Marine heatwave amplifies extreme multi-hazards of extratropical cyclone Babet

Abstract. Climate change is increasing marine heatwave (MHW) frequency and intensity and intensifying rainfall from extratropical cyclones. Storm Babet, the United Kingdom’s most impactful 2023 extratropical cyclone, followed ∼7 weeks of MHW conditions in the North Sea and cooled it by ∼1 °C. The storm produced extreme rainfall, widespread flooding, and severe coastal erosion across northern regions of the UK. Its prolonged southeasterly winds generated above 8 m significant wave heights and up to 60 cm surge in the North Sea coastline of the UK. We show the MHW was generated at the surface by persistent anticyclonic conditions: clearer skies boosted shortwave heating, weaker winds reduced mixing and latent cooling. Subsequent short spells of cyclonic conditions allowed heat to penetrate at depth and be retained throughout the depth of the shallow North Sea. Using a state-of-the-art regional coupled model, we assess how pre-existing marine heatwave conditions influenced the storm’s evolution and associated hazards. The warm shelf seas boosted evaporation, energised the boundary layer and deepened the cyclone. Indeed, the fast, relatively dry and cold southeasterly low-level jet saw amplified latent and sensible heat fluxes over the MHW. These extremely intense air-sea fluxes led to the decay of the MHW whilst loading the storm with additional moisture. Relative to a cooler ocean counterfactual, the MHW amplified multihazard outcomes across the Northeast of the UK, increasing river discharge (12–18 %), coastal wave power (∼9 %), and storm surge (∼20 %). These results highlight the role of MHWs as amplifiers of multi-hazard storm impacts over the shallow Northwest European Shelf, where extra ocean heat cannot escape to depth.