Assessing the impacts of South Atlantic Convergence Zone (SACZ) and atmospheric blockings on rainfall variability in Brazilian biomes using Standard Precipitation Index (SPI)

Climate change is disrupting atmospheric patterns, which, in turn, alters precipitation regimes worldwide. Droughts are becoming more frequent, intense, prolonged, and spatially distributed, posing a threat to water security for millions of people. Drought monitoring is particularly critical in Brazil, a country that encompasses diverse climate regimes and biomes, and where rainfall variability greatly impacts social vulnerabilities, biodiversity, and the economy. To better understand disruptions in rainfall patterns leading to drier conditions in Brazil, we evaluated the correlation between the occurrence of atmospheric blockings and episodes of the South Atlantic Convergence Zone (SACZ) with rainfall variability, particularly for droughts, in various biomes. The Standardized Precipitation Index (SPI) was used to characterize precipitation variability, presenting simple yet robust statistical insights into the distribution, duration and frequency of rainfalls surpluses (positive values) and droughts (negative values). The SPI values for 1, 6 and 12 months were calculated using observed rainfall data from the Brazilian Daily Weather Gridded Data (BR-DWGD) database, from 1961 to 2024. SACZ episodes and atmospheric blocking events were identified using indices developed by LAMMOC/UFF research group, which effectively describe the behaviour of these systems across various regions of the country. The atmospheric blocking index was calculated using ERA5 reanalysis data, while NCEP reanalysis data was the input to the SACZ index. All data were normalized prior to statistical analyses, which included Pearson’s correlation coefficient, Principal Component Analysis (PCA), K-means clustering, Mann-Kendall test, and trend analysis to identify and quantify trends. The results demonstrate that atmospheric blocking events are increasing in all regions of Brazil. Conversely, the SACZ occurrences did not demonstrate a significant trend. The correlation between atmospheric blockings and SPI values exhibit a strong pattern in all evaluated time scales and regions, demonstrating significant positive influence in the Pampa biome within all evaluated time scales, suggesting that blockings, regardless of their position, incur in rainfall surpluses in South Brazil. In the other biomes, blockings show a consistent negative influence, particularly in Cerrado, Pantanal and Amazonia (Central and Northern regions). Cerrado shows correlations of up to -0.5, the highest values observed in the analysis - suggesting atmospheric blockings have an inhibiting effect in precipitation, creating drier conditions that are concerning for wildfire hazard in central Brazil, and also in Southern Amazonia. SACZ and SPI correlation is not as clear, with small to no trend in most biomes, except for the slight negative influence on the Pampa, region where precipitation decreases as active SACZs concentrate rainfall northward. Understanding the correlation between these important atmospheric systems and the precipitation variability observed in Brazil is valuable to drought monitoring and prediction, and may help to identify early warning signals for major droughts, providing insights that can guide mitigation and adaptation strategies to address the impacts of climate change, which affects differently the regions of the country due to the complexity of its diverse climate regimes and biomes, and therefore, water availability and wildfire hazard.