Abstract

SOUZA, E. B. DE  and  AMBRIZZI, T.. ENSO impacts on the South American rainfall during 1980s: Hadley and Walker circulation. Atmósfera [online]. 2002, vol.15, n.2, pp.105-120. ISSN 0187-6236.

The changes in the Hadley and Walker cells and their respective impacts on the South American rainfall during the ENSO episodes observed in the decade of 80, were investigated through cross-sections analyses of the atmospheric circulation in altitude, averaged in the zonal and meridional planes. Such large-scale cells almost inverted their climatological circulation pattern, during El Niño events (1982-83 and 1986-87). In these years, mânifestation of the anomalous descending branch of the Hadley and Walker cells affects most of the north-northeast of South America, which inhibited the convective activity associated to ITCZ and caused drought conditions in the rainy seasons of the Guyana, Surinam, French Guiana, center-east of the Amazon and most of the Northeast Brazil. On the other hand, conditions of excess of precipitation observed in the south-southeast of South America, were favored by the anomalous ascending branch of the Hadley cell. During La Niña events (1984-85 and 1988-89), it was observed an intensification of the ascending and descending branches associated to the Walker and Hadley cells. The anomalous large-scale ascending movement associated to these cells, was extended to the Northeast of Brazil and equatorial South Atlantic, favoring ITCZ to become more active than the normal, which resulted in an above normal rainy season in these areas. An intense subsidence was noticed in the mid latitudes of South America, which inhibited the large-scale convection in the region, explaining the deficient rainy season observed in most of the south-southeast of South America.

Keywords : Hadley circulation; Walker circulation; ENSO; South American Rainfall.