Vertical Characteristics of Marine Heatwaves in Global Coral Reef Regions

Marine heatwaves (MHWs) threaten coral reef ecosystems worldwide, yet their subsurface characteristics remain poorly understood. Using two high‐resolution ocean reanalysis products spanning 1993–2023, we reveal the global spatiotemporal patterns of subsurface MHWs across coral reef regions. MHW annual days decrease with depth, while their cumulative intensity increase, indicating that deep events are stronger but less frequent. Vertical profiles show that severe MHWs frequently extend to 150 m during major climate anomalies, including strong El Niño episodes and the 2020–2023 triple‐dip La Niña. During these periods, cumulative intensities reached up to 160°C‐days at 90–150 m, affecting more than half of all reef regions in the upper 70 m. Intensified subsurface MHWs threaten to undermine the role of mesophotic coral ecosystems as thermal refugia. This reality underscores the critical need to anticipate the intensification of extreme El Niño and multi‐year La Niña events in a warming ocean.