DEFINITION OF THERMOHALINE CIRCULATION

Vertical mixing of seawater driven by density differences caused by variations in temperature

and salinity. Variations in temperature and salinity are found

in waters that occupy different ocean basins, and those found

at different levels in the water column. When the density of

water at one level is greater than or equal to that below that

level, the water column becomes unstable and the denser water

sinks, displacing the deeper, less-dense water below. When the

dense water reaches the level at which it is stable it tends to

spread out laterally and form a thin sheet, forming intricately

stratified ocean waters. Thermohaline circulation is the main

mechanism responsible for the movement of water out of cold

polar regions, and so it exerts a strong influence on global climate.

The upward movement of water in other regions balances

the sinking of dense cold water, and these upwelling

regions typically bring deep water, rich in nutrients, to the surface.

Thus, regions of intense biological activity are often associated

with upwelling regions.

The coldest water on the planet is formed in the polar

regions, with large quantities of cold water originating off the

coast of Greenland and in the Weddell Sea of Antarctica. The

planet’s saltiest ocean water is found in the Atlantic Ocean,

and this is moved northward by the Gulf Stream. As this

water moves near Greenland it is cooled and then sinks to

flow as a deep cold current along the bottom of the western

North Atlantic. The cold water of the Weddell Sea is the

densest on the planet, where surface waters are cooled to

¯35.4°F (¯1.9°C), then sink to form a cold current that moves

around Antarctica. Some of this deep cold water moves

northward into all three major ocean basins, mixing with

other waters and warming slightly. Most of these deep ocean

currents move at a few to 10 centimeters per second.

Ocean bottom topography exerts a strong influence on

dense bottom currents. Ridges deflect currents from one part

of a basin to another and may restrict access to other regions,

whereas trenches and deeps may focus flow from one region

to another.

See also CLIMATE CHANGE; OCEAN CURRENTS.

thermosphere See ATMOSPHERE.

thrust See FAULT.

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