Increased frequency of intense South Atlantic Convergence Zone-related cloud band events by 2100 in convection-permitting simulation

Rainfall intensification due to planetary warming is increasingly impacting nearly all regions of the globe. South America is no exception with unprecedented landslides (São Sebastião, February 2023) and river catchment-scale flooding (Rio Grande do Sul, September 2023 and May 2024) being observed more frequently. Over South America, tropical-extratropical cloud bands in the South Atlantic Convergence Zone (SACZ) produce most of the rainy season precipitation. Droughts can occur in years with fewer SACZ events while intensely raining clusters within the cloud bands can trigger flash floods and landslides. Here, we diagnose the impacts of future precipitation intensification on the frequency and intensity of SACZ tropical-extratropical cloud bands using the first-of-its-kind continental-scale convection-permitting climate simulation. While cloud bands will see a future 20-30\% decrease in their frequency, intense events with a likelihood of 1-in-5 in the present day will become more frequent in the future, with 3-in-5 likelihood, increasing the risk of heavily raining clusters. This tripling in intense cloud band frequency results from intensified mesoscale rainfall structures within the continental-scale cloud bands, a risk better captured by convection-permitting models. Geographically, the intensification of mesoscale rainfall structures is most prevalent in the highly populated coastal regions of Southeastern and Southern Brazil, areas already highly exposed to extreme weather events, floods, and landslides. This increased risk significantly exceeds the projections from traditional climate models with convection parametrizations and highlights the growing risk of intense cloud-band rainfall over South America under warming.