El Niño Southern Oscillation-related precipitation anomaly variability over eastern subtropical South America: Atypical precipitation seasons

Abstract

Precipitation anomalies over subtropical Argentina in eastern subtropical South America (ESSA) show significant signatures of the El Niño Southern Oscillation (ENSO) during the SONDJ (from September to January) season. The correlation maps between seasonal precipitation anomalies and the C-index show a dipole structure with positive correlation over eastern ESSA and negative over South American convergence zone. Based on principal component analysis, precipitation anomalies within the SONDJ season during ENSO events were discriminated into three categories typical, atypical and nontypical, regarding their precipitation response. Typical (atypical) stands for precipitation anomalies similar (inverse) in comparison to the linear expected anomalies for El Niño or La Niña events. In the period 1979–2016, five (five) typical seasons, two (zero) atypical seasons and six (six) nontypical seasons were recorded under El Niño (La Niña) events. During typical SONDJ seasons under El Niño, precipitation over ESSA is mainly modulated by regional tropospheric circulation anomalies induced by quasi-stationary Rossby wave propagation from the western South Pacific towards South America. The precipitation anomalies during the two atypical SONDJ seasons in El Niño were overall partly owing to shifts of the sea surface temperature (SST) gradient in the equatorial Pacific, and changes in the Atlantic basin SST anomalies.