An international research team led by the Hong Kong University of Science and Technology has found that internal waves below the sea surface can cool the temperature of coral reefs and help prevent the bleaching process.
Internal waves are a type of gravity wave that occurs on internal "surfaces" within ocean waters, representing a strata of rapidly changing water density with the increase in depth.
The results suggest an innovative way to protect coral reefs, such as generating artificial waves, assistant ocean science professor Alex Wyatt said.
Coral bleaching is the process of coral reefs losing their colors and turning white. It is caused by the loss of microscopic algae called zooxanthellae that live within the coral, due to environmental changes such as rising temperature as a result of climate changes.
If the temperature stays high and the algae fail to return, the coral dies.
In the research, Wyatt collaborated with scientists from the University of Tokyo, Scripps Institute of Oceanography at the University of California San Diego, US Geological Survey and the Florida Institute of Technology, in performing a quantitative analysis of temperature records influenced by internal waves on coral reefs in the western, central and eastern Pacific Ocean.
The team measured temperatures across depths at coral reef sites in Japan, French Polynesia and Panama for years, capturing heating events associated with the 2015-2016 El Nino.
During their research, they found that internal waves reduced heat by up to 88 percent, while severe heating events likely to totally kill corals were also reduced at some sites - by 36 to 50 percent - or prevented entirely.
The cooling effect increased with depth, as heat was reduced by 20 to 41 percent at the shallowest sites, where water is eight to 10 meters deep, compared to 54 to 88 percent in deeper water of 30 to 40 meters.
The team added that internal waves would help researchers more accurately locate sites of coral bleaching.
Results also suggested that innovative ways may be developed for local protection of coral reefs, Wyatt said, but artificial upwelling can only offer localized, and perhaps temporary, protection from rapid climate heating.