Environmental controls on the detectability of hydrodynamic changes induced by offshore wind farm structures

A combination of model simulations and field observations collected in 2023 and 2024 near Seagreen, Scotland’s largest Offshore Wind Farm (OWF), is used to assess when, where and by how much (physical) changes induced by OWFs structures become detectable and environmentally relevant. Seagreen has the deepest (> 55 m) fixed bottom foundations of any OWF in the world (in 2026), and in offshore deep (> 50 m) waters there is the potential for OWFs structures to perturb natural ocean mixing and modify seasonal stratification. We found that OWF impacts on local hydrodynamics are highly sensitive to environmental conditions, with the strongest effects occurring under weakly stratified conditions, in frontal/transitional zones, when the water column is most responsive to small increases in mixing. Summer average sea surface temperature decreases are generally between -0.1 and -0.3C within the farm and in its wake, but with cooling exceeding -0.5C during the 2023 marine heatwave. Due to high seasonal and monthly variability, OWF impact assessments based on limited observational periods may underestimate or overestimate the range of hydrodynamic changes. This work highlights the value of combining numerical models with observations: model simulations identified the periods and locations of detectable OWF-induced changes, while observations supported the modelled response.