the bottom of the sea and exposed to sunlight in the process called upwelling. This increases marine nutrients leading to the increased nutrient provision to marine life.

Ocean currents sometimes cause the movement of warmer water to colder regions or cold water to water regions. This interferes with the temperature of the water and may affect sensitive marine life in the region like it may end up freezing some marine animals to death. This paper discusses how the ocean currents affect marine animals with particular reference to turtles, sea urchins, and Nektones

Marine turtles rely entirely on ocean currents for their movements. Young marine turtles especially are moved to their pelagic nurseries by ocean currents and these serve as their habitats. The hatching of turtle eggs relies on oceanic tides and more especially on the frontal tides. This reduces the risk of exposing these eggs to predators. The fertilization of the turtle eggs also depends on ocean waves that transport the larvae to allow for fertilization to occur.

During the development of these turtles, their movement is still aided by ocean currents like in the case of searching for food they flow along with the currents to newer regions that could have food to keep them alive. The turtles are cauterized by two major directional movements where one is usually to the feeding area and the other to the nesting area; both two movements are aided by oceanic currents (Luschi, HAys, and Papi 294).

These are families of sea animals that are strong swimmers and large enough to have the strength to propel against ocean currents. Their bodies are streamlined such that they move swiftly. These include fishes, whales, and Dolphins. Ocean currents have such effects on these sea animals as they bring food to them from the shores and other places so the animals can feed on it. Besides the food, they cause the animals to move about and this allows the animals to be away from predators for their survival. During winter, ocean currents cause oceans waters to swirl around which causes a warming effect on the water and this allows the animals to survive the cold weather.

During summers, cold water from the Polar Regions is flown causing a cooling effect in the warmer regions. The currents also allow the animals to migrate or relocate to more accommodating weather conditions. Animals play in water and ocean current give the animals the whirling effect that gives the sea animals especially the large sea animals like Dolphins to whirl out and enjoy the changing weather.

Sea urchins and the starfish are greatly affected by ocean currents just like the other sea animals. Their larvae are transported over long distances to allow for fertilization to occur anywhere in the sea. These currents also aid the movement of these sea urchins. This allows them to have easy access to food, to redistribute to regions that are unoccupied and maybe unexploited. However, it should be noted that sometimes very strong oceanic currents can cause the death of sea urchins (Gray et al. 7)

Ocean currents are water movements in large volumes along a given path in the oceans, seas, or any large water bodies. These movements are usually caused by wind or the upwelling movement in the water bodies. It is important for the sea animals as it causes their movement to food-rich areas or brings feed to the animals. Food is the most essential part of the survival of any creature including those in large water bodies. Ocean currents are therefore inevitable to the survival of sea animals as they cause the flow of food nutrients within the sea.

Gray Eileen, Alexander Ann, Darling Tina, and Sharkey Nelda. “Moving water-Ocean currents and winds.” Drifters , 1998. Web.

Luschi Paolo, HAys Graeme, and Papi Florian. A review of long-distance movement by marine turtles and possible role of ocean currents . Cesenatico: Oikos, 2003.

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IvyPanda. (2022, September 5). How the Ocean Current Affect Animals’ Life in the Sea. https://ivypanda.com/essays/how-the-ocean-current-affect-animals-life-in-the-sea/

"How the Ocean Current Affect Animals’ Life in the Sea." IvyPanda , 5 Sept. 2022, ivypanda.com/essays/how-the-ocean-current-affect-animals-life-in-the-sea/.

IvyPanda . (2022) 'How the Ocean Current Affect Animals’ Life in the Sea'. 5 September.

IvyPanda . 2022. "How the Ocean Current Affect Animals’ Life in the Sea." September 5, 2022. https://ivypanda.com/essays/how-the-ocean-current-affect-animals-life-in-the-sea/.

1. IvyPanda . "How the Ocean Current Affect Animals’ Life in the Sea." September 5, 2022. https://ivypanda.com/essays/how-the-ocean-current-affect-animals-life-in-the-sea/.

Bibliography

IvyPanda . "How the Ocean Current Affect Animals’ Life in the Sea." September 5, 2022. https://ivypanda.com/essays/how-the-ocean-current-affect-animals-life-in-the-sea/.

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An essay on the winds and the currents of the ocean

Introduction. - The earth is surrounded on all sides by an exceedingly rare and elastic body, called the atmosphere, extending with a diminishing density to an unknown distance into space, but pressing upon the earth with a force equal to that of a homogenous atmosphere five and a half miles high. It is also partially surrounded by the ocean, which is of a very variable depth, and known to be, in many places, more than four miles. If the specific gravity of the atmosphere and of the ocean were everywhere the same, all the forces of gravity and of pressure which act upon any part of them, would be in exact equilibrium, and they would forever remain at rest. But as some parts of the earth are much warmer than others, and air and water expand and become rare as their temperature is increased, their specific gravities are not the same in all parts of the earth, and hence the equillibrium is destroyed, and a system of winds and currents is produced. It is proposed in this essay to inquire into the effects which are produced, both in the atmosphere and in the ocean, by this disturbance of equilibrium, and by means of a new force which has never been taken into account in any theory of winds and currents, to endeavor to account for certain phenomena in their motions, which have been a puzzle to meteorology and hydrology. As there are some uncertain data connected with the subject, such as the amount of the disturbing force, the effects of continents, friction, etc, which render a complete solution of the problem impractible, we shall aim at giving a popular explanation of observed phenomena rather than a complete solution of the problem; yet we shall give the result of some calculations, based upon known date, or at least upon very reasonable hypothesis, which will show that the causes which we have given are adequate to the effects which are attributed to them. We shall divide the subject into two parts, and treat, first, of the winds, and secondly, of the currents of the ocean.

The motions of the atmosphere. - From about the parallel of 28° on each side of the equator the winds on the ocean, where they are not influenced by any local causes, blow steadily towards the equator, having also a western motion, producing what are called the north-east and south-east trades. At the meeting of these currents near the equator there is a calm called the equatorial calm-belt, or the doldrums, where the air rises up and flows in the upper regions towards the poles, until it arrives near the 28°, where it is met by an upper current flowing from the poles. The meeting of these upper currents produces an accumulation of atmosphere from under which the air flows out in both directions on account of the increased pressure; a strong and steady current, as we have seen, towards the equator, and another not so strong and somewhat variable over the middle latitudes, towardg the poles, having at the same time an eastern motion, and producing what are called the passage-winds. As this current flows at the surface towards the poles, it gradually rises up and returns in the upper regions towards the equator, meeting the upper current from the equator near the tropics as has been stated.

Such are the general motions of the atmosphere, as laid down by Lieutenant Maury, and as represented in his diagram of the winds. [1] But there are numerous observations which have been made in very high latitudes both in the north and the south, which show that the currents flowing over the middle latitude towards the poles, do not extend to the poles, but that the atmosphere above a certain latitude, has a tendency to flow from the poles, producing another meeting of the air at the surface near the polar circles similar to the one at the equator, except that the currents are comparatively feeble and consequently the belt of meeting not so well defined, and that here also air rises up and flows each way in the upper regions towards the equator and the poles. "Sir J. Ross has shown that there is a large prevalence of north winds over the southern ones in the middle of North America as low down as latitude 70° and longitude 91°58' west, not merely in winter, but in every month in the year. The northern winds were not only more than double as frequent as the southern, but more than double as strong; and he also found southern winds largely to predominate in latitude 77° south." [2] If then there were no continents, or other local causes of disturbance, the motions of the atmosphere would be as represented in the following diagram, in which the direction of the wind is represented by the arrows, and the external part of which represents the motion of the air in the plane of the meridan. This system, however, is found to prevail on the ocean only, and is very much interfered with in other parts on account of local causes of disturbance, especially in the northern hemisphere, where the uniformity of the earth's surface is most interrupted by land.

The pressure of the atmosphere - The atmospere everywhere presses upon the surface of the earth with a force equal to the pressure of a column of mercury of nearly thirty inches in height. This pressure, however, is not the same in all parts of the earth, but varies in different latitudes as well as from an inch lower at the equator, and seems to stand the lowest about the polar circle, where a very remarkable depression has been observed in many places. "It is a singular fact," says Mrs. Somervill, (page 268) "discovered by our navigators, that the mean height of the barometer is an inch lower throughout the Antarctic ocean at Cape Horn, than it is at the Cape of Good Hope, or at Valparaiso." A similar depression has likewise been observed near the sea of Okhotsk in eastern Siberia. It also appears from the tables of the South Sea Exploring Expedition, by Captain Wilkes, that the barometer stands lowest near the polar circle, where it stands at a mean of about twentynine inches, and higher both north and south of it. After examining various authorities and all the circumstances connected with this subject, Professor Espy comes to the conclusion, that "there are three belts where the barometer stands below the mean, with almost constant rain and snow - one near the equator, one near the arctic circle, and one near the antarctic circle, and also that there are, certainly two belts in the outer borders of the trade-winds, where the barometer stands above the mean, and almost certainly two regions more - one around the north pole and the other around the south pole - where the barometer stands above the mean." It also appears for a great many observations made at different places on the Atlantic, at the level of the ocean, that the barometer stands more than half an inch lower at the arctic circle, than it does at the outer limit of the trade winds, and that there is also a considerable depression at the equator. [3]

The forces concerned in producing the motions of the atmosphere. - There are four principal forces which must be taken into account in a correct theory of the winds. The first arises from a greater specific gravity of the atmosphere in some places than others, on account of a difference of temperature and of the dew-point; for, when it becomes heated or charged with vapor in any place to a greater degree than at others, it becomes specifically lighter, and hence, the equilibrium is destroyed. There is a flowing together, then, of the heavier air on all sides, which displaces the lighter air, and causes it to rise up and flow out in a contrary direction. This is the primum mobile of the winds, and all the other forces concerned are dependent on it for their efficiency. A second force arises from the tendency which the atmosphere has, when, from any cause, it has risen above the general level, to flow to places of a lower level. These two preceding forces generally produce counter-currents. Again, when, from any cause, a particle of air has been put in motion toward the north or south, the combination of this motion with the rotatory motion of the earth produces a third force, which causes a deflection of the motion to the east when the motion is to the north, and a deflection to the west when it is toward the south. This is the same as one of the forces contained in La Place's general equations of the tides, the analytical expression of which is 2 n u r sin. l cos. l ; l being the latitude; n , the motion of the earth at the equator; u , the velocity of the particle north or south; and r , the radius of the earth. The fourth and last force arises from the combination of a relative east or west motion of the atmosphere with the rotatory motion of the earth. In consequence of the atmosphere's revolving on a common axis with that of the earth, each particle is impressed with a centrifugal force, which, being resolved into a vertical and a horizontal force, the latter causes it to assume a spheroidal form conforming to the figure of the earth. But, if the rotatory motion of any part of the atmosphere is greater than that of the surface of the earth, or, in other words, if any part of the atmosphere has a relative eastern motion with regard to the earth's surface, this force is increased, and if it has a relative western motion, it is diminished, and this difference gives rise to a disturbing force which prevents the atmosphere being in a state of equilibrium, with a figure conforming to that of the earth's surface, but causes an accumulation of the atmosphere at certain latitudes and a depression at others, and the consequent difference in the pressure of the atmosphere at these latitudes very materially influences its motions. This force is also expressed by one of the terms of La Place's equations, the analytical expression of which is 2 n v r sin. l ; v being the relative eastern or western velocity of the atmosphere.

