An international team of researchers from the Department of Marine Science at the Hong Kong University of Science and Technology have investigated a surprising and paradoxical coral bleaching event. This occurred when coral reefs off the French Polynesian island of Moorea experienced severe and prolonged thermal bleaching from April to May 2019 and most worryingly, happened despite the absence of an El Niño that year.
They discovered that the unexpected event was linked to the passage of an anticyclonic vortex that raised sea levels and concentrated hot water on the reefs, causing the underwater heatwave to be largely hidden from the surface. Most studies of coral bleaching patterns rely on sea surface temperature measurements, which are now known to not fully capture the threat to marine ecosystems. These surface measurements using satellites over vast areas are valuable, but cannot detect the subsurface warming affecting communities living in waters deeper than the shallowest metres of the ocean.
The results were recently published in Nature Communications.
Image: Coral bleaching happens when high water temperatures cause coral to expel its symbiotic algae: Luiz Rocha/California Academy of Sciences