The South American precipitation trends under (or not) El Niño-Southern Oscillation influences and relationship with large-scale circulation

Abstract

The precipitation trend patterns in South America (SA) are determined using trend empirical orthogonal function analysis for the 1951–2016 period. The associated large-scale tropical and extratropical anomalous circulation patterns are also examined. The words “total” and “residual” refer to the monthly anomalies and monthly anomalies without the El Niño–Southern Oscillation (ENSO) effects, respectively. The total precipitation features a positive trend in southeastern SA (SESA, southern Brazil, Uruguay, most of eastern Argentina) and northern Chile, and a negative trend over central-eastern Brazil and central Amazonia. The residual precipitation shows an increased positive trend over most of the coastal extension of northern SA and Colombia; a weak positive trend over southern Brazil, northeastern Argentina, and northern Chile; and a negative trend over central-eastern SA and western Amazonia. The differences between the total and residual precipitation trend patterns in tropical SA is explained as responses to total and residual zonally asymmetric anomalous sea surface temperature (SST) patterns, respectively. The total SST pattern along the equatorial Pacific configures the Pacific Decadal Oscillation, which impacts ENSO variability and as response intensifies the Walker circulation. Without the ENSO, the Walker cell is mainly driven by the tropical Indian and Atlantic Oceans, which configure residual asymmetric anomalous warming. Furthermore, the warming in the equatorial Indian and eastern Pacific Oceans, in the presence of ENSO, induces a Rossby wave train-type anomalous pattern that extends across the South Pacific into SA and modulates the atmospheric anomalous circulation over SESA. In this region, an anomalous anticyclonic accompanied by an intensified South American Low-Level Jet induces a moisture transport to SESA. This anticyclone is also observed in the absence of ENSO but is weaker. The results suggest the importance of ocean warming in the western Pacific-Indian in the modulation of extratropical teleconnections to SESA in the tropical ocean warming scenario. © 2024 Royal Meteorological Society.