Hadley's theory This theory, which is the commonly received theory of the trade-winds, and with which the reader is no doubt familiar, is based upon the first three forces only given above, no account being taken of the fourth. But as it may be seen from the analytical expressions of the last two forces given above, that the latter is greater than the former, and the east and west motion of the atmosphere depends upon the former, we have reason to suppose that the latter also may have a considerable effect, and that it should be taken into account in a correct theory of the winds. Accordingly we see that although Hadley's theory furnishes an explanation of the trade-winds, yet it does not account for many other remarkable phenomena in the motions of the atmosphere, but even requires motions to satisfy it entirely at variance with them. According to this theory, there should be a current on the surface of the earth from the pole to the equator in a kind of loxodromic spiral, and a similar counter-current in the upper regions from the equator towards the poles. The barometer also should stand highest at the poles where the air is coldest and most dense, and gradually fall as it is brought nearer the equator. But both of these, as we have shown, are contrary to observation. The position also of the exterior limits of the trade-winds near the parallels of 28°, the flowing of the atmosphere in the upper regions from both sides towards these parallels, and a low barometer near the polar circles, cannot be explained by this theory, and have not been satisfactorily explained by meteorologists. It is true Professor Espy says the highness of the barometer near the parallels of 28° is owing to the flowing over of the atmosphere which rises up at the equator; but this overflow of atmosphere could have no tendency to accumulate at these parallels, since it evidently would flow on gradually towards the poles to supply the draught caused by the flow towards the equator, as this theory requires. He also assigns as a cause of the low barometer about Cape Horn and the antarctic circle, the abundant rains which prevail there and the consequent disengagement of caloric, which rarefies the atmosphere there. But, if the belt of calms and rains near the equator is caused by the barometer standing lower there then at the outer limit of the trade-winds, which causes a flow of atmosphere there at the surface, and, if rains generally in any region, according to Professor Espy's own theory of clouds and rains, is caused by a low barometer there, and a consequent flowing in from all sides and a rising up of the atmosphere, then the lowness of the barometer in those regions must be the cause of the rains, and not the rain the cause of the lowness of the barometer, as will appear for other reasons.

We shall now undertake to show, that all these phenomena, and others connected with storms, which have never been accounted for by any theory, may be satisfactorily accounted for by taking into account the fourth force given above.

Why the outer limits of the trade-winds are near the parallels of 28°. - If from any cause the atmosphere receives a motion either towards or from the poles, the action of the third force above, causes a deflection of it towards the east, as it moves towards the poles, and towards the west as it moves towards the equator; and as the prime moving cause of the principal currents of the atmosphere has a tendency to cause it to flow towards and from the poles, the general result is, that towards the poles the atmosphere has a motion towards the east, but near the equator towards the west. But from the principle of the preservation of areas, the sum of the products of all the particles of the atmosphere, multiplied into their velocities and their distances from the axis of revolution, cannot be changed be the action of a central force, or by the mutual action of the particles upon each other; hence the sum of the products of each particle into its distance from the axis, and into its relative eastern velocity, must be equal to the sum of similar products, taken with regards to the particles having a relative western motion. But, as the portion of the atmosphere having a relative eastern motion, is nearer the axis than that which has a relative western motion, and as the part having a western motion, inasmuch as it is further from the axis, must be somewhat less than the part comprised between these parallels, in order to make the products equal, unless the relative velocity of the part having an easterly motion is very much greater. Hence, the dividing lines between the portions of the atmosphere having a relative east and west motion, must be within these parallels; and, as the outer limits of the trade-winds depend upon those lines, they must also fall within these parallels; and they are accordingly found to be about the parallels of 28°.

The cause of high barometer about the parallels of 28°, and the low barometer at the polar circles. - The greater pressure of the atmosphere at the parallels of 28° than at the equator and the polar circles can only be caused by an accumulation of atmosphere there. This accumulation results, necessarily, from the action of the new force which we have introduced into the theory of the winds. For, as we have seen, all the atmosphere between the parallels of 28° and the poles has, and, according to theory, must have, a general eastern motion; and this gives such a value to the analytical expression of the fourth force, enumerated above, as to cause the atmosphere to recede from the poles toward the equator. But the western motion of the atmosphere between the parallels of 28° gives that expression a negative value there; and, hence, this force causes the atmosphere to recede from the equator, also. This force then, has a tendency to cause the atmosphere in the upper regions to recede from both the poles and the equator, and to accumulate about the parallels of 28°, and, as it may seem by merely inspecting the expression of this force given above, that for the same value of v , or motion of the atmosphere east or west, this force is much greater toward the poles than it is near the equator, it causes a considerable depression of the atmosphere at the poles, and only a slight one at the equator, as represented in the diagram. The amount of this elevation and depression is not indicated entirely by the barometer, for the height of the barometer depends upon both the height of the atmosphere and its density. Therefore, as the atmosphere is much denser at the polar circles than at the parallels of 28°, on account of its being much colder, the accumulation of atmosphere at these parallels and its depression towards the poles, must be considerable to cause the barometer to stand higher at these parallels than the polar circles.

We shall now give the results of some calculations, based upon a reasonable hypothesis, which show that this new force introduced is entirely adequate to produce an accumulation of atmosphere at the parallels of 28°, and a depression of it at the poles to such an amount, that the difference in the height of the barometer at the parallels of 28°, and a depression of it at the polar circles, may correspond with observations. As friction is a very important element in calculations of the motion of the atmosphere, and its effect cannot be determined, it would be impossible to calculate the motion of the atmosphere from the forces which act upon it, if they were even accurately known. We will therefore assume certain motions of the atmosphere, which are known not to vary much from observation, upon which we will base our calculations. If we assume that the east and west motions of the atmosphere may be represented by the expression 2sin 3/2 p cos. 3/2 p v , p being the polar distance of the place, it would make these motions vanish at the poles and at the parallels of 30°, and we have reason to think, would pretty well represent the motions of the atmosphere east and west at all latitudes except near the equator, where it would make it a little too great. Upon this assumption it may be shown by calculations, the results of which only can be given here, that if v , or the maximum east or west motion of the atmosphere, were only ten miles per hour, it would cause a heaping up of the atmosphere near the parallels of 28° which would make the barometer, if the atmosphere were everywhere of the same density, stand two inches higer here than at the poles. The same hypothesis would also make the depression at the equator only one-ninth that at the poles. If we now suppose that the greater specific gravity alone of the atmosphere at the poles, is sufficient to cause the barometer to stand one inch higher there than at the equator, which would only require a difference of temperature of about 16°, it would still leave a difference in the height of the barometer between the poles and the equator about equal to the observed difference in the southern hemisphere. It would also add a little to the depression of the baromter at the equator, which would make it a little more than one-ninth of that at the poles, and consequently make it correspond with observation. We think therefore it is evident that the observed difference in the height of the barometer in difference latitudes, is owing to the joint effect of a gradual increase of specific gravity from the equator to the poles, and of a heaping up of the atmosphere near the parallels of 28°, caused by the action of this new force that we have taken into account.

Explanation of the passage-winds and calm-belts at the limits of the trade-winds. - As the pressure of the atmosphere, on account of its accumulation there, is greatest about the parallel of 28°, this pressure has a tendency to cause it to rush out from beneath both, towards the poles and the equator. If the motions of the atmosphere were as great at the surface of the earth as in the upper regions, the force which causes a heaping up of the atmosphere about the parallels of 28°, would be as great below as in the upper regions, and would prevent the flowing out of the air below towards the poles. But, on account of friction, the eastern motion of the atmosphere cannot be so great at the surface of the Earth as above, and consequently the accumulation of atmopshere mentioned above, is caused principally by the upper currents, and the pressure wich causes it to flow out below towards the poles, where the barometer, as we have seen, stands much lower, is greater than the force below which causes the accumulation of atmosphere. The lateral pressure then of the atmosphere, and its horizontal motion which has a tendency to cause it to flow at the surface of the Earth, from the poles towards the equator; and secondly, the heaping up of the atmosphere at the outer limits of the trade-winds, which causes it to rush out below, both towards the equator and the poles; thirdly the action of the force depending upon the east or west motion of the atmposphere, wich we have seen, must be greater above than at the surface of the Earth. Between the parallels of 28° and the equator, the first two forces combine against the latter, which is small near the equator, and produce a strong and steady current at the surface of the Earth towards the equator, which, being combined with the rotatory motion of the Earth, gives rise to the tradewinds. Beyond these parallels, the first and third forces are opposed to the second, but it may be seen from the analytical expression of this second force, obtained from our preceding assumption, and which cannot be given here, that this force is very great in the middle latitudes, and consequently it prevails over the two, causing a current towards the poles, which combined with the rotatory motion of the Earth, produces the southwest winds in the northern hemisphere, and northwest winds in the southern hemisphere, called passage-winds. This force has its maximum about the parallels of 48°, and above these decreases rapidly, so that at the polar circles the other two forces begin to prevail over it, and cause a current from the poles. The forces then acting upon the atmosphere at the surface of the earth, causes it to flow in opposite directions, from the parallels of 28° and the poles, and to flow together near the equator and the polar circles. Hence, there is a rising up of the atmosphere at the latter places, and a flowing thence in the upper regions to the former places, where it descends, and thus a system of current is produced as represented by the arrows in the external part of the diagram, which represents a meridional vertical section of the atmosphere. It was shown that about the parallel of 28°, the atmosphere can have no motion east or west, and it has now been shown that these are the parallels also of greatest pressure, whence the currents flow both towards the equator and the poles, consequently there must necessarily be here calm-belts, such as are well-known from observation to exist.

