Submesoscale dispersion of surface drifters in a coastal sea near offshore wind farms

Abstract

Abstract. We analyse relative dispersion of surface drifters released as pairs (6 instances) or triplets (2 instances) during three field experiments in the German Bight in close proximity to wind farms. Drifter pairs can be classified in a remarkably clear way into those with spatial separation growing either exponentially or non-monotonously. There is some tentative evidence that exponential relative dispersion growth rates preferably occur for drifter pairs that are most exposed to the possible influence of a wind farm. Kinetic energy spectra and velocity structure functions suggest that turbulent energy could be injected by tides, possibly also via an interaction between tidal currents and wind turbine towers. Applicability of inertial range turbulence theory, however, can be doubted given distinct peaks of overtides observed in velocity power spectra. More comprehensive studies would be needed to better separate submesoscale effects of wind farms, tides and possibly baroclinic instabilities on observed drifter behaviour in a complex coastal environment.

Relative Dispersion of Surface Drifters in the North Sea: The Effect of Tides on Mesoscale Diffusivity

“…The velocity amplitudes for the southern region are significantly higher with values of 0.35 and 0.19 m s −1 for the zonal and meridional components, respectively. These results are consistent with previous studies in the region (Callies et al, 2019; Meyerjürgens et al, 2019; Port et al, 2011). The inertial period for the study area is computed from the average latitude for all drifters (T inertial = 14.57 hr) and does not show a peak in the FFT analysis.…”

“…The e‐folding times observed in this study are significantly lower compared to other ocean basins, which means that the drifters separate much faster compared to other studies in the submesoscales (Beron‐Vera & LaCasce, 2016; Essink et al, 2019; van Sebille et al, 2015). These results agree with a recent study in the German Bight, which mainly looked for submesoscale dispersion effects in the vicinity of offshore wind farms (Callies et al, 2019). Due to the limited length of the trajectory time series and the limited number of drifter pairs, the study only speculated that nonlocal dispersion occurs in the submesoscales in the German Bight.…”

“…The amount of numerical studies of dispersion which have taken into account tides is limited. The studies which have incorporated baroclinic (Callies et al, 2019; Lynge et al, 2010; Suanda et al, 2018) and barotropic (Orre et al, 2006) tides analyzed dispersion processes in the submesoscales for several hours or days. Suanda et al (2018), for instance, used numerical drifters to show that the inclusion of baroclinic tides in the model induces a 3–4 times larger horizontal diffusivity than barotropic tides in the submesoscales.…”