The South Atlantic Ocean as a moisture source region and its relation with precipitation in South America

Abstract

We explore the South Atlantic as a moisture source for South America and its relationship with the precipitation variability in southeastern South America (SESA) during the 1982–2015 period. Based on diagnostic calculation with a Semi-Lagrangian analytical model, three regions of the South Atlantic acting as moisture sources for South America were studied: the Tropical Atlantic (15° N–5° S), Subtropical Atlantic (30° S–5° S), and Southwestern Atlantic (21° S–50° S; 30° W to the further west). The Tropical and Subtropical Atlantic are important sources of moisture for the Amazon, and occasionally for SESA. The Southwestern Atlantic contributes mainly locally, although in summer it also has a role in increasing precipitation over Uruguay and southern Brazil. Approximately 17% of the observed precipitation over the La Plata basin comes from the three regions identified as moisture source in the South Atlantic. Sea surface temperature variability is related to the moisture contribution from the South Atlantic to the continent. In summer, a significant positive correlation between the sea surface temperature leading-mode and the precipitation contribution from the Tropical Atlantic and La Plata Basin is found. A significant negative correlation between the sea surface temperature leading-mode and contribution in terms of precipitation from the Southwest Atlantic was found, as warm anomalies are associated with an anomalous cyclonic circulation over the Southwest Atlantic that favors moisture transport to SESA. Finally, the study of individual precipitation events identified contributions from the Subtropical and Southwest Atlantic to particular daily precipitation events in SESA. Climatological contributions from the Southwestern Atlantic are low, however, in events such as these, their contribution can increase up to 40% on the synoptic scale.