Avaliando os padrões atmosféricos e oceânicos durante eventos de seca no Brasil: o estudo de caso das bacias do São Francisco e Paraná

Abstract

In recent years, extreme weather events have occurred with greater frequency, intensity and duration, and are projected to increase in future climate. In Brazil, for the period from 1991 to 2012, the natural disaster that affected the largest number of people are droughts. The São Francisco Hydrographic Basin (BHSF) and the Paraná Hydrographic Region (RHP) are among the main hydrographic basins in Brazil, standing out for the generation of energy and consumption, among other ecosystem services. Therefore, as droughts can trigger both economic and social problems, it is important to identify hydrological drought events in these basins and what anomalous weather patterns are associated with these events. Daily precipitation data from the Gauge-Based Analysis of Global Daily Precipitation (CPC) was used to calculate the Standardized Precipitation Index (SPI) at 1, 6 and 12 month scales in order to identify drought events in BHSF and RHP; this index uses historical rainfall series to quantify rainfall deficits, in which negative values are associated with dry episodes. The 1- month scale reflects short-term conditions and can be associated with meteorological droughts, the 6-month scale SPI can be used to show the dynamics of rainfall over different seasons and, in some cases, droughts on this scale can be associated with anomalies in river flows and reservoir levels, as seen in the 12-month scale, in which long-term rainfall patterns have been represented. The 12-month scale was used to identify hydrological drought episodes along the BHSF and RHP between 1979 and 2021. For these episodes, severity, duration, intensity and peak were obtained, and the SPI-1 and SPI-6 were obtained. analyzed during the longest and most severe drought (identified via SPI-12) to identify periods with anomalously dry conditions in the wet season. Anomalous atmospheric and oceanic patterns associated with this episode were also analyzed using data from the European Center for Medium-Range Weather Forecasts (ECMWF) ERA5 Reanalysis. The results revealed that the longest and most severe episode of hydrological drought on the BHSF occurred between 2012 and 2020, while in the RHP of Paraná the event has developed since 2016, still without end until the end of the analyzed period. At BHSF, the anomalous dry conditions observed during the rainy season of this episode were associated with an anomalous high-pressure system acting close to the coast of Southeast Brazil, hindering the formation of precipitating systems. Different large-scale patterns acted during the drought conditions in the RHP in the analyzed periods (2016/2017, 2019/2020 and 2020/2021), but with a common weakening of the low level jets, making it impossible to transport moisture to the basin. Despite the drought being studied in a regionalized way in the country, the results found in the present study indicate recent drought events affecting several regions of Brazil simultaneously.