Seasonal Diagnostic and Predictability of Rainfall in Subtropical South America Based on Tropical Pacific SST

Abstract

The seasonality of the simultaneous relationship between tropical Pacific SST and rainfall, as well as rainfall predictability one season in advance in subtropical South America (258–408S), is studied using different multivariate techniques. This study shows that ENSO-related rainfall anomalies in subtropical South America are restricted mostly to regions on the eastern and western sides of the continent and mainly during the second half of the year. The relationship is almost exclusively of the warm–wet/cold–dry type, but a more widespread impact is found when anomalously warm conditions prevail in the equatorial Pacific. A spatially coherent region with a significant warm–wet/cold–dry signal is detected in southeastern South America during austral spring (October– November), including southern Brazil, southern Paraguay, Uruguay, and eastern Argentina. This signal moves inland toward the west from spring to early summer. During late winter (July–August), a similar SST–rainfall relationship is found in subtropical Chile and southern Brazil. In Chile, a southward propagation of the signal is observed from winter to spring. Most significant ENSO-related rainfall anomalies seem to occur after the maximum in the precipitation annual cycle. The combined analysis of seasonal diagnostics and predictability of rainfall show that the seasonal rainfall predictability in subtropical South America based on tropical Pacific SST to a greater extent is restricted to a specific time of the year and regions that broadly coincide with those where the simultaneous SST–rainfall relationship is significant. This fact suggests that persistence of tropical Pacific SST anomaly is the major source of seasonal rainfall predictability in this region, when SST is used as a predictor.