Multiple Equilibria in Ocean Models as a Side Effect of Convective Adjustment

Multiple equilibria under identical surface forcing have been found in a model for the thermohaline circulation of the ocean (THC). The equilibria are characterized by different convective patterns. With the help of numerical continuation techniques various regions in parameter space are identified where multiple states coexist for the same salinity flux. Robust to variations in surface forcing or to a reduction of the model from fully thermohaline to purely thermal, this multiplicity is related to convective mixing. This finding is in agreement with results from other model studies of the THC. The manifestation of the multiple states depends strongly on grid resolution and details of the parameterization of convective adjustment. The differences between multiple equilibria tend to become smaller for higher vertical resolution, indicating convergence problems. This makes it unlikely that this specific multiplicity represents a real physical property of thermohaline flows. Rather, it appears that at the current resolution, the parameterization of convection interferes with resolved transport processes, mainly that of lateral exchange. Any conclusions about properties of the THC and its role in the earth’s climate that involve this specific multiplity should be verified by grid refinement.