The impact of El Niño on South American summer climate during different phases of the Pacific Decadal Oscillation

Abstract

In austral summer, the observed El Nino (EN) events during warm Pacific Decadal Oscillation (PDO) phases (PDO(+)) exhibited large anomalous upper level wave patterns in response to larger Sea Surface Temperature (SST) anomalies in the Equatorial Pacific and Atlantic Oceans compared with SST anomalies in EN events during cold PDO phases (PDO(−)). The precipitation anomalies in PDO(+) EN are increased over Southeastern South America (SESA) associated with the intensification of the moisture flux convergence in this region. The PDO(−) EN events exhibit positive precipitation anomalies only over southern SESA, while negative anomalies were observed in the north. Downward motion and anomalous divergence over central eastern Brazil may have contributed to the weakening of the northwesterly moisture flux convergence associated with the South American Low Level Jet (SALLJ) over the subtropics. The extratropical cyclones showed higher frequency and lower central pressures in southern Brazil, Uruguay, northeastern Argentina, and Southwest Atlantic Ocean during the PDO(+) EN events compared with the PDO(−) EN events. Such increase in the frequency and intensity of cyclogenesis cases seems to be in accordance with the anomalous moisture flux convergence over the SESA and associated reduction in the Sea Level Pressure observed during PDO(+) EN events. In order to investigate the impact of a canonical El Nino event over South America under different PDO phases, two numerical experiments were done with an Atmospheric General Circulation Model. Global SST and ice sea fields average over years characterized by (a) PDO(+) and (b) PDO(−) were considered as climatologically fields, and a composite of anomalies of SST of all El Nino events observed in 1950–1999 was added in the region 20oS–20oN;120oW–175oW of both “climatologies.” The differences in experiments suggest that a canonical EN may produce significant different anomalous atmospheric patterns associated with distinct PDO climatologies. The more significant differences are simulated over extreme northern and eastern Brazil. Additional numerical experiments isolating the observed variability of SST over several oceanic basins during different PDO phases will be conducted to study their particular role on the South American climate.