We think now, it is manifest, that the introduction of our new force into the theory of the winds, exactly accounts for all the principal motions of the atmosphere, and clears up the difficulties which have heretofore puzzled meteorologists.

Maury's theory of the crossing of the winds. In order to account for the motions of the winds and other phenomena, Lieutenant Maury advances the theory that there is a crossing of the winds or currents at the calm-belts of the equator and the parallels of 28°; that the currents flowing at the surface towards the equator, cross there, each becoming the upper current in the other hemisphere after it crosses the calm-belt at the equator, and then flowing towards the poles until it meets the upper current flowing toward the equator about the parallel of 28°, where there is supposed to be another crossing, each current then becoming again the surface current, and flowing in the same direction as before. [4] He also makes, by his arrangement, the rains in each temperate zone depend upon the vapor received by the winds in their passage to the equator as a trade-wind in the opposite hemisphere. We think there is no necessity for resorting to such an argument to account for the phenomena of the winds, but that they all are satisfactorily accounted for, as above, by tracing out the effects of well known forces without resorting to the mysterious agents of magnetism and electricity. Besides, there is no known principle by which two currents can interpenetrate and cross each other without mingling together, and, especially, is there none by which a current saturated with vapor can pass through a dry current and each one after afterwards retain its distinctive character of a moist or dry current, which this theory requires.

The fact that Ehrenberg has discovered South American infusoria in the blood-rains and "sea-dust" of the Cape Verde Islands and other places, does not prove the crossing of the winds; for, according to the explanation of the winds given above, there are two curents flowing into each calm-belt, and also two flowing out in opposite directions, as it were from a common reservoir, and, consequently, whatever is carried into these belts from either side down flow out again in each direction; and so infusoria in one hemisphere can easily pass to the other without a distinct crossing of the currents. And it even the moist current of the torrid zone could pass though the dry ones to the temperate zones, they could not produce rain there; for Professor Espy has conclusively shown that no descending current, however saturated with moisture, can ever produce rain. [5]

Explanation of the winds at the peak of Tenerife. - We have stated that the greatest atmospheric pressure is about the parallels of 28°, but these cannot be accurately the parallels of the greatest accumulation, for this pressure depends both upon the height of the atmosphere and its density, and, as the density increases gradually with the latitude, there must be an increase of pressure beyond the parallel of greatest accumulation, until the decrease of pressure from the one cause equals the increase from the other. But, as the calm-belts in the upper regions must be where the currents meet, and consequently where there is the greatest accumulation of atmosphere, it follows that the calm-belts are not exactly at the same parallels at the surface of the Earth as in the upper regions, but that they incline above toward the equator, as represented in the diagram. And this explains the peculiarity in the winds at the Peak of Tenerife. This peak stands near the outer limits of the trade-winds, and, as this limit moves north an south with the seasons, the northeast and southwest winds are found to prevail at the base alternatively. But at the top of the peak the southwest winds always prevail, because, when the calm-belt is furthest north, it still leaves the top of the peak north of it, where the southwest winds prevail, where the northwest trades are blowing below. When this belt occupies its most southern position, it leaves both at the top and the bottom of the peak north of it, and, consequently, the southwest wind blows both at the top and the bottom. In the fall, as the calm-belt moves south, more of the peak gradually becomes north of the calm-belt, and hence the southwest winds, which always prevail at the top, should gradually descend lower on the peak until they reach the base, which is exactly in accordance with observation.

The effect of continents - If the surface of the earth were all covered by the sea, uninterrupted by continents, the tradewinds and passage-winds, and also the calm-belts, would extend, without any interruption, entirely round the earth. But continents, and especially high mountain ranges, seem to have a very material effect in changing this regular system of winds. Thus the high table-lands and mountain ranges in Mexico and the western part of the United states, seem to turn the westward current of the trade-winds on the Caribbean sea and the Gulf of Mexico northward over the parallel of the calm-belt into the Unites States, where it arrives at the latitudes where the atmosphere has a general tendency to flow eastward, and thus a kind of aerial gulf-stream is produced. This is evident not only from observations on the general directions of the winds in the Guld of Mexico and the United States, but also from the observed routes of storms, which must be governed very much by the general movements of the part of the atmosphere in which they occur. Instead of the regular trade-winds from the northeast in the Caribbean sea, the prevailing cours of the lower current, is from a point south of east instead of north of east. [6] It is also found from observations at Barbedoes that, while the eastern winds are most prevalent, the southeast winds greatly predominate over the northeast ones. Of a great many hurricanes, also, which had their origin in the Atlantic, east of the Caribbean sea, and whose routes have been determined, by Mr. Redfield, nearly all moved in a direction north of west, until they arrived at the longitude of Florida or the Gulf of Mexico, where they curved around towards the north, and after passing the parallel of the calm-belt, towards the northeast, in the direction of Newfoundland and the northern part of the Atlantic. And this is exactly the route we would suppose the westward currents of the lower part of the atmosphere, interrupted by the high mountain ranges of Mexico, would take. But on the west coast of North America, the eastward current of the northern part of the Pacific, impinging against the range of the Rocky mountains, is turned down towards the equator, and hence the prevailing direction of the wind on the Pacific, west of Mexico, is from the northwest. The eastern coast of Africa also seems to have a similar effect upon the westward current of air in the Indian ocean; for the hurricanes which orginate in that ocean, on approaching that coast, are turned southward and finally towards the southeast into the southern ocean. The typhoons, also, of the China sea seem to be influenced in a similar manner by the eastern part of Asia. These changes of the general direction of the wind which prevail on the open ocean must be caused by the continents.

Hurricanes and storms. - Hurricanes are generally supposed to be produced by the meeting of adverse currents, which produce gyratory motions of the atmosphere at the place of meeting. That they may receive their origin and first impulse in this way, we think is very probable; but that violent hurricanes, extending over a circular area nearly one thousand miles in diameter, and continuing for ten days, and proceeding with increasing violence from the torrid zone to high northern latitudes, depends upon any primitive impuls alone, we think is very improbable. For if even any part of the atmosphere should receive such an impulse as to pruduce a most violent hurricane, friction would soon destroy all motion and bring the atmosphere to rest. Besides, no gradually accelerated motion can depend upon a primitive impulse alone, even where there is no friction. Hurricanes then, and all ordinary storms, must begin and gradually increase in violence by the action of some constantly acting force, and when this force subsides, friction brings the atmospher to a state of rest. This force may be furnished by the condensation of vapor ascending in the upward current in the middle of the hurricane, in accordance with Professor Espy's theory of storms and rains. According to this theory, all storms are produced by an ascending current of warmer atmosphere above by means of the caloric given out of the vapor which is condensed as it ascends to colder regions above. Therefore, as long as this ascending current can be supplied with air saturated with vapor this continual rarefaction must take place, and also the ascent of the air from all sides to supply its place. If, then, all the lower stratum of atmosphere over a large district were saturated with vapor, without some disturbing cause, it might remain undisturbed; but if from any cause an ascending current is produced, either by local rarefaction of air my means of heat, or by the meeting of two adverse currents, which produces a gyratory motion and consequent rarefaction in the middle on account of the pressures being taken away by the centrifugal force, as soon as the air below, saturated with vapor, ascends to the colder regions above, the vapor is condensed and the caloric given out continues to rarefy it so long as the ascending column is supplied with moist air, and consequently the surrounding colder air presses in below from all sides, and thus a hurricane of more or less violence is produced and kept up for ten or twelve days, moving with the general direction of the motion of the atmosphere where it occurs. The violence then of the hurricane, and also its duration, depends upon the quality of vapor supplied by the currents flowing in below. Hence, it is, that the tropical hurricanes which originate in the Atlantic, east of the Caribbean sea, do not abate their violence until they reach a high northern latitude where the atmosphere is cold and dry.

The cause of the gyratory motions of hurricanes. - It has been established by Redfield, Reid, Piddington, and others, that all hurricanes and ordinary storms have a gyroatory motion around a center, and that these gyrations in the northern hemisphere, ar from right to left against the hands of a watch, but in the southern hemisphere from left to right with the hands of a watch. There are some however, amongst whom is Professor Espy, who deny the gyratory character of storm entirely, and contend that there is only a rushing of the air from all sides below towards a centre, without any gyration. We think this gyratory character of storms has been too well established to admit of any doubt. No one, however, has ever given any satisfactory reason why these gyrations in the one hemisphere are always sinistrorsal and in the other dectrorsal. It is true, Mr Redfield [7] endeavors to account for the sinistrorsal gyrations of the hurricanes and storms which proceed from the Gulf of Mexico towards Newfoundland, by means of the pecularities of the aerial currents in the region of the Gulf and the adjacent coast of the Pacific. But if there are the same kind of gyrations over the whole hemisphere, it is evident that the cause which produces them must be as extended as the hemisphere itself. It has also been suggested that this tendency to a distinct kind of gyration in each hemisphere, may be owing to the magnetism of the air. [8]

We shall now undertake to show that there cannot be a rushing of air from all sides towards a center, on any part of the earth except at the equator, without producing a gyration, and that the tendency to a distinct kind of gyration in each hemisphere, is owing, neither to any pecularities of the winds or aerial currents, nor to the mysterious agent of magnetism, bu that it results, as a necessary consequence, form the action, upon the atmosphere, of the four forces which we have taken into consideration in the first part of this essay.

