Hidden heatwaves and severe coral bleaching linked to mesoscale eddies and thermocline dynamics

Hidden marine heatwaves, associated with ocean eddies that modulate undersea internal waves, threaten coastal ecosystems by driving unexpected sub-surface heating and severe coral bleaching and mortality across depths. 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 early 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.

Automated summary

Marine heatwaves hidden beneath the surface, induced by ocean eddies and internal waves, pose a threat to coastal ecosystems by causing unexpected heating and severe coral bleaching and mortality. Previous assessments of marine heatwaves (MHWs) have mainly relied on sea-surface temperature data, without considering heating across ecosystem depths. This study used a unique combination of satellite data, high-resolution in-situ temperature measurements, and sea level anomalies to reveal subsurface MHWs that were paradoxical compared to surface temperature metrics and associated with unexpected coral bleaching.