Synoptic patterns associated with extreme precipitation events over southeastern South America during spring and summer seasons

Abstract

This study focuses on characterizing the diversity of synoptic forcing precursors of extreme precipitation events in southeastern South America (SESA) during the warm season, as well as identifying the differences in the synoptic environment during spring (October–November–December) and summer (January–February–March). The characterization of extreme events was carried out using daily precipitation data from the CPC gridded dataset for the period 1979–2017. Extreme events were defined when the daily precipitation exceeds the 95th percentile of the daily rainy days and covers a minimum area of 100 km2. During the spring months extremes are more intense and are preferably located over the central and eastern part of SESA, while during the summer they occur towards the west and south of SESA. The associated synoptic conditions were identified after applying a principal component analysis to the 850 hPa geopotential height for extreme precipitation days using the ERA-Interim reanalysis. Two distinctive patterns for each season were identified. The anomalous intense wind with a northerly component and a negative geopotential height anomaly over northwestern Argentina emerged as the most common features associated with the occurrence of extreme events. Differences between the two patterns emerge in terms of the southward penetration of the northerly wind and the anomalous circulation off the east coast of South America. A high-level jet was detected in both patterns, although a weaker core and located in a position further south is found for the second pattern compared with the first one. Differences in the synoptic forcing features between spring and summer, such as a more intense low-level northerly wind, a deeper northwestern low, a more intense upper-level jet stream and a more extended area of maximum upper-level wind divergence during spring compared with summer, may explain the differences found in the extreme precipitation in SESA for the two seasons.