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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.

Four monsoon indices (the meridional wind shear index (MWSI), the zonal wind shear index (ZWSI), the 850-hPa zonal wind index (850ZWI) and the 850-hPa zonal and meridional wind index (UVI)) based on the characteristics of the wind circulation are used to identify the onset and the demise dates and the intraseasonal variability of the rainy season over the west central Brazil (WCB) region. All the four-index time series have a high correlation with the precipitation series over WCB. The UVI, MWSI and the 850ZWI represent very well the intraseasonal variability (break and active periods) of the precipitation over WCB and the 850ZWI is also useful for identifying the onset dates.

In an attempt to validate the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) over Brazil, TRMM PR estimates are compared with rain gauge station data from Agência Nacional de Energia Elétrica (ANEEL). The analysis is conducted on a seasonal basis and considers five geographic regions with different precipitation regimes. The results showed that TRMM PR seasonal rainfall is well correlated with ANEEL rainfall (correlation coefficients are significant at the 99% confidence level) over most of Brazil. The random and systematic errors of TRMM PR are sensitive to seasonal and regional differences. During December to February and March to May, TRMM PR rainfall is reliable over Brazil. In June to August (September to November) TRMM PR estimates are only reliable in the Amazonian and southern (Amazonian and southeastern) regions. In the other regions the relative RMS errors are larger than 50%, indicating that the random errors are high.

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[1] The NCEP/NCAR reanalysis data during the period 1958 -1998 show a positive trend in the 100 hPa zonal wind, decrease in the strength of Tropical Easterly Jet (TEJ) during the summer monsoon period of June through September. This is confirmed by the Radiosonde data. The number of observed Tropical Cyclonic Systems (TCS) over Bay of Bengal during the same period shows a decreasing trend. There is a strong negative correlation (À0.549 for the period 1958 -1998, 41 years, significant at 99.9% level by a two sided student t test) between the strength of TEJ and the number of Bay of Bengal TCS. This result has potential for long-range prediction of TCS, which is vital for the prediction of monsoon rainfall.