There are many parallels between the atmosphere and the oceans. Both are composed of fluids, air or water, which are free to flow under the influence of temperature differentials. Like the atmosphere has winds, so the ocean has currents. As with the wind, these currents are a means to redistribute solar energy from one place to another. Just as the atmosphere has surface winds and upper atmospheric circulation, the oceans also have surface currents and deep water currents. Also like the winds, many of these currents have established "permanent" patterns, which can last for tens of thousands of years. Our climate is very much dependent on ocean currents, since the redistribute massive amounts of heat energy from one part of the Earth to another. A disruption of ocean currents would lead to dramatic changes in the climate.
Ocean currents have a serious impact on our lives in other ways as well. They are responsible for the accumulation of nutrients in rich patches, which are prime fishing grounds. Many species of marine life take advantage of ocean currents for their seasonal migrations. Even modern problems such as the accumulation of debris in "garbage patches" on the oceans is driven by currents.
The movement of ships is also impacted by currents - traveling along a current saves fuel, while traveling against it costs more fuel. In the old days of sail ships, this impact could be even more serious -- the Agulhas Current in the southwest Indian Ocean was a serious obstacle to Portuguese sailors trying to reach India.
The volume of water carried by ocean currents is tremendous. It is measured in units called Sverdrup, where 1 Sverdrup is a flow rate of 1 million cubic meters of water per second. To give an idea of how large this circulation is, the total flow of fresh water from all the rivers in the world is about 1 Sverdrup. Meanwhile the flow of just one single ocean current - the Gulf Stream - varies from 30 Sverdrup to 150 Sverdrup, depending upon its location. This is why an oceanic current which is just a few degrees warmer or colder than the surrounding water can carry enormous amounts of heat energy from one location to another.
Typically, ocean currents are divided into two types: surface currents (which usually extend no more than about 400 meters below the surface), and deep water currents (also known as the thermohaline circulation) which occur in much deeper layers of the ocean.
Causes of Ocean Currents
There are several causes for ocean currents, including:
This is the single most important cause. The Sun provides the bulk of the energy which drives the circulation of water in the oceans, either directly or indirectly (through winds). The uneven distribution of solar energy across the globe (highest at the equator, decreasing towards the poles) produces an uneven heating of water in the ocean. Like air, hot water expands. The differential heating is so pronounced that sea level at the equator is about 8 cm (3.15 inches) higher than at temperate latitudes.
The equatorial bulge of the oceans caused by the expansion of water under equatorial heat creates a slope, and water tends to run downhill under the force of gravity. This is one of the major reasons for surface water flow from the equator towards higher latitudes. Compare this to the flow of air (winds) at the surface, also from the equator to the tropics, which is described here (the trade winds).
Winds produce a flow of water at the ocean surface due to frictional coupling between the wind and the surface of the oceans. Since the oceans are largely flat expanses unobstructed by topography, winds can blow for long "fetches" or distances, for prolonged periods of time. Friction between the air and the surface of the water is sufficiently high that a wind blowing for about 10 hours can produce a surface current in the water at about 2% of the wind velocity. So a steady wind blowing in a certain direction at 20 miles per hour for about 10 hours will produce a surface water current at about 0.4 miles per hour. The direction of the water current is not the same as that of the wind flow. The direction of the water current is affected by a phenomenon known as Eckman Transport.
Briefly, a column of water can be thought of as consisting of many layers. Wind friction affects the topmost layer, pulling the water in the direction of wind flow. This top layer of water tends to pull layers of water beneath, but because of the Coriolis force (described in the section below), the water actually moves at an angle to the side. In progressively deeper layers, the sideways movement is enhanced, so the entire water column can been thought of as moving in a spiral. The net flow of water is almost at right angles to the direction of the wind.
