Abstract
Abstract. The ENSO phenomenon is one of the key factors that influence the interannual variability of precipitation over Southern South America. The aim of this study is to identify the regional response of precipitation to El Niño/La Niña events, with emphasis in drought conditions. The standardized precipitation index (SPI) was used to characterize precipitation variabilities through the 1961–2008 period for time scales of 3 (SPI3) and 12 (SPI12) months. A regionalization based on rotated principal component analysis allowed to identify seven coherent regions for each of the time scales considered. In order to identify the regional influence of El Niño and La Niña events on the SPI time series, we calculated the mean SPI values for the El Niño and La Niña years and assessed its significance through bootstrap analysis. We found coherent and significant SPI responses to ENSO phases in most of the seven regions considered, mainly for the SPI12 time series. The precipitation response to La Niña events is characterized with regional deficits, identified with negative values of the SPI during the end of La Niña year and the year after. During El Niño events the precipitation response is reversed and more intense/consistent than in the case of La Niña events. This signal has some regional differences regarding its magnitude and timing, and the quantification of these features, together with the assessment of the SST composites during drought conditions provided critical baseline information for the agricultural and water resources sectors.
Highlights
The El Niño-Southern Oscillation (ENSO) phenomenon is the dominant mode of coupled atmosphere-ocean variability on interannual time scales in several regions of the world (Trenberth and Stepaniak, 2001)
The regionalization of the 56 SPI3 time series resulted in a total of seven homogeneous regions: CES (CentralEast South); P + PN (Pampas and Northern Patagonia); CW (Central-West); CEN (Central-East North); NE (Northeast); SP (Southern Patagonia) and NW (North-West) (Fig. 3)
The ENSO phenomenon has a strong impact on precipitation in Southern South America (SSA), both at seasonal to interannual time scales, associated to its two phases: El Niño and La Niña
Summary
The El Niño-Southern Oscillation (ENSO) phenomenon is the dominant mode of coupled atmosphere-ocean variability on interannual time scales in several regions of the world (Trenberth and Stepaniak, 2001). A pioneer study performed by Ropelewski and Halpert (1987) on global scale, based on their previous research for North America (Ropelewski and Halpert, 1986), identified a clear ENSO signal in precipitation patterns over SSA This signal was characterized with an increase in precipitation over central-east Argentina, Uruguay and Southern Brazil during the summer following the development of El Niño conditions. With focus in South America and surrounding oceans, Aceituno (1988) showed that the SO-related changes in the large-scale circulation lead to a vast diversity of anomalous regional precipitation regimes This result was further verified by Grimm et al (2000), who conducted an analysis of precipitation variability associated with El Niño and La Niña phases through a regional approach. This study showed a high degree of regional variabil-
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