Rainfall patterns in the Southern Amazon: a chronological perspective (1971-2010)

The aim of this study is to characterize rainfall patterns in a vast transition zone between the Amazon and the Cerrado Biomes. The analysis is focused on annual and seasonal tendencies, mainly about the onset and offset of the rainy season, its length and shifts. More than 200 Rain Gauges (RGs) were analyzed in the study area using Pettitt's and Mann-Kendall's non-parametric tests allied to a Linear Regression Analysis over the period 1971-2010. The onset and offset dates of the rainy season and its duration are also identified for 89 RGs. Pettitt's test indicates ruptures in 16 % of the rainfall time series while Mann-Kendall's monthly test indicates that 45 % of the RGs had negative trends, mainly in the transition seasons (spring and austral autumn). Linear Regression Analysis indicates negative trends in 63 % of the time series concomitant to the rainy season onset and offset analysis, which confirmes a delay for the onset of the rainy season in 76 % of the RGs and a premature demise for 84 % of the RGs. Identification of the tendencies for rainy season duration indicates that the rainy season has become shorter at 88 % of the RGs. There were recurring patterns in the results displaying drier conditions in RGs localized in deforested areas opposed to forested locations.