The frequency of surface cyclogenesis over South America (approximately the area enclosed by 15°–50°S and 30°–90°W) has been calculated using, 10 years (1979–1988) of data. The frequency of cyclogenesis is more in winter than in any other season. Highest frequency (139) is found in the month of May and the lowest frequency (71) is found in the month of December. In addition to seasonal, the frequency of cyclogenesis shows interannual variation. The occurrence of cyclogenesis is more during the years of negative Southern Oscillation index (El Niño years) and less during the years of positive Southern Oscillation index. The years of higher (lower) cyclogenesis are found to be associated with higher (lower) rainfall. This explains the negative correlation between the precipitation over southern Brazil and the Southern Oscillation index. Abstract The frequency of surface cyclogenesis over South America (approximately the area enclosed by 15°–50°S and 30°–90°W) has been calculated using, 10 years (1979–1988) of data. The frequency of cyclogenesis is more in winter than in any other season. Highest frequency (139) is found in the month of May and the lowest frequency (71) is found in the month of December. In addition to seasonal, the frequency of cyclogenesis shows interannual variation. The occurrence of cyclogenesis is more during the years of negative Southern Oscillation index (El Niño years) and less during the years of positive Southern Oscillation index. The years of higher (lower) cyclogenesis are found to be associated with higher (lower) rainfall. This explains the negative correlation between the precipitation over southern Brazil and the Southern Oscillation index.

We studied rainfall characteristics of Brazil for the period 1979–2011. This is an update for an earlier study with data from 1958 to 1978. We compared the three consecutive wet and dry months in the two data sets. In the northern most part rainy season earlier was in April to July and now it is occurring later. In a large part of the south central region the rainy season in austral summer and dry season in austral winter did not change. However, to the east of this region rainy season in the latter data occurs earlier. Calculation of linear tendencies showed, that over the northern Amazon region there is a significant increase of rainfall. This agreed with previous results. Over this region dry season is becoming drier and wet season wetter. In the west of Northeast Brazil (NEB) there is an increase of rainfall. In southeast Brazil there is a region of highly significant decrease of rainfall in both wet and dry seasons. This is confirmed in our analysis on river discharges in Sao Paulo and Minas Gerais states. The negative tendencies in rainfall have great impact on the hydroelectric generation not only in this region but also at the national level because the hydroelectric dams furnish energy on a national level. The increase in rainfall in Northern Amazon is probably associated with Atlantic warming that resulted in higher moisture transport from east. The decrease of rainfall in southeast Brazil seems to be associated with the plummet of rain producing systems from south due to increase of mean sea level pressure in the south.

Using the data for the period 1914–83, rainy and dry season of eastern Northeast (ENF) Brazil are determined. The principal rainy season is April through July (60% of the annual rainfall), and the principal dry season is September through December (10% of the annual rainfall). The fall and winter rains over ENE Brazil are associated with southeasterly winds blowing perpendicular to the coast. The subtropical high in the South Atlantic modulates the strength and direction of winds along the coast that have a determining influence on the interannual variations of rainfall over ENE Brazil. The possible link between the rainfall over ENE Brazil and SST anomalies in the Atlantic is examined. The SST anomalies of the season January, February, March, April (JFMA) in the southeast Atlantic are positively correlated with the rainfall anomalies of ENE Brazil. The sea surface temperature (SST) anomaly pattern frequently becomes established by January/February over southeast Atlantic suggesting the predictiv... Abstract Using the data for the period 1914–83, rainy and dry season of eastern Northeast (ENF) Brazil are determined. The principal rainy season is April through July (60% of the annual rainfall), and the principal dry season is September through December (10% of the annual rainfall). The fall and winter rains over ENE Brazil are associated with southeasterly winds blowing perpendicular to the coast. The subtropical high in the South Atlantic modulates the strength and direction of winds along the coast that have a determining influence on the interannual variations of rainfall over ENE Brazil. The possible link between the rainfall over ENE Brazil and SST anomalies in the Atlantic is examined. The SST anomalies of the season January, February, March, April (JFMA) in the southeast Atlantic are positively correlated with the rainfall anomalies of ENE Brazil. The sea surface temperature (SST) anomaly pattern frequently becomes established by January/February over southeast Atlantic suggesting the predictiv...