INTRODUCTION

There has been a good deal of scientific, economic, and even political interest in studying potential changes in our global climate and their influences on our environment. The search for an understanding of climate change, both past and present, has led directly to the ocean and in particular to the oceans' thermohaline circulation. The ocean, with its large thermal stability and its potential to store both anthropogenic and natural greenhouse gases, also serves as an important regulator of climate. It is the buffer that moderates temperature fluctuations during the course of a day, from season to season and even from year to year. One only has to compare the maritime climate of Victoria, British Columbia (48°25'N, 123°22'W), which has average temperatures of 4°C in January and 16°C in July, with the continental climate of Winnipeg, Manitoba (49°54'N, 97°14'W), which has average temperatures of - 18°C in January and 20°C in July, to see the moderating effect of the ocean. The ocean also acts as a large-scale conveyor that transports heat from low to high latitudes, thereby reducing latitudinal gradients of temperature. Much of the oceanic heat transport is thought to be associated with the thermohaline circulation. In the North Atlantic, intense heat loss to the overlying atmosphere causes deep water to be formed in the Greenland, Iceland, and Norwegian seas. These sinking regions are fed by warm, saline waters brought by the thermohaline circulation from lower latitudes. No such deep sinking exists in the Pacific. Again, if one compares the climates of Bodö, Norway (67°17'N, 14°25'E), which has an average January temperature of - 2°C and an average July temperature of 14°C, to that of Nome, Alaska (64°30'N, 147°52'W), which has an average January temperature of - 15°C and an average July temperature of 10°C (both being at similar latitudes and on the western flanks of continental land masses), one sees the impact of this oceanic poleward heat transport.

The thermohaline circulation is driven by the flux of buoyancy through the ocean surface. This buoyancy flux can be broken down into two competing component—heat and fresh-water fluxes. High-latitude cooling and low-latitude heating tend to drive a poleward surface flow, high-latitude sinking, and a deep equatorward return flow, whereas high-latitude excess precipitation over evaporation and low-latitude excess evaporation over precipitation (except in a relatively narrow belt at the Intertropical Convergence Zone) tend to brake this thermally driven overturning. The existence of multiple equilibria and the stability and variability properties of the thermohaline circulation depend fundamentally on the competing properties of temperature (T) and salinity (S) in the net surface-buoyancy forcing of the ocean, and in particular on the fundamental difference in the coupling of T and S between the ocean and the atmosphere. Variations in the stability or variability properties of the ocean's thermohaline circulation, and hence its associated poleward transport of heat, would have significant impact on both local and global climate.

The introduction of a new generation of fast supercomputers and workstations has allowed researchers to undertake long-time integrations of coarse-resolution ocean general-circulation models (OGCMs). These integrations have revealed numerous intriguing results pertaining to the stability and variability of the ocean's thermohaline circulation. In particular, during integrations of uncoupled (ocean-only) GCMs, self-sustained variability of the models' thermohaline circulation has been found on time scales ranging from decades to millennia. The purpose of this paper is to review some of these recent OGCM studies in order to illustrate the types of spontaneous thermohaline variability that may arise. Furthermore, some new experiments are discussed in which a coarse-resolution North Atlantic model is driven by annual mean Levitus (1982) restoring temperatures, the annual mean Schmitt et al. (1989) North Atlantic fresh-water flux field, and the annual mean Hellerman and Rosenstein (1983) wind-stress field.

The structure of this paper is as follows. First, some observations of century-to-millennial time scale variability in the air-sea ice climate system are discussed. A few observations of climate variability on the shorter (decadal to interdecadal) time scales are also briefly summarized. The nature and time scales of the internal, self-sustained variability of the thermohaline circulation found in coarse resolution OGCMs are then reviewed. Here, particular attention is focused on the relative importance of fresh-water flux, thermal, and wind forcing in driving the variability. The results of some recent experiments conducted in an idealized coarse-resolution North Atlantic basin driven by realistic forcing fields are then described. Finally, a summary and discussion are presented.

OBSERVATIONS OF CENTURY-TO-MILLENNIAL CLIMATE VARIABILITY

On the basis of ice-core records for the last glacial period, Oeschger et al. (1984) suggested that the climate system had two quasi-stable modes of operation between which the system oscillated in the transition between glacial and postglacial times. Broecker et al. (1985) further postulated that the two modes described by Oeschger et al. (1984) were characterized by the presence or absence of significant North Atlantic Deep Water (NADW) formation. There is indeed much evidence that during glaciations deep ocean temperatures were colder (Labeyrie et al., 1987) and that more Antarctic Bottom Water (AABW) flowed into the North Atlantic (Duplessy et al., 1988), while NADW formation was substantially reduced (Boyle and Keigwin, 1987), all of which tend to support the idea that the present-day thermohaline circulation is not unique. Furthermore. Ruddiman and McIntyre (1977) found that the surface waters off



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