Unravelling the drivers of the April–May 2024 extreme precipitation event in Rio Grande do Sul

Abstract

This study examines the extreme precipitation event that caused unprecedented flooding in Rio Grande do Sul (RS), Brazil, during April–May 2024, leading to record-breaking floods in Porto Alegre. Using data from the European Centre for Medium-Range Weather Forecasts ERA5, the high-resolution Multi-Source Weighted-Ensemble Precipitation dataset, a Lagrangian moisture tracking approach, and an analogue-based analysis, we identified the atmospheric drivers and moisture sources fueling this event. Results show that a persistent, quasi-stationary dipole configuration, consisting of a high over the South Atlantic and a deep low over southern South America, drove this extreme event. This configuration, reinforced by heat wave conditions over central and southeastern Brazil, favoured a sustained, atmospheric river-like moisture transport from the Amazon region, channelled by a strengthened South American Low-Level Jet east of the Andes, configuring a compound event. We also found that the highest moisture contribution occurred predominantly 1–3 days before the precipitation over RS. The analogue-based analysis revealed that while the mean sea level pressure (MSLP) pattern was uncommon but not unique within the 1991–2020 reference period, the RS extreme event exhibited significantly enhanced moisture transport and uptake from the Amazon region compared to similar past occurrences. Additionally, we found lower predictability and persistence of the MSLP pattern during this event than that of its analogues. Overall, this research underscores the critical role of specific large-scale atmospheric patterns and sustained anomalous moisture supply in driving extreme precipitation, reinforcing the need for an improved understanding of climate-weather interactions and the development of mitigation strategies to address intensifying extreme precipitation events in a changing climate.