Surface heat flux parameterization and the response of ocean general circulation models to high-latitude freshening

Abstract

A global ocean general circulation model (OGCM) is forced using mixed boundary conditions (i.e. a restoring condition on the upper level temperature but using a fixed, specified surface salt flux). A freshwater 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 found to be dependent upon the details of the parameterization of the surface heat flux: In particular the “coupling strength” or Haney relaxation 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 parameterization is therefore required to properly model the evolution of the response. It is uncertain that the restorative condition is sufficiently realistic, especially in cases where a significantly different climatology is obtained. It is possible, for example, that the evolution could move from an unstable trajectory to a stable one if the parameters in the heat flux formulation are also allowed to evolve. This might help to explain why OGCMs under mixed boundary conditions are more sensitive 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 provided 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 Ni±o, Southern Oscillation.