Threefold increase in most intense South Atlantic convergence zone events by 2100 in convection-permitting simulation

Abstract Unprecedented rainfall extremes resulting from global warming are becoming more frequent each year, including over South America. In this region, tropical-extratropical cloud bands in the South Atlantic Convergence Zone (SACZ) produce most of the rainy season precipitation. In this study, we diagnose the impacts of warming on the frequency and intensification of SACZ tropical-extratropical cloud bands. The cloud bands are identified using a feature-detection algorithm applied to a suite of convection-permitting simulations produced by the UK Met Office. Intensely raining clusters embedded within these large-scale cloud bands are diagnosed in order to identify the most intense events. Although the total number of cloud-band days will see a 20-30% decrease in their frequency under high-emission global warming, the present day 1-in-5 most intense cloud-band days will happen every 3-in-5 cloud-band days in the future. Therefore, despite fewer cloud-band days occurring in a given year, when they form they will frequently be more intense than is typical in the current climate. This increase is primarily due to warming-driven intensification of rain rates within the heavily raining clusters embedded in these weather systems. These results highlight the growing risk of intense SACZ rainfall over South America under warming, increasing the likelihood of flash floods, landslides, and unprecedented catchment-scale fluvial flooding.