Week Five: The Major Ocean Currents

Here we review the major ocean currents of the world ocean. Driven by the force of the winds, bent by the spin of the Earth, and spurred by the passion of poets and sailor, the major ocean currents bring a host of good and bad influences on human affairs. Influencing our global climate , providing routes for ships in global trade, and creating fascinating whorls of water we are just beginning to understand, the ocean currents continue to confound and amaze us. The bird's-eye view afforded by our ventures into outer space have opened a new era in the study of ocean currents and led to a much greater appreciation for the structure of the oceans as a whole.

5.1 General Circulation

As the wind blows across the surface of a body of water, an amazing thing happens: the water begins to move. First, small capillary waves  are formed; tiny ripples of waves which appear like a brushstroke across the water's surface. If the wind continues to blow, larger waves appear, momentum is transferred to the water, and the water begins to move. Moving water is subject to the Coriolis effect , just like the wind, and it begins to bend. In the Northern Hemisphere, water is deflected to the right of the direction of the wind; in the Southern Hemisphere, water is deflected to the left. The net effect is that surface currents move at a 45 degree angle to the wind.

Differential heating  of the globe is what gives rise to global wind patterns. Because they are persistent, these winds create "permanent" movements of water, which we know as currents. The wind patterns, the Coriolis effect , and the arrangement of landmasses are responsible for large-scale patterns of wind-driven surface currents in the world ocean.

These wind-driven surface currents can be compared and to the density-driven deep water circulation. Surface currents are much swifter and driven by the wind. Density-driven currents are slow, and result from changes in seawater density which occur as a result of temperature and salinity changes. Surface currents are subject to large deflections due to the Coriolis effect ; density-driven currents are less influenced by the Coriolis effect because they are so slow moving. Finally, landmasses create barriers to the flow of surface currents whereas the landscape of the ocean floor , especially ridges and basins, impede the flow of deep-water currents.

In general, each hemisphere of an ocean has its own gyre , a circular motion of water. Gyres  in the northern hemisphere rotate clockwise and gyres in the southern hemisphere rotate counterclockwise. Let's take a look at these ocean currents and become familiar with their names and movements.

The current that runs southward along our coast is known as the California Current . This current moves cold water from the Gulf of Alaska  along our coast and is partially responsible for the relatively cold water we experience when we go to the beach. The California Current feeds into the North Equatorial Current , which heads west along the equator at about 5 degrees N. As this current turns northward, it becomes the Kuroshio Current , which flows along the eastern coast of Japan . As bends east under the Coriolis effect , it becomes the North Pacific Current, which complete the loop. (see the web site for the best map.)

Another major current feeds off the North Pacific Current in the Gulf of Alaska . This is the Alaskan Current  which loops northward along the coast of Alaska and flows west along the Aleutian Islands . Another important current in this region is the Oyashio Current , which flows southward from the Bering Straits and converges with the Kuroshio Current  before becoming part of the North Pacific Current. Note also that both the Kuroshio and California Current partially result from the accumulation of water masses  driven by the trade winds . Thus, these currents are both wind-driven and gravity-driven, or geostrophic .

South of the equator, the South Equatorial Current  draws water from the South Pacific  Ocean and flows westward at a latitude of about 5 degrees S, in the same direction as the North Equatorial Current . Note that as a result of the "piling up " of water in the eastern Pacific , another current is formed, the Equatorial Countercurrent , which flows in between the North and South Equatorial Currents and returns water eastward, where it hits the coast of Mexico  and diverges to the north and south. This northern divergence, which runs northward along the coast of southern California in a direction opposite to that of the California Current , is known as the California Countercurrent .

As the South Equatorial Current  flows westward, it bends south (left) under the influence of the Coriolis effect . Here, it is split into separate flows by the various islands of the South Pacific . Collectively, these currents are known as the East Australian Current  because they flow along the east coast of Australia . However, they may assume more regional names based on the landmasses near where they flow. At one point or another, all these currents flow into the West Wind Drift , which circulates around the continent of Antarctica . The West Wind Drift is the only surface current which literally travels around the globe. Along the western coast of South America , the Peru  Current  flows northward and completes the circulation of the Southern Pacific .

