Cloudy weather may have saved Society Island reef corals during the 1998 ENSO event

During the 1998 El Nino-Southern Oscillation (ENSO) event, mass coral bleac hing in French Polynesia was patchy at a scale of 100s of km, Bleaching was extensive in parts of the Tuamotu archipelago (creating up to 99% coral mo rtality) but extremely mild in the Society Islands (Tahiti, Moorea), ca 350 km to the south-west, despite sea surface temperature (SST) anomalies bein g of similar magnitude to previous years in which mass bleaching occurred. We examine whether environmental variables account for this unexpected pauc ity of bleaching using a 50 yr record of SST, a 17 yr record of daily wind and cloud cover, and a 17 yr record of monthly sun hours, Records from Tahi ti reveal that exceptionally high cloud cover significantly reduced the num ber of sun hours during the summer of 1998, Quadratic discriminant analyses of annual bleaching occurrence based on up to 3 predictors (cumulative deg ree heating months, wind speed, and cloud cover during periods of elevated summer SST) only predicted the correct bleaching scenario for 1998 when clo ud cover was added to the function. The results demonstrate that the intera ctive effect of cloud cover can reverse the bleaching predictions of such s tatistical models. We suggest that reduced radiative stress, resulting from high cloud cover, may have prevented large-scale coral bleaching in 1998.