Chasing the offshore wind farm wind-wake-induced upwelling/downwelling dipole

Abstract

The operational principle of offshore wind farms (OWF) is to extract kinetic energy from the atmosphere and convert it into electricity. Consequently, a region of reduced wind speed in the shadow zone of an OWF, the so-called wind-wake, is generated. As there is a horizontal wind speed deficit between the wind-wake and the undisturbed neighboring regions, the locally reduced surface stress results in an adjusted Ekman transport. Subsequently, the creation of a dipole pattern in sea surface elevation induces corresponding anomalies in the vertical water velocities. The dynamics of these OWF wind-wake induced upwelling/downwelling dipoles have been analyzed in earlier model studies, and strong impacts on stratified pelagic ecosystems have been predicted. Here we provide for the first time empirical evidence of the existence of such upwelling/downwelling dipoles. The data were obtained by towing a remotely operated vehicle (TRIAXUS ROTV) through leeward regions of operational OWFs in the summer stratified North Sea. The undulating TRIAXUS transects provided high-resolution CTD data which enabled the characterization of three different phases of the ephemeral life cycle of a wind-wake-induced upwelling/downwelling dipole: development, operation, and erosion. We identified two characteristic hydrographic signatures of OWF-induced dipoles: distinct changes in mixed layer depth and potential energy anomaly over a distance < 5 km and a diagonal excursion of the thermocline of ~10–14 m over a dipole dimension of ~10–12 km. Whether these anthropogenically induced abrupt changes are significantly different from the corridor of natural variability awaits further investigations.

Projected cross-shore changes in upwelling induced by offshore wind farm development along the California coast

“…Further, no measurable change was observed within the highly productive 10 km coastal upwelling zone at either area of interest. However, around 35°N, the changes exhibit a dipole-like pattern 8,13 , where a decrease in upwelling is seen on the inshore side of the wind farm and an increase in upwelling on the offshore side of the wind farm, in response to the polarity of the wind-stress curl. While the total upwelling strength, when calculated over a 100 km zone, shows little reduction, the pronounced change in the cross-shore structure of upwelling is in excess of natural variability.…”

“…Wind farm wakes are able to increase the magnitude of pycnocline displacements 11,12 , and modulations in the pycnocline displacement can change the spatial-temporal patterns in coastal upwelling. Most recently 13 , the presence of an upwelling/downwelling dipole was measured in the German Bight and characterized in terms of changes in mixed layer depth and potential energy. These studies focused on European wind farms, which are often in shallower water than is likely to be the case off the U.S. west coast and are not specifically located in a region such as the California Current System where contributions to wind-driven upwelling include both the wind stress curl and coastal components, both of which can potentially be affected by wind stress reductions.…”

Cross-shore changes in upwelling from offshore wind farm development in California

“…Further, no measurable change was observed within the highly productive 10 km coastal upwelling zone at either area of interest. However, around 35 • N, the changes exhibit a dipole-like pattern [9,12], where a decrease in upwelling is seen on the inshore side of the wind farm and an increase in upwelling on the offshore side of the wind farm, in response to the polarity of the wind-stress curl. While the total upwelling strength, when calculated over a 100 km zone, shows little reduction, the pronounced change in the cross-shore structure of upwelling is in excess of natural variability.…”

“…Wind farm wakes are able to increase the magnitude of pycnocline displacements [10,11], and modulations in the pycnocline displacement can change the spatial-temporal patterns in coastal upwelling. Most recently [12], the presence of an upwelling/downwelling dipole was measured in the German Bight, and characterized in terms of changes in mixed layer depth and potential energy. These studies focused on European wind farms, which are often in shallower water than is likely to be the case off the U.S. west coast, and are not specifically located in a region such as the California Current System where contributions to wind-driven upwelling include both the wind stress curl and coastal components, both of which can potentially be affected by wind stress reductions.…”

Insights from the management of offshore energy resources: Toward an ecosystem-services based management approach for deep-ocean industries

“…The environmental risks of offshore energy production have been assessed mainly for shallow water but remain uncertain in the deep sea. Increased turbulence around the submerged structures and shifts in Ekman transport causing local upwelling/downwelling may increase upper-ocean mixing (Broström, 2008;Floeter et al, 2022). Variability in vertical mixing at the mesoscale has been shown to be a significant, but generally overlooked factor controlling the distribution of heat, nutrients, and carbon in the ocean (Busecke and Abernathey, 2019).…”