An alternative method for coupling atmosphere and ocean models is used to examine the response of the tropical climate system to enhanced atmospheric CO2. The Equilibrium Asynchronous Coupling method (EAC) accelerates coupled model integrations and allows clearer distinction among forcing mechanisms. EAC involves a sequence of quasi-equilibrium integrations of an atmospheric model driven by fixed SST's, whose output is used to force an ocean model. Although the solution in this iterative procedure should be the same as that of a synchronously coupled atmosphere-ocean model, EAC's decomposition of air-sea interaction into separate, one-way forced components allows the final climatic response to be partitioned between the atmospheric- and oceanic-driven portions.
In this study, EAC is used in conjunction with a low-resolution (T31) atmospheric GCM (GENESIS 2.0) and ocean GCM (MOM 1.0) to diagnose changes in tropical climate resulting from a tripling of atmospheric CO2. Previous modeling studies by others have identified the tropics, especially the equatorial Pacific, as being sensitive to anthropogenically forced future climate changes. The results here indicate that the initial atmospheric response of the warmer climate is a reduction in the strength of the Walker circulation. The associated relaxation of the tropical easterly trade winds causes the ocean to produce weaker equatorial surface currents and reduced upwelling. The most pronounced expression of these oceanic changes occurs in the central equatorial Pacific, where the SST warms by up to 1 oC more than other tropical locations. This warming pattern represents an eastward shift in the western Pacific warm pool and causes the subsequent atmospheric model integration to generate much more cloudiness and precipitation in this region than in the previous integration. The resulting SST and atmospheric patterns resemble those during El Nino, except that the most enhanced warming is concentrated in the central equatorial Pacific, with little change in the SST difference between the eastern and western equatorial regions