modes: through open ocean convection in polynyas (areas of open water that are surrounded by sea ice), in geographically suitable semi-enclosed seas (Ross and Weddell Seas), along the shelf of Australia, and driven by plumes of dense water descending from the shallow Antarctic shelf and slope, and interacting strongly with the overlying permanent ice. In contrast with the North Atlantic, the overturning that feeds the deepest waters is fed by cool mid-depth waters that first upwell near the Antarctic Circumpolar Current. Thus, the total heat transport associated with formation of dense waters in the Antarctic is not as dominant a part of the global heat budget as is the heat transport associated with the North Atlantic THC. However, variations in Antarctic overturning, and possibly surface warming, can affect freshwater budgets, as evidenced in large-scale freshening in the lower latitude oceans of the Southern Hemisphere (Wong et al., 1999; Johnson and Orsi, 1997). Both intermediate-depth and deeper branches of the THC form complex arteries of sinking and recirculating in the Southern Ocean (e.g., Orsi et al., 1999); both are important to the time-constants of response of the global ocean to abrupt change.

The possible alteration of northern and southern sinking in the global THC, known as the see-saw, arises from model simulations (Stocker et al., 1992; Crowley, 1992) and from comparison of climate anomalies in Greenland and Antarctic ice-core records (e.g., Blunier and Brook, 2001; see Plate 2). Instrumental data are scarce, but the inventory of CFC and other transient tracers can be interpreted to suggest a recent weakening of sinking around Antarctica, in comparison with the longer-term signal seen in steady-state tracers related to nutrients and dissolved oxygen (Broecker, 1999). A suggested link is proposed between this oceanic behavior and the Little Ice Age, one of the major climate events of the past 1,000 years in the northern hemisphere.

Identifying global connections in the climate system is a particularly important goal of modern observations. Toggweiler and Samuels (1995) have found that, using global ocean models, the THC is sensitive to forcing by westerly winds in the Southern Ocean (particularly those near the relatively narrow Drake Passage, south of South America). In their simulations the THC volume transport, as far north as the subpolar Atlantic, shifted in response to changes in this distant wind forcing.

In 1972, a large hole opened in the ice cover of the Weddell Sea. This polynya lasted until 1974, and its progress was tracked by the early Nimbus satellites. The polynya was supported by the breakdown of the usual stratification leading to open-ocean convection (Gordon and Comiso, 1988).

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