The figure on the right shows Eckman Transport in the northern hemisphere, under the influence of the westerlies, which are the prevalent winds over most of the continental United States. The westerlies or anti-trade winds, blow from southwest to northeast in the northern hemisphere. When flowing over oceans or large lakes, surface water is pushed further eastwards, so it flows in a direction about 45° east of the wind direction. In deeper layers, the direction is deflected even further to right, and in the very deepest layers (at about 100 - 150 meters, which is about the maximum depth affected by the wind), it is deflected so far to the right from the wind's direction that it is actually flowing south, almost opposite to the direction of the wind. However, if all these vectors are summed up, the net direction of flow is about 90° to the right of the wind direction.
The duration of the wind is very significant. Since water is much heavier than air, it also has much more inertia. Short duration winds only produce turbulence at the water's surface. It takes winds blowing over a longer duration to produce a sustained movement of water in the wind's direction. However, as we described here, there are many long-duration wind patterns (such as the trade winds or the westerlies) which blow for sustained periods of weeks or months in the same direction over vast stretches of ocean, so wind driven ocean currents are a very significant factor in ocean circulation.
Coriolis Force and Ocean Gyres
This is a pseudo force resulting from the Earth's rotation from west to east about its axis. Because of the Earth's rotation, any movement away from the equator (in both the northern and southern hemispheres) is deflected eastwards, while movements towards the equator are deflected westwards. This effect is very pronounced in movements that happen within a fluid medium (atmosphere and oceans), and over long distances. The Coriolis Effect is described in more detail here, in the section relating to wind patterns.
Because of the Coriolis Effect, currents tend to flow in curves rather than in straight lines. When the space for movement is restricted (such as by land bounding the oceans), these curves can close in on themselves, and cause a circular flow of water around a center. Such circular flows are called oceanic gyres. There are many permanent gyres in the world's oceans. Their locations are dictated by the temperature of the water and the geography (the ocean-land boundary).
The figure to the right shows an ocean gyre. The central bulge is caused by heat induced expansion of the water. These bulges can be huge - hundreds of thousands of square kilometers in size over the expanse of the open oceans. Under the force of gravity, water flows down from the top of the bulges to the bottom. However, because of the Coriolis effect, the water does not take a straight path. Instead, it's curved east or westwards depending upon its flow direction. The figure on the right shows the current flowing in a clockwise direction. This would only happen if water flowing southward was deflected east by the Coriolis Effect, while water flowing northwards was deflected west. This indicates that the equator must be south of this gyre, since the direction of deflection would be opposite in the southern hemisphere. Therefore, this gyre is in the northern hemisphere. In fact, all ocean gyres have clockwise circulation in the northern hemisphere and anticlockwise circulation in the southern hemisphere.
Gyres are usually bounded by the shallow waters of continental shelves. There are five major gyres in the world's oceans, which are delimited by the continents around them.
These gyres are responsible for much of the world's surface currents. As you can see in the map above, much of the eastern coast of Africa has a current going from north to south, part of the Indian Ocean Gyre. This current was a great problem to early European navigators, trying to go around the Cape of Good Hope (the southern tip of Africa) to find a trade route to India. Early sailing ships tended to hug the coast, where the currents are strongest, and they didn't have a lot of motive power in the days of sail. Even today, ships use these currents to save fuel, since making way against the current is costly. Debris floating in the ocean also tends to converge in certain zones because of these currents. The North Atlantic Garbage Patch and the Great Pacific Garbage Patch are places where a lot of trash dumped into the oceans has aggregated.
These ocean currents affect marine life as well. The migrations of many organisms in the sea follows various currents in the oceans. Traveling along the path of a current is energy efficient and fast. They also affect the fishing industry. Obviously, fishing along the migration path of a commercial species can be a good idea, if implemented at the right time. Also, currents have an effect on nutrient levels in the ocean, which can affect the density and diversity of sea life. In areas where these surface currents move away from the coast, there is an upwelling of deep sea water (to replace the water caught and dragged along by the current). Deep sea waters are rich in nutrients, and so these places are very rich in marine life. Some of the world's best fishing areas are located in places where a current moves away from a shore.
