Climatological studies of the influences of El Niño Southern Oscilation events in the precipitation pattern over South America during austral summer

Recent studies have indicated that the precipitation pattern over South America, in particular over some regions in Brazil are strongly influenced by the extremes of the Southern Oscillation (SO). Using seasonal and monthly rainfall anomalies and Outgoing Longwave Radiation (OLR) data during El Niño/La Niña events during the austral summer (December-January-February), analysis of the lower and upper level circulation, the associated convection and rainfall features is made for some events representative of the extremes of the SO: The El Niño events of 1982/83, 1986/87 and 1991/92, and the La Niña events of 1984/85, 1985/86 and 1988/89.

This study uses rainfall observations from 2138 meteorological stations over South America, with few of them going back in time to 1853 until 1996. The precipitation dataset implemented by Xie and Arkin for the period from 1979 to 1995 is also used on this study, together with the NOAA OLR, and the (NCEP/NCAR) Reanalisys, for the period from 1968 to 1996.

Preliminary results indicate that the atmospheric response and consequently the summertime precipitation pattern over South America to El Niño events of 1982/83, 1986/87 and 1991/92, ehibits large variability from one event to another: The central part of the South America shows enhanced precipitation while the southern section of South America did not experience significant changes in rainfall. On the other hand, the north part of South America presented large negative anomalies during these events, which are related to changes in the meridional cell of Walker during the El Niño, where the anomalous heating in the middle equatorial Pacific modies the zonal and meridional circulation in the tropics, and extending to other regions around the world via a teleconnection pattern.

The La Niña events of 1984/85, 1985/86 and 1988/89 also indicates a large variability from one event to another, however, the variability over the south part of South America was less significant. The central sector of South America experiences decrease in the precipitation pattern. On the other hand, a rainfall distribution pattern for north South America during La Niña contrasts with that typical of El Niño, with enhanced rainfall over that area.

Another important finding is that the precipitation pattern are important changes in the position and intensity of the Inter Tropical Convergence Zone (ITCZ) and the South Atlantic Convegence Zone (SACZ), that exhibit opossite meridional migrations during both El Niño and La Niña events.