A THERMODYNAMIC-EQUILIBRIUM CLIMATE MODEL FOR MONTHLY MEAN SURFACE WINDS AND PRECIPITATION OVER THE TROPICAL PACIFIC

Diagnosis of the dynamic and thermodynamic balances using observed cli matological monthly mean data reveals that 1) anisotropic, latitude-de pendent Rayleigh friction coefficients lead to much improved modeling of the monthly mean surface wind field for a given monthly mean sea le vel pressure field, and 2) the annual variation of the vertically aver aged lapse rate is important for modeling sea level pressure. Based on the aforementioned observations, a thermodynamic equilibrium climate model for the tropical Pacific is proposed. In this model, the sea lev el pressure is thermodynamically determined from sea surface temperatu re (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-indepen dent correction. The surface winds are then computed from sea level pr essure gradients through a linear surface momentum balance with anisot ropic, latitude-dependent Rayleigh friction coefficients. The precipit ation is finally obtained from a moisture budget by taking into accoun t the effects of SST on convective instability. Despite its simplicity , the model is capable of simulating realistic annual cycles as well a s interannual variations of the surface wind, sea level pressure, and precipitation over the tropical Pacific. The success of the model sugg ests that the tropical atmosphere on a monthly mean time scale is, to the lowest-order approximation, in a thermodynamic equilibrium state i n which sea level pressure is primarily controlled by SST and the effe cts of dynamic feedback on sea level pressure may be parameterized by an empirical SST-lapse rate relationship. Further studies are needed t o establish a firm physical basis for the proposed parameterization.