High-Resolution Modelling of Offshore Wind Farm Impacts in the Baltic Sea with Ocean Reanalyses

Offshore wind farms can influence the physical marine environment in several ways. During construction, they may increase suspended sediment concentrations and sedimentation. During operation, they can alter wave dynamics and hydrodynamics through turbine foundations, wake-induced wind reduction, and potential discharges of warmer, saline water (brine) from on-site hydrogen production. Assessing these effects is essential for responsible planning, but remains challenging due to limited operational experience, scarce large-scale and long-term observations, and the complex interplay of atmospheric, hydrodynamic, and sedimentary processes.

To overcome these challenges, this contribution demonstrates how freely available Copernicus Marine Service (CMEMS) ocean reanalyses data [1], combined with Atmospheric Re-Analysis 5 (ERA5) data developed by the European Centre for Medium Range Weather Forecasts (ECMWF) [2], can support efficient modelling offshore wind farm impacts in the Baltic Sea. We present a systematic workflow that employs these datasets to characterize baseline hydrodynamic conditions and setup locally refined, project-specific numerical models (DHI MIKE).

The reanalysis data is applied to identify seasonal patterns, stratification regimes, and interannual variability,  guide decisions on appropriate model complexity (e.g., 2D versus 3D, barotropic versus baroclinic), provide initialization and boundary forcing of the project-specific numerical hydrodynamic models, and enable the validation of numerical models through comparison of three-dimensional current, temperature, and salinity fields where spatially dense measurements are missing.

Our calibrated models reveal how wind farm layouts influence wind, wave, and current patterns, as well as salinity and temperature. For a planned offshore wind farm in the Bay of Bothnia, key findings include:

• Wind: annual mean wind speed reduction of ~0.1 m/s within a 10–15 km radius,
• Waves: significant wave height reduction of ~5% within the farm and ~1.5% up to 20 km beyond,
• Currents: mean surface current changes of −0.015 m/s inside the farm and +0.006 m/s outside,
• Salinity: variations <0.05 PSU (<1% of natural variability),
• Temperature: annual changes within ±0.25°C, with summer surface warming (+0.25–0.50°C) and subsurface cooling (−0.25 to −0.50°C) under stratified conditions.

This workflow illustrates how publicly available ocean reanalyses can support robust, cost-effective impact assessments, enabling more reliable planning for offshore wind farm development.

Datasets used:

[1] E.U. Copernicus Marine Service Information (CMEMS). Marine Data Store (MDS). Multiple products accessed:
Baltic Sea Physics Analysis and Forecast (BALTICSEA_ANALYSISFORECAST_PHY_003_006, https://doi.org/10.48670/moi-00010),
Baltic Sea Physics Reanalysis (BALTICSEA_MULTIYEAR_PHY_003_011, https://doi.org/10.48670/moi-00013),
Baltic Sea Wave Analysis and Forecast (BALTICSEA_ANALYSISFORECAST_WAV_003_010, https://doi.org/10.48670/moi-00011),
Baltic Sea Wave Hindcast (BALTICSEA_MULTIYEAR_WAV_003_015, https://doi.org/10.48670/moi-00014)
Baltic Sea Biogeochemistry Reanalysis (BALTICSEA_MULTIYEAR_BGC_003_012, https://doi.org/10.48670/moi-00012),
Baltic Sea Biogeochemistry Analysis and Forecast (BALTICSEA_ANALYSISFORECAST_BGC_003_007, https://doi.org/10.48670/moi-00009).

[2] Hersbach, H., Bell, B., Berrisford, P., Biavati, G., Horányi, A., Muñoz Sabater, J., Nicolas, J.,Peubey, C., Radu, R., Rozum, I., Schepers, D., Simmons, A., Soci, C., Dee, D., Thépaut, J.-N. (2023). ERA5 hourly data on single levels from 1940 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS). https://doi.org/10.24381/cds.adbb2d47