An improved linear model of tropical surface wind variability

Abstract

For coupled modelling studies, the atmospheric component is required to simulate the large-scale part of the observed surface winds accurately, as only this part is responsible for driving low-frequency interannual variability in the oceans. A simple linear model of the tropical atmosphere is developed in which surface winds are viewed as the response of the atmospheric boundary layer to pressure perturbations produced 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 given by the first two empirical orthogonal functions (EOFs), it also successfully simulates the equatorial surface wind anomalies and their evolution during the entire period 1974-1991. Certain interesting differences between precipitation forcing and SST induced forcing of surface winds is brought out in this study. The precipitation forcing was found to be dominant over the central and western Pacific, while the SST forcing was found to be dominant over the eastern Pacific. The resultant wind response was also found to reflect this behaviour. The most interesting result is the change in balance of the precipitation-related and SST-gradient-related terms in the forcing of anomalous and climatological winds. Our results indicate that convective heating predominates over SST-gradient-induced effects in forcing anomalous winds, but the balance reverses in the case of the climatological winds. We also find that SST-gradient effects are critical in the simulation of the climatological wind field.