Topics Covered in this Lecture:
Video: Currents Lab
- The Major Ocean Currents
- Ekman Spirals
- Eddies and Rings
- Ocean Waters of the World
In this lecture, we will 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. TThe bird's-eye view images and video captured from 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. Still, there is nothing like a good rubber "ducky" to measure the ocean currents, and in 1990, a container ship carrying rubber NIKES (yes, the sneaker) inadvertently performed a most interesting study in ocean currents. We will learn more about this at the close of today's lecture.
The Major Ocean Currents
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.
As discussed in a previous lecture, differential heating of the globe 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 contrasted to the density-driven deep water circulation we talked about earlier. 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.
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 Straights and converges with the Kuroshio Current before becoming part of the North Pacific Current. Note also that both the Kuroshio and California Currents 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, the 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 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. Be sure to read your textbook for a more complete description of the global wind patterns that give rise to these currents.
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.
Your book provides a figure that illustrates an Ekman spiral. 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. Your book illustrates some of the basic physical processes that cause upwelling.
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 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. We will return to fishes of the sea towards the end of the semester. For now, let's move on with our study of the physical oceanography of currents.
Rings and Eddies
Our discussion of this topic 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 "tornado-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. Your book has a wonderful illustration of the formation of 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 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.
Ocean Waters of the World
The physical processes that change the density of seawater differ in very distinct ways across the world ocean. Recall the patterns of evaporation and precipitation we talked about in a previous lecture. As a result of these two processes, different regions of the world ocean give rise to distinctive masses that can be identified based on their temperature and salinity characteristics. In addition, the formation of these waters drive the circulation patterns of deep-sea, or abyssal, waters. Because this circulation depends on the density differences between water masses, driven by changes in the temperature and salinity of seawater, this type of deep-ocean circulation is called thermohaline circulation.
Water masses can be divided into five parts:
The end result is a layered ocean about which we can form some generalities and for which we can derive individual names for the particular water masses that form in a given region.
Let's start with the Atlantic Ocean. A water mass formed off the coast of Iceland, known as the North Atlantic Deep Water, appears to drive the major deep ocean currents of the world ocean. This region of the world is characterized by cool temperatures and lots of precipitation, resulting in a water mass with a salinity of approximately 34.9 ppt and a temperature of 2 to 4 degrees C. North Atlantic Deep Water sinks to the bottom of the ocean and flows south along the east side of the Atlantic Ocean. Eventually, as the story goes, this water loops its way into the Indian Ocean and surfaces in the equatorial upwelling zone in the Pacific Ocean!
This circulation of deep water in the oceans is known as the "global conveyor belt", a concept developed by Wallace Broeker at Lamont-Doherty Earth Observatory, where I did my post-doctoral research. Take a look at the figure in your book to get some idea of how this process works. Although North Atlantic deep water is not the most dense water formed in the world ocean, it is by far the most abundant, and, for this reason, it has been suggested that North Atlantic deep water drives the circulation of the deep ocean.
The densest water in the world is produced in the Antarctic Ocean, primarily in the Weddell Sea in the winter, along the edge of the Antarctic continent. The combination of high salinity and cold temperatures leads to water with the highest density of any water formed in the ocean. Antarctic bottom water, as it is called, has a temperature of approximately -0.5 degrees C and a salinity of around 34.8 ppt. Although Antarctic bottom water has the highest density, it is formed in small quantities. For the most part, according to your book, it is limited to the bottom regions along the coast of South America. Here the Mid-Atlantic ridge keeps it from extending into the eastern side of the Atlantic. However, according to other sources, Antarctic Bottom Water has been found in the Pacific Ocean at the equator (a journey of 1,000 years) and as far north as the Aleutian Islands (another 650 years from the equator). In the Atlantic Oceans, Antarctic Bottom Water has been found as far north as 40 degrees N, a journey of 750 yeas. Thus, at times, sufficient quantities may form to creep far from its origins. Its sluggish movement and confinement to the deep sills of the southern oceans means that this water mixes slowly and allows Antarctic bottom water to retain its characteristics for up to 1,600 years!