This upwelling and down welling of water at coastlines can be seen in the figure to the left. Since the net movement of water is at 90° to the right of the wind direction, it can either push the water towards the shore or away from the shore, depending upon the wind's direction.
The figure shows the situation in the northern hemisphere (note that the equator is at the bottom of the figure). The westerlies (anti-trade winds) blow from southwest to northeast, pushing the water to the right (eastwards). This produces a down welling, as shown in the lower half of the figure. The water is pushed towards the coast, has nowhere to go except down. So it flows towards the coast in the surface layers (which are influenced by wind) and away from the coast in deeper layers. This is under eastern boundary conditions, meaning the land is towards the east of the water. Under the same winds at at the same latitudes, the conditions at the western boundaries would be reversed. The same winds would push water away from the coast, and deeper water would therefore rise to take its place, creating an upwelling.
In the trade winds zone, the wind blows from the northeast to the southwest. Water is therefore pushed away from the coast under eastern boundary conditions, and towards the shore in western boundary conditions.
A similar situation exists in the southern hemisphere. In order to figure out where the water will go, just remember that the movement of water is 90° to the right of the wind direction. Then, based on the pattern of prevailing winds, and depending upon whether you are looking at eastern or western boundary conditions, you can figure out whether there is an upwelling or down welling of water at the coast. This is quite important for fishermen, since upwellings create rich fishing conditions as nutrients are brought up from deep water to the surface. Also remember that these are local conditions, since in addition to these coastal conditions created by the winds, there are also the larger movements of water up and down the coastlines, driven by the oceanic currents.
Returning to the oceanic currents, you can see from the previous map that the gyres are located in large patches of open ocean. The distribution of these gyres differs somewhat from the distribution of Hadley Cells in the atmosphere, shown in the map in this section. As you can see, the wind map is much more complex than the map of oceanic gyres. This is understandable, since wind blows over the entire surface of the Earth, both land and water, and is therefore influenced by differential heating of land and water. The oceanic gyres exist only in the oceans, where the situation is more uniform.
The distribution of oceanic gyres can be best understood if we think of the equator as the zone of maximum expansion and upwelling of water. This equatorial band, about 5 degrees north and south of the equator, is the zone of maximum heat, and is known as the doldrums. Water upwelling in this band has nowhere to go except north or south, so it flows downhill in those directions. As it flows north and south, it is subject to the Coriolis Effect, and curves westwards in the northern hemisphere, and eastwards in the southern hemisphere. However, at the edges of these gyres, it hits the landmass of the continents, and is forced to follow the contour of the continental edges.
Sea Surface Temperatures measured by satellite in May, 2001. From NASA.
Meanwhile, at the distal ends of the gyres (distal meaning the sides farthest from the equator), water is flowing downwards from the big central bulge of each gyre. Since this water is flowing away from the equator, it curves eastwards in the southern hemisphere and westwards in the northern hemisphere. This completes the full circle of each gyre.
The five ocean gyres are the source of most of the ocean currents in the world. However, there are smaller currents that are produced as the result of local heating or cooling, or because of the presence of land forms (such as undersea mountain ranges, the presence of island chains, etc.) which cause the main currents to split into two. Here's a map of the major ocean currents:
Major Ocean Currents of the World. From Wikipedia.
