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Natural Climate Variability on Decade-to-Century Time Scales
ments of the Southern Ocean is used to indicate the changing content of the North Atlantic Deep Water (NADW) in the abyssal circulation over the past 6,000 to 18,000 years. Since Circumpolar Deep Water consists of both NADW with a high d13C content and Indo-Pacific Deep Water with low d13C, the increase in d13C content at the end of the Ice Age is thought to reflect the rapid resumption of full NADW production at that time.
Whether or not we believe that NADW production was stopped or merely much reduced during the last glaciation, it is plain that the glacial-interglacial signal was reflected in the amount of NADW in circulation. Since the dense overflows across the Greenland-Scotland Ridge (plus consequent entrainment) constitute the largest two of the four main sources of NADW (see Figure 17, taken from Swift
d13C record of benthic forams in Southern Ocean core, indicating the changing production of North Atlantic Deep Water, 6,000-18,000 BP. (From Charles and Fairbanks, 1992; reprinted with permission of Pergamon Press.)
Potential temperature vs. salinity diagram for North Atlantic deep waters colder than 4°C, with the characteristics of NADW shown by hatching. (From Swift, 1984b; reprinted with permission of Pergamon Press.)
(1984b)), it is equally plain that a time-varying exchange between the sub-Arctic seas and the deep open Atlantic has had a global importance in modulating the thermohaline circulation (THC) on 100- to 1000-year time scales.
The less obvious question concerns the stability or instability of the thermohaline circulation on decade-to-century time scales, which form the focus of this symposium. Such shorter-term changes have often been invoked, most frequently to provide the feedback mechanism required in "recurrent GSA" models. Typically these conceptualizations include:
The suppression of deep convection in the Greenland and Iceland seas (e.g., Mysak and Power, 1991). As Aagaard and Carmack (1989) point out, the main convective gyres in these seas are rather delicately poised with respect to their ability to sustain convection, requiring only a modest redistribution of a small part (about 150 km3) of the vast Arctic fresh-water reservoir (approaching 100,000 km3) to achieve shutdown.
The effect of suppressed deep convection on the strength of the thermohaline circulation. This process has been suggested at many scales up to and including the "haline catastrophe theory" of Broecker et al. (1985b), in which runoff from the rapid melting of continental ice sheets provokes a radical renewed suppression of NADW production during deglaciation.
Compensatory changes in the influx of heat and salt to the Norwegian Sea due to changes in the strength of the thermohaline outflow to the North Atlantic. A direct relationship was demonstrated, for example, in the simulation by Manabe and Stouffer (1988).
From these and similar arguments, the thermohaline circulation has indeed been associated with the decadal-scale of climate change—for example, by Weaver et al. (1991) and, most recently, by Yang and Neelin (1993 and personal communication). In the latter model, a recurrent 13.5-year periodicity is obtained using a GSA feedback loop local to the North Atlantic in which brine rejection from heavy ice production stimulates the thermohaline circulation and increases the poleward heat flux in compensation, thus melting ice, suppressing convection, and weakening the thermohaline circulation once again.
A number of objections to the simplifications of such models remain:
Without realistic bathymetry (the Yang/Neelin model has a constant depth of 4000 m from 70°S to 70°N, for example), the strength of the thermohaline circulation really is just a function of the effectiveness of deep convection. In reality, the presence of a Greenland-Scotland Ridge with a sill depth in the Denmark Strait of only 600 m drives home the point that it is intermediate water production that drives the global overturning cell,