Southern hemisphere eastern boundary upwelling systems emerging as future marine heatwave hotspots under greenhouse warming.

Marine heatwaves (MHWs) exert devastating impacts on ecosystems and have been revealed to increase in their incidence, duration, and intensity in response to greenhouse warming. The biologically productive eastern boundary upwelling systems (EBUSs) are generally regarded as thermal refugia for marine species due to buffering effects of upwelling on ocean warming. However, using an ensemble of state-of-the-art high-resolution global climate simulations under a high carbon emission scenario, here we show that the MHW stress, measured as the annual cumulative intensity of MHWs, is projected to increase faster in the Southern Hemisphere EBUSs (Humboldt and Benguela current systems) than in their adjacent oceans. This is mainly because the additional warming caused by the weakened eastern boundary currents overwhelms the buffering effect of upwelling. Our findings suggest that the Southern Hemisphere EBUSs will emerge as local hotspots of MHWs in the future, potentially causing severe threats to the ecosystems.