It has been shown that when a particle of air receives a motion toward the poles it is deflected toward the east, as in the passage-winds, but when it revieves a motion toward the south, there is a force which also turns it toward the west, as in the tradewinds. It has likewise been shown that when the air has a relative motion east, it has a tendency, on account of the greater centrifugal force, to move also towards the south, but that when it has a relative motion west, it has a tendency, on account of the diminished centrifugal force, to move also towards the north. If, then, we suppose that the air at M , N , O and P on the diagram (page 8), has a tendency, on account of rarefaction or for any other reason, to flow towards c , from what has been stated, the air at M would not move equally towards c , but would be deflected northward a little towards m . In like manner the air at N when there is any force which tends to make the surrounding air flow towards a center, the resultants of all the forces which act upon it must cause it to receive a gyratory motion, and that this motion in the northern hemisphere must be sinistrorsal, but in the southern hemisphere the contrary.

It may be observed here that these gyrations are not cucular but spiral, gradually approaching the center; for the forces which tend to produce these gyrations depend for their efficacy upon a motion from all sides towards the center. First, the force of which we have already treated tends to give the atmosphere a gyratory motion, as soon as it begins to converge towards a center; and secondly, these gyrations, however slight, being once produced, the centripetal force, which causes the air to flow towards the center, accelerates these gyrations as they approach the center, upon the principle by means of a string, are rapidly accelerated as the string becomes shorter. Hence if the first of these forces be only sufficient to produce a very slight gyration, the latter or centripetal force may cause very rapid gyrations near the center. And it is only upon this principle that the rapid motion of the air in a hurricane can be produced, and any theory which does not take this principle into account is defective. This centripetal force is caused by the superior pressure of the denser atmosphere on the borders of the hurricane or storm, and consequently prevails only in the lower part of the atmosphere. In the upper regions it has a tendency to recede from the center for two reasons; first, on account of the gyratory motion wich it has received below in appraoching the center, which it still, in some measure, retains after ascending to the regions above, where the surrounding pressure does not prevail, and consequently the centrifugal force resulting from the gyrations, causes it to recede from the center; secondly, because the ascending current causes an accumulation of atmosphere above the general level, which gives it a tendency also to flow out in all directions form the center. These motions however, are not distinctly towards and from the center, but in spirals, so that the currents below may be at right angles with the currents above; and hence it is that in our ordinary storms attended with rain, the clouds in the lower part of the atmosphere frequently move in a direction at right angles with the direction of those above.

It has been stated that the gyrations below approach the center in a spiral, but this approach must be slow towards the center; for, at a certain distance from the center, the gyrations becoms so rapid that the centrifugal force nearly equals the centripetal, produced by the external pressure of the atmosphere, and then the further appoach towards the center in a great measure ceases,and consequently the force which produces the gyrations. Hence, at a certain distance from the center the hurricane has the greatest violence, and within this circumference, friction in a great measure destroys the gyrations, so that the middle of our most violent hurrianes is a calm. The extent of this calm is a circle, varying generally from five to thirty miles in diameter.

If we examine the analytical expressions of the forces which produce these gyrations, we will see that at the equator they have no value, and hence no hurricane can have its origin exactly on the equator. Accordingly, of all the hurricanes which have originated within the tropics, none have been traced back to the equator, but always to some region from 10° to 20° from it.

The reason why the hurricanes which originate east of the Caribbean sea pass northward to the east of the United States, may be owing to the direction of the wind here and on the Caribbean sea, which generally blows north of west, as has been observed in the former part of this essay. If the general direction of the trade-winds prevailed here, they would be carried on towards the equator, as those without doubt are which originate at other places in the same latitude.

We come now ot the second part of our subject, the currents of the ocean.

The general motions of the ocean. - Inasmuch as the atmosphere and the ocean are both fluids somewhat similarly situated, except that there is a similarity of their general motions. This is known from observation to be the case, except that the continents interfere more with the motion of the ocean than with those of the atmosphere. The general motion of the ocean in the torrid zone, where it is not interrupted by continents, is toward the west with an average velocity of about ten miles in twenty-four hours. Towards the poles the motion, in general, is towards the east, which is a necessary consequence of the preservation of areas; for if one part have a western motion, another part must have an eastern one, as was shown with regard to the atmosphere. If, then, there were no continents, there would be a general flowing of all the tropical parts of the ocean westward, and of the remaining parts toward the east. But when the tropical or equatorial current impinges agains the eastern sides of the continents as in the Atlantic, a part is turned along the eastern side towards each pole. Likewise, when the eastern flow towards the poles, strikes against the western sides of a continent, it is deflected towards the equator. Hence the northern parts of both the Atlantic and Pacific, have a tendency to a vortical motion, their tropical parts moving westward, and then turning northward on the eastern sides of the continents and joining the eastern flow, and south again towards the equator on the western sides of the continents. And it is evident from observation, that the southern parts of these oceans, and also the Indian ocean, have a tendency, in some measure, to the same kind of motions, except that the continents do not extend so far south, and consequently only a part of the eastern flow is turned towards the equator, the rest flowing on and producing the general eastern motion of the waters observed in the southern ocean.

The forces which produce the motions of the ocean - The primum mobile of the motions of the ocean, as of the atmosphere, depends principally upon the difference of temperature between the equatorial and polar regions. The temperature of the ocean, on the surface at the equator, is about 80°, and it has a temperature above the mean temperature of the earth, which is 39°.5, to the depth of 7200 feet. Towards the poles it is below the freezing point, and continues below the mean temperature at the parallel of 70°, to the depth of 4500 feet.* As water expands about 0.000455 of its bulk for every degree of increasing tempeature, and sea-water contracts down to the temperature of 28°, calculations based upon these data, supposing the temperature to increase or decrease in proportion to the depth, make the specific gravity of the part at the equator, so much less than that at the poles, that it would have to rise about ten feet above the general level of the equator to be in equilibrium at the bottom of the sea, with the part at the poles. But then the equilibrium at the surface would be destroyed, and the waters would flow there towards the poles, where the superior pressure at the bottom over that of the equator, would cause a current to flow back at the bottom of the sea, towards the equator. Hence, if this cause of disturbance existed alone, ther would be a current at the bottom of the sea from the poles to the equator, moved by a force equal to the pressure of a stratum of water of about five feet, and one at the surface from the equator towards the poles, moved by an equal force. But this motion, combined with the rotatory motion of the earth, gives rise to other forces, just as in the case of the atmosphere, which greatly modify these motions, as will be shown hereafter.

The preceding are the principle forces concerned in giving motion ot the waters of the ocean. Lieutenant Maury, however, lays little stress upon these, and seems to think that the principle agencies concerned in these motions, arise from evaporation, the saltness of the ocean, galvanism, &c.* But we think it may be shown that these agencies can have no perceptible effect.

First, Lieutenant Maury supposes excessive evaporation to take place within the tropics and this vapor to be carried away and precipitated in extra-tropical regions, and infers that this would have, at least, a very sensible effect in producing the currents of the ocean. He puts the amount of evaporation of a stratum of one-half of an inch per day. Now if a stratum of water one-half of an inch in thickness is evaporated in twenty-four hours in one place and precipitated in another, it produces a difference of level of one inch between the two places, and the currents which it produces must be such as are sufficient to restore this level in the same space of time. Now we may judge how exceedingly small a current this would produce when we consider that there is a rise of about two feet in the open ocean at one place and a fall of the same amount at another every six hours, caused by the tides, and yet the flowing of the water from the one place to the other place to produce this rise at one place, and fall at the other, it is well known does not produce any sensible currents in the open sea. Again, this matter can be easily reproduced by calculation. If a stratum of water, one-half of an inch in thickness were taken up by evaporation from the torrid zone, and none of it precipitated there but all conveyed to the temperate and polar zones, it may be demonstrated upon the supposition that the ocean is four miles in depth, that the flow of water towards the equator to restore the equilibrium in the same time would not amount to a velocity of one foot per hour.

We think it may be likewise shown by calculations based upon reasonable hypothesis, if not entirely upon well-known data, that the salts of the sea also can have but little influence in producing currents. Lieutenant Maury makes a similar hypothesis in treating of the influence of the salt of the ocean, which he does in treating of the influence of evaporation, and supposes that the excess of salt left in the torrid zone by the excess of evaporation there, and the great precipitation in the temperate and polar regions produces such a difference in the specific gravity as to destroy the equilibrium of the sea, and to have a very sensible influence in producing current, and especially the Gulf-stream.

With regard to the latter, he supposes that the water of the Gulf of Mexico has a much greater specific gravity than the water in the Atlantic, on account of the great evaporation to which it has been exposed in its passage from the coast of Africa across the Atlantic ocean and through the Caribbean sea, and that, consequently, it is forced out into the Atlantic by its greater pressure. Now, suppose it takes the water a year, which is about the actual time, to pass from the coast of Africa to the Gulf of Mexico; in this time, according to the hypothesis, there is evaporated a stratum of water fifteen feet in depth, and, as the salt contained in this stratum cannot be evaporated, it remains in the part left, and increases it saltness. But sea-water contains only about three per cent of saline matter, and consequently the amount of salt contained in this stratum of fifteen feet only increases the weight of the rest to an amount equal to the weight of a stratum of water about six inches deep. Hence, it only gives the water of the Gulf a tendency to flow out into the Atlantic with a force equal to the force with which a homogenous fluid would flow out with its surface six inches above the general level of the Atlantic. This is much less than the opposing force arising from the great specific gravity of the water in the northern part of the Atlantic on account of its lower temperature, as we have shown by calculations. The same reasoning may be applied to any other part of the ocean; for, if the salt of the ocean has any influence in producing currents, it must be to produce an undercurrent from the torrid zone, where evaporation is supposed to be in excess, toward the poles, and consequently, a counter-current at the surface from the poles toward the equator. But, upon any reasonable hypothesis, the water at the surface cannot lose by evaporation in passing from the poles to the equator, a stratum of water of such a depth, that the amount of salt contained in it can increase the specific gravity at the equator as much as the lower temperature increases it toward the poles; hence, if the salt of the sea has any sensible influence, it is only in opposition to a greater influence, and, consequently, it has a tendency to diminish, rather than increase, the currents of the ocean. We think it, therefore, manifest that neither evaporation nor the salts of the sea can have much influence in producing currents, even upon Lieutenants Maury's hypothesis, that evaprotation is greatly in excess of precipitation in the torrid zone. But is this a true hypothesis? Although there is a great evaporation in the torrid zone, there is also great precipitation; for, with few exception, more rain falls at the equator than in any other part of the earth, and it is only the amount of evaporation over precipitation that should be taken into account, which we have reason to think is very small, and, if professor Espy's theory is correct, it can not be anything; for, according to this theory, no vapor can pass from the torrid to the temperate zones and produce rain, since the current bearing it there would be a descending current, and consequently could not produce it.

