The system of surface and subsurface ocean currents that is driven by temperature and salinity differences that create density gradients between adjoining water masses. They are responsible for moving great amounts of thermal energy around the globe.
The thermohaline circulation is a density-driven, global ocean circulation. Sometimes called the Conveyor Belt, the thermohaline circulation moves warmer waters poleward and cold bottom waters equatorward.The North Atlantic Ocean (near the Greenland and Norwegian Seas) and the Antarctic Ocean (near the Weddell Sea) are prime spots for density-induced downwelling, which drives thermohaline circulations. The North Atlantic Thermohaline Circulation has many effects on weather patterns and events. It is linked with the North Atlantic Oscillation (NAO), a multi-decadal oscillation in pressure (or sea surface temperature) in the same vicinity. Current research on thermohaline circulation has exploded as more and more impacts on humans are found.
Atlantic Deep Water (NADW)
Cold and salty water from the North Atlantic that, because of its greater density, sinks to the bottom of the sea and drives a great underwater "conveyor belt" current around the world's oceans that ultimately returns via the surface current of the Gulf Stream.
A thermohaline circulation is fueled by density changes (Henrichs, 2000).Density is a function of temperature and salinity (and pressure, which is usually neglected).This explains why the circulation is termed thermohaline (Appenzeller, 1997). Ocean water density is directly proportional to salinity, but inversely proportional to sea surface temperature (SST).SST anomalies are much more pronounced than salinity variations in the open oceans, and are thus more responsible for density variations in ocean waters than are salinity changes. However, circulation centers in the Mediterranean Sea and the Red Sea are salinity-induced (Gorton, 2000). Salinity decreases when water is added to the system in the following ways: precipitation, sea ice melting, or land runoff. Evaporation and ice formation cause salinity levels to increase.
The thermohaline circulation has numerous effects on a coupled ocean and atmosphere. Since warm waters are driven northward, this contributes to the unusually mild European winters. This effect from the thermohaline Conveyor Belt is said to be twice as important as the Gulfstream (Appenzeller, 1997).In addition, the shifts in temperature and salinity affect marine ecology in the North Atlantic.Colder waters are nutrient-rich and contain a great deal of dissolved carbon dioxide, whereas warmer waters are oxygenated and stripped of nutrients. The Conveyor Belt is also a major forcing on hurricane activity in the North Atlantic basin (Gray et al., 2000). Hurricanes are comparable to heat engines, feeding off of warm SSTs. The stronger the thermohaline circulation, the more favorable tropical cyclone activity becomes, since a larger area has warm SSTs.
The thermohaline circulation has been modeled to gain further insight into its impacts. One idea that baffles scientists is the lack of a thermohaline circulation in the Pacific Ocean (Zachos, 1999).Also, there is speculation that the thermohaline circulation may cease to exist if carbon dioxide levels continue to rise, causing extensive global warming. This would have a tremendous effect on global climate. Finally, significant research is being conducted to completely understand the link between the North Atlantic Oscillation and the Conveyor Belt.
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Appenzeller, Christof. “Draft Fact-sheet
Thermohaline Circulation.” 1997. http://www.climate.unibe.ch/~christof/div/fact4thc.html
(April 2, 2000)
Gorton, Arnold L. “Thermohaline Circulation.” Updated 2000. http://www.britannica.com/bcom/eb/article/3/0,5716,115013+12,00.html (April 2, 2000)
Gray, et al. “Early April Forecast of
Atlantic Seasonal Hurricane Activity and U.S. Landfall Strike Probabilities for
2000.” April 1, 2000. http://typhoon.atmos.colostate.edu/forecasts/2000/april2000
(April 15, 2000)
http://www2.sfos.uaf.edu:8080/msl111/notes/den.html (April 2, 2000)
Wormuth, John. “Thermohaline Circulation.” Updated 1999.
http://www-ocean.tamu.edu/~wormuth/thermohaline.html (April 2, 2000)
Zachos, James. “Lecture #15: Polar
Oceans, Ice, and Thermohaline Circulation.”
(A pril 2, 2000)
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