Among physical oceanographers, there is general agreement that the South Pacific  Ocean actually consists of two gyres. Because this ocean is so large (the largest, in fact, of all the ocean basins), its currents tend to be slow and somewhat complex. For the purposes of our discussion, we will treat the currents here as one giant gyre , but you should be aware that this treatment is a simplification.

Let's move on to the Atlantic  Ocean. Perhaps the most famous of all currents is the Gulf Stream , recognized by Ben Franklin (and his cousin!) as a powerful stream of water that could be used to assist the ships that delivered mail and other goods from Europe to America. The Gulf Stream, which flows northward along the East Coast of the United States, is the most powerful and swift of the ocean currents. Flows in the Gulf Stream have been clocked at 5 miles per hour, moving 55 million cubic meters of water per second, 300 times the volume of water moved by the Amazon River . Its average width is 43 miles with a depth reaching 1,500 feet. Water in the Gulf Stream can move more than 100 miles in a day. In fact, standing on a beach in Florida  (where I spent much of my time as a young boy), the Gulf Stream is evident as a powerful train of standing waves moving northward in the distance. Look for it if you ever get to West Palm Beach .

The Gulf Stream  moves water northward as it flows from the Florida Current , which circulates through the Gulf of Mexico  and the straits of Florida, and the North Equatorial Current , which flows westward along the equator. To the north, the Gulf Stream feeds into the North Atlantic Current , which splits in northern and southern directions along the coast of Ireland . The southward flow turns into the Canary Current , named for the Canary Islands  off the coast of southern Morocco  in northern Africa . Water flowing north along the coast of England  becomes the Norwegian Current , as it moves north along the coast of Norway . Other currents in this region include the East Greenland Current , which flows south along the east coast of Greenland, and the West Greenland Current , which flows north along the west coast of Greenland.

As with the Pacific  Ocean, the equatorial currents are the same; the North Equatorial Current  flows westward north of the equator and the South Equatorial Current  flows west south of the equator. Between these currents, the Equatorial Countercurrent  flows eastward, becoming the Guinea Current  along the horn of Africa , where it converges with the northward flowing water of the Benguela Current . On the opposite side of the South Atlantic , the Brazil  Current  flows south along the east coast of South America  and turns eastward at the Falkland Islands . An extension of the West Wind Drift  flows through the Falkland Islands here and converges with the Brazil Current. This current is known as the Falkland Current .

We complete our study of the currents in the Indian Ocean . Because of its small size, the gyres  here are reduced in size. In fact, the North Indian Ocean, that small portion of the Indian Ocean north of the equator, has a circulation pattern that consists primarily of the flow of the North Equatorial Current  westward along the southern tip of India  and the flow of the eastward-moving Equatorial Countercurrent  just south of the equator. In the southern Indian Ocean, the South Equatorial Current  predominates, becoming the Agulhas Current  along the east coast of Africa . The Agulhas Current flows south around the Cape of Good Hope  where it converges with the Benguela Current  and the West Wind Drift . To complete the circulation, the West Wind Drift carries water eastward to the coast of Australia , where it flows north in a current known as the West Australia Current .

The flows of these major currents can give rise to very complicated but interesting ocean features. In fact, as man traveled into space and began to take pictures of the ocean, it became apparent that the flows of ocean currents are anything but simple circular movements of water. Nonetheless, these major ocean currents have historical and oceanographic significance, and help us understand the general movements of water about the globe.

