Assessment of a zoomed global model for the North Seaby comparison with a conventional nested regional model

摘要

Assessment of marine downscaling of global model simulations to the regional scale is a prerequisite forunderstanding ocean feedback to the atmosphere in regional climate downscaling. Major difficulties arise fromthe coarse grid resolution of global models, which cannot provide sufficiently accurate boundary values for theregional model. In this study, we first setup a stretched global model (MPIOM) to focus on the North Sea byshifting poles. Second, a regional model (HAMSOM) was performed with higher resolution, while the openboundary values were provided by the stretched global model. In general, the sea surface temperatures (SSTs) inthe two experiments are similar. Major SST differences are found in coastal regions (root mean square differenceof SST is reaching up to 2℃). The higher sea surface salinity in coastal regions in the global model indicates thegeneral limitation of this global model and its configuration (surface layer thickness is 16 m). By comparison, theadvantage of the absence of open lateral boundaries in the global model can be demonstrated, in particular for thetransition region between the North Sea and Baltic Sea. On long timescales, the North Atlantic Current (NAC)inflow through the northern boundary correlates well between both model simulations (R~0.9). Afterdownscaling with HAMSOM, the NAC inflow through the northern boundary decreases by ~10%, but thecirculation in the Skagerrak is stronger in HAMSOM. The circulation patterns of both models are similar in thenorthern North Sea. The comparison suggests that the stretched global model system is a suitable tool for longtermfree climate model simulations, and the only limitations occur in coastal regions. Regarding the regionalstudies focusing on the coastal zone, nested regional model can be a helpful alternative.