The map above shows the major oceanic currents, color coded to show the warm and cold currents which affect the climate of nearby landmasses. Some of the currents which are important in maintaining the climate of inhabited regions include:
This is part of the north Atlantic gyre. This is a warm water current. Much of the warm water actually begins in the Gulf of Mexico, and exits to the Atlantic through the Florida strait. There it meets warm water from the Caribbean, and together, this warm water flows northwards along the east coast of the US. Leaving the Florida strait, the current is about 30 Svedrup, or 30 million cubic meters per second (contrast this to the outflow of all the rivers on Earth, which have a combined flow of about 1 Svedrup). As it gathers more warm water from the Caribbean and proceeds up the east coast of the US, the current increases to about 150 Svedrup As the continental shelf curves eastwards near the Maritime Provinces of Canada, the current deflects eastward and becomes the North Atlantic Drift, which heads towards Europe. At a latitude of about 30° W, it splits into two branches. The southern branch, known as the Canary current, flows southeast towards the Canary islands off the coast of Africa. This current is then deflected southwards by the west coast of Africa, where it continues until at the southern end of the north Atlantic gyre, it turns westwards as the North Equatorial Current, and thus completes the circuit. The other branch of the North Atlantic Drift, however, heads northwards along the west coast of northern Europe, as the Norwegian Current. This is what warms much of western Europe, far warmer than similar latitudes in Asia or Canada. This has allowed long growing seasons in Europe, extensive farming, and the development of thriving population centers so far north.
Although its influence on Europe is more famous, it is also an important factor in the climate of Florida. The warm winters we see in Florida are due to the Gulf Stream. Other places at the same latitude as Miami, which are landlocked and far from any warm current can reach near freezing temperatures in winter, while Miami has an average high of 75 °F and an average low of 60 °F even in January.
This is a cold water current off the southern half of the west coast of the US, as well as Baja California in Mexico. It's part of the North Pacific Gyre. This keeps the climate of San Francisco, Los Angeles and San Diego cooler than it would normally be for those latitudes. The California current is an eastern boundary current. Boundary currents are those which are influenced by a shoreline. The eastern and western edges of all gyres are different, because of the direction of the trade winds and the westerlies. Since this area lies in the westerlies zone of the northern hemisphere, the direction of prevailing winds is northeast. This drives ocean current south east (at an angle of 90° to the wind flow, due to the Eckman Effect described earlier). The eastward component of the water flow causes an upwelling of water at the coast (sea levels on the west coast of the US are therefore higher than on the east coast). This upwelling of water brings up cold water from deeper parts of the sea, which is rich in nutrients. Marine life is therefore plentiful, and these are good waters for fishing.
This is a cold current, part of the "Viking" gyre. If you recall reading above in the Gulf Stream section, the North Atlantic Drift breaks into two branches as it approaches the coast of Europe. The southerly branch rejoins the North Atlantic Gyre, but the northerly branch (the Norwegian Current) shoots off northwards along the northern coast of Europe. As it nears Iceland, much of the water is deflected westwards, where it joins the East Greenland Current, which continues westwards until it reaches cold waters drifting southward off the northern coast of Canada, and becomes the Labrador Current.
The Gulf Stream and the Labrador Current
The Labrador current is a cold water current and therefore keeps the east coast of Canada (at least in the north) cold and sparsely inhabited. In fact, the treeline on the east coast of Canada stops at about 53° to 56° N latitude (there are no trees north of that). Compare that to some places in central Siberia, where the tree line extends as far north as 72°, a difference of about 15° of latitude.
An interesting thing happens when the cold Labrador Current meets the warm Gulf Stream off the coast of Newfoundland and the Maritime Provinces. This meeting of a cold and warm current produces heavy fogs in the Grand Banks, as well as very rich fishing grounds. Normally, since currents on the east coast of North America are western boundary currents, the coastal regions are not as good for fishing as the west coast. However, because of the meeting of these two currents, the Grand Banks has some of the richest fishing regions in the world. They extend to the Flemish Cap, which is an underwater plateau off the Grand Banks. These shallow waters (as shallow as 400 feet in some spots) were free of glaciation during the last ice age, and possibly became a sanctuary for marine life during the ice age. They are still rich in many commercial species, including swordfish and halibut.
The Labrador current tends to carry down stuff from northern waters. Sometimes, this consists of icebergs, which can cause a shipping hazard in far southerly waters where icebergs are normally not expected. There are agencies specially dedicated to monitoring icebergs drifting south on the Labrador current.
