Thermohaline Circulation
Overview
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) head polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. They then return towards the equator as deep sea currents. This process, known as "thermohaline circulation", is controlled by the temperature and salinity of the water.
Mechanism
The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. On occasion, it is used to refer to the MOC, which is the larger scale, density-driven part of the oceanic circulation, driven by differences in temperature and salinity. In these cases, it is distinguished from the wind-driven currents of the surface ocean.
Components of Thermohaline Circulation
The thermohaline circulation is composed of two main parts: the wind-driven circulation and the thermohaline circulation. The wind-driven circulation is caused by the wind patterns on the surface of the Earth, while the thermohaline circulation is driven by differences in temperature and salinity in different parts of the ocean.
Wind-Driven Circulation
The wind-driven circulation is the part of the oceanic circulation that is driven by the wind. This includes the Ekman transport, which is the net motion of fluid as the result of a balance between Coriolis and turbulent drag forces. In the physical oceanography, the Ekman transport is part of the wind-driven currents that are set in motion by the force of the wind on the surface of the sea.
Thermohaline Circulation
The thermohaline circulation is driven by differences in the density of sea water. The density of sea water is controlled by its temperature and its salinity: warm water is less dense than cold water and fresh water is less dense than salty water. Thus, if there is a region where water is made denser by cooling or by evaporation, then this denser water will sink beneath the surrounding water. Conversely, if there is a region where the sea water is made less dense through heating or through the addition of fresh water, then this less-dense water will rise to the surface. This process of water sinking in certain regions and rising in others creates the thermohaline circulation.
Impact on Climate
The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the global climate. The warm surface waters carried poleward by the thermohaline circulation release heat to the atmosphere in the cold polar regions, and this heat loss to the atmosphere makes the water denser, causing it to sink. This sinking water then flows back towards the equator as deep sea currents. This process of transporting heat from the equator to the poles is a key component of the Earth's heat budget.
Potential Disruption of Thermohaline Circulation
There is some concern that global warming could lead to a shutdown or slowdown of the thermohaline circulation. This could occur if global warming leads to an increase in the amount of fresh water entering the ocean in the polar regions, either through increased precipitation, increased melting of glaciers and ice caps, or both. Because fresh water is less dense than salty water, this increase in fresh water could prevent the formation of the dense water that drives the thermohaline circulation.