Active and Break Phases in the South American Monsoon System

Abstract

The South American monsoon system (SAMS) refers to the austral summer season features of deep convective activity and large-scale circulation. This study examines intraseasonal variations in the low-level wind circulation in the Amazon and their modulating effects on active and ''break'' phases in SAMS. Daily averages of outgoing longwave radiation (OLR), NCEP-NCAR reanalysis, and gridded rainfall station data in Brazil are used from 1 November to 28 February 1980-99. The direction of wind anomalies (10-70 days) in the Rondonia State, Brazil, is used to classify periods of westerly (W) and easterly (E) low-level wind regimes. Composites of W regime show low-level wind anomalies crossing the equator southward and closing in a cyclonic anomalous circulation off the coast of Argentina and Uruguay. Broad areas of enhanced convection and rainfall are observed in central and southeast Brazil. Suppressed convection is observed over the Bolivian Altiplano and in northern South America. In contrast, in the E regime, opposite patterns are observed in the low-level circulation, con- vection, and rainfall anomalies. The duration of active (W regimes) and break (E regimes) periods are quite similar, with median values of 4 and 5 days, respectively. Further investigation showed that the region of convection and rainfall anomalies over Venezuela and northwest Brazil is observed only in the 10-30-day band. Comparison of the results shown here with previous studies indicates the importance of intraseasonal variations in the activity of SAMS.