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

Abstract. Marine heatwaves are 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 northwest European Shelf, which is a critically important marine ecosystem. We use an ensemble of five dynamically downscaled hydrodynamic models under the high emissions scenario RCP 8.5. Heatwaves were defined as events lasting at least 5 days 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 projected to be in heat wave 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 with 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 variation in temperature outside the summer. The impacts of marine heatwaves on shelf seas are therefore likely to be much more complex than anticipated heretofore, when taking a view beyond the ocean surface and summer.