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Elucidation of Thermal Bleaching of Coral Reefs by Internal Waves

A collaborative team of researchers led by the Hong Kong University of Science and Technology (HKUST) have discovered that internal waves are able to provide cooling of coral reefs, relieving it of the bleaching effects of high temperatures.

Since the year 2016, half of the corals in the Great Barrier Reef have suffered the effects of thermal bleaching. Many other coral reefs around the world are threatened by pan-tropical bleaching events caused by rising seawater temperature related to drastic climate change and extreme conditions like El Niño. These are the side effects of global warming which have damaging effects on natural ecosystems in the sea.

At present, predictions of bleaching patterns are based on surface estimates of seawater temperatures gathered by satellites. This makes it difficult to obtain an accurate prediction especially in deep waters. While satellite observations are important for understanding large-scale patterns and studying remote locations, they can only provide temperatures at the very surface of the ocean and calculate averages over relatively large scales.

In the research by HKUST, Professor Alex Wyatt, Assistant Professor at HKUST’s Department of Ocean Science, collaborated with scientists from the University of Tokyo, Scripps Institute of Oceanography at the University of California San Diego, U.S. Geological Survey and the Florida Institute of Technology, to perform 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 multiple years, capturing in situ heating events associated with the 2015-2016 El Niño.

The novel filtering approach developed by the team was able to extract internal wave signals form the observed temperatures and used to compare heating in the presence and absence of internal waves. The results showed that internal waves reduced heating by up to 88 percent during the 2015-2016 El Niño event. The duration of severe heating events likely to wipe out corals was also reduced at some sites – by 36 to 50 percent – or prevented entirely at others.

The team also demonstrated that natural internal wave cooling increases with depth. Heating was reduced by 20 to 41 percent at the shallowest site (8 to 10 meters water depth), compared to 54 to 88 percent in deeper water (30 to 40 meters water depth). Shedding light on the natural cooling effect of internal waves that is naturally able to reduce coral bleaching.

Alternatively, this also proves evidence that the absence of natural internal waves or there is a reduced frequency or intensity due to climate change, thermal bleaching of coral reefs will be even more severe.

Prof Wyatt said the results suggested that there might be innovative ways to adapt this information for local protection of coral reefs. “Active management approaches such as artificial upwelling may reduce the worst impacts of heating across a small scale over at-risk coral communities identified for special protection. However, artificial upwelling can only offer localized, and perhaps temporary, protection from rapid climate heating. Urgently addressing the underlying causes of climate change are critical for the future survival of coral reefs.”

The study was published in November 2019 on the scientific journal Nature Geoscience[APBN]