MJO Impact on South America Monsoon Precipitation: Observations and S2S models

One of the main features of the South American summer monsoon is the South Atlantic Convergence Zone (SACZ), a northwest-southeast band of enhanced convection extending from central South America over Southeast Brazil and into the subtropical Atlantic Ocean. The SACZ affects very densely populated areas in Southeast Brazil, including Rio de Janeiro and Sao Paulo. During the austral summer this region is strongly affected by landslides and floods associated with active SACZ. Extreme precipitation events in the SACZ region receive contribution from synoptic and intraseasonal variability. To explore extended-range predictability of these events, the Madden Julian Oscillation (MJO) impacts on the frequency of extreme events in South America are determined with rain gauge data for the period 1979-2009. The MJO cycle is divided into 8 phases according to the temporal evolution of the first two modes of multivariate EOF analysis of tropical convection and zonal winds. The teleconnections associated with these impacts are analyzed with simulations and influence functions of a simple model. The results show that two of the MJO phases strongly enhance the extreme events in the SACZ region and indicate the responsible mechanisms, lending these events a higher degree of predictability on subseasonal time-scales. Therefore, it should be possible to extend the forecast range of extreme events in this region, which is crucial to prevent/mitigate socio-economic impacts. In selecting models to build a subseasonal-range forecasting scheme for extreme precipitation events, a necessary step is the assessment of their skill in reproducing the observed impacts of the MJO on South America. Two well-known models of the S2S Project, CFS-v2 and ECMWF, are analyzed. Their reforecasts for weeks 1, 2, 3, 4 are separately projected onto the first two modes of tropical convection and zonal wind variability in order to identify the MJO phases. The skill of the models in predicting these phases is good and extends to week 4, with some differences between the models. However, the skill is lower in reproducing the MJO influence on the SACZ, due to differences between the observed and the models´ convection anomalies in the central Pacific Ocean and associated teleconnections to South America.