Short-term impact of offshore wind farms on the regional ocean turbulence and stratification in the North Sea and Danish coastal waters

Sustainable multi-use offshore infrastructure has been installed in the North Sea and Baltic Sea coastal regions as part of the OLAMUR initiative. Large offshore wind farm aggregations are being combined with low-trophic aquaculture to enhance fish and shellfish production. A key requirement of this initiative is to assess the impact of these wind farms on local wind and waves, ocean currents and turbulence, and the variability of nutrient and carbon uptake. In this work, we use a hydrostatic HIROMB-BOOS Model (HBM) setup to investigate the short-term (20 days) impact of Danish and North Sea wind farms on the regional ocean turbulence and stratification. While previous modeling studies have used unstructured grids to resolve monopile geometry, our approach employs a structured, submesoscale-resolving grid, and the turbine impact is being represented through a subgrid frictional drag increment to the prognostic equations of the k-omega turbulence closure model used in the HBM. We conduct short-duration simulations, both with and without wind farm forcing, for the summer and winter seasons. This enables an assessment of seasonality and the spatial reach of wind-farm-induced anomalies over a 20-day window. Our analysis focuses on four regions: Helgoland, the Southern North Sea, Kriegers Flak, and Anholt. We examine changes in the vertical structure using potential energy anomaly (PEA) and compare them with kinetic energy differences in both resolved and subgrid space. The tidally active Southern North Sea exhibits a strong increase in stratification during summer, with PEA anomalies ranging between 4% and 6% over multi-day periods, whereas Helgoland shows a smaller response (on the order of 1%). In contrast, the Danish coastal regions (Kriegers Flak and Anholt) display PEA values one to two orders of magnitude smaller (0.2 %) and more intermittent behavior, consistent with weaker tidal signals and stronger eddy-induced turbulence. We interpret the North Sea response as wind farm drag extracting energy from a tidally dominant regime, thereby reducing shear-driven flow and allowing stratification to persist. Far-field regions in the Skagerrak and Kattegat channels show strong anomalies at later stages in the simulation, which is attributed primarily to the background submesoscale turbulence caused by cross-flow exchange between North Sea and Baltic Sea waters.