Surface energy balance errors in AGCMs: Implications for oceanatmosphere model coupling

The oceanic poleward heat transport (T-O) implied by an atmospheric General Circulation Model (GCM) can help evaluate a model's readiness for coupling with an ocean GCM. A well known problem is that the sub-tropical Southern Hemisphere T-O implied by many models is equatorward, contrary to most observationally-based estimates. By correcting the T-O of a diversity of models with satellite-derived observations, an earlier study demonstrated that the dominant T-O problems could be explained by model errors in cloud radiative effects. Such errors were argued to be primarily responsible for the erroneous Southern Hemisphere T-O. However, systematic evaluation of one model in a later study suggested that the equatorward Southern Hemisphere T-O might also be attributable to biases in the ocean surface latent heat flux. In this study we revisit the problem with a more recent suite of simulations, and demonstrate that collectively models have improved, but only slightly. We then use ocean surface fluxes estimates to examine the problem from a surface point of view. Our results clarify that common model errors in the surface latent heat flux can afflict T-O as dramatically as errors in cloud radiative effects. To illustrate the relative importance of the two errors, diagnostic tests are introduced that should become increasingly useful as estimates of ocean surface fluxes are improved.