An international research team led by the Hong Kong University of Science and Technology (HKUST) has demonstrated that cooling by internal waves could potentially create thermal refuges for coral reefs, and may help prevent and more accurate predict locations of coral bleaching.
Coral reefs around the world are threatened by pan-tropical bleaching events caused by rising seawater temperatures linked to ongoing climate change and extreme conditions like El Niño. However, bleaching patterns can be hard to predict, especially in deeper water. Currently, most bleaching predictions are based on surface estimates of seawater temperatures gathered with satellites. While satellite observations are important for understanding large-scale patterns and studying remote locations, they are only able to detect temperatures at the very surface of the ocean and provide averages over relatively large scales.
In this research, Prof. 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.
Using a novel filtering approach developed by the team, internal wave signals were extracted from the observed temperatures and used to compare heating in the presence and absence of internal waves. This showed that internal waves reduced heating by up to 88% during the 2015-2016 El Niño. The duration of severe heating events likely to totally kill corals was also reduced at some sites – by 36 to 50% – or prevented entirely at others.
The team also showed that natural internal wave cooling increases with depth. Heating was reduced by 20 to 41% at the shallowest sites (8 to 10m water depth), compared to 54 to 88% in deeper water (30 to 40m water depth). Internal waves may thus be an important process for naturally reducing coral bleaching across coral reef depths and ocean locations. Conversely, the results also suggest that in the absence of internal waves, or if internal waves frequency and intensity decrease with climate change, the heating threatening coral reefs could become 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 team’s findings were recently published in the scientific journal Nature Geoscience.
About The Hong Kong University of Science and Technology
The Hong Kong University of Science and Technology (HKUST) (www.ust.hk) is a world-class research university that focuses on science, technology and business as well as humanities and social science. HKUST offers an international campus, and a holistic and interdisciplinary pedagogy to nurture well-rounded graduates with global vision, a strong entrepreneurial spirit and innovative thinking. HKUST attained the highest proportion of internationally excellent research work in the Research Assessment Exercise 2014 of Hong Kong’s University Grants Committee, and is ranked as the world’s best young university in Times Higher Education’s Young University Rankings 2019. Its graduates were ranked 10th worldwide and top in Greater China in Global Employability University Survey 2019.
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