Cross-Equatorial Flow and Seasonal Cycle of Precipitation over South America

The relationship between South American precipitation and cross-equatorial flow over the western Amazon is examined using the 15-yr (1979-93) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. A meridional wind index, the V index, is constructed to represent the variability of the crossequatorial flow, based on area-averaged (5S-5N, 65-75W) daily 925-hPa meridional winds. The V index displays large submonthly, seasonal, and interannual variabilities, and correlates well with precipitation over South America. Two circulation regimes are identified, that is, a southerly regime and a northerly regime. Linear regression shows that when the V index is southerly, precipitation is mainly located to the north of the equator. When the V index is northerly, precipitation shifts toward the Amazon basin and subtropical South America. The V index is predominately southerly in austral winter and northerly in summer. The onset (demise) of the Amazon rainy season is led by an increase in the frequency of the northerly (southerly) V index. The relation between the V index and upper-level circulation is consistent with the seasonal cycle of the South American monsoon circulation. Hence, the V index is a good indicator for precipitation change over tropical and subtropical South America.