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In project E1.13 a coupled atmosphere-ocean-ice system is studied to assess the impact of the North and Baltic Seas and the Mediterranean Sea on the climate in central Europe. Due to differences in domain areas, grid sizes and time steps between the atmosphere model COSMO-CLM and the ocean model NEMO, the OASIS3 coupler is used to interpolate the exchanged data in space and time.

COSMO-CLM used in this study is a non-hydrostatic regional climate model. The model is set up based on the CORDEX Europe domain with a horizontal grid size of 50 km (0.44 degree). For the lateral boundaries, the 6-hourly ERA-Interim reanalysis data are used.

The NEMO ocean model (version 3.3) includes a sea ice model (LIM3) and is set up for North and Baltic Seas. It has a horizontal resolution of 2’, with 619 x 523 grid cells and 56 vertical layers. The adjustment to the North and Baltic Seas is provided by the Swedish Meteorological and Hydrological Institute (SMHI).

The simulation period for the uncoupled atmosphere model as well as for the coupled atmosphere-ocean system is from 1979 to 2010.

The OASIS3 coupler sends the effective precipitation, the atmospheric wind stress, the solar and non-solar radiation and the sea level pressure to the ocean model, which sends the sea surface temperature and the fraction of ice via the OASIS3 coupler to the atmosphere (Figure 1). These fields are exchanged every 3 hours, the interpolation method is distance weighted.

fig. 1

Figure 1: Coupling between atmosphere and ocean.

The first results from the coupled models show a slight increase of 2m temperature in central Europe compared with the stand-alone atmosphere model (Figure 2). In the South West of Europe, temperature tends to be lower in the coupled system because of the cooling effect of the ocean.

fig. 2

Figure 2: Annual average temperature at 2 meter in Kelvin in 1989, difference between coupled system and uncoupled atmosphere model.

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