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Natural Climate Variability on Decade-to-Century Time Scales
Thirty-day means of near-bottom current speed and direction in the Denmark Strait Overflow, taken from successive deployments of instruments set at similar depths on the East Greenland Slope. The "87", "88", and ''89" instruments are from the Angmagssalik Array. The "90" instruments are from equivalent depths on the TTO Array, 160 km upstream. (From Dickson and Brown, 1994; reprinted with permission of the American Geophysical Union.)
TALLEY: Is it possible that the Great Salinity Anomaly, singular though it was, could have circled around the basin a few times before dying away?
DICKSON: Our rather poor records of salinity were at their worst just at that point, so we didn't observe that. It's certainly possible— it has a subpolar gyre to go 'round in, and remember Meincke's long studies of the flow across the mid-Atlantic ridge and what gets turned back—but would it make any difference once it's been diluted?
GHIL: Where did you get the periodicities you showed, like the 7-year North Atlantic oscillation?
DICKSON: Well, there were quite a few. The 6.7-year peak you mention was from Rogers. Jerry Namias and I have long talked about another, which is the relationship between conditions at the U.S. eastern seaboard and the high pressures in Greenland. We used air and sea temperature and calculated the seaboard baroclinic gradients for cold and warm groups of years. Then we looked at the storm frequency for those periods, and found an increase along a line up the coast where the gradient was heightened, and a corresponding periodic retraction of the Iceland low.
SARACHIK: You've made the case for the decoupling of convection and genesis and overflow. Would you care to speculate about what can turn off the thermohaline circulation and the production of deep water in the Atlantic?
DICKSON: Knut Aagaard's papers show that it's very delicately poised; the fresh-water flow into the Greenland Sea can stop convection dead. It would take 31 years of the same trend to make a difference at the depth of the Denmark Strait Sill, but during the Ice Age there could have been something quite dramatic. My feeling is that if something so light were formed it would set up a sort of shallow thermohaline circulation. We don't really know yet how intermediate and deep-water formation mesh.
KUSHNIR: It seems possible that very cold temperatures in the central North Atlantic trigger certain atmospheric conditions that pull ice and fresh-water from the Arctic Sea.
MCGOWAN: Bob, was the increased cod population a matter of improved survival or different spatial distribution?
DICKSON: Well, at first people thought it was increased survival because of the improved climate on the West Greenland Banks. But that's not true of haddock, and the haddock catch mirrors the change in cod, so we now think it's a time-varying conveyor belt that relocates larvae.