Characteristics of rainfall in the Brazilian Amazon Basin

Using records of daily rainfall from a large number of stations located within the Brazilian Amazon Basin, spatial and temporal variabily on many scales is examined. The most fundamental of these is the annual cycle. The northwest (Brazilian) Amazon has a relatively short period when rainfall is above its annual average, and rainfall is relatively equally distributed throughout the year. By contrast, both the northern and southeastern Amazon have distinct wet seasons, with little precipitation for several months of the year. The regions with a distinct annual variation of rainfall receive more rainfall during the rainiest three months than do areas where rainfall is more equally distributed throughout the year, even though total rainfall is less in the former areas. Simultaneous correlations between seasonal averages of rainfall and the Southern Oscillation Index and or surface temperatures (SSTs) in the eastern Pacific Ocean reveal a strong relationship with El Nino/Southern Oscillation in only parts of the basin. Regressions show that in the region of strong correlations, a 1 standard deviation warming in the vicinity of Nino 3-4 is associated with about a 35% reduction in seasonal rainfall totals. Atlantic SST anomalies are not associated with variations of rainfall in the Amazon Basin. In some parts of the basin, however, variations in onset and end of the rainy season are related to SST anomalies in both the Pacific and Atlantic oceans. The sign of this relationship is such that warm SST in the Atlantic south of the equator and cold SST in the eastern Pacific is associated with an early onset in the Amazon that lies south of the equator. Finally, distributions of daily rainfall amounts are examined for dry and wet rainy seasons. These results show that there is little difference in the distribution of rainfall events during dry and wet years (e.g., no change in the frequency of 'extreme' events). Rather, the difference is accounted for by a change in the number of days without rainfall.