Rôle of the oceanic heat transport in climate dynamics A sensitivity study with an atmospheric general circulation model

Estimating meridional ocean heat transport from the present generation of atmospheric general circulation models, assuming energetic equilibrium, leads to a large variety of results, depending on the model. The current uncertainty on such an important process may cause significant errors in coupled atmosphere/ocean models. To determine the possible nature of these errors, we investigate how the prescription of the oceanic heat transport can affect the results of a coupled surface ocean/atmosphere model where the ocean is limited to thermodynamics and turbulent fluxes but sea-ice is included. In particular, we study the response of the surface fluxes and atmospheric transport to a reduction of the ocean transport. We focus on the initial phase, where these feedback effects begin to develop while the model is still realistic. The model response is strongly dependent on a combination of features: changes in the Hadley cell circulation, the atmospheric heat transport, the radiative and turbulent fluxes at the surface, changes of the radiative fluxes at the top of the atmosphere. In this study, we examine the partitioning between these different effects. It is shown that the atmosphere partly takes up the missing ocean transport, but that this leads to a change in the cloud/radiative equilibrium of the ITCZ region.