The role of lateral ocean physics in the upper ocean thermal balance of a coupled ocean-atmosphere GCM

The role of lateral ocean physics in the upper ocean thermal balance of a coupled ocean-atmosphere GCM Abstract The sensitivity of the upper ocean thermal balance of an ocean-atmosphere coupled GCM to lateral ocean physics is assessed. Three 40-year simulations are performed using horizontal mixing, isopycnal mixing , and isopycnal mixing plus eddy induced advection. The thermal adjustment of the coupled system is quite di€erent between the simulations, con®rming the major role of ocean mixing on the heat balance of climate. The initial adjustment phase of the upper ocean (SST) is used to diagnose the physical mechanisms involved in each parametrisation. When the lateral ocean physics is modi®ed, signi®cant changes of SST are seen, mainly in the southern ocean. A heat budget of the annual mixed layer (de®ned as thè`bowl'') shows that these changes are due to a modi®ed heat transfer between the bowl and the ocean interior. This modi®ed heat intake of the ocean interior is directly due to the modi®ed lateral ocean physics. In isopycnal di€usion, this heat exchange, especially marked at mid-latitudes, is both due to an increased e€ective surface of di€usion and to the sign of the isopycnal gradients of temperature at the base of the bowl. As this gradient is proportional to the isopycnal gradient of salinity, this con®rms the strong role of salinity in the thermal balance of the coupled system. The eddy induced advection also leads to increased exchanges between the bowl and the ocean interior. This is both due to the shape of the bowl and again to the existence of a salinity structure. The lateral ocean physics is shown to be a signi®cant contributor to the exchanges between the diabatic and the adiabatic parts of the ocean.