Rainfall Regime and Interannual Variability of South American Summer Monsoon

By analyzing the reanalysis data from the Goddard Earth Observing System (GEOS) and the National Center for Environmental Prediction (NCEP) and the rainfall data from the CPC (Climate Prediction Center) Merged Analysis of Precipitation (CMAP) the spatial regime of the South American summer monsoon (SASM) rainfall is investigated. From austral spring to summer, the precipitation band over the South American continent migrates from the northwest toward the southeast following the seasonal progression of the solar heating. Accompanying the change of the large-scale circulation, the precipitation band jumps more than 10-degree from tropics to subtropics in November, which signifies the onset of summer monsoon. Throughout the summer season (DJF), the monsoon rainfall regime is spatially confined between 15°S and 35°S, where the peak regional rainfall occurs.

The interannual variability of the monsoon rainfall is studied. A composite analysis reveals that during El Niño years an enhanced monsoon circulation with poleward shift of the monsoon rainfall regime and increased monsoon rainfall amount occurs. The situation is reversed in La Niña years. The temporal evolution of SASM circulation and rainfall exhibit a strong biennial tendency between 1997/98 and 1998-99.