Physical drivers of the November 2023 heatwave in Rio de Janeiro

Abstract. Because extreme heat has not historically been a major hazard for the city of Rio de Janeiro, the November 2023 Heatwave magnitude and timing were staggering. Here we conduct a case study of reanalysis data and high-resolution projections to explore the event drivers and characterize the evolving extreme heat risk in the city of Rio de Janeiro. We find that the heatwave was associated with atmospheric blocking, potentially linked to the 2023–24 El Niño event. Soil moisture declines increased surface sensible heat flux, and elevated sea surface temperatures reduced coastal cooling. The heatwave was preceded by weeks of suppressed precipitation and terminated by the onset of rain. We also find a significant historical increase in the frequency of high heat days throughout Brazil and a lengthening of the heat season in the city of Rio de Janeiro. The frequency of the city's austral spring heat extremes is expected to increase further in the future, highly dependent upon our future emissions pathway. These results emphasize the rapidly emerging risk for extreme heat in the city of Rio de Janeiro.