A numerical study on the coupling between sea surface temperature and surface evaporation

The feedback between sea surface temperature (SST) and surface evaporation is an important issue in the study of climate change. To understand this fe edback and its interaction with surface wind in the tropical Pacific Ocean (30 degrees N - 30 degrees S), and in particular over the warm pool region, a dynamic tropical atmospheric circulation model is used. The model consis ts of a two-layer free troposphere and a well-mixed boundary layer. It invo lves active interactions between the boundary layer flow, forced by an SST gradient, and the free atmospheric flow, forced by SST. Various SST fields (representing climatology, El Nino, and La Nina conditions) are used to dri ve the model. It is found that the binned averages of evaporation and wind speed increase with SST for up to about 300-301 K. From that point on they decrease with SST. In addition, negative SST anomalies correspond to excess latent heat flux and wind speed. These results are in agreement with relev ant observations. To understand the thermodynamic versus dynamic effects of SST on surface evaporation, in one of the experiments we impose a sudden p ositive SST perturbation on the climatological SST field during the model i ntegration. It is shown that while surface evaporation is initially enhance d in response to the SST change, as the atmospheric circulation gradually " feels" the SST perturbation, its dynamic effect through the circulation cha nge becomes more apparent over the SST perturbation region. Overall, the re sults of our study show that in the low SST regime the behavior of evaporat ion is dictated by thermodynamics, whereas in the high SST regime it is dic tated by atmospheric dynamic considerations.