Modelling the role of Atlantic air–sea interaction in the impact of Madden–Julian Oscillation on South American climate

Abstract

This study addresses the role of Atlantic air–sea interaction in the remote influence of the Madden–Julian Oscillation (MJO) on eastern South American climate during austral summertime. To disentangle the different processes involved, reanalysis data as well as a regional climate model run in coupled mode and as a stand-alone atmosphere are used. The simulations are able to represent the observed influences of the MJO in precipitation and surface air temperature. In particular, in both setups the model is able to represent adequately the atmospheric teleconnections associ- ated with the MJO, which involves the development of a barotropic cyclonic anomaly over South America between 30oS and 60oS, which favours a southwards shift of the South Atlantic Convergence Zone (SACZ) and a warming in eastern Brazil. Moreover, model simulations support the hypothesis that air–sea interaction is important to set up the strength of the rainfall response in the SACZ. That is, the development of a local warm SST anomaly forced by heat flux anomalies associ- ated with the direct MJO impact in turn feeds back into the atmosphere generating a stronger surface convergence that shifts the SACZ southwards. In the absence of this SST-forced response the SACZ still shifts southwards, but anomalies are much weaker and less extensive. We also found that the coupled model represents more adequately the remotely forced MJO temperature signal over eastern Brazil, proba- bly due to a too strong response of the stand-alone model to prescribed sea surface temperature.