A numerical Model is developed to study the thermodynamics of the Arabian Sea. The model consists of a surface mixed layer of thickness hm and temperature Tm, embedded in a dynamic layer of thickness h≥hm. Entrainment and detrainment in the mixed layer are determined by wind stirring and cooling at the oceanic surface, as in the Kraus and Turner (1967) model. The dynamic layer is a 11/2layer model which allows entrainment due to shear instability at the base of the layer, similar to the Pollard et al. (1973) model. Monthly climatological data, along with the model sea surface temperature (SST), are used to determine the wind stress and heat fluxes forcing the model. The model is very successful in simulating the observed SST patterns, generally differing by no more than 0.5¿C. During the Southwest Monsoon (June--September), Tm decreases markedly near Somalia and the Arabian peninsula owing to the entrainment of cool subsurface water at the coast and its subsequent advection offshore; farther offshore in the central Arabian Sea it decreases owing to increased evaporative cooling. During the Northeast Monsoon (December--March), the decrease in Tm is caused in part by reduced solar radiation and by increased evaporative cooling, with additional cooling by entrainment in the northern Arabian Sea. An interesting observed feature is that there is a net annual surface heat flux into the Arabian Sea. In the western Arabian Sea this flux is a direct result of coastal upwelling during the Southwest Monsoon, the cold water reducing the latent heat flux out of the ocean. In the central and eastern Arabian Sea it is caused by the southward advection of this upwelled water toward the equator. The annual heat and mass budgets are closed by equatorial currents: a westward undercurrent is the source of cool subsurface water that is entrained in the Arabian Sea, and a warm eastward surface current removes the excess heat and entrained water from the upper layer. ¿ American Geophysical Union 1989