AN IMPROVED LINEAR-MODEL OF TROPICAL SURFACE WIND VARIABILITY

For coupled modelling studies, the atmospheric component is required t o simulate the large-scale part of the observed surface winds accurate ly, as only this part is responsible for driving low-frequency interan nual variability in the oceans. A simple linear model of the tropical atmosphere is developed in which surface winds are viewed as the respo nse of the atmospheric boundary layer to pressure perturbations produc ed by deep convection and sea surface temperature (SST) gradients. An empirical parametrization of total convection as a nonlinear function of total SST is adopted. The large-scale variability of the simulated surface winds is examined for the period 1974-1991. The model not only captures the large-scale low-frequency part of the surface winds give n by the first two empirical orthogonal functions (EOFs), it also succ essfully simulates the equatorial surface wind anomalies and their evo lution during the entire period 1974-1991. Certain interesting differe nces between precipitation forcing and SST induced forcing of surface winds is brought out in this study. The precipitation forcing was foun d to be dominant over the central and western Pacific, while the SST f orcing was found to be dominant over the eastern Pacific. The resultan t wind response was also found to reflect this behaviour. The most int eresting result is the change in balance of the precipitation-related and SST-gradient-related terms in the forcing of anomalous and climato logical winds. Our results indicate that convective heating predominat es over SST-gradient-induced effects in forcing anomalous winds, but t he balance reverses in the case of the climatological winds. We also f ind that SST-gradient effects are critical in the simulation of the cl imatological wind field.