A Thermodynamic Equilibrium Climate Model for Monthly Mean Surface Winds and Precipitation over the Tropical Pacific

Abstract

Diagnosis of the dynamic and thermodynamic balances using observed climatological monthly mean data reveals that 1) anisotropic, latitude-dependent Rayleigh friction coefficients lead to much improved modeling of the monthly mean surface wind field for a given monthly mean sea level pressure field, and 2) the annual variation of the vertically averaged 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 level pressure is thermodynamically determined from sea surface temperature (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-independent correction. The surface winds are then computed from sea level pressure gradients through a linear surface momentum balance with anisotropic, latitude-dependent Rayleigh friction coefficients. The precipitation is finally obtained from a moisture budget by taking into account the effects of SST on convective instability. Despite its simplicity, the model is capable of simulating realistic annual cycles as well as interannual variations of the surface wind, sea level pressure, and precipitation over the tropical Pacific. The success of the model suggests that the tropical atmosphere on a monthly mean time scale is, to the lowest-order approximation, in a thermodynamic equilibrium state in which sea level pressure is primarily controlled by SST and the effects of dynamic feedback on sea level pressure may be parameterized by an empirical SST-lapse rate relationship. Further studies are needed to establish a firm physical basis for the proposed parameterization.