The Agulhas Current off the east coast of Africa may be one of the largest western boundary currents. Its source is in the warm waters of the Arabian sea, off the west coast of India. The island of Madagascar forms a natural eastern boundary to the Indian Ocean Gyre, leaving room for this channel of warm water to flow west of Madagascar. It keeps the east coast of Africa warm, even as far south as South Africa. The west coast of Africa is much colder at the same latitudes. At the very southern tip of Africa (Cape Agulhas), some interesting things happen. The southern tip of Africa has several currents in close proximity. The Benguela cold water current (which is the eastern arm of the South Atlantic Gyre) wends its way northward past the southern tip of Africa. The water of the Benguela current comes from the southern arm of the South Atlantic Gyre - the South Atlantic Current. But not all of the water of the South Atlantic Current is deflected northward as the Benguela - some escapes and continues eastwards where it hits the Agulhas Current. And farther below these two currents, the Antarctic Circumpolar Current flows eastwards, in its circuit around the continent of Antarctica.
The Agulhas Current at the southern tip of South Africa, showing retroflexion and the formation of the Return Agulhas Current. From Peterson and Stramma, 1991.
This doesn't leave the Agulhas Current much room, and it actually retroflexes, or bends backwards towards the Indian Ocean, as the Agulhas Return Current. A lot of eddies are created at these meetings of currents, as shown in the image to the right. These eddies are periodically pinched off, and taken up by other currents in the area. Every two months or so, a large eddy breaks off and enters the Benguela current. However, the majority of the water does in fact return to the Indian Ocean, and in fact allows the circuit to be completed, maintaining the current.
This current was specially important historically, because it flows down the east coast of Africa. Early sailors from Europe, trying to find a way around the southern tip of Africa failed and died in large numbers due to this current.
One obvious reason is because the current heads southwards along the east coast of Africa, in exactly the opposite direction to where the sailors wished to go when rounding the cape and heading towards India. Since they had a habit of hugging the shore for safety, this current was a big problem as it slowed down travel and even pushed them back on days when they were becalmed.
The second reason is not so obvious, but it is the reason why the southern tip of Africa has some of the worst maritime weather in the world, and is a graveyard of ships. The meeting of different currents, along with the steady anti-trade winds blowing from northwest to southeast, creates some of the worst waves in the world. Since we started monitoring for rogue waves via satellite, a very large concentration of such rogue waves has been found near the tip of south Africa. Rogue waves are waves that are fantastically tall (60 - 80 feet, perhaps even 100 feet) that appear out of the blue, apparently at random, in otherwise calm waters. They can capsize even large ships, and cause much damage. These rogue waves, as well as the regular severe weather and storms in these waters are generated by the unusual combination of these currents meeting in the shallow waters of the African continental shelf, where the Indian Ocean meets the Atlantic Ocean, and the prevailing anti-trade winds.
Deep Water Currents
The currents we have talked about so far are surface currents, driven mostly by temperature, winds and gravity, and modified by things such as the Coriolis Effect, and by continental shorelines. "Surface" is a bit of a misnomer here. While it's true that wind action alone doesn't much affect the water below a depth of 150 meters, the other factors mentioned (temperature, gravity, etc.) can make these currents larger and deeper than just wind alone. Even so, for the most part they affect only the surface of the ocean. There are a few exceptions, specially where the water is shallow, as on the continental shelves. For instance, the Gulf Stream in certain areas near the east coast of the US, reaches all the way from the surface to the very bottom of the ocean floor. This is because the continental shelf extends far out into the ocean, and the ocean floor is only a few hundred feet deep in spots. There are other examples of such "surface" currents reaching the sea floor, again only in areas where the ocean is quite shallow.
However, in addition to these surface currents, there is also a deep water circulation in the oceans, referred to as the thermohaline circulation, or sometimes the "Oceanic Conveyer Belt". As the word "thermohaline" indicates (thermo = heat, haline = salts), this circulation is driven mostly by temperature and salinity of the water.
