Eventos ENOS com consequências anômalas na Bacia Hidrográfica Mirim São Gonçalo, RS, Brasil

Abstract

Rio Grande do Sul State (RS) is located in a region with irregular topography, conducive to the action of several meteorological phenomena that represent the state's weather and climate. Climatic oscillations influence precipitation on the continent, such as the El Niño Southern Oscillation (ENSO) phenomenon, which influences extreme events, associated with social and economic impacts. Several studies show that in southern Brazil, the El Niño phenomenon is associated with precipitation above the climatological normal, while La Niña is associated with precipitation below the climatological normal. The research is inserted in the Brazilian part of the Mirim-São Gonçalo Hydrographic Basin, one of the main transboundary hydrographic basins in South America, which has an important environmental and economic value for the State of Rio Grande do Sul and for Uruguay. Therefore, the objective is to analyze cases (exceptions) in which El Niño did not bring more precipitation to the basin, as well as cases in which La Niña did not bring drought. The analysis of anomalous cases (consequences on the precipitation regime) in the Mirim-São Gonçalo Hydrographic Basin was carried out by observing whether other climatic oscillations interfered in the anomalous cases. The methodology consists of using observed data from the 1981-2020 historical series of precipitation from meteorological stations, available from the National Institute of Meteorology (INMET) and the National Agency for Water and Basic Sanitation (ANA), as well as the ONI index (NOAA, 2022) and by the SAM and IASAS indices (SOUZA and REBOITA, 2021). Among all the rainfall stations in the basin, 118 anomalous precipitation cases were identified, 65 corresponding to cases of precipitation deficit under the influence of the El Niño and 53 to excess during the La Niña phase. It was observed that Arroio Grande and Ponte Cordeiro de Farias were the pluviometric stations that presented precipitation anomalies in both phases. In general, the season of the year that most presented precipitation anomaly was summer with 41 cases, and the one that presented the least was spring, with 12 cases. During the period under study, there were 47 ENSO events and, from these, in 31 were detected of at least one case of anomalous precipitation in one of the 15 stations of the basin. It was found that the intensity of ENSO in most cases is weak. The SAM and IASAS indices could influence in 20 cases, however 11 ENOS events with anomalous consequences neither the SAM nor the IASAS can explain.