Statistical analysis of upper tropospheric vortices in the vicinity of northeast Brazil during the 1980-1989 period

Abstract

Daily European Centre for Medium Range Weather Forecasts (ECMWF) analyses for the 1980-1989 period are used to study cyclonic vortices at the upper tropospheric levels (CVULs) in the vicinity of northeast Brazil. The occurrence of CVULs shows significant interannual and seasonal variability, with the highest frequency found during the Southern Hemisphere summer months. CVULs develop mostly over oceanic areas, with the closed circulation at 200-hPa and the cold core at 300-hPa, and remain confined to the middle and upper troposphere. The CVUL mean lifetime ranges from 4 days in June to 11 days in February. Approximately 57% of the Southern Hemisphere summer CVULs originate accordingly to the formation mechanism so far known, hi 27% of the Southern Hemisphere summer cases, the vortex originates associated with the occurrence of an upper level anticyclonic circulation over southwestern Atlantic and southeastern Brazil, in turn associated with stationary cold fronts (South Atlantic Convergence Zone-SACZ). CVULs can present regular or irregular trajectories. Midlatitude trough associated with frontal systems and an inter-hemispheric bifurcation of the upper level westerlies in the northwest of South America can cause such irregular displacements. For the CVULs with regular displacement, it is common to observe an amplification of the ridge axis in the southeastern sector of the Bolivian High, which can cause the upper level anticyclone in the southwestern Atlantic and southeastern Brazil.