Although we talk about two separate circulations - surface and deep water, it's important to remember that the two systems are connected. Since sea levels around the world are pretty much constant, water moving into one area by some current must be removed by another current, otherwise it would accumulate and one ocean would empty into another. Obviously, this does not happen; the world's oceans are in equilibrium. Surface currents and deep water currents are connected by the upwelling and sinking of water at certain spots on the Earth, mostly near the poles. This vertical circulation (rising and sinking water) connects surface currents to deep water currents.
Over billions of years, the oceans have accumulated vast quantities of salt, from the effect of rivers washing salts down into the sea. Sea water evaporates under the influence of heat, leaving the salt behind, and falls on land as rains, which flow into rivers and back to the sea, bringing more salts to the sea. This process has been going on for billions of years, and has steadily raised the ocean's saltiness.
Graph showing the thermocline in tropical waters. Data from the Woods Hole Oceanographic Institution Atlas Series, Volume 1.
One might suppose that over time, the salt in the sea has mixed evenly, and that sea water is homogenous and equally salty no matter where you sample it from. However, this has not happened, except on the surface, where wind turbulence mixes the water up very thoroughly. Deeper down, the water column is stratified, with distinct layers of temperature and salinity, and relatively sharp boundaries between them.
This stratification is mostly because of the limited penetration of sunlight in water. The intensity of sunlight diminishes very rapidly as you go deeper. Only about 5% of the sunlight reaches a depth of 100 meters. However, since the human eye is very adaptable to low light, we would not see it get very dark until about 200 meters, where the intensity reaches about 1% of that at the surface. These top 200 meters are known as the euphotic zone. This is the maximum extent of photosynthesis, though the bulk of photosynthesis occurs within the top 50 meters.
Depths between 200 meters and 1000 meters are referred to as the "dysphotic" or twilight zone, where visibility is close to zero. Below 1000 meters, ocean depths are in the aphotic zone, where no sunlight ever reaches.
The warming effect of sunlight on ocean waters drops off even faster. As can be seen in the graph on the left, ocean temperatures fall very sharply between depths of 100 meters and 200 meters. After that, the fall is much more gradual. So it's easy to see that the top layers of the ocean are much warmer than the lower layers, and there is a stratification based on temperatures with a relatively sharp boundary. This temperature based boundary is known as a thermocline. It's important not only for marine life, but also for fishing and the military. The thermocline tends to bounce acoustic and sonar signals because of the abrupt change in impedance. The graph on the left only goes to 1000 meters, but deeper waters are even colder. Temperatures near the bottom are around zero. Note that at average ocean salinity levels, water freezes at about -1.8 °C to -2.3 °C, not 0 °C, so it's quite possible for sea water to be at zero and not frozen. Also, remember that unlike fresh water, sea water does not reach its maximum density at 4 °C. It continues to get denser all the way to its freezing point (-1.8 °C to -2.3 °C, depending on its salinity).
Similarly, there is also stratification based on salinity. Salt water is heavier than fresh water, and therefore tends to sink. Unless there is violent agitation of the water, layers of differing salinity will stay separate. This is easy to see at home, with a simple experiment. Take a clear drinking glass, and fill it part way with salty water. Let it sit for a minute for the water to stabilize, then slowly add some fresh water down the side of the glass, taking care not to splash the water too much. You should see the fresh water floating on top of the salty water. You can see the boundary between the two as a hazy line, if you look at it from the right angle. Boundaries between layers of different salinity are called haloclines.
The same thing happens in the oceans, with layers of differing salinity floating on top of each other. There are various reasons why one layer might have a different salinity than another. In warm areas, surface layers are heated by the Sun, and water evaporates, leaving salt behind. These layers therefore have a higher salinity than the layer below. Since higher salinity also means heavier water, these layers would normally sink, but they are held up by the thermocline below them. Recall that there is a thermocline about 100-200 meters below the surface, with the warm water on top and much colder water below. Cold water is denser and heavier than warm water. So within certain limits of salinity of water above the halocline, and certainly temperature differentials of water below the thermocline, the more salty water stays on top of the less salty but colder water below, maintaining the stability of the water column. Of course, there are limits to this stability, and salinities or temperatures beyond those limits would cause a massive overturning of the water, with water from the surface sinking below, and water from the depths rising to the top. Such overturnings are not infrequent. Often they result in phytoplankton blooms, since deeper waters bring up a lot of nutrients when they rise to the top.
