Decadal Variability in the South Atlantic Convergence Zone and Changes in Precipitation over Southeastern Brazil

Abstract:

The present study examines the role of the South Atlantic Convergence Zone (SACZ) in the observed extreme precipitation changes along the southeast coast of Brazil (-18°S to -25°S). This area is under the influence of the South American Monsoon System (SAMS) and its main feature, the SACZ, which together are related to extreme precipitation events during the wet season (Oct-Mar). Previous studies suggest that one possible change in convergence zones behavior due to climate change is an increase in precipitation intensity at their central portion at the expense of moisture advected from their margin. By analyzing extremes in daily precipitation during the wet season based on more than 70 years of rain gauge data, we identified a positive trend in the intensity of precipitation and a reduction in the number of rainy days over the north portion of southeast coast. Further south, the tendency was for an overall increase in total precipitation. This spatial pattern in extreme precipitation trends along the coast is likely linked to changes in the characteristics of the SACZ. In this study we investigate decadal variations in the SACZ intensity, location and persistence using a multivariate daily index obtained with combined EOF analysis. The variables considered are zonal and meridional winds, temperature and specific humidity at low levels, derived from two different reanalysis datasets: NCEP/NCAR Reanalysis 1 and CFSR. The extent and spatial variability of the SACZ is investigated with daily Outgoing Longwave Radiation (OLR). The focus of this analysis is on intense SACZ events according to the SACZ index. Mechanisms associated with the decadal variability of the SACZ are also explored and include changes in sea surface temperature, the low–level Jet east of the Andes, and changes in circulation and moisture transport over South America and the South Atlantic Ocean.