Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SST) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proven a good predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale, which are only poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. In spite of a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large amplitude internal waves (LAIW) alleviated heating and mitigated coral bleaching and mortality in shallow LAIW-exposed waters. In LAIW-sheltered waters, by contrast, bleaching susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW, which are ubiquitous in tropical stratified waters, benefit coral reefs during thermal stress and provide local refugia for bleaching susceptible corals. The swash zones of LAIW may thus be important, so far overlooked, conservation areas for the maintainance of coral diversity in a warming climate. The consideration of LAIW can significantly improve coral bleaching predictions and can provide a valuable tool for coral reef conservation and management.

CalMarO - Calcification by marine organisms, PEOPLE-2007-1-1-ITN Marie Curie Action: 215157BioAcid - German Ministry for Education and Research Project (Bmbf), Grant number 03F0608B, Bioacid 3.2.3, Coral Calcification in marginal reefsORCAS - Ocean Reef Coupling in the Andaman Sea - German Research Foundation (DFG), Grant number RI 1074/7-1

Supplement to: Wall, Marlene; Putchim, Lalita; Schmidt, Gertraud; Jantzen, Carin; Khokiattiwong, Somkiat; Richter, Claudio (2015): Large-amplitude internal waves benefit corals during thermal stress. Proceedings of the Royal Society B-Biological Sciences, 282(1799), 20140650