Seasonal predictability of an indicator of mass coral bleaching between the Pacific ocean and the East China sea with a large ensemble climate model

In this study, we investigated the seasonal predictability of Degree Heating Week, a widely used indicator of coral thermal stress and a strong predictor of coral bleaching. We used a 108-member seasonal prediction system to analyze four seasons (March–May, June–August, September–November, and December–February) from nine initialization dates (February 1–9, May 1–9, August 1–9, and November 1–9) between 1982 and 2022. We found that the model skillfully predicts Degree Heating Weeks for most coral reef areas, including southern Japan in summer. Through mixed-layer heat budget and statistical analyses, we found that La Niña and negative Indian Ocean Dipole events contributed to suppressing tropical cyclone activity and thus building up the Degree Heating Week by weakening cooling via vertical diffusion in southern Japan. Successful prediction of the Degree Heating Week, as well as of mass coral bleaching events, at least a few months in advance, could help reduce the risk of coral reef collapse by facilitating the implementation of necessary adaptation measures.