The importance of regional ocean modelling for sea-level projections in the north-west european shelf region

Abstract

Regional sea-level projections are of interest, because they can strongly deviate from the global mean sea-level change. In a previous study, dynamical downscaling was found to give more realistic simulations in the Northwestern European Shelf (NWES) region than coarse-resolution global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Dynamical downscaling uses high-resolution regional climate models (RCMs) to refine the GCM results. Here we investigate the performance of dynamically downscaling sea-level projections in the NWES region with the regional ocean model (ROM) ROMS. The simulations are performed over the period from 1980 to 2098, on a 1/4 ֯ by 1/4 ֯ horizontal grid resolution. ROMS is forced at the lateral boundaries with the CMIP5 GCM HadGEM2-ES ocean component. The ROMS surface boundary is forced by the dynamically downscaled HadGEM2-ES atmosphere component with the regional atmosphere model (RAM) RCA4 from the EURO-CORDEX database. The downscaled sea-level projections are compared to the HadGEM2-ES data and the results from (Hermans, Tinker, et al., 2020), where HadGEM2-ES was dynamically downscaled with the ROM NEMO AMM7 (with a horizontal grid resolution of 1/15 ֯ by 1/9 ֯ ).

We find that dynamical downscaling with ROMS leads to a reduced sea-level rise in the North Sea for the end of the twenty-first century compared to HadGEM2-ES (13 cm for the Belgian, Dutch and Danish coast). The results from ROMS on the shelf compare well with the higher-resolution NEMO results. Only a difference of 0.77 cm is found before the Belgian, Dutch and Danish coast. Our results show that with using the relatively low resolution ROM ROMS comparable improvements as with the higher resolution ROM NEMO can be obtained for the GCM sea-level projections in the coastal regions of Belgium, the Netherlands and Denmark. Allowing dynamical downscaled sea-level projections for the coastal area of Belgium, The Netherlands and Denmark with a relatively low resolution ROM reduces computational time.