Climatology of upper level cyclonic vortices over northeastern South America

The Palmer type upper level (25 hPa) cold core vortices are frequently observed over the Northeast region of Brasil and the adjoining Atlantic in the spring and summer seasons (Oct - March) with maximum frequency in January and February. They form and persist in the region 20-45W, 0-20S. Intensification of the trough, east of the anticyclone centered over Bolivia, gives rise to a closed vortex. The intensity and position of the Bolivian high has a strong influence on the position of the vortex. . The positive and negative anomalies in the rainfall associated with the location of the vortices affect the economy of the Northeast region of Brazil and therefore their study is important.

In this study the frequencies of these cold upper level vortices are obtained in the period 1987-99 in the three longitudinal bands, weat of 40W, from 40W to 30 W and east of 30 W. They are expressed in terms of number of days of the presence of the vortex center with vorticity values equal or less than 6x10-5 s-1. The effect of the vortices over the rainfall characteristics at different stations of the Northeast Brazil are obtained by comparing the monthly rainfall series at coastal and interior stations with the vortex frequencies. The differences between the El Niño and La Niña years are studied are significant especially during the prerainy season of the Northeat Brazil.