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

• Published in: International journal of climatology 2019-02, Vol.39 (2), p.1104-1116
• Main Authors: Barreiro, Marcelo, Sitz, Lina, de Mello, Santiago, Franco, Ramon Fuentes, Renom, Madeleine, Farneti, Riccardo
• Format: Article
• Language: English
• Subjects: Air temperature; Air-sea interaction; Anomalies; Atmosphere; Atmospheric models; Barotropic mode; Climate; Climate models; Computer simulation; Convergence; Convergence zones; Coupled modes; Heat flux; Heat transfer; Madden-Julian oscillation; MJO; Modelling; Precipitation; Rain; Rainfall; regional climate model; Regional climate models; Regional climates; Sea surface; Sea surface temperature; South America; South Atlantic Convergence Zone (SACZ); Surface temperature; Surface-air temperature relationships; teleconnections; Temperature (air-sea); Temperature effects
• ISSN: 0899-8418; 1097-0088
• DOI: 10.1002/joc.5865

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 associated with the MJO, which involves the development of a barotropic cyclonic anomaly over South America between 30°S and 60°S, 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 associated 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, probably due to a too strong response of the stand‐alone model to prescribed sea surface temperature. During phases 2 and 3 the MJO generates atmospheric teleconnections that induce a weakening and southwards shift of the SACZ in eastern Brazil. The changes in surface heat flux anomalies associated with the SACZ rainfall anomalies induce a warm sea surface temperature anomaly off Brazil between 20°S and 30°S during the following phases 4 and 5. While the direct MJO impact on the SACZ continues, the regional air–sea interaction strengthens the surface circulation anomalies and rainfall changes in the SACZ during phases 4 and 5. In the absence of air–sea interaction the southwards SACZ shift also occurs but is weaker. We suggest that surface winds accelerate over the warm SST anomaly due to increased downwards flux of momentum in a less stable atmospheric boundary layer. This increases the wind divergence (convergence) in the northern (southern) portions of the SACZ, thus strengthening rainfall anomalies.