From Anomalous SST Gradients in the Indian Ocean to Floods and Tornadoes in Southern Brazil: The Multiscale Nature of a Historical Precipitation Event

Abstract From late April to early May 2024, a persistent episode of high precipitation in Brazil’s southernmost state of Rio Grande do Sul (RS) produced a regional-scale flood of historical proportions, representing the largest geohydrological disaster in the country’s history in terms of affected area. Adding to the copious amount of rain, tornadoes were reported in RS during the same period, while a downslope windstorm was observed in central RS in the week following the high precipitation event. This multitude of hazards shared a setup that resulted from a combination of mechanisms belonging to a vast range of atmospheric scales. This article provides a view from researchers who experienced a direct hit from the historical weather event in RS, describing its extension in a general timeline from 27 April to 8 May 2024. Ranging from the planetary scale to the smaller scales, the forcing mechanisms included interbasin SST gradients in the southern oceans, anomalous deep convection east of Africa, planetary Rossby wave trains across the Southern Hemisphere, persistent blocking high over the southeastern Pacific, migratory troughs crossing the southern Andes, and frequent lee cyclogenesis over northern Argentina. This cascade of oceanic and atmospheric features culminated in a persistent northerly low-level jet that continuously supplied RS state with unseasonably large amounts of moisture, acting as an exceptionally strong atmospheric river. Finally, this same feature also created favorable conditions to the formation of tornadic storms and to a downslope windstorm in central RS following the extreme rainfall event. Significance Statement A historical precipitation episode impacted Rio Grande do Sul (RS), the southernmost state of Brazil, from late April to early May 2024, resulting in the most extensive flood-related geohydrological disaster ever recorded in that country. In addition to the catastrophic flooding, the event was also accompanied by several tornadoes and succeeded by a downslope windstorm. This study presents a multiscale analysis of the precipitation episode ranging from planetary to storm scales and highlights the areal extent and a timeline of the sinister. The cascade of planetary- to continental-scale circulations produced an anomalously moist, strong, and long-lasting atmospheric river which fueled high-precipitation storms, tornadoes, and ultimately windstorms. Future research should address how atmospheric rivers can influence phenomena other than extreme rainfall.