Eddy-Induced heat transport in a coupled ocean-atmospheric anomaly model

The midlatitude meridional heat transport in the ocean can be partitioned into a transport by the mean flow and an eddy transport. This heat transport has been studied in several ocean-only models. Surprisingly, it was found that eddy-resolving and coarse-resolution models have similar total heat transport. This is caused by a compensation mechanism in which poleward eddy transport is counterbalanced by an eddy-induced meridional circulation. Recently it was shown that this compensation depends on details of the atmospheric forcing and, in fact, only occurs for weak thermal coupling, where thermal coupling is defined as the rate of change of the surface heat flux with respect to the sea surface temperature (SST). The thermal coupling varies with the spatial scale of the SST anomaly. To study the actual strength of this coupling on the eddy length scale the authors have coupled an isopycnic ocean model (with embedded mixed layer) to an atmospheric anomaly model. By comparing coarse-resolution and eddy-resolving simulations it is found that 1) the thermal coupling is strong on the eddy length scale and 2) the aforementioned compensation does not occur. Consequences for the temperature boundary condition in ocean-only models, the Gent and McWilliams eddy parameterization, and climate modeling are discussed.