Seafloor marine heatwaves outpace surface events in the future on the northwestern European shelf

Abstract

Marine heatwaves are becoming increasingly frequent across the world's oceans. As a result, there are growing impacts on marine ecosystems due to temperatures exceeding the thermal niche and historical exposure of many species. Anticipating the future frequency and severity of marine heatwaves is necessary. Here, we provide the first projections of future marine heatwaves for the sea surface and seafloor across the northwestern European shelf, which is a critically important marine ecosystem. We use an ensemble of five dynamically downscaled hydrodynamic models under the high-emission scenario Representative Concentration Pathway 8.5 (RCP 8.5). Heatwaves were defined as events lasting at least 5 d where temperatures exceed the 90th percentile of a historical baseline period. The frequency of marine heatwaves at the surface and seafloor is projected to increase significantly during the 21st century under RCP 8.5, with most of the year being projected to be under heatwave conditions by the end of the century. Critically, we find that marine heatwaves are projected to increase in frequency to a greater extent at the seafloor compared to at the sea surface due to their lower levels of natural temperature variation. Similarly, we find that the severity of summer heatwaves at the surface is projected to be lower than that of heatwaves during the rest of the year due to lower climatological variations in temperature outside the summer. The impacts of marine heatwaves in shelf seas are therefore likely to be much more complex than previously thought.

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