The role of synoptic and intraseasonal anomalies in the life cycle of summer rainfall extremes over South America.

Abstract

The main goal of this study is to describe the role of synoptic and intraseasonal anomalies during the life cycle of summer rainfall extremes over South America. Eastward-propagating synoptic-scale midlatitude waves are the main drivers of extreme precipitation events south of the Amazon and their interaction with intraseasonal anomalies over South America is important for heavy rainfall over the South Atlantic convergence zone (SACZ) region and the La Plata basin. Madden&ndashJulian Oscillation (MJO) convective activity in the western Pacific (phases 6 and 7) leads 31 out of 81 extremes over the SACZ region by nearly 10 days. The connection between the MJO and rainfall extremes in other regions is less robust. During El Niño seasons extremes are more frequent in the La Plata basin, with decreased importance of intraseasonal anomalies. Precipitation extremes over the La Plata basin tend to be less frequent and also shorter during La Niña summers and, consequently, less hazardous. In the SACZ and the southeastern Brazilian coast, heavy rainfall is also more frequent under El Niño conditions, while La Niña episodes also increase extreme events in the southeastern coast. Extremes over the southeastern coast during El Niños are favored by strong intraseasonal anomalies flanking the subtropical jet, while during La Niñas intraseasonal anomalies are not significant.