The Relationships Between El Niño Southern Oscillation and Climate Extremes in Paraná River Basin, Brazil

The basin of Paraná River is located in the southeast and center-south of Brazil and in the center-east of South America. It is the second largest hydrographic region of Brazil and it has great importance in the national context, since it concentrates more than 32% of the Brazilian population, and due to the high rate of industrialization, presents the highest energy demand of the country. It is worth highlighting that the generation of energy, which occurs mostly from the water flow, has a close relationship with weather conditions. Hence, it presents a worrying vulnerability to climate variability and climate change. Thus, a better understanding of rainfall behavior is vital, mainly, for those regions where the hydroelectric power is essential to satisfy the energy demands. In this sense, the objective of this study is to evaluate the possible influence of the El Niño Southern Oscillation (ENSO) phenomenon in wet and dry events in Paraná River basin. The study was performed based on daily rainfall data of 986 stations of the hydrometeorological network managed by the National Water Agency (Agência Nacional de Águas) and Department of Water and Electrical Energy (Departamento de Águas e Energia Elétrica), from 1975 to 2014. In order to characterize the ENSO, the climate indices named Niño (Niño 1 + 2, Niño 3, Niño 3.4 and Niño 4) obtained from the Climate Prediction Center (CPC) of the United States National Oceanic and Atmospheric Administration (NOAA) (http:/www.cpc.ncep.noaa.gov/data/indices) were used. The dry and rainy events were characterized by the Standard Precipitation Index - SPI, which is commonly used for monitoring conditions associated with drought and excess of rain. The SPI can be calculated for different time scales, according to the period which the precipitation values were accumulated. In this study, we used SPI-3, corresponding to the cumulative rainfall periods of 3 months, and correlated it with the quarterly anomalies of the Niño indices (Niño 1 + 2, Niño 3, Niño 3.4 and Niño 4). The significance level (of 5%) of the correlation coefficients was defined using the Student's t-test. The results obtained suggest that the ENSO events influence the dry and wet conditions in the Paraná River basin, mainly in the autumn and austral winter, periods with greater occurrence of dry events in all the regions of the basin. The Niño 1 + 2 and Niño 3 indices had the highest correlation coefficients and the Niño 4 had the lowest values. The correlations with significant coefficients at 5% were positive, showing an direct relation between the climate indices and the SPI, that is, the Sea Surface Temperature anomalies of the Pacific regions were positive (negative) when the SPI were positive (negative). As the ENSO phenomenon is characterized by anomalies, positive (El Niño) or negative (La Niña), this result suggests that o El Niño (La Niña) contributes to the excess (lack) of rainfall in the region.