The summer monsoon regime in South America is associated with the rainy season in most of the continent. Although the classical seasonal reversal of the surface winds is not apparent, there are climatic features that are characteristic of a monsoon regime. El Niño/La Niña (EN/LN) events affect some important components of this regime, in view of its impact on the tropical heat sources and global circulation. The present study seeks a comprehensive view of the LN impact on summer precipitation over Brazil and the associated mechanisms. This impact has not been well established through seasonal analysis because it shows significant sub-seasonal variations. A consistent picture is obtained with a monthly resolution analysis.

The data used are monthly precipitation totals from more than 1000 stations in the period 1956-92 and NCEP/NCAR Reanalysis circulation, temperature and humidity data. The impact of LN events on precipitation is represented at each station by percentiles of gamma distributions for each month of the summer and for whole season (DJF). The consistency of the impact is assessed with a test using the hypergeometric distribution. The mechanisms of precipitation anomalies are analyzed through the perturbations that LN events impose on the essential ingredients of precipitation: moisture convergence, and the force to lift the moist air to the condensation level. For this, composites of LN related anomalies are presented of the rotational and divergent wind at 850 and 200 hPa, of moisture flux and its divergence, and of the atmospheric thermodynamic structure.

The monthly analysis shows coherence between precipitation and circulation anomalies and enhances anomalies that are important during only part of the summer. There is a reversal of precipitation and circulation anomalies over most of Brazil in January when compared to the other summer months. It is not clear whether surface-atmosphere and land-ocean interactions favor the reversal of some regional circulation anomalies in January or whether the atmospheric basic state in January is conducive to perturbations different from the other months in summer.