Southern hemisphere circulation anomalies and impacts over subtropical South America due to different El Niño flavours

Abstract

ENSO exhibits different flavors with worldwide impacts. However, the associated teleconnections with subtropical South America (SSA) are still controversial and modelling studies are needed. Here, we analyze the Southern Hemisphere (SH) circulation anomalies and the impacts over SSA during the austral summer due to different El Niño patterns (Canonical and El Niño Modoki). The analysis is performed considering reanalysis data and two different Atmospheric General Circulation Models (SPEEDY and UCLA - AGCM).
Results from reanalysis show that positive precipitation anomalies develop over SSA during Canonical El Niño events. These anomalies are induced through an increase of upper level cyclonic vorticity advection and a stronger low-level southward moisture transport. However, in El Niño Modoki events, rainfall anomalies are observed over SSA only for the strongest events.
Both models are able to reproduce the precipitation signal over SSA in the Canonical El Niño case, although the underlying physical mechanism depends on the model. In SPEEDY, the increased rainfall is due to an increase of the moisture transport toward SSA, while in UCLA - AGCM it is related to both, an increase of the low-level moisture transport toward SSA and the increase of upper level cyclonic vorticity advection. The precipitation signal associated with El Niño Modoki is more controversial. While UCLA - AGCM suggests a rainfall increase over SSA, SPEEDY, in agreement with observations, does not show any statistically significant signal. However, the upper level circulation anomalies reproduced by UCLA – AGCM are more consistent with reanalysis than those from SPEEDY, which makes UCLA – AGCM to be more reliable. This result suggests increased rainfall over SSA during El Niño Modoki.