Thermohaline Circulation

THERMOHALINE CIRCULATION IS global oceanic circulation generated by buoyancy fluxes resulting from heat and freshwater exchange between the ocean, atmosphere, cryosphere, and land. External forcing leading to an increase in water density (i.e., cooling or salinity rise) causes the sinking of more dense water (so-called thermohaline convection) and compensating transport of more light shallower waters of the upper mixed layer and thermocline. This process forms thermohaline overturning, which is one of the principal mechanisms of meridional heat transport in the world's ocean and global coupled ocean-atmosphere system.

Thermohaline circulation is characterized by two regimes, as was first pointed out by Henry Stommel. They are caused by thermal and haline effects and, in turn, account for large-scale temperature and salinity distribution in the World Ocean and, hence, influence the global climate. In general, in recent climate conditions, just thermal overturning circulation prevails in the world oceans, because global thermohaline circu lation is formed mostly by the sinking of cold high-latitude waters and the compensating transport of warm shallower water. In general, there are two principal sources of deep and bottom waters. They are in the North Atlantic and Antarctic regions, respectively. These sources produce North Atlantic deep water (NADW), the core of which is at 1.2 to 1.5 mi. (2 to 2.5 km.), and Antarctic bottom water (its core deepens below 2.5 mi. [4 km.]). Haline circulation prevails in some specific regions of the world oceans, such as in the semiclosed Black and Red seas. The effect of salinity changes on the density field is also enhanced in subtropical oceanic regions, especially in the subtropical Atlantic, which is close to the Sahara desert. There, the upper mixed layer depth is mostly controlled by thermohaline convection as a result of salinity effects.

Thermohaline and superimposed wind-driven forcing has caused recent large-scale general oceanic circulation. The relative importance of these two sources for integral volume transport of principal large-scale oceanic currents varies from one region to another. In the North Atlantic, for instance, thermohaline and wind-driven shares in the general circulation of the upper 1.5 mi. (2 km.) layer are discussed in a recent study by Alexander Polonsky.

Global warming may cause, in principle, a change in circulation regime as a result of ice/glacier melting and increased freshwater input into the polar zone of the North Atlantic. This may lead to surface water lightening and blocking of thermohaline overturning. The Gulf Stream should dramatically weaken as a result of that. Such a regime has been called thermohaline catastrophe because it should be accompanied by strong climate shift in the North America and Europe. It is expected that a new climate will be much more severe and will be accompanied by much more frequent and strong North Atlantic cyclones because even just eddy meridional heat transport prevails in the midlatitude atmosphere, and it must now compensate for the reduced meridional thermohaline heat transport in the ocean after thermohaline catastrophe. However, as follows from recent multimodel simulations published by Ronald Stouffer and coauthors, the likelihood of thermohaline catastrophe happening in the next 100 years is quite small, taking into account recent tendencies of ice/glacier melting.

As follows from the simulation results of Stefan Rahmstorf, during the Last Glacial Maximum (about 21,000 years ago), thermohaline circulation was char acterized by more shallow meridional cell and reduced meridional heat transport in the North Atlantic. The core of NADW was at about 1 km. In general, this was the result of severe ice conditions in the North Atlantic, where NADW has being produced. Different paleodata analyzed recently by Jean Lynch-Stieglitz and coauthors (2007) confirm in part such a scenario.

There is some evidence that blocking thermohaline circulation in the North Atlantic occurred about 8,200 years ago, just after the last glacier period. Most likely, it was a result of a plume of freshwater from juvenile lakes that rose in the end of a glacier period. Another possibility is the relatively fast melting of an armada of icebergs spreading from a Greenland glacier. However, it has not been proven by the analysis of deep ocean sediments provided by Christopher Ellison and coauthors (2006).

sEE ALso: Abrupt Climate Changes; Modeling of Ocean Circulation; Modeling of Paleoclimates; Mixed Layer; Thermocline.

BIBLIoGRAPHY. Christopher Ellison, et al., "Surface and Deep Ocean Interactions During the Cold Climate Event 8200 Years Ago," Science (v.312/5782, 2006); Jean Lynch-Stieg-litz, et al., "Atlantic Meridional Overturning Circulation During the Last Glacial Maximum," Science (v.316/5821); Alexander Polonsky, Role of the Ocean in the Climate Change (Naukova Dumka, 2007); Stefan Rahmstorf, "Rapid Climate Transitions in a Coupled Ocean-Atmosphere Model," Nature (v.372/6501); Henry Stommel, "Thermohaline Convection With Two Stable Regimes of Flow," Tel-lus (v.13, 1961); Ronald Stouffer, et al., "Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes," Journal of Climate (v.19/8, 2006).

Alexander Boris Polonsky

Marine Hydrophysical Institute, Sebastopol

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