MULTIPLE EQUILIBRIA OF AN ASYMMETRIC 2-BASIN OCEAN MODEL

An ocean general circulation model is used to examine the role of mode l geometry and surface buoyancy and wind stress forcing in the asymmet ry of the global thermohaline circulation. The model domain is a highl y idealized Atlantic and Pacific, linked by a circumpolar ocean in the south, and the integrations are performed under mixed boundary condit ions diagnosed from spinups under various temperature and salinity pro files constructed from the present-day climatology. The model exhibits a tendency to favor either a ''conveyor''-type circulation with sinki ng in the northern North Atlantic and upwelling in the North Pacific, or a ''southern sinking'' state with deep sinking in the Antarctic onl y. This bias is not dictated solely by the hydrological cycle, nor app arently by the greater northern extension of the Atlantic basin, but p resumably by the overall asymmetry of the geometry. Equilibria with no rthern sinking in both basins can appear, however, when the winds in t he Southern Ocean are reduced or the horizontal or vertical mixing in that region is tampered with. Three alternate two-basin geometries als o have both ''northern sinking'' and ''inverse conveyor'' solutions. T he common characteristic of all the North Pacific sinking states is th e appearance of a very fresh halocline in the Southern Ocean that stro ngly reduces the Antarctic Circumpolar Current and reverses the sign o f the normal pole-to-pole surface density contrast in the Pacific. A l inear relationship is in fact found between the North Atlantic overtur ning and the meridional gradient of depth-integrated steric height, in good analogy with the simple box models of the thermohaline circulati on where the overturning circulation is parameterized as linearly prop ortional to a meridional density difference. Model results suggest the existence of multiple conveyor-type equilibria with different strengt hs of the North Atlantic overturning. The southern overturning is in c ontrast quite stable except in states with very strong sinking in the Northern Hemisphere.