The ocean not level. - As it has been shown in the case of the atmosphere, that the resultant of the forces causes an accumulation about the parallels of 28°, so as the motions of the ocean are somewhat similar, and it is acted upon by the same forces, it may be shown that there must be a slight accumulation about those parallels in the ocean also. Whatever may be the causes of the motion of the ocean, we know that in the torrid zone it has a small western motion, and in the other parts a slight motion towards the east. The great equatorial current of the Atlantic moves about ten miles in twenty-four hours, but if we suppose that the average motion of the water in the torrid zone is five miles only per day, and that the mximum velocity of the water eastward in the extra-tropical regions is the same, using the same hypothesis we did with regard to the atmosphere, the forces which result from these motions must cause an accumulation of more than forty feet about the parallel of 28°, above the level of the sea at the poles, and about five feet above the level of the equator. This however, would be the amount of accumulation to produce an equilibrium of the forces at the surface, but as this accumulation would then produce greater pressure there upon the bottom than towards the poles and at the equator, it would produce, as in the case of the atmosphere, a flowing out from beneath this accumulation towards the poles and the equator, and settling down of the surface above, below the state of equilibrium, sufficient to cause a counter-current at the surface from the poles and the equator to supply the currents below. The accumulation there would be only about one-half that stated above, and there would be a flowing of water at the surface from both sides towards the parallel of 28°, and below a current in both directions from these parallels, similare to the motions in the atmosphere.

That the water of the ocean has such a motion as has been stated, appears from observations of its motions and other circumstances. Says Lieutenant Maury, "There seems to be a larger flow of polar water into the Atlantic than of waters from it, and I cannot account for the preservation of the equilibrium of this ocean by any other hypothesis than that which calls in the aid of undercurrents." It is well-known that in Baffin's bay there is a strong surface-current running south, and a strong counter-current beneath running north. Another evidence of this general tendency of the waters, is, that icebergs, in both hemispheres, are drifted from the poles towards the equator, and in the south Atlantic and Pacific oceans,there are large collections of drift and sea-weed about the parallels of 28°, so thickly matted that vessels are retarded in passing through them. [9]

These collections can only be formed by the flowing of the water at the surface from both sides to these parallels. It has been supposed that these collections are owing to the slight vortical motion of these oceans, it being supposed that any floating substances on the surface would have a tendency to collect at the vortex. This, however, would not be the case, for on account of friction at the bottom, the surface would have a greater vortical motion than the bottom, and consequently the water would be driven very slowly at the surface by the centrifugal force towards the sides, where it would cause a slight elevation and increase of pressure, which would cause the water to return towards the vortex at the bottom and not at the top; and hence floating substances at the surface could have no tendency to collect at the vortex.

We have corroborated these deductions from theory by numerous experiments made with a vessel of water with light substances on the surface. When the vessel is first receiving a vortical motion, the substances collect in the middle; for, as it is the vessel which gives motion to the water by means of the friction, the vessel, and consequently the bottom of the water, has then a greater motion than the top; and hence the reverse of what is stated above takes place; but, if the vessel is now stopped, and the water within allowed to continue its motion, the vortical motion at the bottom is retarded faster than at the top, and soon has a slower motion there, when the light substances on the surface are seen to recede from the vortex towards the sides, and if there are any light substances on the bottom they collect in the centre, all of which proves, that the water recedes from the middle at the surface, and returns to it at the bottom, and exactly agrees with the deductions from theory. These collections of sea-weed, then, cannot be caused by the vortical motions of the ocean, but must be the result of a general tendency of the surface water, to flow from both the equator and the poles towards these parallels; and, as it is prevented from collecting on these parallels near either side, on account of the slight vortical motion of the ocean, it collects only in the middle.

Explanation of the Gulf-stream - We come now to the Gulf-stream, which has been a puzzle to philosophers ever since it was discovered. Many explanations have been given, and all known forces which can have any influence, have been brought in to account for this wonderful phenomenon. The most usual explanation is, that it is the escaping of the waters which have been forced into the Caribbean sea and the Gulf of Mexico by the trade-winds, which have been supposed to raise their surface above the general level, and thus afford a head as it were for the stream. This, without doubt, has a very considerable effect, but it has not generally been deemed adequate alone to account for the phenomenon, nor does it, in connection with all other known influences, afford a satisfactory explanation. "What is the cause of the Gulf-stream." says Lieutenant Maury, "has always puzzled philosophers. Modern investigations are beginning to throw some light upon the subject, though still all is not yet clear."

We shall now endeavor to show that the additional force which we have taken into account in explaining both the winds and the currents of the ocean, and which seems to have been overlooked heretofore, will at least throw much additional light upon the subject, if not afford a complete explanation. We have shown that this force, which results form the eastward flow of the water in extra-tropical regions, and from the western motion within the tropics, has a tendency to drive the water from the poles towards the equator, and also slightly from the equator towards the poles, and to produce an accumulation of at least twenty feet on the parallel of 28° above the level at the poles, upon the supposition that the maximum of this east and west flow is only five miles per day. But if, from any cause, the force which results from this eastward flow should be cut off at any place, the water would flow northward at that place with a force equal to that which would result from a head on the parallels of 28°, at least twenty feet above the level towards the poles. Now, it may be seen from the configuration of the coast of the United States, that this force is actually cut off along that coast; for this force depends upon the eastward flow of the water there, which it cannot have, inasmuch as it must flow in both ways along the coast to fill up the vacuum which such a motion would produce. As the Gulf of Mexico, therefore, and the adjacent parts of the Atlantic, lie in the parallel of greatest accumulation, the water must flow from these parts along the coast with a force equal to that stated above. In addition to this, the momentum of the water flowing westward in the torrid zone, with a motion depending upon the prime moving course, due to a difference of specific gravity between the poles and the equator, in connection with the rotatory motion of the earth, and being independent of the effect of the trade-winds, must force the water in the Caribbean sea and the Gulf of Mexico considerably above the general level and add to the preceeding force. When we consider that the motion of the water which produces tides on our coasts, is in general imperceptible in the open ocean, and yet, on account of the sloping bottom of the ocean, which causes a smaller volume of water to receive the momentum of a larger one, it causes considerable rise of the water along the coast, we have reason to think that the general tendency of water westward in the torrid zone may keep the water in the Gulf considerably above the general level, since its water and that in the Caribbean sea, if the bottom of the ocean be sloping, must in great measure receive the momentum of the whole body of the water moving westward in the adjacent part of the Atlantic. The eastern tendency of the water in the northen part of the north Atlantic, due to the prime moving force mentioned above and independent of the winds which prevail there, causes the surface of the ocean in the latitude of Newfoundland to be somewhat depressed below the general level next to the coast, which also adds to the force of the Gulf-stream. All these forces, taken in connection withthe influence of the trade-windst, to which this phenomenon has been mainy attributed, we think, furnish a complete and satisfactory explanation of that great wonder and mystery of the ocean, the Gulf-stream.

The Greenland and other currents. - The general eastward motion of the waters of the ocean in the northern part of the Atlantic, and consequent depression next the coast of North America, also furnish an explanation of the cold current of water flowing between the Gulf-stream and the coast of the United States, called the Greenland current. On account of the rotatory motion of the earth, the water of the Gulf-stream in flowing northward, tends to the east, and for the same reason the water flowing from Greenland and Baffin's bay to supply the eastern flow, tends towards the west, and consequently flows in between the Gulf-stream and the coast of the United States.

There must be a motion of the waters somewhat similar to that of the Gulf-stream and the Greenland current, wherever the great equatorial current impinges against a continent, and the eastward flow towards the poles is cut off. Hence, on the eastern coast of South America, there is a warm Brazilian current towards Cape Horn, and on the eastern coast of Africa, the Mozambique current which at the Cape of Good Hope is called the Agulhas current. Also, on the eastern coast of Asia, there is the warm China current, flowing towards the north, similar to the Gulf-stream, and the cold Asiatic current, insinuating itself between it and the coast, like the Greenland current.

On the western side of the continents a motion somewhat the reverse of this must take place. Hence, instead of a warm stream flowing towards the north, there is a cold current flowing towards the equator. On the west of Portugal, and the northern part of Africa, there is a flow of colder water towards the equator, both to join the great equatorial current flowing across the Atlantic. On the west coast of South America, is Humboldt's current, 8° or 10° colder than the rest of the ocean in the same latitude, both tending towards the equator to there join the great western current across the Pacific, and to fill up, as it were, the vacuum which this current has a tendency to leave about the equator, on the western coast of America.

NASHVILLE, October 4, 1856.

• ↑ Physical Geography of the sea, page 70.
• ↑ Professor Espy's Third Report on Meterology, § 101.
• ↑ See Kaenitz' Meteorology. by C. Walker, page 277.
• ↑ See "Physical geography of the Sea" § 106.
• ↑ Third report on Meteorology §§ 68, 69.
• ↑ See W. C. Redfield Esq. on three several hurricanes, etc., in Silliman's Journal, second series, vol. I., page 13.
• ↑ Silliman's Journal, second series, Vol II., page 325.
• ↑ Maury's Physical Geography of the Sea § 224.
• ↑ Humbold's Cosmos, Vol. 2 page 278

This work was published before January 1, 1929, and is in the public domain worldwide because the author died at least 100 years ago.

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Weaker ocean currents lead to decline in nutrients for North Atlantic ocean life during prehistoric climate change

by Georgia Institute of Technology

Researchers at Georgia Institute of Technology have finished investigating how the prehistoric weakening of a major ocean current led to a decline in ocean nutrients and negative impacts on North Atlantic ocean life. The results support predictions about how our oceans might react to a changing climate—and what that means for ocean life.

