THE ROLES OF VERTICAL MIXING, SOLAR-RADIATION, AND WIND STRESS IN A MODEL SIMULATION OF THE SEA-SURFACE TEMPERATURE SEASONAL CYCLE IN THE TROPICAL PACIFIC-OCEAN

The climatological seasonal cycle of sea surface temperature (SST) in the tropical Pacific is simulated using a newly developed upper ocean model. The roles of vertical mixing, solar radiation, and wind stress are investigated in a hierarchy of numerical experiments with various combinations of vertical mixing algorithms and surface-forcing product s. It is found that the large SST annual cycle in the eastern equatori al Pacific is, to a large extent, controlled by the annually varying m ixed layer depth which, in turn, is mainly determined by the competing effects of solar radiation and wind forcing. With the application of our hybrid vertical mixing scheme the model-simulated SST annual cycle is much improved in both amplitude and phase as compared to the case of a constant mixed layer depth. Beside the strong effects on vertical mixing, solar radiation is the primary heating term in the surface la yer heat budget, and wind forcing influences SST by driving oceanic ad vective processes that redistribute heat in the upper ocean. For examp le, the SST seasonal cycle in the western equatorial Pacific basically follows the seminanual variation of solar heating, and the cycle in t he central equatorial region is significantly affected by the zonal ad vective heat flux associated with the seasonally reversing South Equat orial Current. It has been shown in our experiments that the amount of heat flux modification needed to eliminate the annual mean SST errors in the model is, on average, no larger than the annual mean uncertain ties among the various surface flux products used in this study. Where as a bias correction is needed to account for remaining uncertainties in the annual mean heat flux, this study demonstrates that with proper treatment of mixed layer physics and realistic forcing functions the seasonal variability of SST is capable of being simulated successfully in response to external forcing without relying on a relaxation or da mping formulation for the dominant surface heat flux contributions.