An overview is presented of the role of the Atlantic thermohaline circulation (THC) in climate variability and change, and the factors which can influence the THC. The THC has a potentially important effect on Atlantic climate through its meridional transport of heat and freshwater. On decadal to centennial time scales, fluctuations in these transports can have a substantial impact on Atlantic sea surface temperatures, as well as possible impacts on the climate of adjacent continental regions. Recent modeling work has suggested that THC changes can affect the meridional position of the Intertropical Convergence Zone, thereby potentially altering tropical convection and large-scale atmospheric circulation.

Modeling studies have shown that the North Atlantic Oscilation (NAO) exerts a strong influence on the THC by modulating the surface buoyancy fluxes over the North Atlantic. A variety of factors (including greenhouse gases and tropical SSTs) can influence the NAO, and hence the THC. The Arctic may also exert a substantial influence on the THC by modulating the export of fresh water and sea ice from the Arctic. Changing fresh water amounts in the regions of deep water formation in the North Atlantic serve to modulate oceanic stratification, and therefore the THC.

Greenhouse gas-induced global warming may have a substantial influence on the future behavior of the THC, but there remains considerable uncertainty in model-based projections of this potential change.