Multiple Equilibria of an Asymmetric Two-Basin Ocean Model

An ocean general circulation model is used to examine the role of model geometry and surface buoyancy and wind stress forcing in the asymmetry of the global thermohaline circulation. The model domain is a highly idealized Atlantic and Pacific, linked by a circumpolar ocean in the south, and the integrations are performed under mixed boundary conditions diagnosed from spinups under various temperature and salinity profiles constructed from the present-day climatology. The model exhibits a tendency to favor either a “conveyor”-type circulation with sinking in the northern North Atlantic and upwelling in the North Pacific, or a “southern sinking” state with deep sinking in the Antarctic only. This bias is not dictated solely by the hydrological cycle, nor apparently by the greater northern extension of the Atlantic basin, but presumably by the overall asymmetry of the geometry. Equilibria with northern sinking in both basins can appear, however, when the winds in the Southern Ocean are reduced or the horizontal or vertical mixing in that region is tampered with. Three alternate two-basin geometries also have both “northern sinking” and “inverse conveyor” solutions. The common characteristic of all the North Pacific sinking states is the appearance of a very fresh halocline in the Southern Ocean that strongly reduces the Antarctic Circumpolar Current and reverses the sign of the normal pole-to-pole surface density contrast in the Pacific. A linear relationship is in fact found between the North Atlantic overturning and the meridional gradient of depth-integrated steric height, in good analogy with the simple box models of the thermohaline circulation where the overturning circulation is parameterized as linearly proportional to a meridional density difference. Model results suggest the existence of multiple conveyor-type equilibria with different strengths of the North Atlantic overturning. The southern overturning is in contrast quite stable except in states with very strong sinking in the Northern Hemisphere.