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Climate Science: Investigating Climatic and Environmental Processes
Thermohaline Circulation

Driven by the sun's heat absorbed by tropical oceans and impacted by variations in salt content in the water, the Thermohaline Circulation or THC is a powerful force on the world's climate system. As heat from the tropics is carried by the Gulf Stream into the North Atlantic where it is vented into the atmosphere, a deep convection of ocean waters is caused by surface cooling, with the flow of water then sinking to depths and then upwelling back to the surface at lower latitudes.

When frozen, water releases salt, and thus when it melts it is salt-free. This factor and the heavier density of salty water is particularly important in polar regions where the convergence of fresh and saline waters influences ocean currents.

The THC is dynamic and has been known to dramatically shift, as it appears to have done just after the last Ice Age and perhaps during episodes of abrupt climate change. Because such shifts in the THC's "conveyor belt" of ocean heat can cause major changes in climate over relatively short-time scales (10-20 years) which in turn can have enormous impact on human activities, understanding the THC has become a major focus for climate research. As we examine in Climate Science 10,000, scientists are looking at how influxes of freshwater into the North Atlantic can impact the THC. See Gordon, 1986 and Broecker, 1987 for more on the Thermohaline Circulation system.

See figure below for a generalized overview of the THC.

Image of Thermohaline Circulation system
Graphic from USGCRP
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