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Abrupt Climate Change: Inevitable Surprises
The conceptual discussion above has been framed entirely in terms of Stommel’s hemispheric model of the THC, but the likely importance of the bipolar see-saw alone indicates that this view must be broadened. Two seemingly unrelated questions help to focus the discussion: First, what is the role of convective mixing in the THC? Second, if salinity is so important in determining the global THC pattern, why does the North Atlantic gain its high-latitude surface density mainly through heat loss (Schmitt et al., 1989)? The first question is motivated by the strong observational (Munk, 1966; Munk and Wunsch, 1998) and numerical (e.g., Bryan, 1987; Marotzke, 1997) evidence that the strength of the THC is strongly controlled by the vigor of vertical mixing in the stratified regions of the ocean. This is because the deepwater that upwells in these regions must be heated by mixing to maintain the near-surface higher temperatures. In contrast, the THC strength is quite insensitive to the efficiency of convective mixing (that is, the speed with which the ocean eliminates dense water overlying lighter water), and the THC could even be strong in the absence of convective mixing, according to the single-hemisphere GCM of Marotzke and Stott (1999).
The answer to the first question leads the way to answering the second. Marotzke (2000) has argued that the results from single-hemisphere models should be viewed as applying to the global integral of the various THC branches and that the magnitude of the global integral obeys different laws from the distribution of the grand total over various competing deepwater formation sites. A number of idealized and more realistic ocean GCMs have shown that varying the freshwater flux forcing leaves the global integral of deep sinking nearly constant but the strength of North Atlantic sinking considerably changed (Tziperman, 1997; Klinger and Marotzke, 1999; Wang et al., 1999). Hence, a reduction in Northern Hemisphere THC would be associated with an increase in Southern Hemisphere THC. Thus, the globally integrated THC (sum of all branches) is rate-limited by vertical mixing and the gross pole-equator density contrast, which is dominated by the pole-equator temperature contrast (not by salinity). It follows that convection is basically driven by heat loss and that it occurs predominantly at high latitudes.
That so much oceanic deepwater is formed in the North Atlantic, and not in any of the other competing high latitude regions, is determined by the North Atlantic’s high surface salinity and hence high surface density. These, in turn, are strongly influenced by the freshwater flux forcing; moreover,