Follow the energy: Why using more offshore wind power weakens ocean dynamics and impacts marine ecosystems

Wind turbines harvest the kinetic energy of the winds in the lower atmosphere to generate electricity, thereby reducing the wind speed locally.  Over oceans, this can have direct consequences for the wind energy input into the oceanic mixed layer, as well as associated wave dynamics, ocean mixing, and the wind-driven circulation.  To estimate the potential relevance of these effects, we consider a scenario of 150 GW of installed capacity, as formulated by the Esbjerg declaration, which we distribute over a hypothetical area of 100 000 km² in the North sea.  To do so, we budget kinetic energy fluxes within the lower atmosphere to estimate the impacts of wind energy use on frictional dissipation near the surface, and then use relationships inferred from the ERA-5 reanalysis to link frictional dissipation to different aspects of ocean dynamics.  Our first-order estimates show that the mean wind speed is reduced by 12% in this scenario.  Using the mean wind speed as an example case, such a reduction is associated with a more substantial reduction of surface friction by about 33%.  In this case, the ocean impacts are reflected in reduced wave heights by 14%, reductions in Stokes drift velocities at the ocean surface by 18%, wave power by 27%, and ocean mixing by 35%.  These basic energetic arguments highlight the need to assess and quantify the extent of impacts large-scale offshore wind power use is likely to have on ocean dynamics that need to be considered with the anticipated expansion in the coming decades.  This is necessary in order to mitigate potentially substantial and detrimental impacts on marine ecosystems.