involve reduced solar intensity, low carbon dioxide, ocean heat transport, and dynamics of sea ice (Hoffman et al., 1998; Poulsen et al., 2001; Hyde et al., 2000).

ABRUPT CLIMATE CHANGE AND THERMOHALINE CIRCULATION

The previous section explored a variety of mechanisms that might be involved in abrupt climate change. However, because sudden change in the THC stands as the only well-developed theory to explain abrupt climate changes, such as the Younger Dryas and the Dansgaard/Oeschger and Heinrich events, we now investigate this phenomenon in greater detail. This is not intended to imply that other mechanisms in this rapidly evolving field will not be found that could also contribute substantially to abrupt climate change, but only that models for other such mechanisms are not as mature as for THC changes.

Processes Driving the Thermohaline Circulation

The global THC consists of: cooling-induced deep convection, brine rejection, and sinking at high latitudes; upwelling at lower latitudes; and the horizontal currents feeding the vertical flows. Contrary to widespread perception, convection and sinking are neither the same nor co-located (Marotzke and Stott, 1999) because when rotational effects are strong, flow tends to be around a patch of maximal surface density (characterizing convection) rather than into it (Marshall and Schott, 1999). In the North Atlantic, where much of the deep sinking occurs (Gordon, 1986; Ganachaud and Wunsch, 2000), the THC is responsible for the unusually strong northward heat transport; part of this heat is imported from the Southern Hemisphere. Much of this heat is given off to the atmosphere over the Gulf Stream, from where it is transported northeastward by the atmosphere. This part of the heat loss is typical of all subtropical gyres and is not associated with the global overturn (Talley, 1999). The enhanced heat transport has been believed by many to contribute to the relative mildness of western European climate, particularly that of Scandinavia. However, as described earlier in this chapter, the relative contributions to European climate of the THC, the wind-driven ocean circulation, atmospheric transport associated with land/ocean contrasts, atmospheric planetary waves, and so on, remain uncertain.



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