Synoptic analysis of an intense rainfall event in Paraíba do Sul river basin in southeast Brazil

Abstract

The aim of this study was to analyse the characteristics of an intense rainfall event that occurred in the Paraíba do Sul river basin, southeast Brazil, in early 2000. Rainfall data from conventional meteorological stations and automatic weather stations, satellite images and datasets from the Climate Forecast System Reanalysis, National Centers for Environmental Prediction, were employed. After the arrival of a cold front in the state of São Paulo on 1 January, the first episode of the South Atlantic convergence zone (SACZ) of the year was established, lasting for 8 days. A dipole blocking pattern over the state of Rio Grande do Sul and Uruguay, with a cold core cyclone and a warm core anticyclone in the upper troposphere, weakened the westerly flow from the Pacific, favouring slow displacement of the SACZ. Intense moisture fluxes from the southern Amazon and tropical South Atlantic converged over southeast Brazil providing copious amounts of moisture for convective activity during the first 4 days of January 2000. The most notable aspect of this event was the development of a cyclonic vortex embedded in the SACZ (CVES) in the Atlantic Ocean, with a warm core in the lower troposphere and a cold core at higher levels, which intensified rain in the study region. After the weakening of the blocking on 4 January, the CVES moved southeastward in the Atlantic. The symmetry of the CVES and conditions of the thermal gradient in the lower and higher troposphere suggest that the system presented subtropical characteristics during the first few days and became an extratropical cyclone later.