The mean atmospheric circulation over South America for the rainy season over west-central Brazil (60W-50W; 20S-10S) is investigated for a 18-year period (July 1979 to June 1997). The NCEP/NCAR reanalysis, pentad outgoing longwave radiation (OLR) and daily gridded precipitation analyses from NCEP are used in this study. The pentad (5-days) 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 d-1) is from December to February. There are distinct wet (seven months) and dry (five months) seasons, with approximately 90% of the total annual rainfall occurring during October-April. Then the onset and demise of the rainy season over the west-central Brazil (60W-50W; 20S-10S) are identified for each season of the 1979/80 to 1996/97. We define the onset (demise) of the rainy season as the first occurrence of 850-hPa westerly (easterly) winds along 60ºW in the band 10ºS-20ºS together with rainfall rates greater (less) than 4 mm d-1 for at least 75% of the subsequent 8 pentads. Based on these criteria the onset occurs between the pentads centered on September 15 (1982) and November 14 (1986) and the demise occurs between the pentads centered on April 3 (1986) and May 3 (1985).

Composites of atmospheric fields over South America were constructed relative to onset and demise dates. The OLR composite maps for the onset period show that the deep convection starts over the northwest portion of the South America with a connection to the Pacific ITCZ. During the withdrawal phase, the deep convection overall is more active than during the onset phase, and it shows a connection to the Atlantic ITCZ.

The composite of 850-hPa wind circulation shows that before the onset wind flow is directed more perpendicular to the Andes Cordillera to the north of 10S. At the onset the flow near the Andes is directed more to the Southeast. This change is enough to increase the specific humidity in the study region. During the demise, the situation is the opposite but specific humidity deceases gradually in contrast to the rapid increases in specific humidity during the onset. This suggests that it takes some time to dry out the vegetation and evaporate the moisture accumulated during the rainy season. At the upper levels, prior to onset, the flow is primarily westerly over tropical South America south of the equator. At onset an anticyclonic circulation begins to develop near 65W and 5S. This anticyclone intensifies and propagates to the south coming to rest over Bolivia. Towards the end of the rainy season the anticyclone weakens and shifts northeastward. At the demise of the rainy season the anticyclone loses its character and no long features a closed circulation.

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 over Paraguay and northern Argentina, and easterly anomalies over west-central Brazil.