Changes in the South American Monsoon System and Impacts on Extreme Precipitation

The South American Monsoon System (SAMS) is the most important climatic feature in South America and affects millions of people. SAMS is characterized by pronounced seasonality in rainfall, circulation and moisture transport. The South Atlantic convergence zone (SACZ) is a component of SAMS and plays significant role for the variability of precipitation in highly populated areas of South America. Using NCEP/NCAR reanalysis (1948-2010) and several distinct precipitation datasets we show evidence that the duration and amplitude of SAMS and the amplitude of the SACZ have increased in the last 62 years. The metrics used to investigate interannual to decadal variations of SAMS and the SACZ are based on a large-scale index obtained from combined Empirical Orthogonal Functions (CEOF) of circulation, temperature and specific humidity. The extensive warming of tropical South America and adjacent oceans has played a significant role for changes in SAMS characteristics. Due to the equatorward large geographical extent of South America, the warming of recent decades has been more pronounced in the lower troposphere of tropical regions compared with the subtropics. Nevertheless, the warming is not uniform and large rates of increase in temperature are observed over eastern Brazil. These changes have affected land-ocean temperature gradients and the intensity of the trade winds. As a consequence, the easterly vertically integrated moisture transport over eastern Amazon has weakened and moisture convergence has been replaced by moisture divergence in recent decades. On the other hand, the northerly moisture transport and moisture convergence have intensified over central and southeastern Brazil, resulting in the strengthening of SAMS. The interplay of local and remote forcings are likely responsible for the intensification of the SACZ. We use several precipitation datasets with different spatial resolution and show that these changes have significantly affected patters of extreme rainfall over South America.