Assessment of surface and bottom marine heatwaves along the Spanish coast

Marine heatwaves (MHWs) are discrete yet persistent events of anomalously warm ocean temperatures which are becoming a hot topic in climate change research due to their extensive disruption of marine ecosystems worldwide. As a consequence, surface MHW events (SMHWs) and their drivers have been characterised worldwide typically under a consolidated common methodology. However, subsurface and bottom events of MHW (BMHWs) are less known due to the limited availability of data. Furthermore, recent advances suggest an improved MHW definition to distinguish the extreme event from the long-term ocean warming. Here, we use high-resolution GLORYS12V1 reanalysis data from 1993 to 2022 to characterise both SMHWs and BMHWs along the Spanish Marine Demarcation (SMD) areas defined within the Marine Strategy Framework Directive. We also broadly analyse their interconnections and ultimately generate a regional assessment of the integrated exposure of SMD to MHWs. We find that both SMHWs and BMHWs were more intense, longer-lasting, and widespread over the last 15 years. We also find that while SMHWs exhibit spatial variation following heat fluxes anomalies in the ocean surface layers, BMHWs roughly scale with ocean bottom depth and persist longer than their surface counterparts. Further, in shallower coastal regions where the mixed layer extends to the ocean bottom, average BMHW intensities can be comparable or even higher than those concurrently overlaying at the surface. Finally, we also demonstrate that both SMHWs and BMHWs are more likely to co-occur with high cumulative intensities in coastal SMD areas, with the 69% of their spatial extent categorised as highly exposed to MHWs. This highlights the imperative need for analysing and integrating SMHWs and BMHWs, especially in coastal zones, when assessing and addressing present and future impacts on wildlife and economies under the expected climate change scenario.