On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing

An ocean general circulation model coupled to an energy-moisture balance atmosphere model is used to investigate the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing. The climate sensitivity of the coupled model using three different parametrisations of subgrid scale mixing is 3 degrees C for a doubling of CO2 (6 degrees C for a quadrupling of CO2). This suggests that the ocean has only a weak feedback on global mean surface air temperature although significant regional differences, notably at high latitudes, exist with different sub-grid scale parametrisations. In the experiment using the Gent and McWilliams parametrisation for mixing associated with mesoscale eddies, an enhancement of the surface response in the Southern Ocean is found. This enhancement is largely due to the existence of more realistic sea-ice in the climatological control integration and the subsequent enhanced ice-albedo feedback upon warming. In accordance with earlier analyses, the Gent and McWilliams scheme decreases the global efficiency of ocean heat uptake. During the transient phase of all experiments, the North Atlantic overturning initially weakened but ultimately recovered, surpassing its former strength. This suggests that in the region around the North Atlantic the ocean acts as a negative feedback on local warming during the transient phase but a positive feedback at equilibrium. During the transient phase of the experiments with a more sophisticated and realistic parametrisation of sub-grid scale mixing, warmed Atlantic water was found to penetrate at depth into the Arctic, consistent with recent observations in the region.