Unit 3: Oceans // Section 4: Thermohaline Circulation
The thermohaline circulation is often referred to as the "global conveyor belt" because it moves large volumes of water along a course through the Atlantic, Pacific, and Indian oceans. Cold, salty water sinks in the Norwegian Sea and travels south to the Antarctic, then east to the Pacific. Here water warms and rises, then reverses course and follows an upper-ocean course back through the Indian Ocean and around Cape Horn to the Atlantic (Fig. 7). The current has a flow equal to that of 100 Amazon rivers.
Figure 7. Thermohaline circulation
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Source: © United Nations Environmental Programme/GRID Arendal.
The thermohaline circulation is driven by buoyancy differences in the upper ocean that arise from temperature differences (thermal forcing) and salinity differences (haline forcing). As noted in the previous section, ocean temperatures are lower in polar regions and higher at the equator because the latter receives more radiant energy from the sun. Cold water sinks in polar areas and warmer, less dense water floats on top.
In contrast, salinity differences are caused by evaporation, precipitation, freshwater runoff, and sea ice formation. Evaporation rates are highest in subtropical regions where Hadley circulation loops produce descending currents of warm, dry air (for more details on Hadley circulation, see Unit 2, "Atmosphere"). When sea water evaporates or freezes, most of its salt content is left behind in the ocean, so high rates of evaporation in the subtropics raise salinity levels. Sea water is relatively less saline at higher latitudes because these regions have more precipitation than evaporation. In addition, melting sea ice returns fresh water to the oceans (Fig. 8).
Figure 8. Sea surface salinity (SSS) values
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Source: © National Aeronautics and Space Administraton.
Putting these two factors together, water cools in the North Atlantic and becomes dense enough to sink. Surface currents carry warm, salty water poleward to replace it. In the Indian and Pacific oceans, deep water returns to the surface through upwellings.
As sea ice forms and melts at the poles, it influences ocean circulation by altering the salinity of surface waters. When sea water freezes into ice, it ejects its salt content into the surrounding water, so waters near the surface become saltier and dense enough to sink. This process propels cold waters to depth and draws warmer waters northward in their place. Two factors can make polar waters less salty and reduce this flow. First, warmer temperatures on land increase glacial melting, which sends higher flows of fresh water into the oceans. Since fresh water is less dense than salt water, it floats on the ocean's surface like a film of oil, reducing vertical mixing and slowing the formation of deepwater. Warmer air temperatures also reduce the formation of sea ice, so less salt is ejected into northern waters.
Within the ocean, distinct water masses with physical properties that are different from the surrounding water form and circulate, much like air masses in the atmosphere. Several important water masses help to drive the thermohaline circulation. North Atlantic Deep Water (NADW), the biggest water mass in the oceans, forms in the North Atlantic and runs down the coast of Canada, eastward into the Atlantic, and south past the tip of South America. NADW forms in the area where the North Atlantic Drift (the northern extension of the Gulf Stream) ends, so it helps to pull the Gulf Stream northward. If the NADW were to slow down or stop forming, as has happened in the past, this could weaken the Gulf Stream and the North Atlantic Drift and cool the climate of northwest Europe. (For more details, see Unit 13, "Looking Forward: Our Global Experiment.")
Another cold water mass, Antarctic Bottom Water (AABW), is the densest water mass in the oceans. It forms when cold, salty water sinks in the seas surrounding Antarctica, carrying oxygen and nutrients with it, and flows northward along the sea floor underneath the North Atlantic Deep Water, displacing the waters above it and helping to propel the Thermohaline Circulation (Fig. 9).
Figure 9. Cold water masses
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Source: © Wikimedia Commons. Creative Commons Attribution Share-A-Like 1.0 license.
This circulation pattern is not constant or permanent. Studies of Earth's climate history have linked changes in the strength of the thermohaline circulation to broader climate changes. At times when the conveyor belt slowed down, temperatures in the Northern Hemisphere fell; when the circulation intensified, temperatures in the region rose. Current analyses suggest that as the oceans warm in response to global climate change, the Thermohaline circulation could weaken again, possibly shutting down completely in extreme scenarios. (For more details, see Unit 13, "Looking Forward: Our Global Experiment.")