Multiple Equilibria in a Global Ocean General Circulation Model

Abstract

A global ocean general circulation model is forced using various upper boundary conditions (BCs) on temperature and salinity. Solutions are obtained under restoring and mixed BCs (i.e., a restoring condition on the upper-level temperature but using a fixed, specified surface salt flux). Salt flux anomalies are temporarily applied under mixed BCs, and solutions are obtained in which overturning associated with deep-water formation is either present or absent in the North Atlantic and either vigorous or weak in the North Pacific. A comparison between these solutions helps to clarify the role North Atlantic deep-water formation plays in maintaining the current climate. The surface heat fluxes differ substantially between the solutions, and their very existence is dependent upon these differences. As a result they are not multiple equilibria of the ocean model alone. Instead, they should be regarded as multiple equilibria of a very crude coupled atmosphere-ocean system. As the global ocean is substantially altered between the equilibria, it is unreasonable to expect that the parameters in the heat flux formulation will remain unchanged. Consequently, the calculated heat flux anomaly may be in error and there is, therefore, no guarantee that the additional equilibria will exist in more sophisticated models. If multiple equilibria do actually exist in such models, they could be quite different from those obtained under the restorative condition.