Drivers of marine heatwaves and their decadal suppression in the southwest Indian Ocean

Anthropogenic warming has intensified marine heatwaves across most of the global ocean. In the southwest Indian Ocean, however, the marine heatwave characteristics and their drivers remain largely unknown. Here, we investigate these issues using satellite observations, reanalysis data, and climate model simulations spanning 1982-2023. We find pronounced marine heatwave activity during 1982-2023, with a marked suppression from 1992 to 2004. The marine heatwave events exhibit large spatial scales, extending across the southwest Indian Ocean. Their main drivers are surface wind anomalies associated with middle-latitude atmospheric intraseasonal Rossby waves that intrude into the subtropics. These wind anomalies weaken the mean wind speed and increase specific humidity by advecting moist tropical air into the subtropical southwest Indian Ocean. These processes reduce latent heat loss, shoal the surface mixed layer, enhance shortwave heating, and ultimately trigger marine heatwave events. Volcanic eruptions combined with modes of internal variability jointly drove the decadal suppression. Our findings highlight the important interplay between external forcing and internal climate variability in shaping regional extremes. Marine heatwaves in the southwest Indian Ocean are driven by surface wind anomalies and suppressed by volcanic activity, according to a combination of satellite observations, reanalysis data, and climate model simulations.