Impact of Dynamical Downscaling on Sea Level Projections in the North‐East Atlantic Ocean and Regional Seas

Abstract Practitioners and policy makers require sea‐level (SL) change estimates at regional‐to‐local scales to inform adaptation measures against sea‐level rise (SLR). Ocean dynamic SL (DSL) changes are a dominant driver of regional patterns of SL projections provided by the global climate models (GCMs). However, there is medium confidence in DSL changes due to the relative coarse resolutions of GCMs. In the north‐east Atlantic and adjacent regional seas (NEA‐RS), dynamical downscaling studies are sparse, often limited to one or two GCMs and focus on long‐term trends, while changes in DSL variability at seasonal and interannual timescales may also be important. Here, a 4‐member ensemble of downscaled DSL projections is produced for 3 climate change scenarios over the NEA‐RS using a high‐resolution regional ocean climate model (RCM). The impact of downscaling on projected DSL changes at 20‐year mean interannual and seasonal timescales is evaluated to inform on the reliability of GCM‐based estimates. Results show substantial impacts of downscaling on 20‐year mean and seasonal DSL changes. The maximum monthly‐mean SL attained when compounding the three timescales are up to 8% lower and 48% higher in the North Sea and western Mediterranean Sea respectively in the RCM compared with the GCM estimates. Outside these regions, the confidence in GCM‐based estimates is increased due to good alignment of RCM and GCM projected DSLs. Our findings underscore the value of dynamical downscaling for building confidence in GCM projections of DSL in the NEA‐RS, revealing high sensitivity to downscaling for long‐term trends and variability along marginal seas.