Exchange-grid coupling approach for the IOW Earth System Model (version 1.04.00) of the Baltic Sea region

Abstract. In this article the development of a high-resolution Earth System Model (ESM) for the Baltic Sea region is described. In contrast to conventional coupling approaches, the presented model features an additional (technical) component, the flux calculator, that calculates fluxes between the model components on a common exchange grid. This approach automatically ensures conservation of exchanged quantities, a locally consistent treatment of the fluxes and facilitates interchanging model components in a straightforward manner. The main purpose of this model is to downscale global reanalysis or climate model data to the Baltic Sea region since typically global model grids are too coarse to resolve the region of interest sufficiently. The regional ESM consists of the Modular Ocean Model 5 (MOM5) for the ocean and the COSMO model in CLimate Mode (CCLM, version 5.0_clm3) for the atmosphere. The bi-directional ocean-atmosphere coupling allows for a realistic air-sea feedback which outperforms the traditional approach of using uncoupled standalone models as typically pursued with the EURO-CORDEX protocol. In order to address marine environmental problems (e.g. eutrophication and oxygen depletion), the ocean model is internally coupled to the marine biogeochemistry model ERGOM set up for the Baltic Sea's hydrographic conditions. The regional ESM can be used for various scientific questions such as climate sensitivity experiments, reconstruction of ocean dynamics, study of past climates and natural variability as well as investigation of ocean-atmosphere interactions. Therefore, it can serve for better understanding of natural processes via attribution experiments that relate observed changes to mechanistic causes.