From April to May 2019, coral reefs near the French Polynesian island of Moorea in the central South Pacific Ocean experienced prolonged and severe thermal bleaching, despite the absence of El Niño conditions this year- there, a phenomenon that has puzzled oceanologists around the world.
Now, a team of scientists led by the Hong Kong University of Science and Technology has investigated this startling and seemingly paradoxical event, and found it was linked to the passage of anticyclonic eddies that raised sea levels and concentrated warm water on the coral reef. , resulting in a massive underwater marine heatwave that was invisible from the sea surface.
Most studies of coral bleaching rely on measurements of sea surface water temperature, a method that cannot fully capture the threats of ocean warming to various ecosystems, including coral reefs. . By analyzing data collected in Moorea over a 15-year period (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 Hong Kong experts found that passing high-seas high-pressure eddies near the island caused sea levels to rise, pushing internal waves toward deeper waters. As a result, internal wave cooling was halted in early 2019, leading to unexpected reef warming and large-scale coral bleaching and mortality.
Interestingly, unlike the situation in 2019, reefs near Moorea did not experience significant bleaching mortality in 2016, regardless of the El Niño conditions that decimated many shallow reefs around the world. Given that relying solely on sea surface temperature data would have predicted only moderate bleaching in 2016 and 2019, the new findings underscore the importance of collecting temperatures across the full range of depths occupied by coral reefs.
“The present study highlights the need to consider environmental dynamics across depths relevant to threatened ecosystems, including those due to the passage of submarine ocean weather events. This type of analysis depends on in situ data at long-term measured across ocean depths, but these data are generally lacking,” said the study’s lead author, Hong Kong ocean science expert Alex Wyatt.
“Our paper provides a valuable mechanistic example for assessing the future of coastal ecosystems in the context of changing ocean and climate dynamics.”
The study is published in the journal Nature Communication.
By Andrei Ionescu, Editor of Earth.com
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