SURFACE HEAT-FLUX PARAMETERIZATION AND THE RESPONSE OF OCEAN GENERAL-CIRCULATION MODELS TO HIGH-LATITUDE FRESHENING

A global ocean general circulation model (OGCM) is forced using mixed boundary conditions (i.e., a restoring condition on the upper level te mperature but using a fixed, specified surface salt flux). A freshwate r flux anomaly is then applied over the western half of the sub-polar gyre in the northern North Atlantic. The response of the model is foun d to be dependent upon the details of the parameterization of the surf ace heat flux: In particular the ''coupling strength'' or Haney relaxa tion time is crucial. Responses range from a halocline catastrophe at short relaxation times (strong coupling) to a very modest perturbation at longer relaxation times (weaker coupling). An accurate parameteriz ation is therefore required to properly model the evolution of the res ponse. It is uncertain that the restorative condition is sufficiently realistic, especially in cases where a significantly different climato logy is obtained. It is possible, for example, that the evolution coul d move from an unstable trajectory to a stable one if the parameters i n the heat flux formulation are also allowed to evolve. This might hel p to explain why OGCMs under mixed boundary conditions are more sensit ive than the observations suggest they should be. When a recovery does occur it does so on decadal time scales. It is therefore tempting to speculate that the positive feedback on the initial perturbation provi ded by the heat flux response plays a central role in the dynamics of North Atlantic variability, in a manner that is analogous to the wind- stress feedback in the El Nino, Southern Oscillation.