Hidden heatwave led to massive coral bleaching event

From April to May 2019, the coral reefs close to the French Polynesian island of Moorea in the central South Pacific Ocean suffered prolonged and severe thermal bleaching, despite the absence of El Niño conditions that year, a phenomenon which intrigued oceanologists around the world.

Now, a team of scientists led by the Hong Kong University of Science and Technology has investigated this surprising and apparently paradoxical event, and discovered that it was related to passage of anti-cyclonic eddies which elevated sea levels and concentrated hot water over the coral reef, leading to a massive underwater marine heatwave that was invisible from the sea surface.

Most studies of coral bleaching rely on sea-surface measurements of water temperatures, a method that cannot fully capture the threats from ocean heating to various ecosystems, including coral reefs. By analyzing data collected at Moorea over a period of 15 years (from 2005 to 2019) through a combination of remotely sensed sea-surface temperatures and high-resolution, long-term in-situ temperatures and sea level anomalies, the Hong Kong experts found that the passage of anti-cyclonic eddies in the open ocean near the island raised sea levels, pushing internal waves down into deeper water. As a result, internal wave cooling was shut down in early 2019, leading to unexpected heating over the reef and large-scale coral bleaching and mortality.

Interestingly, in contrast with the situation in 2019, the reefs near Moorea did not undergo significant bleaching mortality in 2016, regardless of the prevailing El Niño conditions which decimated many shallow reefs worldwide. Since relying solely on sea-surface temperature data would have predicted only moderate bleaching in both 2016 and 2019, the new findings stress the importance of collecting temperatures across the whole range of depths that coral reefs occupy. 

“The present study highlights the need to consider environmental dynamics across depths relevant to threatened ecosystems, including those due to the passage of underwater ocean weather events. This kind of analysis depends on long-term, in situ data measured across ocean depths, but such data is generally lacking,” said study lead author Alex Wyatt, an expert in Ocean Science at Hong Kong.  

“Our paper provides a valuable mechanistic example for assessing the future of coastal ecosystems in the context of changing ocean dynamics and climates.”

The study is published in the journal Nature Communications.

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By Andrei Ionescu, Earth.com Staff Writer

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