The North Atlantic ocean is a hub of biological activity, due in large part to the Gulf Stream, which supplies a rich current of nutrients. Scientists have speculated that our changing climate may lead to a decline of nutrients and biological activity in the North Atlantic due to a weakening of the ocean circulation—but this theory has previously been supported only by models.

Now, by studying sediments buried at the Gulf Stream's origin, the team has conducted a first-of-its-kind investigation into the impact of a similar climate-induced decline nearly 13,000 years ago, when Earth exited the last ice age.

The paper, "A Diminished North Atlantic Nutrient Stream During Younger Dryas Climate Reversal" was published in Science this week. Led by Jean Lynch-Stieglitz, a professor in the School of Earth of Atmospheric Sciences at Georgia Tech, the team also included Lynch-Stieglitz's past students: Tyler Vollmer, Shannon Valley, and Eric Blackmon, along with Sifan Gu (Jiao Tong University School of Oceanography), and Thomas Marchitto (University of Colorado, Boulder).

"The research tests a concept that has previously only been explored in theory and models," Lynch-Stieglitz says. "The large-scale Atlantic overturning circulation provides the nutrients that underly biological productivity in the North Atlantic."

Since the current is expected to continue weakening over the next century as a result of greenhouse gas emissions, researchers anticipate that the North Atlantic will receive fewer and fewer nutrients.

"This concept has real-world implications for the future health of the oceans and fisheries," Lynch-Stieglitz explains. Impacts range from a decline in fish populations to potentially impacting the amount of CO 2 the ocean can uptake.

"The dramatic climate changes the Earth has experienced in the past can help us understand what parts of the Earth system are vulnerable to change, and help us evaluate ideas about the impacts of the ongoing climate change," she adds.

An unlikely mystery

The team studied the Younger Dryas, a period of time during the transition out of the last ice age when there was a weakening of the Atlantic circulation. By examining how the nutrient stream changed when circulation weakened in the past, the researchers hoped to better understand what we may expect from today's warming oceans.

However, the team didn't initially set out with this goal in mind—the work began as an undergraduate research project with an intriguing mystery. Eric Blackmon, then a student in Lynch-Stieglitz's lab, was interested in investigating the disappearance of a species of plankton from the North Atlantic Ocean during the last ice age.

"The outcome of this study was puzzling," Lynch-Stieglitz recalls. The team decided to use a rarely used technique to better understand the results. The method of reconstructing seawater oxygen concentration produced an unusually clear record of how oxygen concentration in the seawater had changed through time.

"Our team realized that when combined with an earlier reconstruction of seawater chemistry, the technique provided key information on the history and mechanisms of nutrient delivery into the North Atlantic Ocean," Lynch-Stieglitz says. "We set out to answer a small question, and along the way discovered our data has broader implications than we anticipated."

Beautiful tiny shells

With this new technique, the team analyzed layers of sediment in the Florida Straits, a narrow passage between the Florida Keys and Cuba, where the Gulf of Mexico and the Atlantic Ocean meet. By coring into these layers and taking a cylindrical sample, "the layers of accumulating sediments provide an environmental history at the site," Lynch-Stieglitz explains. In this instance, "we looked at how the shells of single-celled organisms called foraminifera changed with time." Because foraminifera live on the ocean floor , their shells accumulate within each layer of sediment, preserving important chemical signatures that can be used to reconstruct the chemistry of the ocean in which they resided.

"It is pretty amazing that ocean chemistry of the past can be reconstructed in such detail using beautiful, tiny shells," Lynch-Stieglitz says.

The research showed that during the Younger Dryas, as the overturning circulation weakened, nutrients in the Gulf Stream decreased and the amount of oxygen in the Florida Straits increased. The team also found that as the nutrient stream decreased, the amount of biological productivity in the North Atlantic decreased as well.

"The study represents an important development of the carbon isotope-based proxy for past oxygen concentrations," Lynch-Stieglitz says. "The record is very clean, and the magnitude and timing of the changes in dissolved oxygen are mirrored to an astonishing degree in the phosphate reconstruction."

Beyond climate

Beyond these findings about how the ocean works, the team's study of foraminifera also provides new ways to understand how nutrients are cycled around the ocean, and how we investigate this. These windows into how Earth's oceans changed in the past provide a critical tool for testing models, letting us better predict how our oceans and the resources they provide may respond to climate change in the future.

"The physical changes in the earth system can have profound changes on life in the ocean, and far-reaching impacts," Lynch-Stieglitz notes. "Climate change is about more than climate."

Journal information: Science

Provided by Georgia Institute of Technology

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For microscopic organisms, ocean currents act as 'expressway' to deeper depths, study finds

S ome of the ocean's tiniest organisms get swept into underwater currents that act as a conduit that shuttles them from the sunny surface to deeper, darker depths where they play a huge role in affecting the ocean's chemistry and ecosystem, according to new research.

Published in the Proceedings of the National Academy of Sciences and based on fieldwork during three research cruises spanning 2017 to 2019, the study focuses on subtropical regions in the Mediterranean Sea. It uncovered how some microscopic single-celled organisms that are too light to sink beyond 100 meters or so—like phytoplankton and bacteria—end up going deeper into the ocean where there's not enough sunlight for these photosynthetic organisms to grow, live and eat.

"We found that because these organisms are so small, they can be swept up by ocean currents that then bring them deeper than where they grow," said Mara Freilich, an assistant professor in Brown University's Division of Applied Mathematics and Department of Earth, Environmental and Planetary Sciences who launched the work as a Ph.D. student a joint program at MIT and the Woods Hole Oceanographic Institution. "It's often a one-way trip for these organisms, but by taking this trip, they play a critical role in connecting different parts of the ocean."

Freilich conducted the research during her Ph.D. with Amala Mahadevan, senior scientist at Woods Hole, in a close collaboration with senior scientist at the Marine Biological Laboratory Alexandra Z. Worden and her team.

The currents the team found are called intrusions, and by sweeping up the tiny organisms, they help change the types of food available in the deeper layers of the ocean while also transporting a significant amount of carbon from the water surface. This helps feed other organisms in the ocean's food chain and increases the complexity of the ecosystem at deeper depths, influencing how life and chemistry work underwater.

Altogether, the study challenges conventional understanding of how carbon, which is turned into organic matter by photosynthesis in the sunlit layer of the ocean, gets transported to depth.

"The majority of photosynthesis—by which light is converted into organic carbon, a food source for living organisms—happens in the upper 50 meters of the ocean, so the question has always been: How does the carbon that gets fixed through photosynthesis get into the deep ocean?" Freilich said.

"The sinking of carbon-rich particles has always been thought to be the only answer to this question. But what we found is that tiny, single-celled organisms get caught in the oceanic flow to form intrusions… Such intrusions are significant features of the subtropical ocean—while they extend tens of kilometers laterally, they also descend hundreds of meters in the vertical, bringing cells and carbon with them. This mechanism has been unaccounted in previous estimates of carbon transport."

The researchers found that the intrusions occur year-round and originate in areas rich with biomass, including where the plant-like organisms are at their highest concentrations. Previously, ocean currents were only thought to carry carbon to depth seasonally. The researchers suggest these intrusions are widespread in the world's subtropical oceans. They provide conduits for the continual transport of carbon and oxygen from the sunlit ocean to depth.

"We observed microbial communities that looked just like surface microbial communities down to 200 meters," Freilich said. "In other regions, we think this could be a lot deeper. To our surprise, we found that the majority of microbes in the intrusions were bacteria that feed on carbon fixed by the photosynthesizing cells. This showed that the bulk of the biomass transported from the sunlit layers comprised of non-photosynthetic microbes."

Within a collaboration among the US, Spain and Italy, the scientists went on three trips to the subtropical Mediterranean ocean for the study. They used special tools to measure properties like water temperature, salinity and the abundance of the tiny organisms at different depths. The analyses, conducted in collaboration with microbial ecologist Alexandra Worden at the Marine Biological Laboratory, helped show the differences between intrusion samples and background waters.

Seeing that the microbial communities in the deeper intrusion samples resembled surface microbial communities showed that they were being transported to depth. The researchers also used computer models to simulate ocean currents to reveal how the communities of tiny plants and bacteria moved in the water.

"With the strong data from the Mediterranean establishing this process of three-dimensional conduits as a mechanism for bringing surface microbes to the dark ocean in warm waters, we have been able to see traces of similar export in major open ocean regions," said Worden.

Along with underscoring the ecological importance of intrusions in shaping oceanic biodiversity, the study also touches on how intrusions could be affected by climate change. It is thought that as the Earth's oceans get warmer, the proportion of carbon in tiny cells would increase and the transport in intrusions may not be as affected as other mechanisms that carry carbon to depth. Intrusions change our understanding of how carbon moves around in the ocean and could help regulate carbon storage and microbial dynamics in the deep ocean.

"There's so much more to explore now that we've found this," Freilich said. "What's next is taking what we've learned here and determining if we can use this to predict how changes in the microbial community composition would affect the transport of carbon and the global carbon cycle in a changing climate."

More information: Mara A. Freilich et al, 3D intrusions transport active surface microbial assemblages to the dark ocean, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2319937121

Provided by Brown University

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How Changing Ocean Temperatures Could Upend Life on Earth

Is the world’s climate close to a tipping point.

This transcript was created using speech recognition software. While it has been reviewed by human transcribers, it may contain errors. Please review the episode audio before quoting from this transcript and email [email protected] with any questions.

From “The New York Times,” I’m Katrin Bennhold. This is “The Daily.”

[MUSIC PLAYING]

Many of the effects of climate change are already with us — heatwaves, droughts, wildfires, flooding. But some of the most alarming consequences are hiding beneath the surface of the ocean. Today, my colleagues David Gelles and Raymond Zhong on just how close we might be to a tipping point.

It’s Tuesday, May 7.

David, you’ve been writing about different aspects of climate change for years and are definitely no stranger to distressing news about a warming planet. But something about ocean temperatures seems particularly distressing. What’s going on?

