The atmospheric model COSMO-CLM is a non-hydrostatic regional climate model. The model setup complies with CORDEX-EU in the CORDEX framework (Coordinated Regional climate Downscaling Experiment) (Giorgi et al. 2006). The domain covers the whole of Europe, North

Africa, the Atlantic Ocean and the Mediterranean Sea (Figure 1a). The horizontal resolution is 0.44° (approximately 50 km) and the time step is 240 seconds; it has 40 vertical levels. COSMO-CLM Oligomycin A clinical trial applies a ‘mixed’ advection scheme, in which a positive-definite advection scheme is used to approximate the horizontal advection while vertical advection and diffusion are calculated with a partially implicit Crank-Nicholson scheme. In COSMO-CLM, several turbulence schemes are available; in our experiments, we used the so-called 1-D TKE-based diagnostic closure, which is a prognostic

turbulent kinetic energy (TKE) scheme. It includes the interaction of air with solid objects at the surface (roughness elements). We modified the model code to adapt it to the coupled mode. Originally, COSMO-CLM did not have sub-grid scale ice; a grid over the ocean is either fully covered with ice or fully open-water. Thus, a grid size of 50 × 50 km2 implies a rather coarse approximation of real ocean conditions. In addition, COSMO-CLM does not have an ice mask over the ocean; an ocean grid is handled as sea ice or open water depending on the SST. If the temperature is below the freezing point of water, which is −1.7 °C PF-02341066 concentration in COSMO-CLM, the surface is considered to be sea ice. When the temperature is equal to or higher than the freezing point, COSMO-CLM C-X-C chemokine receptor type 7 (CXCR-7) handles the surface as open water. However, a freezing point of water of −1.7 °C is applicable to sea water with a salinity of approximately 35 PSU

(Practical Salinity Units). In contrast, brackish sea water like the Baltic Sea has a much lower salinity than the average salinity of the World Ocean. At the centre of the Baltic Sea, the Baltic Proper, the salinity is only 7–8 PSU, and this decreases even further northwards to the Bothnian Sea, Bothnian Bay and Gulf of Riga (Gustafsson 1997). The freezing point of this brackish water should therefore be higher than −1.7 °C. When the freezing point is so low, the sea ice cover in the Baltic Sea in COSMO-CLM will be substantially underestimated. Therefore, when coupling COSMO-CLM with the ocean model NEMO, the sea ice treatment is modified in the surface roughness and surface albedo schemes. In the current albedo calculation scheme, COSMO-CLM attributes fixed albedo values to the water surface (0.07) and the sea ice surface (0.7) for the whole grid cell. In the coupled mode, as COSMO-CLM receives the ice mask from NEMO, it can now calculate the weighted average of the albedo based on the fraction of ice and open water in a grid cell. The surface roughness length of the sea ice and open-water grid is calculated in the turbulence scheme of COSMO-CLM.