The Role of South American Convective Cycles on Tropical Intraseasonal Oscillations

The eastward propagation of intraseasonal anomalies associated with the Madden-Julian Oscillation (MJO) is described as a feedback involving variations over the Indo-Pacific warm pool and over Tropical South America. Intraseasonal convection over the eastern hemisphere tropics enhances convective activity in the western hemisphere tropics a few days later, and vice versa. However, strong intraseasonal convection over one region does not necessarily translates into strong anomalies over the other. We show that large part of this disagreement is due to extratropical anomalies linked to the intraseasonal cycle and to orographic impacts on the eastward propagation of intraseasonal anomalies. Although the amplitude of convective anomalies over South America is smaller, lead-lag correlations indicate that the feedback is significant and of similar magnitude and in both direction. Strong intraseasonal events over the Indian Ocean are preceded by westward moving low pressure anomalies from Africa to South America, resulting in weak convective events over eastern Brazil and the generation of a new eastward propagating wave. On the other hand, strong intraseasonal convection over eastern Brazil is a consequence of dry phases over the Indian Ocean. These strong South American events lead to weak Indian Ocean events around two weeks later. We describe how this relationship changes in association with ENSO events. The location of the rising branch of the Walker circulation determines the characteristics of Rossby wave trains emanating from the Pacific to South America. It also modulates the position and strength of the South American subtropical jet and the negative zonal stretching deformation region over South American subtropics. These changes in the climatological background affects the propagation of extratropical system equator ward east of the Andes and the subsequent formation and structure of the intraseasonal anomalies over eastern Brazil. 11-17-2011-->