Changes in temperature and salinity of the water column with depth.
One effect of thermoclines or haloclines is to prevent the mixing of water from different layers. This can have important effects on oxygen levels. Deeper waters have low oxygen levels because organisms living in those waters use up the oxygen, and the thermocline or halocline prevents surface water (which is rich in oxygen) from passing into the layers below.
There are certain areas in the oceans where exchange between the deep and surface layers takes place on a continuous basis. These spots are located mostly near the poles. The air in polar regions is very cold, much colder than the water. As a result, it can cool the surface waters to very low temperatures, to the point where the density of this water increases beyond the limits that the halocline below can support. This causes this cold water from the surface to sink to the bottom, and this is what feeds the deep water circulation. Sinking of cold water to the deeper parts of the ocean is the main trigger for the thermohaline circulation.
Besides the cold air of the polar regions, there are a couple more reasons for the sinking of cold water at the poles:
Polar winds are strong and steady most of the year. Since the poles are centers of high air pressure, polar winds blow away from the poles, southwards or northwards depending on the pole, as can be seen in the figure here. This causes evaporative cooling of the water. Evaporative cooling is especially marked in the Norwegian Sea, between the coasts of Norway and Iceland. This further reduces the temperature of surface waters.
Polar regions have a lot of ice, in the form of icebergs, and in the case of the north pole, the polar ice cap. Since pure water freezes preferentially over salty water, polar ice is much less salty than ocean water. This exclusion of salts in the ice increases the salinity of the water. Salinity increases the density of the water, which makes these waters even more dense.
The map below shows the main thermohaline currents in the oceans. Note that there are 3 major zones of down welling, where cold water sinks vertically to the ocean floor. Two are located in the northern hemisphere (in the Norwegian Sea, and just south of Greenland), and one is located in the southern hemisphere, in the Weddell Sea, off the coast of Antarctica.
Thermohaline circulation in the world's oceans
These 3 areas of sinkage are the sources of the cold water currents in the deep ocean. In the southern hemisphere, the down welling occurs in the Weddell Sea. Brine exclusion (the formation of sea ice, with the exclusion of salt) is one of the major causes of the down welling in this area. Due to the peninsula that juts out to the west (and the underwater Endurance ridge that continues beyond the peninsula), the westward path is blocked. So the cold water that sinks in the Weddell Sea flows eastwards along the sea floor. This current circles the entire continent of Antarctica. Cold water from this current flows northwards towards to Indian Ocean (along the east coast of Africa), and into the Pacific. In the Indian Ocean, there is strong heat induced expansion of surface waters, and the exits are blocked on 3 sides - by the African continent to the west, the Arabian peninsula to the north, and India to the east. Therefore, surface waters have nowhere to go except southwards, which causes the strong Agulhas and Mozambique currents mentioned earlier. The upwelling caused by this outflow, as well as the fact that the continent of Asia blocks the cold deep water flow from the Antarctica, causes the overturning of water in this region. The cold Antarctica water rises to the surface, where it is warmed and then continues southwards. The southwards flow breaks into two channels near the southern tip of Africa - one channel joining the warm water circumpolar current, and the other channel bending around the tip of Africa, into the Atlantic.
This second channel of warm water heads towards the Caribbean and the Gulf of Mexico, and then towards northwestern Europe. Note that there is a surface current which parallels this flow - the Gulf Stream and the North Atlantic Drift. This is not the same current as the Gulf Stream, this is a deeper current which is part of the thermohaline circulation.
