NUMERICAL SIMULATION OF THE TROPICAL INTRASEASONAL OSCILLATION AND THE EFFECT OF WARM SST

An atmospheric general circulation model is used in a series of three experiments to simulate the intraseasonal oscillation in the tropical atmosphere.Analyses of the model daily data show that various physical variables,from sever-al different regions,exhibit fluctuations with a spectral peak between 30 and 60 days.This represents a 30-60 day oscillation in the tropical atmosphere and possesses several features which are consistent with observations.These include a horizontal structure dominated by zonal wavenumber 1 and a vertical structure which is predominantly baroclinic.The effect of warm SST (sea surface temperature) anomalies on the 30-60 day oscillation in the tropical atmosphere is also simulated by prescribing global SST as observed in 1983.This has the effect of weakening the oscillation while at the same time the vertical structure becomes less baroclinic.The importance of cumulus convection to the propagational characteristics of this oscillation is demonstrated by a comparison of results based on different parameterizations for convection.In one case,where the maximum convection over the Pacific is simulated to be too far east,the simulated 30-60 day oscillation shows evidence of westward propa-gation.In the second case,where the convection maximum is located near the observed position in the western Pacific,there is more clearly evidence of eastward propagation.Both results suggest that the location of maximum convection in the Pacific can have an important influence on the strength,structure and propagation of the 30-60 day oscillation.