Tropical climate modes control strength and distribution of thermal stress mitigation in a coral reef refugia.

Tropical coral reefs are threatened by rising seawater temperatures. However, cold-water intrusions to coral reefs by oceanic processes such as upwelling or internal waves, which propagate along the thermocline, can offer refugia from global warming. How reef temperatures and coral responses vary in such locations remains poorly understood due to limited in-situ observations. We present monthly subsurface temperature and coral response records based on coral skeletal geochemistry from the Andaman Sea, a known thermal refugia. Coral Sr/Ca-temperature indicates subsurface reef cooling varied at interannual to decadal timescales, driven by large-scale wind and thermocline changes, and was most pronounced during the combined positive Indian Ocean Dipole and El Niño event of 1997/98. Coral carbon isotopes indicate relative shifts from autotrophy to heterotrophy during most coral bleaching events observed in the Andaman Sea, except during the 1998 event when we found strong and widespread subsurface reef cooling by thermocline shoaling leading to enhanced influence of internal waves. Our results indicate tropical climate modes such as the Indian Ocean Dipole and the El Niño-Southern Oscillation controlled the strength and spatial distribution of thermal stress mitigation by oceanic processes through time, suggesting some coral reef refugia will be variable under future climate change.