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… Over time, the extraction of energy by offshore wind farms results in extensive areas of reduced wind speed and subsequently the decrease of the shear-driven forcing at the sea surface boundary. As this reduces the momentum transfer from the atmosphere into the ocean, horizontal velocities and turbulent mixing initially decrease several tens of kilometers around offshore wind farms. Thereby the induced perturbations imply significant changes for the residual currents in the respective areas. Furthermore, convergence and divergence of water masses lead to the formation of sea surface elevation dipoles, which over time merge into large coherent structures. As shown here, these large-scale anomalies in the sea surface elevation are one of the main drivers of wake-related processes in the ocean. In addition to the general reduction of turbulent mixing, the large- scale sea level alterations trigger lateral and vertical changes in the temperature and salinity distribution and affect the hydrodynamics in areas covered by offshore wind farms. However, the magnitude of these changes is rather small compared to the long-term variability of temperature and salinity and can hardly be distinguished from the interannual variability. A severe overall impact by the wake effects on the ocean’s thermodynamic properties is thus not expected but rather large-scale structural change in the stratification strength and unanticipated mesoscale spatial variability in the mean current field. Nevertheless, further investigations are necessary to assess possible feedback on the air-sea exchange and thus potential impact on the regional atmospheric conditions, since surface heating along with the reduction in turbulent mixing influences the upward heat and momentum fluxes from the ocean into the atmosphere.
In this study, the structural changes in stratification become noticeable in a couple of ways. Firstly, we observed large dipole-related changes in the potential energy anomaly, as the geostrophic and baroclinic changes alter the temperature and salinity distribution. Secondly, the reduction of mixing at offshore wind farms results in the enhancement of the stratification strength, in particular, during the decline of the summer stratification. While the structural changes in stratification are minor in shallow mixed waters, the pronounced alterations in stratified waters can translate to the mixed layer depth, which likely increases or decreases depending on the respective stratification changes. This, in turn, might be crucial for marine ecosystem processes (Sverdrup, 1953). During the stratified summer months, the mixed layer depth is acting as barrier for nutrients and phytoplankton and plays a major role for the ecosystem dynamics. Therefore, induced fluctuations of the mixed layer depth can entail the intrusion of nutrients from the pycnocline into the surface mixed layer or the spreading of the nutrient-poor surface layer, respectively. The alterations in the nutrient availability, in turn, might affect local primary production and the nutrient balance. Thus, further studies are required to elucidate the impact on marine ecosystems and organisms in the North Sea, with regard to current and future wind farm scenarios. …
Nils Christiansen, Ute Daewel, and Bughsin Djath, Institute of Coastal Systems, Helmholtz-Zentrum Hereon, Geesthacht, Germany
Corinna Schrum, Center for Earth System Research and Sustainability, Institute of Oceanography, Universität Hamburg, Germany
Download original document: “Emergence of Large-Scale Hydrodynamic Structures Due to Atmospheric Offshore Wind Farm Wakes”
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