Britain were of the order of 7°C cooler during glacial times than today, which, as noted by Broecker (1989), would be consistent with the absence of the Atlantic thermohaline conveyor and associated poleward heat transport.

Recently much attention has been given to trying to understand the climate oscillations that have taken place between the last glacial period and the present interglacial period. One example of such an oscillation is the Younger Dryas cold event, which took place between about 11,000 and 10,000 years before the present (BP). Keigwin et al. (1991) found that during the Younger Dryas (and at three other times since the last glaciation, about 14,500, 13,500, and 12,000 years BP) NADW production was substantially reduced or even eliminated, which tends to support the hypothesis of Broecker et al. (1985) regarding the existence of more than one quasi-stable mode of operation of the thermohaline circulation. The transitions into and out of these mini-glaciations are thought to have been very rapid. For example, Dansgaard et al. (1989) suggest that the Younger Dryas event ended abruptly within a 20- to 50-year period leading to the present interglacial period. Broecker et al. (1990) have proposed that during glacial times, when the northern end of the Atlantic Ocean is surrounded by ice sheets, a stable mode of operation of the conveyor belt for NADW is not possible. They propose the following millennial-time-scale oscillation: When the NADW conveyor is shut down and there are growing ice sheets, there is little oceanic salt export from the Atlantic to the other world basins. If a net evaporation over the North Atlantic is assumed, the salinity continues to increase. When a critical salinity is reached, deep convection and subsequently the conveyor turn on, transporting and releasing heat to the North Atlantic and thereby melting the ice sheets. The flux of fresh-water into the North Atlantic from the melting ice sheets eventually shuts off the conveyor, and the process begins anew.

Variability in the earth's climatic system on the century time scale is also evident in oxygen-isotope and other proxy records such as those of Dansgaard et al. (1970) at Cape Century in northwest Greenland (see Stocker and Mysak, 1992, for more details). Over the last 10,000 years (since the Younger Dryas event) Dansgaard et al. found that the dominant climatic variability exhibited an energy peak at the 350-year period. This variability may well be linked to fluctuations of the thermohaline circulation over its overturning time scale (Mikolajewicz and Maier-Reimer, 1990).

OBSERVATIONS OF DECADAL-TO-INTERDECADAL CLIMATE VARIABILITY

On shorter time scales the air-sea-ice climate system also exhibits decadal-to-interdecadal variability. For example, signals of decadal-to-interdecadal time scales are exhibited by global surface air temperatures (Ghil and Vautard, 1991), sea surface temperature (SST) anomalies (Loder and Garrett, 1978), West African rainfall and the landfall of intense hurricanes on the U.S. coast (Gray, 1990), properties of NADW formation (Dickson et al., 1988; Lazier, 1980; Roemmich and Wunsch, 1984; Schlosser et al., 1991), temperature and salinity characteristics and circulation of the North Atlantic (Greatbatch et al., 1991; Levitus 1989a,b,c; Levitus, 1990), Arctic sea-ice extent (Mysak and Manak, 1989; Mysak et al., 1990), runoff from the Eurasian land mass (Cattle, 1985; Ikeda, 1990), and global sea-level pressure (Krishnamurti et al., 1986). While many of these studies have been restricted to relatively short time series, the Greenland ice-core data of Hibler and Johnsen (1979) clearly show a 20-year oscillation in the North Atlantic for oxygen isotope records spanning the years 1244 to 1971.

The source of this variability may once more be linked to internal fluctuations of the thermohaline circulation. Indeed, this hypothesis was originally put forward by Bjerknes (1964) in his attempt to explain decadal-to-interdecadal changes in long time series of SST in the subpolar North Atlantic (see Bryan and Stouffer, 1991, for a more complete discussion of Bjerknes' paper).

VARIABILITY OF THE THERMOHALINE CIRCULATION IN COARSE-RESOLUTION GENERAL-CIRCULATION MODELS

Over the past few years it has become evident that the thermohaline circulation may not be static, and that it may indeed undergo natural, internal variability on the decadal-to-millennial time scale. Numerous OGCM simulations have found such natural variability under steady, imposed surface-boundary conditions (strictly speaking, although the wind and fresh-water flux are time-invariant, the surface heat flux may vary with time if the sea surface temperature changes, as only the restoring temperature is time-invariant).

Variability of the thermohaline circulation found in coarse-resolution OGCMs can be roughly classified according to fundamental time scale: diffusive, overturning, and horizontal advection. This classification is used below in a brief review of recent modeling efforts aimed at understanding the variability properties of the thermohaline circulation in OGCMs.

Mixed Boundary Conditions

The heat and fresh-water flux coupling between the ocean and the atmosphere occur on different time scales and involve different physical processes. The lag of SST behind the seasonal cycle of insolation, which is on the order of 6 weeks (Bretherton, 1982), is conventionally parameterized in ocean models as a response to changing atmospheric conditions. The dependence of long-wave emission, sensible heating, and atmospheric humidity (and hence latent heat



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