Marine heat waves in vulnerable coral reef regions are skillfully predictable on multi-year timescales

Marine heat waves (MHWs) increasingly threaten coral reefs, but their predictability beyond seasonal timescales remains poorly understood. Here we assess the seasonal-to-multi-year predictability of MHW frequency, cumulative intensity, and duration across 12 coral reef regions. Using Community Earth System Model version 2 simulations, ocean-assimilated experiments and five-year hindcasts initialized annually from 1981 to 2020, we show that frequency and cumulative intensity are predictable for several months to multiple years across many ocean basins, with skill varying strongly by region and season. Anthropogenic warming is the dominant source of multi-year skill across most hotspots, while internal variability limits predictability in the equatorial Pacific. El Niño-Southern Oscillation and Atlantic Multidecadal Oscillation further modulate regional forecasts through coupled ocean-atmosphere processes. These findings reveal strong regional contrasts in the sources and limits of forecast skill, identifying where advance warning can support timely, targeted regional preparedness, coral reef conservation and adaptation to escalating thermal stress. The study shows that marine heatwave frequency and cumulative intensity across vulnerable coral reef regions are skillfully predictable from months to years ahead, with long-term anthropogenic warming accounting for most multi-year forecast skill, while internal variability, particularly El Niño, limits predictability in the equatorial Pacific.