The South America Monsoon Circulation and Its Relationship to Rainfall over West-Central Brazil

Abstract

The mean atmospheric circulation for the rainy season over west-central Brazil (20°–10°S, 60°–50°W) is investigated for a 21-yr period (July 1979–June 2000). The NCEP–NCAR reanalysis, pentad outgoing longwave radiation (OLR), and daily gridded precipitation analyses from NCEP are used in this study. The analysis presented here provides an improved description of the mean annual cycle based on daily and pentad (5-day) rainfall and atmospheric circulation data. The pentad precipitation averaged for the study region shows a rapid increase during the spring months (September–November) and a rapid decrease in April. The rainiest period (more than 8 mm day−1) is from December to February. There are distinct wet (7 months) and dry (5 months) seasons, with approximately 90% of the total annual rainfall occurring during October–April. The rainy season begins following the period when temperature is at its seasonal maximum. The pre-rainy-season warmth acts to destabilize the atmosphere and to create a reversal in the low-latitude temperature gradient. This results in a reversal of the vertical shear of the zonal wind from increasing (decreasing) westerly (easterly) winds with height to decreasing (increasing) westerly (easterly) winds with height. The examination of the vertical structure of the average zonal wind over the area reveals that prior to the onset of the rainy season easterlies prevail in the lower troposphere (up to 600 hPa) and westerly winds prevail at upper levels with maximum speed in the layer 200–150 hPa. During the rainy season the vertical structure of zonal wind is opposite to that of winter (June–August). Within each rainy season, west-central Brazil experiences anomalously wet and dry periods. Wet periods are associated with low-level northwesterly and westerly flow east of the Andes that converges on west-central Brazil. Dry periods are associated with enhanced northwesterly flow over Paraguay and northern Argentina, and easterly anomalies over west-central Brazil.