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Natural Climate Variability on Decade-to-Century Time Scales
but we also don't see a strong signal in the surface layer where we might expect it. We're just going to have to look at these complex dynamics more carefully.
LEHMAN: I noticed too that your data set indicates no diminution of the strength of the overflow or western boundary current. Before the shutoff of deep ventilation, the formation rate was about one-half a sverdrup, which means it would take 30 years to fill the basin. The overflow is controlled by the damming effect of the sills, so at 10 years after the shutoff we should now be one-third of the way through a cycle that should affect the overflow.
SCHLOSSER: But the overflow seems to be regulated to a very large extent by intermediate-depth convection, as Bob Dickson has pointed out. We may be seeing a phenomenon that is largely decoupled from the global circulation, but can give us insight into regional dynamics.
MYSAK: Isn't it in the Icelandic Sea rather than the Greenland Sea that the GSA caused the shutdown?
SWIFT: In my opinion, the overflows haven't changed; it's only the type of water that has changed. The current continues with whatever's at hand. The Arctic Ocean also outflows through the Denmark Strait, so you get high-salinity pulses from its intermediate waters as well.
RIND: Can you tell from the tracer data how much of the vertical mixing is associated with small-scale convection and how much with larger-scale overturning?
SCHLOSSER: We can only look at rates at present, not processes. We'd need a more elaborate model to do that.
TALLEY: Could you say something about the relative importance of sea-ice formation in open-ocean convection or in ventilating the Greenland, Norwegian, or Icelandic Seas?
SWIFT: In the conceptual model of Rudels and Quadfasel, which has the cyclic overturn, convection proceeds until the surface layer freezes. Then the brine released from further ice formation penetrates the warmer, saltier water underneath, bringing up heat, melting the ice, and starting another cycle. Deep-water formation appears to occur in the regions where the surface salinity and the heat and salt underneath are balanced so that this cycle can continue. The ice-formation process has to be parameterized as well.
WEAVER: I don't quite understand this concept of a western boundary current with constant magnitude but changing temperature and salinity. Wouldn't there be feedbacks that would force it to vary?
SWIFT: Well, that question is related to something I'd wanted to mention earlier. The models I've seen here seem to favor deep-ocean convection from the surface—the classic large-scale overturn. This focuses attention on "chimneys", which are relatively small-scale instabilities. But in the ocean the processes that prevail will be those that produce the densest water the fastest. For example, where there are continental shelves or ice shelves at high latitudes the local density can be greatly enhanced, partly because cooling is concentrated on a smaller total volume and dense products can accumulate, and you get the spreading Peter mentioned. This process may account for half the "deep convection" in the Arctic mediterranean seas. The point is that the environmental sensitivities of the continental-shelf processes are different from that of open ocean convection, which could affect the sensitivities used in modeling deepwater formation.
I guess that doesn't really answer your question, but I think we need to keep it in mind.