The influence of nonlinearities and different SST spatial patterns on the summertime anomalies in subtropical South America during strong ENSO events

Abstract

In this study we analyze the impacts of the strongest ENSO events on subtropical South America (SSA) rainfall during the austral summer in the period (1979–2014). We consider reanalysis data and model experiments. For the strongest El Nino events, the precipitation pattern over SSA presents a monopole structure. The reanalysis shows a short Rossby wave train that causes an increase of the baroclinicity and advection of cyclonic vorticity toward SSA. Together with an increase in the moisture transport by the South American Low Level jet (SALLJ), they induce strong positive precipitation anomalies over SSA. The model reproduces this rainfall anomaly as consequence of a strengthening of the upper level jet and of the SALLJ, although it does not simulate the short wave train from the Pacific. For the strongest La Nina events, the precipitation pattern presents a tripole like-structure with centers located over: (1) Uruguay, (2) the continental part of the South Atlantic Convergence Zone (C-SACZ) and (3) the northwestern Argentina (NW-Arg). In both cases, reanalysis and model experiments, the tripole is associated with a weakening of the SALLJ that increases (reduces) rainfall anomalies over C-SACZ (Uruguay), and with an anomalous increase of the lower level moisture convergence over NW-Arg that increases precipitation over there. Differences in precipitation responses to the strongest ENSO events are mainly associated to changes in the spatial pattern of the sea surface temperature anomalies, however, there is also a contribution of nonlinearities.