The 2017 Record Marine Heatwave in the Southwestern Atlantic Shelf

Marine heatwaves (MHWs), considered as one when temperatures warmer than the 90th percentile based on a 30-year historical baseline period are registered for five or more consecutive days, can have devastating ecosystem and socioeconomic consequences. MHWs were studied in the Southwestern Atlantic shelf (32-38 degrees S) using daily sea surface temperatures between 1988 and 2017. More than half of the days with MHWs occurred since 2014. The most intense event happened in austral summer 2017, reaching temperatures of 26.8 degrees C, 1.7 degrees C above previous maximum. Regionally, an unprecedented combination of persistent extremely high air temperature and low wind speed probably led to an intense loop of heat gaining and stratification in the Rio de la Plata plume water's influence. At upper levels, the atmospheric circulation had a wavenumber of 3 pattern before the event and a blocking configuration during the event. Madden-Julian Oscillation seems to have played a leading role in setting this planetary scale configuration. Plain Language Summary The area between Rio Grande (Brazil) and Mar del Plata (Argentina) up to 200km off the coast experienced the most intense warming of the last 30years of sea surface temperature satellite record between 16 February and 9 March 2017. This anomalous warming, categorized as a marine heatwave, reached 26.8 degrees C, 1.7 degrees C above previous maximum and caused in the capital city of Uruguay fish mass mortalities and a toxic algae bloom, leading to the closure of recreational beaches for some days. Regionally, an unprecedented combination of persistent extremely high air temperature and low wind speed probably led to an intense loop of heat gaining and vertical stratification of the water column. The tropical Madden-Julian Oscillation seems to have played a leading role in setting this atmospheric configuration. Our results stress the importance of deepening the study of marine heatwaves in this region, as they are becoming more frequent since 2014.