Enhanced mixing by floating wind farms in stratified shelf seas

The drive to achieve net zero carbon has motivated the development of offshore wind into deeper waters further from shore. The relatively weak tidal currents and deep water of future development sites means that infrastructure will, for the first time, be deployed at scale in seasonally stratified waters.   Current designs for floating turbines have sub-structures which penetrate this stratification.  Flow past such substructures generates turbulent wakes which can regionally enhance the very low levels of internal mixing observed in the seasonal thermocline. 

These low natural mixing rates drive nutrient fluxes which sustain phytoplankton growth at the subsurface chlorophyl maximum through the summer months and are responsible for 50% of the primary production in shelf seas.  Since this production supports the marine food web, changes to the physical drivers will fundamentally impact the marine food web.  Therefore, an anthropogenic source of turbulent mixing at the seasonal thermocline, has the potential to cause fundamental biogeochemical changes, impacting ecosystems, and fisheries in shelf seas. 

We present new measurements of strongly elevated turbulence within wakes at a shallow water wind farm.  Strongly enhanced mixing is observed in the wake, and across the wider wind farm area and is associated with reduced stratification.    These observations and our estimates for deeper water wakes suggest that mixing from these structures can be significant, and further research is essential to quantify the impact of this new source of anthropogenic mixing.