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Abrupt Climate Change: Inevitable Surprises
the deepwater formation is intimately related to the very deep convective mixing occurring there.
This two-step procedure—considering first the global integral of THC strength and then its distribution over different regions—helps to sort out a number of conceptual questions, but others arise. Most important is the role of the Antarctic Circumpolar Current, especially the wind-induced upwelling there (Toggweiler and Samuels, 1995); this has only recently begun to be addressed theoretically (Gnanadesikan, 1999). The interaction of the North Atlantic THC with the other oceans is poorly understood, both conceptually and from observations (e.g., Whitworth et al., 1999).
Extending one’s perspective beyond the Stommel model also calls into question the long-held tenet that freshwater forcing necessarily weakens the THC. Rooth’s (1982) interhemispheric box model suggests that the Atlantic THC actually increases with increased freshwater flux (Rahmstorf, 1996; Scott et al., 1999; Marotzke, 2000). This is confirmed for the Atlantic branch of the THC in an idealized global GCM, as long as one considers the equilibrium response (Wang et al., 1999). Under a faster increase, as would be expected with increased greenhouse gases or might have occurred with outburst floods or ice-sheet surges in the past, the THC did weaken.
If the equilibrium response can be interpreted as reflecting the THC’s response to very slowly varying atmospheric moisture flux, as might plausibly have happened during the glaciations, the assumed overall glacial weakening of the THC, as opposed to the shorter-lived and stronger proposed weakening of the THC in specific events during the glacial, could be explained.
Given confirmation that the North Atlantic surface densities and the resulting convective activity do matter, the question arises whether we understand, and can observe, what they are influenced by. The drivers could be oceanic transports of freshwater (e.g., Aagard and Carmack, 1989) and heat, local surface fluxes, or remote influences, such as the water-vapor transport from the Atlantic to the Pacific (Zaucker and Broecker, 1992; Schmittner et al., 2000; Latif et al., 2000). In addition, one should consider the evolution of surface density in the Southern Ocean, in particular the Weddell Sea, because it is not well observed and the processes that link it with North Atlantic densities are not well understood.
There is a huge gap in our conceptual understanding linking changes in convective activity, in the North Atlantic or elsewhere, to the THC and the northward heat transport. Regionally, changes in properties appear to occur rapidly but are poorly understood (e.g., Sy et al., 1997). On a larger