Abstract
As part of the transition to more sustainable energy generation and the reduction of anthropogenic greenhouse gas emissions, offshore wind energy has developed rapidly over the past decade. As a result, the economic use of the coastal ocean is continuously increasing, and with it the interactions between anthropogenic impacts and the marine environment. In light of offshore wind development, this dissertation investigates the physical effects of offshore wind farms on the hydrodynamics of the North Sea, a global hotspot for offshore renewable energy. Offshore wind farms affect the physics in the atmosphere and ocean by extracting energy and disturbing incoming horizontal winds and currents. The associated effects occur on a variety of horizontal scales, from local mixing at turbine foundations to largescale wind speed reductions. This thesis demonstrates how these wind farm effects influence hydrodynamics on regional and seasonal scales, providing essential knowledge about the implications for ocean physics. Using three-dimensional numerical modeling, the thesis presents new and existing parameterization approaches to account for wind speed reduction and additional structure-induced mixing from offshore wind farms in regional hydrostatic models. Sectioned into three individual studies, this dissertation illustrates the physical implications of surface wind speed reduction and underwater structure drag on winddriven processes and local mixing, respectively. In this context, the so-called wind wakes are shown to associate with changes in wind-induced currents and mixing, whereas the oceanic wakes particularly influence the local turbulence and horizontal circulation. Thereby, the changes in ocean physics do not remain local, but propagate through advection and baroclinic currents into the far field of offshore wind farms. The emerging large-scale anomalies translate to current speeds, surface elevation or vertical velocities. Eventually, both wind farm effects alter the vertical density stratification and cause regional perturbations of the seasonal pycnocline of about ±5-10 % on average. This dissertation emphasizes that physical implications from wind speed reduction and underwater structure drag can emerge on similar magnitudes, although being driven by different mechanism and originating on different spatial scales. The monthly-mean wake effects in the atmosphere and ocean are shown to cause large-scale restructuring and spatiotemporal redistributions of ocean physics within natural variability. In this context, the wake effects, particularly wind wake effects, show strong variability and sensitivity to local conditions such as tidal stirring, which can disturb initial signals from wind speed reduction and attenuate wake effects by 50 % or more. While the outcomes advance the knowledge about regional implications from offshore wind energy production, changes in the physical environment indicate potential consequences for physically determined ecosystem dynamics. Questions remain open about the interaction of the wind farm effects and possible mitigation strategies or beneficial use of physical implications from offshore wind farms.
