Extreme precipitation events in southeastern South America and large-scale convective patterns in the South Atlantic convergence zone

The occurrence of daily extreme precipitation events in southeast South America (Sao Paulo, Brazil) and the spatial features of convective activity in the South Atlantic convergence zone (SACZ) are investigated. Precipitation data from surface stations in Sao Paulo state from 1979 to 1996 are used to determine the frequency of occurrence of extremely heavy daily precipitation events. Daily averages of outgoing longwave radiation (OLR) are examined to characterize convective activity in the SACZ. OLR features are identified with factor analysis. Two factors explain similar to65% of the total variance of the convective activity patterns in tropical South America and characterize events according to the intensity and extent of the OLR features over the Atlantic Ocean. The combination of factors indicates that 35% of extreme precipitation events occurred when convective activity in the SACZ was intense over large parts of tropical South America, which includes Sao Paulo, but with less extent toward the Atlantic Ocean. Warm SST episodes (El Nino) seem to modulate the occurrence of extremes associated with intense convection in the SACZ displaced northward of Sao Paulo and toward the Atlantic Ocean. The remaining events associated with weak convective activity in the SACZ suggest the role of transient systems producing extreme precipitation in Sao Paulo. The important contribution of the present work is the documentation of the role of orographic features for the regional distribution of extreme precipitation in Sao Paulo. It is shown that the regional distribution of extreme precipitation depends on both the intensity and form of the convection in the SACZ.