Contributions from sea level variability changes to extreme sea level projections in western Europe

Future changes in extreme still water levels (ESWL) will play a critical role in shaping coastal flood risk across western Europe. Yet most large-scale assessments, including recent IPCC reports, estimate ESWL changes using static approaches that account only for long-term sea-level rise while treating other sea-level components as stationary (static approach). Here, we use a regional 3D ocean model to dynamically downscale four CMIP6 GCMs to quantify how changes in sea-level variability - in tides, storm surges, the seasonal cycle, and dynamic sea-level anomalies - modify ESWL projections (dynamic approach) through the 21st century under SSP1-2.6 and SSP5-8.5. Using a transformed-stationary extreme value analysis, we evaluate changes in future ESWL return levels and the contributions of individual sea-level components to these changes. Across western Europe, dynamically simulated changes in the 10-year ESWL average to 39 cm (SSP1-2.6) and 57 cm (SSP5-8.5), but regional deviations reach ±20 cm. Dynamic estimates can locally amplify ESWL by 30–40% relative to static ones, particularly in the southern North Sea, northern Irish Sea, and western Mediterranean, with similar impacts for the 100-year event. In many regions, differences relative to static estimates result from compensation effects between changes across sea-level components. Changes in dynamic sea level anomalies and the seasonal-cycle dominate dynamic contributions to ESWL changes in the Mediterranean and Atlantic façade south of 47°N. Tidal changes dominate in the English Channel and UK/Irish coasts, while storm surges dominate in the southeastern North Sea. However, the reported contributions exhibit a large inter-model spread and cannot be readily attributed to either forced or internal variability. Our results show that future ESWL changes are shaped by multiple still water level variability drivers in addition to long-term trends, underscoring the limitations of simplistic static approaches. They further reveal strong regional differences in dominant drivers and compensations among them, which can only be comprehensively resolved using 3D ocean models that represent coastal processes.