Intermediate waters are those water masses that form a layer above deep and bottom waters, and generally represent the top of the permanent thermocline in the oceans of the world. Atlantic Intermediate Water forms as a result of the mixing of cold waters from the north and saline waters from the south. The resultant intermediate water floats as a layer on top of the North Atlantic Deep Water and flows south. Its counterpart, the Antarctic Intermediate Water, forms in a similar way, although its density is not as great as Atlantic Intermediate Water. This water flows north towards the equator at a depth of approximately 500-1000 meters.
In the Pacific Ocean, Intermediate Water masses are again dominated by Antarctic Intermediate Water below the equator and formed as intermediate water, known as North Pacific Intermediate Water, in the North Pacific. In the Indian Ocean, the intermediate waters consist of Antarctic Intermediate Water.
Central waters are those water masses that form directly above the permanent thermocline. Generally, they are confined to regions closer to the equator. In all three oceans, central water masses correspond to the regions where they exist. In the Atlantic Ocean, we have the North Atlantic Central Water and the South Atlantic Central Water. In the Pacific, the North Pacific Central Water and South Pacific Central Waters are present. In the Indian Ocean, central waters consist of Southern Indian Central Water and Equatorial Central Water.
A few "specialized" water masses should be noted. In the Atlantic Ocean, water coming out of the Mediterranean and flowing over the sill at Gibraltar forms water known as Mediterranean Water. Traces of Mediterranean Water have been found as far as 2,500 miles away from its source! This water is very distinct and imparts a greater complexity to deep water circulation in the Atlantic Ocean. In the Indian Ocean, the Red Sea serves a similar role, providing a distinct characteristic water mass to deeper waters in the northern Indian Ocean.
Identification and characterization of these water masses is one of the favorite pastimes of physical oceanographers. As the role of the oceans in controlling the climate of our globe has become better understood, the need to understand the factors which govern the formation and circulation of these water has become increasingly important. Understanding the properties of these water masses and defining their distribution across the globe are one means to better understand how they are formed.
The "take-home" message here is simple, yet profound. The oceans consist of distinct layers or "lenses" (or "blobs") of water that move about the globe and retain their characteristics for hundreds, if not thousands, of years. This "layer cake" model of the world ocean serves to define the "structure" of the oceans. This structure of the sea functions much in the same way that the "structure" of a forest functions; it creates distinct zones and regions on which organisms can depend and to which these organisms can adapt. It creates a type of framework on which the tapestry of ocean ecosystems can be weaved.
A Few Final Thoughts
Clearly, the ocean is not a smooth and homogeneous place! With the advent of satellite and space-shuttle oceanography, our view of the oceans has changed dramatically. How can we explain these structures in the context of the simple models of wind-driven circulation of the surface currents, or the density-driven currents of the deep ocean? How can we hope to understand the nature of the interactions of the physical, chemical, and biological processes within these complicated movements of water? These are just a few of the formidable challenges facing oceanographers.
Finally, you should now be getting a better picture of how physical, geological, chemical, and biological processes interact in the ocean. Think about how the physics of the ocean creates rings and eddies, how it creates upwelling centers, how it creates a layered structure that changes over the seasons. Think about how landmasses and the topography of the bottom divert or trap the surface and deep currents; these large-scale geological features create different patterns of flow like the squashed flow of currents in the northern Indian Ocean or the two-gyre expanded system of the South Pacific Ocean. Think about the evolution of chemical characteristics within each of these physical structures; cold-core rings having centers of nutrient-rich water and warm-core rings being like deserts. Think about how these rings and eddies, upwelling centers and layered oceans create habitats for marine organisms; the structure and evolution of these features create plankton blooms, provide conditions well-suited for seaweeds and fish, and are persistent enough over the course of the seasons and years to attract the attention of the smartest animals of the sea, the whales.
Take these notes, go down to the beach, make yourself comfortable, and stare at the sea. Read a few paragraphs, look at the sea; read a few more paragraphs, look again at the sea. You will begin to understand how interconnected are all these processes in the sea. You will begin to appreciate the vast and complex drama that is played out in the oceans everyday. You will begin to feel the call of the sea, your own primordial rhythms urging you back to the birthplace of your ancient ancestors. Once felt, you will never look at the sea in the same way again. But once felt, you will experience a profound and heartfelt awareness that you too, are an important part of this precious cycle, and that you too, can make a difference. Congratulations! You are now beginning to understand the remarkable world of oceanography.