Hidden heatwaves and severe coral bleaching linked to mesoscale eddies and thermocline dynamics
Alex S.J. Wyatt1*, James J. Leichter2, Libe Washburn3,4, Li Kui3, Peter J. Edmunds5, Scott C. Burgess6
1Department of Ocean Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. 2Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA. 3Marine Science Institute and 4Department of Geography, University of California, Santa Barbara, California, USA. 5Department of Biology, California State University, Northridge, California, USA. 6Department of Biological Science, Florida State University, Tallahassee, Florida, USA.
The severity of marine heatwaves (MHWs) that are increasingly impacting ocean ecosystems, including vulnerable coral reefs, has primarily been assessed using remotely sensed sea-surface temperatures (SSTs), without information relevant to heating across ecosystem depths. Here, using a rare combination of SST, high-resolution in-situ temperatures, and sea level anomalies observed over 15 years near Moorea, French Polynesia, we document subsurface MHWs that have been paradoxical in comparison to SST metrics and associated with unexpected coral bleaching across depths. Variations in the depth range and severity of MHWs was driven by mesoscale (10s to 100s of km) eddies that altered sea levels and thermocline depths and decreased (2007, 2017 and 2019) or increased (2012, 2015, 2016) internal-wave cooling. Pronounced eddy-induced reductions in internal waves during summer 2019 contributed to a prolonged subsurface MHW and unexpectedly severe coral bleaching, with subsequent mortality offsetting almost a decade of coral recovery. Variability in mesoscale eddy fields, and thus thermocline depths, is expected to increase with climate change, which, along with strengthening and deepening stratification, could increase the occurrence of subsurface MHWs over ecosystems historically insulated from surface ocean heating by the cooling effects of internal waves.