5.2 Ekman Spirals

When the wind blows across the surface of the water, the water begins to move. As we learned above, this moving begins to deflect to the right (in the Northern Hemisphere) or the left (in the Southern Hemisphere) as a result of the Coriolis effect . As the surface water moves, it exerts drag on the layer of water immediately below it, causing this layer of water to move. As this layer moves, it also begins to deflect. Each successive layer of water beneath the surface layer moves in this way; flowing a bit slower than the layer above it but deflecting under the influence of the Coriolis effect. As a result of the successive movements and deflections of these layers of water, a spiral pattern is created, such that at some depth, the water is actually flowing in a direction opposite to that of the surface waters . This spiral pattern is called an Ekman spiral , named after the man who developed the mathematical formulations to describe this phenomena.

An illustration of the Ekman spiral  is shown below. Note that the average direction of water flow, the net transport, is 90 degrees to the direction of the wind. This contrasts with the 45 degree angle that surface water moves in relation to the wind. You should also realize that the water flows slower as depth increases. The transfer of wind energy throughout the water column is not 100% efficient. The frictional forces between successive layers of water remove energy from the flow at each successive depth.

Ekman spirals  may persist as deep as 100 to 150 meters. Their biggest effect takes place on our California coast (and other regions of the world) in the form of a ocean phenomenon known as upwelling . The net movement of water at a 90 degree angle to the wind, known as Ekman transport , forces surface water away from our coastline when the wind is blowing from the north (as it often does during the winter). Confirm this for yourself by pointing a pencil towards the south along a map of our coastline and determining the direction of water flow. As surface water moves away from the coast, it must be replaced from somewhere. A current pattern is set up whereby cold, deeper water is transported to the surface to replace the warm surface water that has been swept seaward by the wind. This process is called upwelling.

The figure below shows upwelling  for the southern hemisphere. Why? In what direction must wind blow along the coast of California for upwelling to occur?

This upwelling  of cold, nutrient-rich, deeper water has a profound impact on the plankton community. In fact, the most productive regions of the world ocean occur at upwelling "centers", areas where upwelling occurs. There are several places along our coast where periodic upwelling "episodes" create blooms of plankton. The Whale Watching Point at Point Vicente  on the Palos Verdes Peninsula is one place where upwelling occurs regularly. Perhaps this is why whales visit here so often!

Another effect of Ekman transport  is to drive water towards the center of a major ocean gyre . If you follow a major current on its path, you will quickly understand that the net movement of water is directed towards the center of the gyre . This forces water to "pile up" in the center of gyres, creating a "mound" of water perhaps 3 feet higher than the "surrounding" water. These words are placed in quotes because in actuality you couldn't go out in the middle of a gyre  and see a mound of water. The elevation change due to the piling up of water happens gradually over hundreds of miles. However, this piling up of water causes it to flow "downhill" due to the force of gravity. There is a "butting of forces" as water wants to move inwards (as a result of the Coriolis effect ) towards the center of the gyre  and outwards as it wants to flow downhill (as a result of gravity). At some point, these forces balance themselves out, and create a circular pattern of flow around the mound. This balance between Coriolis driven Ekman transport and gravity-induced flow is called geostrophic flow .

A good example of a "habitat" created by geostrophic flow  is the Sargasso Sea . This mound of water in the middle of the North Atlantic  Ocean used to frighten sailors because of the large mats of Sargassum seaweed  found in this region. Sailors believed that sea monsters lurked beneath the seaweed, and the hot, listless climate  did little to dispel those fears.

The Sargassum seaweed  lives its entire life in the Sargasso Sea  and probably accumulates there as a result of the geostrophic flow  that creates the Sargasso Sea. This seaweed has learned to adapt to the warm, saline water here, and, at one time, Sargassum covered this region in profusion. In recent years, there has been some concern about the lack of Sargassum in the Sargasso Sea. Scientists are working to understand whether man-made factors or natural factors have caused reductions in the populations of Sargassum in this sea.

