thermohaline circulation

thermohaline circulation (THC), the component of general oceanic circulation controlled by horizontal differences in temperature and salinity. It continually replaces seawater at depth with water from the surface and slowly replaces surface water elsewhere with water rising from deeper depths. Although this process is relatively slow, tremendous volumes of water are moved, which transport heat, nutrients, solids, and other materials vast distances. Thermohaline circulation also drives warmer surface waters poleward from the subtropics, which moderates the climate of Iceland and other coastal areas of Europe.

A brief treatment of thermohaline circulation follows. For full treatment, see Ocean current: Two types of ocean currents: Thermohaline circulation.

The general circulation of the oceans consists primarily of wind-driven ocean currents. These, however, are superimposed on the much more sluggish circulation driven by horizontal differences in temperature and salinity—namely, thermohaline circulation. Wind-driven circulation, which is strongest in the surface layer of the ocean, is the more vigorous of the two and is configured as large gyres that dominate an ocean region. In contrast, thermohaline circulation is much slower, with a typical speed of 1 centimetre (0.4 inch) per second, but this flow extends to the seafloor and forms circulation patterns that envelop the global ocean.

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Water and its Varying Forms

In some areas of the ocean, generally during the winter season, cooling or net evaporation causes surface water to become dense enough to sink. Convection penetrates to a level where the density of the sinking water matches that of the surrounding water. It then spreads slowly into the rest of the ocean. Other water must replace the surface water that sinks. This sets up the thermohaline circulation. The basic thermohaline circulation is one of sinking of cold water in the polar regions, chiefly in the northern North Atlantic—where the circulation is often referred to as the Atlantic Meridional Overturning Circulation (AMOC)—and near Antarctica. These dense water masses spread into the full extent of the ocean and gradually upwell to feed a slow return flow to the sinking regions. The theory for the thermohaline circulation pattern was first proposed by Henry Stommel and Arnold Arons in 1960.

Some scientists believe that global warming could shut down this ocean current system by creating an influx of freshwater from melting ice sheets and glaciers into the subpolar North Atlantic Ocean. Since freshwater is less dense than saline water, a significant intrusion of freshwater would lower the density of the surface waters and thus inhibit the sinking motion that drives large-scale thermohaline circulation. It has also been speculated that, as a consequence of large-scale surface warming, such changes could even trigger colder conditions in regions surrounding the North Atlantic. Experiments with modern climate models suggest that such an event would be unlikely. Instead, a moderate weakening of the thermohaline circulation might occur that would lead to a dampening of surface warming—rather than actual cooling—in the higher latitudes of the North Atlantic Ocean. See also ocean current.