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

Abstract

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 cross-equatorial flow, based on area-averaged (5°S–5°N, 65°–75°W) 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. The singular value decomposition (SVD) analysis suggests that the V-index-related variation represents 92% of the total covariance between the low-level meridional wind and precipitation over South America. It also represents 37% of the seasonal variance of precipitation. On the seasonal timescale, the V index appears to correlate well with the meridional migration of the Hadley cell globally. On submonthly timescales, the change of V index is not correlated with the meridional wind over the adjacent oceans except in the South Atlantic convergence zone, suggesting a control by more localized and higher-frequency dynamic processes. The SVD analysis also suggests that during spring and fall precipitation changes over the equatorial eastern Amazon are associated with the seasonal variations of sea surface temperature in the Pacific and the Atlantic Oceans.