In the Norwegian Sea, surface waters are very cold due to the high latitude and also because of a high degree of evaporative cooling from polar winds. This causes a vertical movement of water downwards, which causes the thermohaline circulation to loop around and head southwest again, this time as a cold water current. The cold water current flows past the northwest coast of Canada (compare to the Labrador current, which is a cold water surface current in the same region), and then heads south along the continental margins of north and south America to joining the Antarctica circumpolar current.
There is a second area of down welling off the northern coast of Canada, which short circuits some of the warm water flowing towards the Norwegian Sea, taking it deeper to join the return circuit current from the Norwegian Sea.
As you can see, there are some similarities between the paths of the thermohaline circulation and the surface currents described earlier, specially at continental margins. This is to be expected, since continents are an absolute barrier to all currents, whether at the surface or in deep water. Continents cause deflection of both surface currents and deep water currents.
In other ways, the surface currents and the deep water thermohaline circulation are very different. Surface currents tend to follow narrow channels and are much faster. Complete circuits of water along the major ocean gyres only takes 3-6 years. This is why a message in a bottle tossed into the ocean can cross the Atlantic in 2-3 years, if it happens to find a good path along the surface currents.
Deep water currents, on the other hand, are very wide and kind of slowly seep along the ocean floors, wending their way between sea floor topography. They are much slower in comparison. A complete circuit around a deep water current could take 2000 years.
Deep water currents are thought to be important in maintaining the Earth's energy balance over the long term, and therefore have a very long term impact on climate. They tend to move heat from the equator to the polar regions, and are probably the most important source of heat to the poles. This in turn regulates the amount of polar ice, which may in turn have long term effects like ice ages and warmer periods between ice ages. Since they are also the major means of exchanging water between the surface layers and the deep ocean, they are also very important for the Earth's carbon dioxide balance. The oceans are major carbon sinks, and the sinking of ocean waters carries a lot of dissolved carbon dioxide to the cold sea floor, where it tends to get sequestered.
The exact role of deep water currents on climate is not well understood, although the consensus among scientists seems to be that it is very significant. One example that is often quoted is the Younger Dryas, which was a sudden short (about 1300 years in duration) period of sudden rapid cooling that happened between 12,800 and 11,500 years ago, in the middle of an otherwise warming trend. This event is believed to have happened due to the melting of ice walls that held back the glacial Lake Agassiz, near the border between the US and Canada. This was an immense glacial lake, holding more water than all the 5 great lakes today combined. Due to gradual warming since the end of the last ice age, the walls holding Lake Agassiz (which were glacial walls in parts, made of ice) melted, causing an out flux of an enormous amount of fresh lake water northwards into the Arctic Ocean. The sudden influx of so much fresh water decreased the salinity of the ocean, stopping the deep water circulation. Recall that deep water circulation is caused by the sinking of water into the depths. Fresh water is less dense and lighter than sea water, and it tends to float on top instead of sinking.
This shutdown of the northern deep water circulation is believed to have caused the Younger Dryas, greatly cooling much of Europe and the middle east. This may have led to the agricultural revolution. Human populations had been steadily increasing since the Magdalenian, 17,000 - 13,000 years ago. This was not a problem when the climate was warming, and vegetation was abundant. The Younger Dryas signalled the return of cold weather, perhaps putting pressure on a population grown too large for the colder years. This may have been the incentive to engage more seriously in agriculture (which had probably been known for thousands of years before the "agricultural revolution" actually started), leading to a dramatic change in lifestyles and the beginnings of civilization.
Events of this magnitude show that shutting down or changing the deep water circulation can have very serious impacts on the Earth's climate. However, the role of smaller fluctuations is not well understood. There are some studies that show that global warming today is causing increased melting of polar ice, which has freshened the water at the poles and led to a decrease in down welling, and therefore a decrease in global deep water circulation. However, the climate has not cooled (as happened in the Younger Dryas). We don't know if this is because the effect is too small (compared to breaking the dams on Lake Agassiz), or because other factors are raising temperatures to compensate. This is still a hot area of research.