At any rate, these large floating mats of Sargassum make a wonderful hiding place for a whole variety of marine organisms. Some organisms spend their entire lives in the mats of seaweed. One animal in particular is the Sargassum fish . This rather strange looking fish has taken on the appearance of the seaweed, having brown-and-yellow seaweed-like colors and knobby and leafy protrusions that act as camouflage . Although this fish attains a length of only a few inches, it is a voracious predator. Using "wait-and-pounce" tactics, this little fish hangs out beneath the mats of seaweed and when another small fish gets close, the Sargassum fish pounce with the might of a lion and swallows the other fish whole with its large, extensible mouth. When I was a lad, I made the mistake of putting a Sargassum fish into my aquarium with twelve other fish. Every day another fish was gone, a puzzle I couldn't solve. After a couple of weeks, I had the answer. The only fish left in the tank was the Sargassum fish. Such is the life of a young oceanographer!

Other fishes, like Dolphin Fish, Florida Sailfish, and Mola mola , the Giant Sunfish, like to lounge beneath the Sargassum mats, finding food, shade, and shelter. An entire ecological community has developed here around the mats of Sargassum, and they tell a fascinating story if you ever get a chance to read about it.

5.3 Rings and Eddies

Our discussion of ocean currents would not be complete if we did not mention another of the major ocean "structures" created as a result of the major ocean currents. These are the "hurricane-like" structures known as rings and eddies .

As currents flow along a coastline or in the middle of the ocean, they encounter resistance from landmasses or water with different properties. As a result of this resistance, the current meanders and pinches off into a "whirlpool ". If you have ever observed water flowing in a stream, you have seen whirlpools from around the backsides of rocks or other impediments in a stream. The process is essentially the same in the ocean. The only difference is that the "whirlpools" or eddies  that form in the ocean are much larger and generally more intense in terms of energy and momentum.

We don't have time to review all the processes that lead to the formation of eddies , but I want to introduce you to the major features of the eddies that form as a result of the Gulf Stream . Below is a wonderful satellite sea-surface temperature image of the Gulf Stream that illustrates the two types of eddies, or rings, as they are now called: these are the warm core rings  and the cold core rings . As the Gulf Stream flows north, it encounters the Labrador Current  flowing south along the banks of Cape Hatteras. As these two currents meet, they begin to meander (they wind back and forth like a snake). Eventually, these meanders "pinch off" from the main flow and become independently rotating structures, known as rings.

One set of rings, which contains the cold water of the Labrador Current , are known as cold core rings . These cold core rings  spin off from the Gulf Stream  and are propelled eastward into the North Atlantic  Ocean. Their movements may take them quite far from the Gulf Stream and, depending on their size, they may retain their characteristics for months. As they spin off from the Gulf Stream, they have a certain rotational velocity, a net direction, and a characteristic temperature structure. Because their centers contain cold nutrient-rich water, a plankton bloom  develops in the middle of these rings. The development of the plankton bloom , the development of organisms that feed on the plankton, and the eventual "death" of the ring as it mixes with the surrounding water is a fascinating study in the ecological succession  of plankton communities. In recent years, entire oceanographic studies have been devoted to understanding the nature of the formation and evolution of these rings, and the biological communities that develop as a result of these rings.

On the other side of the Gulf Stream , warm water pinches off into structures called warm core rings . These rings typically spin off west and north of the Gulf Stream, and travel against the flow of the Labrador Current . Because their centers are composed of warm, nutrient-poor water, conditions are not ripe for plankton blooms . As such, these rings typically don't develop the kinds of biological communities we observe in cold-core rings .

The satellite image of sea surface temperature reveals quite nicely the differences between cold-core and warm-core rings. In the image shown, reds, oranges, and yellows are warm water, and greens and blues are cold water. Take a look at the center of the picture. You should be able to make out swirling masses of water associated with the northernmost part of the Gulf Stream . Can you see two dots of green in the middle of the red and yellow water? These are cold core rings . Just above them, on the other side of the Gulf Stream, is a large swirl of yellow water floating in the middle of green water. This is a warm-core ring. Note also the meanders of the Gulf Stream as it bends towards the east. It is these meanders that give rise to these rings.

Wouldn’t it be spooky if the ocean always looked that color?