SENSITIVITY OF EDDY-INDUCED HEAT-TRANSPORT TO DIABATIC FORCING

Compensation of the poleward eddy heat transport by the heat transport of an eddy-induced mean meridional overturning cell is a common featu re in many eddy-resolving ocean models. It has been argued that this i s the result of the weak thermal driving of the ocean. As the actual a ir/sea coupling is scale dependent, it might be questioned whether the approximation of weak thermal driving is relevant for the oceanic edd y field. In this paper the role of diabatic forcing in modifying eddy- mean flow interaction is investigated. Emphasis has been placed on the sensitivity of the eddy-induced change in heat transport to the sea s urface thermal boundary condition. Experiments have been performed wit h a multilayer isopycnic primitive equation model of an idealized Nort h Atlantic subtropical and subpolar gyre. For different values of the air/sea coupling, solutions with and without transient eddies have bee n compared. The air/sea coupling mostly affects the upper ocean therma l and velocity fields. A decrease of the coupling timescale pushes the separation point of the midlatitude jet further northward and induces a tight recirculation southwest of the separated jet. These effects a re enhanced by the eddies. In the present model there is compensation of the eddy heat transport for sea surface temperature (SST) relaxatio n times longer than 150 days; a breakdown of the compensation occurs f or SST relaxation times shorter than 50 days (the average upper layer depth is 200 m). In between is a transition regime. For strong thermal driving the eddy-induced change in total heat transport is of the sam e order as the eddy heat transport.