Moisture budget of the bimodal pattern of the summer circulation over South America

Submonthly variations in warm-season (January-February) precipitation over South America, in particular over the Amazon basin, central southwest Brazil, north Argentina, and Paraguay are studied. Two distinct regimes of lower tropospheric winds (westerlies and easterlies) were observed in Rondonia during the Wet Season Atmospheric Mesoscale Campaign (WETAMC) component of the Large-Scale Atmosphere-Biosphere Experiment in Amazonia (LBA) and the Tropical Rainfall Measuring Mission (TRMM) field campaign. The westerly (easterly) winds were associated with strong (weak) convective activity over the South Atlantic Convergence Zone (SACZ). The period of this study (January and February of 1999) was divided into SACZ and no SACZ (NSACZ) regimes. The vertically integrated moisture fluxes over South America obtained from the National Aeronautics and Space Administration/Goddard Data Assimilation Office (NASA/DAO) assimilation system show that during the SACZ (NSACZ) period, strong (weak) convergence occurred over the Amazon basin with divergence (convergence) over southwestern Brazil, northern Argentina, and Paraguay. These moisture budgets also indicated that moisture transport from the tropics to the extratropics in the South American sector occurs more efficiently during the SACZ regime than during the NSACZ regime.