Interdecadal variability and extreme precipitation events in South America during the monsoon season

This study aims to clarify the impact of interdecadal oscillations on the frequency of extreme precipitation events over South America, in the monsoon season (austral spring and summer), and determine the influence of these oscillations on the daily precipitation frequency distributions. Interdecadal variability modes of precipitation during the monsoon season are provided by a continental-scale rotated empirical orthogonal function analysis using a longer data set (1950-2009) than used in a previous study (Grimm & Saboia 2015, J Clim 28: 755-775). The disclosed modes are consistent with those previously obtained, confirming their robustness. Oceanic and atmospheric anomalous fields associated with these modes give indications about their physical basis and mechanisms of their impact. The significant anomalies of the extreme event frequency in opposite phases of the interdecadal oscillations display spatial patterns very similar to those of the corresponding modes. In addition, the modes of extreme event frequency bear similarity to the modes of seasonal precipitation, although a complete assessment of this similarity is not possible with the daily data available. The Kolmogorov-Smirnov test is applied to the daily precipitation series in positive and negative phases of the interdecadal modes, in regions with high factor loadings. It shows, with significance level better than 0.01, that daily precipitation from opposite phases pertains to different frequency distributions. Further analyses disclose clearly that there is much greater relative impact of the interdecadal oscillations on the extreme ranges of daily rainfall than in the ranges of moderate and light rainfall. This impact is more linear is spring than in summer.