Synoptic scale features of the tropospheric circulation over tropical South America during the WETAMC TRMM/LBA experiment

Abstract

This paper analyzes the synoptic scale features of the upper and lower level tropospheric circulation over tropical South America during the first Atmospheric Mesoscale Campaign in the Wet Season (WETAMC) of the Large Scale Biosphere – Atmosphere Experiment in Amazonia (LBA). This campaign occurred in January and February, 1999 and concomitantly with the validation of the Tropical Rainfall Measuring Mission (TRMM) satellite over Brazil. National Centers for Environmental Predictions (NCEP) grid point data were used to analyze the winds and its associated divergence fields and define the prevailing patterns of the tropospheric circulation in that region. TRMM images were used to depict the rain patterns. The results show at least four distinct patterns in the upper air winds fields during the WETAMC-TRMM/LBA, involving mainly the Bolivian anticyclonic circulation and cyclonic vortices in the vicinity of Northeast Brazil. The results also suggest that, in general, the convective activity over vast areas of South America is dynamically supported by upper level divergence. Two out of a total of six vortices were observed to have anomalous displacements into the Amazonia during the study period. At lower levels, the South Atlantic Convergence Zone (SACZ), except for the first half of January, was not active but equatorward incursions of midlatitude frontal systems disturbed the convective activy in the Southwestern Amazon basin. Besides, during the period of the study, the low-level moisture divergence as derived from NCEP data does not show convergence where the TRMM rain field clearly reveals the existence of precipitation areas.