9. the South American Monsoon System

The main characteristics of the South American monsoon system are reviewed. According to some diagnostics, the wet season in tropical South America begins in early October over the Brazilian highlands and spreads northward. Wet season rain rates are somewhat smaller than those in other continental monsoons. The annual cycle of precipitation is most pronounced in the southern Amazon, where some of the largest seasonal rainfall occurs. Amazonian rainfall extends to the southeast in the South Atlantic convergence zone. Precipitation in this area is out of phase with that farther to the south and is driven synoptically, although intraseasonal variations are evident, including associations with the Madden-Julian oscillation. The El Niño /Southern Oscillation, the largest known forcing of interannual variability, results in decreased precipitation near the Equator and increased precipitation in east-central South America. Within an ENSO cycle, large subseasonal changes also occur as a result of surface feedback. In the Amazon Basin, long-term trends of precipitation are small. Deforestation of the Amazon is likely to reduce rainfall, but its influence on the large-scale circulation is unclear. The effect of increasing CO2 on precipitation is also unclear. Although improving, the present-day climate in coupled models is still not well-simulated and remains an impediment to improved climate forecasts.