Starting last year, scientists started noting something alarming happening in oceans all around the world. It was warm in the oceans, but it started to get really hot in ways that they had never seen before. And for the last year-plus, I’ve been checking in with scientists. And they are increasingly concerned, perplexed even, with what they’re seeing.

And when you say oceans are getting hotter, can you just give me a sense of how much the oceans are warming and how fast?

Well, if you look at a chart that shows, say, the last two or three decades of average sea surface temperatures, you’ll see a gradual warming trend. But starting last March, we didn’t see another gradual increase. We saw this big jump. And from March of 2023 on, it stayed hot. And it’s just getting hotter. And we began 2024 at this much higher level than we’ve ever seen before.

And we’re still there. And in many parts of the world, the temperatures are still going up as we head into summer. And that has scientists really concerned.

And you said earlier that none of the scientists that you’ve been checking in with and none of these very sophisticated climate models that they’re operating with can explain this big jump. And I guess I have to ask at this point, why are these scientists so surprised? I mean, we’ve seen record-breaking heat waves for the past several years. It seems like every single year is hotter than the last. Why is it any surprise that the ocean is no different?

Well, they’re not surprised that the ocean is warming. They have understood for many years now that the overall man-made global warming that we’re experiencing all over the world in all these different ways is going to affect the oceans. Water is very absorbent for heat, and a lot of the extra heat that we’re producing from the burning of fossil fuels, a lot of the extra heat that’s being caused in the atmosphere as a result of that is just getting sucked in to the ocean. And bit by bit, over the last many decades, the oceans have gotten warmer.

So even after the air cools, say, in winter or something, the ocean holds on to a portion of that heat?

That’s right. Even when it’s cold outside, the oceans, year after year, have been getting a little warmer as a result of climate change.

But that on its own does not account for the kind of warming that we’re seeing right now?

Not even close.

So what else do they think could be going on?

Well, for the last year or so, the Pacific Ocean has been going through an El Niño cycle, which is when a lot of excess heat is released from the ocean. And in addition to making the Pacific Ocean hotter, it has sort of an overall warming nudge, a little boost for warmth in ocean heat around the world. But even that doesn’t explain the big jumps we’ve seen. And there’s another counterintuitive factor that scientists believe is playing a role here as well, and that has everything to do with the pollution being emitted by big ocean liners, by big ships traveling across the Atlantic.

So in 2020, some shipping regulations changed. And they required that the emissions from the fuel being used in big ocean tankers become much cleaner. And as a result of that, there was less sulfur dioxide in the shipping emissions. That’s a good thing for many reasons. Sulfur dioxide is a pollutant. It can have really adverse health effects on humans. But one of the things it was also doing was refracting sunlight away from the oceans.

And so what we’ve seen over the last recent years is, as there was less sulfur dioxide in the North Atlantic, as less and less of that particulate matter was in the atmosphere and able to bounce the sun’s energy back into space, more and more of the sun’s energy, more and more of the sun’s heat was making its way into the oceans. And that, scientists are now understanding, likely played a warming role as well.

So basically, lower emissions in these shipping lanes means less kind of smoggy cloud cover and, therefore, more direct sunlight hitting the ocean surface and heating it up, which actually kind of rings a bell. Because we talked about this in the show recently in relation to scientists wanting to artificially create more potent cloud cover to cool down the planet. It’s a little ironic that scientists are now trying to engineer the very thing we were trying to stop doing. Talk about unintended consequences.

That’s right. And this is one of those instances where there are these very tough trade-offs, right? It’s great, yes, that we have less pollution. That’s going to be good for public health. On the other hand, it may have allowed yet more warming in the oceans that’s having cascading effects across all sorts of ecosystems that we’re only just beginning to understand.

Yeah. Let’s talk about that for a moment. What are the effects of this mysterious warming? You mentioned ecosystems.

Well, there are a lot of effects when the oceans get this hot this fast. But perhaps the most immediate concern among people all over the world is the fact that in dozens and dozens of countries all over the world, we’re experiencing a record wave of coral bleaching, which is to say that coral reefs, these vitally important parts of the ocean ecosystems, are just dying at a rate we’ve never really seen before. And that’s going to have all sorts of negative effects for fisheries all over the globe.

Why are coral reefs so important?

Well, they matter for a lot more than just snorkeling. Yes, they’re beautiful. And yes, they are the center of a lot of tourist activity. But they’re also sort of this fundamental, foundational part of the marine food chain. And so when we think about all of the life that coral reefs sustain, that all allows for smaller fish to flourish and go out into the ocean. And those smaller fish contribute to the lives and the ecosystems of bigger fish.

And when we think back to the food chain that we learned about when we were in elementary school, coral reefs are right at the bottom of that. And if we lose that, it’s going to have devastating effects for marine ecosystems all over the world.

Is there any hope to save the coral at this point, or are they doomed?

My colleague Catrin Einhorn has been doing a lot of great reporting on the situation with corals. And she’s highlighted some of the efforts underway to try to save them, to grow more resilient corals. But the truth is, the scale we’re seeing, with bleaching events happening all around the world in something like 56 countries, it’s just not possible to stop it entirely at this point.

So it’s one of these dominoes in our ecosystem that can set off a whole range of problems and actually result in major losses to biodiversity.

And just to come back to the mystery of it all, we just don’t know what happens when those dominoes start to fall. And it’s not just corals that are being impacted here. The weather could well change as a result of these warmer oceans as well. And I’m particularly thinking about the upcoming Atlantic hurricane season. Warm water is a key ingredient to hurricanes. When you think about hurricanes and how strong they get and how fast they intensify, one of the most important factors in both of those dynamics is how hot the water is.

And when we see all this warm water hanging out in the Atlantic Ocean, that is leading forecasters to predict a potentially record-breaking hurricane season that is right around the corner in the North Atlantic. And that could affect the Caribbean, North America, and beyond.

What’s striking to me is that we know that the oceans are getting much hotter. We don’t fully know why, and we can’t fully explain what the impact will be, which really doesn’t sound great. But if we don’t fully understand what’s going on, then how can we even hope to do something about it?

Well, I’ve been asking scientists this very question. Like, what is there to be done? And the tough answer is, there’s no easy way to turn down the thermostat of the oceans. This warming is happening. And our job now is to live with it as best we can.

And we haven’t even talked about what some people regard as the biggest threat of all. As the oceans warm, they’re contributing to the melting of glaciers and the loss of Arctic sea ice. And as that happens, many people are worried not just about rising sea levels but also about the disruption to a vitally important ocean current. And if that happens, it could have massive ramifications for the entire planet. It could change just about everything we know about life on Earth.

After the break, my colleague Raymond Zhong talks about the possibility of that ocean current collapsing.

We’ll be right back.

Raymond, before the break, our colleague David Gelles told us all about this alarming trend of the ocean heating up and some of the very worrying consequences of that. But you are here to tell us about something else that may be happening in the ocean, something to do with ocean currents. And if I’m honest, it sounds like something straight out of a science-fiction movie. Can you explain?

Sure, Katrin. And actually, you’re right. It is something from a science-fiction movie.

I’m here at the global warming conference in New Delhi, where, if you can believe your eyes, it’s snowing.

About 20 years ago, a movie called “The Day After Tomorrow” came out.

The only force strong enough to affect global weather is the sun.

What about the North Atlantic Current? I got a call last night from Professor Rapson at the Hedland Center. He thinks the current has changed.

[MURMURING]

Oh, come on, Jack. How could that be?

And the plot of that movie is that there’s this major ocean current in the Atlantic that — it suddenly stops moving. It collapses. And it sets off this cascade of natural disasters —

In Nova Scotia earlier today, the ocean rose by 25 feet in a matter of seconds.

What you’re seeing are two actual tornadoes striking Los Angeles International Airport.

It’s a mob scene here at Grand Central Station. Over half the platforms are flooded, and service has suspended on all trains.

— that ultimately plunge the planet into a new ice age.

What can we do?

Save as many as you can.

I guess my question to you, Raymond, given what we’re talking about here is, which part of that movie is science fiction, and which part is actual science?

I’d say most of the movie is pretty purely fiction. But there is a kernel of truth in the science, which is that this vital ocean current in the Atlantic is very real. And just like in the movie, scientists are worried about what happens if it shuts down.

OK. So tell me about this current.

Scientists have given it a very, very unwieldy name.

They call it the Atlantic Meridional Overturning Circulation. But most people just use the acronym AMOC. AMOC.

Think of it as a giant conveyor belt of water that loops around the Atlantic Ocean. And it starts near the equator, goes up through the Caribbean, around the East Coast of the US, up toward Northern Europe, and then back again.

Scientists have come to realize how important AMOC is for a lot of the climate that we enjoy today. A lot of Northern Europe — Britain, Iceland, Scandinavia — is habitable today, really, because of AMOC. It’s because this system transports heat from the equator and very generously drops it off in Northern Europe — that even though it’s so far from the equator, it’s not as uninhabitable as, say, far Northern Canada or Siberia.

And let me just ask you, is this AMOC current the same thing as the Gulf Stream, which people write about and are worried about a lot these days?

Yeah, the Gulf Stream is a similar system of currents. But AMOC is the full loop. The Gulf Stream is just one part of it, but AMOC is really what’s important for the climate.

OK. So before we talk about AMOC shutting down or collapsing, can you actually kind of give me a quick science 101 explanation of how this thing works? Like, there’s this massive loop of water, like, I guess, an underwater river that you described which kind of transports warm water up towards the north and then comes back as cold water. What keeps this thing running? How does a current just stay in constant motion and loop around like that?

It has to do primarily with differences in temperature and salinity. Basically, fluids want to keep moving in a particular direction. And it’s driven by this balance between warm water, cold water, salty water, fresh water. Heavier water wants to sink. Lighter water wants to rise. And so the temperature and salinity of the water is sort of what determines how dense it is. And this density and differences in density keeps this giant loop moving.

It’s something that explorers noted in the Atlantic hundreds of years ago. They noticed that deep water was very cold. It was unexpected. And so I think as people have started studying the oceans more and science has advanced, they’ve realized there’s a very delicate system of differences in temperature and salinity that keeps this conveyor belt moving.

