Madden-Julian Oscillation impacts on South American summer monsoon season: precipitation anomalies, extreme events, teleconnections, and role in the MJO cycle

The impacts of the Madden Julian Oscillation (MJO) on the South American monsoon season are analyzed in the global context of the MJO propagating anomalies of convection and circulation. Unexplored aspects, such as the continental-scale daily precipitation anomalies in the MJO frequency band and changes in the frequency of extreme rainfall events, are disclosed throughout its cycle. Among other effects, the MJO increases the average daily precipitation by more than 30% of the climatological value and doubles the frequency of extreme events over central-east South America (SA), including the South Atlantic Convergence Zone (SACZ). The evolution of the most intense precipitation anomalies depends on the interplay between tropics-tropics and tropics-extratropics teleconnections, and the topography over central-east SA seems to play a role in enhancing low-level convergence. The maximum anomalies are produced by a tropics-extratropics wave train. It not only favors precipitation anomalies over the SACZ and subtropical SA, but also strengthens the anomalies over tropical SA when the system propagates northeastward. Influence function analysis and simulations of the responses to different components of upper-level anomalous divergence associated with the MJO anomalous convection indicate the probable origin of the anomalous circulation leading to the main precipitation anomalies over SA. It is triggered by secondary anomalous convection, while the main tropical anomalous circulation is produced by the strongest equatorial convection anomalies. There are indications that MJO-related anomalies over SA contribute to the impacts on other regions and to the initiation of the MJO in the Indian Ocean.