VERTICAL MOTION CHANGES RELATED TO NORTHEAST BRAZIL RAINFALL VARIABILITY - A GCM SIMULATION

The atmospheric structure over north-east Brazil during anomalous rain fall years is studied in the 11 levels of the outputs of the Laboratoi re de Meteorologie Dynamique atmospheric general circulation model (LM D AGCM). Seven 19-year simulations were performed using observed sea-s urface temperature (SST) corresponding to the period 1970-1988. The en semble mean is calculated for each month of the period, leading to an ensemble-averaged simulation. The simulated March-April rainfall is in good agreement with observations. Correlations of simulated rainfall and three SST indices relative to the equatorial Pacific and northern and southern parts of the Atlantic Ocean exhibit stronger relationship s in the simulation than in the observations. This is particularly tru e with the SST gradient in the Atlantic (Atlantic dipole). Analyses on 200 hPa velocity potential, vertical velocity, and vertical integral of the zonal component of mass flux are performed for years of abnorma l rainfall and positive/negative SST anomalies in the Pacific and Atla ntic oceans in March-April during the rainy season over the Nordeste r egion The results at 200 hPa show a convergence anomaly over Nordeste and a divergence anomaly over the Pacific concomitant with dry seasons associated with warm SST anomalies in the Pacific and warm (cold) wat ers in the North (South) Atlantic. During drought years convection ins ide the ITCZ indicated by the vertical velocity exhibits a displacemen t of the convection zone corresponding to a northward migration of the ITCZ. The east-west circulation depicted by the zonal divergent mass flux shows subsiding motion over Nordeste and ascending motion over th e Pacific in drought years, accompanied by warm waters in the eastern Pacific and warm/cold waters in northern/southern Atlantic. Rainfall v ariability of the Nordeste rainfall is linked mainly to vertical motio n and SST variability through the migration of the ITCZ and the east-w est circulation.