That’s really fascinating. I mean, I’ve always known we had these currents and that they were really important, but I never really appreciated and understood what it takes to keep them going. So, OK, tell me why scientists are so worried about AMOC shutting down.

As the planet warms, there’s something that might be significantly affecting this delicate balance. And that’s the melting of the Greenland ice sheet. With the melting of the ice, there’s this big infusion of fresh water into the northern Atlantic. And because that affects the salinity of the northern Atlantic, it sort of changes the balance of salinity and temperature, potentially enough to knock this loop off course. And there’s signs already that this is happening, that at least the current is slowing down. And one major piece of evidence is this cold blob that’s appeared in the northern Atlantic.

A cold blob?

A cold blob, that’s right. With most almost everywhere on the planet getting warmer because of the greenhouse gases that we’re putting into the atmosphere, there’s a conspicuous blob in the North Atlantic, near Greenland, that is getting cooler. And it’s exactly the place that scientists would expect to be getting cooler if AMOC were slowing down.

OK. So the Greenland ice sheet is melting due to climate change, and that seems to be disrupting this current. So as a result, less warm water is being transported north, and that’s why we have this cold blob that scientists have noticed in the North Atlantic.

That’s right.

And do we know what would happen if the current collapsed today?

So the best source of information we have about what happens when AMOC collapses comes from about 12,800 years ago —

— which is, as far as scientists know, the last major time this happened. Basically, the climate changed really, really quickly, at least by geological time. It was sort of less than 100 years, as far as scientists know. Much of the Northern Hemisphere got cold again. The temperature in parts of Greenland probably fell by about 18 degrees Fahrenheit.

Forests were replaced by tundra. Ice sheets grew again. As far away as California and the Southwestern US, you had evidence of cooler, drier conditions. And as far as scientists can tell, this may even have contributed to the disappearance of some of our early hunter-gatherer civilizations. Some of our ancestors were probably pushing into new territory as the ice sheets retreated and were suddenly confronted with another blast of cold.

Wow. OK. So the last time this happened, this current shutdown, the world was basically plunged into an ice age and wiped out part of humanity. Is that the scenario we’re looking at today?

I think scientists are careful about not being too precise about what it would look like if it happened again. The world, after all, is still warming. And so in a lot of ways, the climate is already quite different from the one that was around at the time. But certainly, I think scientists expect Northern Europe — the UK, Iceland, Scandinavia — to become a much colder place even than it was 200 years ago, before the Industrial Revolution, before humans started adding greenhouse gases to the atmosphere.

How cold are we talking? You mentioned the UK. I’m in the UK right now. I mean, would Britain suddenly look like the frozen tundra?

It would be significantly colder. It’s far north enough to really be almost Arctic. And I think an AMOC collapse could bring much stronger winter storms, deep cold in the winter that would be extremely dangerous. But at the same time, that cold around the North Atlantic doesn’t just translate straightforwardly into cold everywhere. Basically, if you don’t have AMOC, you have more heat that stays around the tropics and the equator. A warm ocean around the equator gives the fuel to storms around the equator and rainfall around the equator.

So it’s not just about getting that warm water from the tropics to Europe. It’s also getting that warm water away from the tropics to avoid things like extreme weather events.

That’s right. And it’s not just about the temperatures people would experience. It’s also about agriculture. And especially in vulnerable places like Africa, big shifts in rainfall, big shifts in temperature could really affect food chains, food supplies, our ability to feed ourselves.

And what does that mean for humans living in these places? Like, you were saying that, 13,000 years ago, Northern Europe basically wasn’t inhabitable anymore. Would humans still be able to live in these places?

We, as a civilization, as a species, obviously have some ability to withstand a range of temperatures, a range of weather conditions. But we really haven’t seen, in our recent history, at least, changes as fast as the ones scientists imagine an AMOC collapse would bring. It’s really hard to say, is Northern Europe ready for a much colder climate than it is right now? If, for instance, sea level rise accelerates on the East Coast of the US, are cities there prepared? That’s another consequence of AMOC that scientists are worried about.

And it’s obviously already happening. And you can see the effects of sea level rise in places like the Gulf Coast, North Carolina, Florida, even New York. But certainly, it would be a test for countries, societies around the northern Atlantic that they haven’t seen before.

OK. That’s a very scary prospect with everything that goes with it — mass floods and displacement of people, climate migration — everything that you can imagine. When do scientists predict that this could actually happen based on the data they have and assuming, of course, current human behavior and emissions don’t change?

As best as scientists can tell right now, they know AMOC is weakening. They expect it to continue weakening. Whether a collapse is imminent, whether it’s far away is still really, really hard to say. But it couldn’t quickly be reversed. Once you started on this process, the system just keeps moving in that direction.

Like a point of no return?

So if we’re already seeing signs that this current is weakening, does that mean we’re possibly already past that point of no return? Is it no longer a matter of if but when?

The really short answer is, we just don’t know. The best guess is that it’s not going to shut down this century. But plenty of scientists are worried. It’s something that a lot of them are focused on very intensely, just because we do think the consequences, if AMOC did shut down, could be so catastrophic.

If that current collapsed, though, is there anything we could do to bring it back? Like, it did come back 13,000 years ago after that last ice age. What happened?

Short answer again, nobody is quite sure. It seems to have abruptly grown warm again over 40, 50 years. But it’s pretty unclear why. And I think as far as what would happen today to bring it back, scientists would still say cutting greenhouse gas emissions and preventing the planet from overheating is probably the only thing we have in our control that could influence the climate on that scale.

I mean, it’s sort of where we land at the end of every episode about climate change. And it’s sort of interesting. I mean, probably like a lot of people listening to you, I find all this pretty scary and dystopian and worrying. It’s kind of crazy to think, actually, that scientists have warned about this for decades, that there was even a science-fiction movie made 20 years ago, which, in some ways, predicted what might be the real consequences of this current shutting down. It’s not like we humans lack the imagination of all the terrible things we’re risking here. But when it comes to protecting ourselves and the planet against this existential threat, we are clearly unwilling to do what it takes.

Yeah. Scientists have been thinking about AMOC and had a pretty good grasp of what AMOC looked like decades ago. As early as the ‘80s and ‘90s, scientists made this connection between the warming that humans were bringing about and consequences like an AMOC collapse and other things as well.

It was one of the scientists who wrote about AMOC in the ‘80s who said, “The climate system is an angry beast, and we are poking it with sticks.” And that’s still true today.

Well, Raymond, thank you very much.

Thank you, Katrin.

Here’s what else you need to know today. On Monday, the judge in Donald Trump’s hush money trial held the former president in contempt for repeatedly violating a gag order and threatened to jail him. The judge told Trump that the last thing he wanted to do is to put him in jail, but at the end of the day, he had a job to do and would seriously consider it. And Israel stepped up its attack on the southern city of Rafah in Gaza hours after Hamas said it was ready to accept a ceasefire proposal.

The proposal was put forward by Egyptian and Qatari mediators. But Israel responded by saying that it failed to meet its demands. The prime minister’s office said it would still send a delegation to talk about how to reach an acceptable deal.

Today’s episode was produced by Carlos Prieto, Michael Simon Johnson, Alex Stern, and Diana Nguyen. It was edited by Devon Taylor, contains original music by Rowan Niemisto and Marion Lozano. Our theme music is by Jim Brunberg and Ben Landsverk of Wonderly.

That’s it for “The Daily.” I’m Katrin Bennhold. See you tomorrow.

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• May 7, 2024   •   27:43 How Changing Ocean Temperatures Could Upend Life on Earth
• May 6, 2024   •   29:23 R.F.K. Jr.’s Battle to Get on the Ballot
• May 3, 2024   •   25:33 The Protesters and the President
• May 2, 2024   •   29:13 Biden Loosens Up on Weed
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• April 30, 2024   •   27:40 The Secret Push That Could Ban TikTok

Hosted by Katrin Bennhold

Featuring David Gelles and Raymond Zhong

Produced by Carlos Prieto ,  Michael Simon Johnson ,  Alex Stern and Diana Nguyen

Edited by Devon Taylor

Original music by Rowan Niemisto ,  Marion Lozano and Dan Powell

Engineered by Alyssa Moxley

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While many of the effects of climate change, including heat waves, droughts and wildfires, are already with us, some of the most alarming consequences are hiding beneath the surface of the ocean.

David Gelles and Raymond Zhong, who both cover climate for The New York Times, explain just how close we might be to a tipping point.

On today’s episode

David Gelles , who reports for The New York Times Climate team and leads The Times’s Climate Forward newsletter .

Raymond Zhong , a reporter focusing on climate and environmental issues for The New York Times.

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Scientists are freaking out about ocean temperatures.

Have we crossed a dangerous warming threshold? Here’s what to know .

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The Daily is made by Rachel Quester, Lynsea Garrison, Clare Toeniskoetter, Paige Cowett, Michael Simon Johnson, Brad Fisher, Chris Wood, Jessica Cheung, Stella Tan, Alexandra Leigh Young, Lisa Chow, Eric Krupke, Marc Georges, Luke Vander Ploeg, M.J. Davis Lin, Dan Powell, Sydney Harper, Mike Benoist, Liz O. Baylen, Asthaa Chaturvedi, Rachelle Bonja, Diana Nguyen, Marion Lozano, Corey Schreppel, Rob Szypko, Elisheba Ittoop, Mooj Zadie, Patricia Willens, Rowan Niemisto, Jody Becker, Rikki Novetsky, John Ketchum, Nina Feldman, Will Reid, Carlos Prieto, Ben Calhoun, Susan Lee, Lexie Diao, Mary Wilson, Alex Stern, Dan Farrell, Sophia Lanman, Shannon Lin, Diane Wong, Devon Taylor, Alyssa Moxley, Summer Thomad, Olivia Natt, Daniel Ramirez and Brendan Klinkenberg.

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Katrin Bennhold is the Berlin bureau chief. A former Nieman fellow at Harvard University, she previously reported from London and Paris, covering a range of topics from the rise of populism to gender. More about Katrin Bennhold

David Gelles reports on climate change and leads The Times’s Climate Forward newsletter and events series . More about David Gelles

Raymond Zhong reports on climate and environmental issues for The Times. More about Raymond Zhong

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