Diagnostic Studies of Rainfall Anomalies in Northeast Brazil

Abstract

The large-scale atmospheric circulation of the Brazil-Atlantic sector is studied in relation to extreme rainfall anomalies in two large regions of Northeast Brazil (Nordeste). Long-term rainfall series, aerological records of stations in South America, and ship observations over the tropical Atlantic form the data base for this study. Departure patterns of meteorological elements over the Atlantic are investigated for composites of extremely dry and wet years in the southern and northern Nordeste. Southern Nordeste's peak rainy season is around November/December. The wet years in the southern Nordeste are marked by negative pressure departures over the South Atlantic, weak onshore southeast trades and anomalously cold waters along the south Brazil coast. These features appear to be related to Southern Hemispheric frontal systems. During the dry years, departure patterns are nearly reversed to those of the wet years.Northern Nordeste receives its maximum rainfall in March/April. Rainfall variations are modulated by the latitudinal displacement of the baric trough and confluence axis over the equatorial Atlantic and concomitant sea surface temperature anomalies. Case studies of recent extreme years indicate the possible existence of a local meridional circulation. The more northerly position of the convergence band over the Atlantic, the anomalously cold waters to the south of the equator and the subsidence in the southern portion of the thermally-induced meridional circulation cell over the Nordeste, characteristic of drought years, are all unfavorable for rainfall in Northeast Brazil.