Atmospheric circulation features associated with rainfall variability oversouthern Northeast Brazil

The atmospheric circulation features associated with rainfall variability d uring the main rainy period of the region of southern Northeast Brazil (sNE B) are determined through principal components analysis and cluster analysi s. Daily rainfall, NCEP reanalysis, and OLR data are used from 1979 to 1997 . The analyses are performed considering a large area including South Ameri ca and the Pacific and Atlantic Oceans and also considering a regional area over tropical South America. The influence of synoptic systems. as well as the phenomena associated with their variability, such as El Nino-Southern Oscillation (ENSO) and baroclinic waves over the Pacific Ocean and South Am erica are discussed. Positive (negative) rainfall anomalies over sNEB are r elated to cold (warm) episodes in the tropical Pacific. On the intraseasona l timescale there are interrelationships among the South Atlantic convergen ce zone (SACZ), Bolivian high, and the northeast upper-cyclonic vortex. It is noted that rainy periods in sNEB are associated with a northward displac ement of the SACZ. an upper-level cyclonic vortex situated over the Atlanti c Ocean, intense convection over the eastern Amazon, and an eastward shift in the position of the Bolivian high. At low levels the flow is directed fr ont the Amazon toward the sNEB and there is a shift of the South Atlantic s ubtropical anticyclone eastward with a weakening of the Southeast trade win ds over the northeast coast. The dry pattern is associated with a southward displacement of the SACZ. an upper-level cyclonic vortex either close to o r over the continent, and a westward displacement of the Bolivian high. In this case, the low-level flow is directed from the Amazon it) the south and there is a strengthening and/or westward shift of the South Atlantic antic yclone with an intensification of the southeast trade winds along the north east coast.