From: Frontiers in Marine Science 27 July 2022 Public documentChasing the offshore wind farm wind-wake-induced upwelling/downwelling dipole. Authors:Jens Floeter*Jens Floeter1*Thomas PohlmannThomas Pohlmann2Andr HarmerAndré Harmer1Christian MllmannChristian Möllmann
A. 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 windwake 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 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.
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B. From:Introduction
Offshore wind farms (OWFs) convert kinetic wind energy into electricity, creating regions of reduced wind speed and high atmospheric turbulence intensity downstream of wind turbine arrays.
Christiansen and Hasager (2005); Christiansen and Hasager (2006) were the first to describe these wind-wakes by synthetic aperture radar (SAR)-derived wind speed images and well-known wind farm wake photographs (Hasager et al., 2013).
Numerical analyses by Broström (2008) triggered a series of modeling studies (Paskyabi and Fer, 2012; Paskyabi, 2015; Ludewig, 2015), which all predicted that a wind speed of 5–10 m s-1 generates so-called upwelling/downwelling dipoles in a stratified ocean with vertical velocities exceeding 1 m day-1.
The generated oceanic response is predicted to extend several kilometers around the OWFs and to be strong enough to influence the local pelagic ecosystem, especially the surface mixed layer (SML).
These studies formulated prerequisite conditions for the generation of an OWF wind-wake-induced upwelling/downwelling dipole: the characteristic width of the wind-wake has to be at least the internal radius of deformation (Broström, 2008), which is fulfilled for OWFs in the German Bight of the North Sea, as both are ~10 km (Chelton et al., 1998; Platis et al., 2018).
An almost constant wind direction for at least ~8–10 h with moderate speeds (5–10 m s-1) is the second condition which needs to be met (Ludewig, 2015).
Other theoretically derived factors likely to influence the vertical velocities in OWF wind-wake-induced upwelling/downwelling dipoles are the size of the wind farm (Broström, 2008), surface waves and tidal advection (Paskyabi and Fer, 2012), and atmospheric stability (Platis et al., 2018).
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In June 2016, we deployed a high-speed remotely operated towed vehicle (ROTV) to investigate the offshore wind farm wind-wake-induced upwelling/downwelling dipole.
C. Discussion
Our working hypotheses of observing three different phases of an upwelling/downwelling dipole life cycle, i.e., early development, established and eroding phases, were generally confirmed. The coexistence of a tidal mixing front on June 29–30 in our investigation area prevented the observation of an undisturbed OWF wind-wake-induced development and decay of dipoles.
* Wind-wakes
In situ wind and turbulence measurements of far-field OWF wakes (Platis et al., 2018) confirmed previous model- (Dörenkämper et al., 2015) and SAR-derived results (Li and Lehner, 2013; Djath et al., 2018; Djath and Schulz-Stellenfleth, 2019) that higher atmospheric stability, i.e., the absence of thermally produced turbulence, increases wake dimensions. However, we do not know the atmospheric stability during our survey.
SAR images quantifying wind-wake dimensions in a specific situation are rare, as the repeat cycle of the satellite is about 11–12 days (Platis et al., 2018), but promising (Elyouncha et al., 2021).
Airborne observational data showed that in the German Bight stable atmospheric situations are most probable for southwesterly wind directions, as in our survey period, from which Platis et al. (2018) inferred that this is the most likely direction producing long wakes.
However, even in unstable and neutral atmospheric situations, Platis et al. (2018); Platis et al. (2020); Platis et al. (2021) frequently observed wind-wakes with lengths between 5 and at least 35 km in our study area.
Therefore, we deduced that the southwesterly wind with speeds >4 m s-1 which prevailed during our study generated wind-wakes at the leeward regions of BARD and GTI, with lengths between 5 and ~35 km.
Observations of absolute (Figure 10) and normalized (Supplementary Figure 9) wind speeds recorded at FS Heincke revealed for June 27 a clear wind deficit leeward of BARD only for T1 and T2—the transects with identified upwelling/downwelling dipoles. The wind speed deficit was ~3 m s-1 or 30%, which is well in the range reported by Platis et al. (2018) for a similar-sized neighboring OWF.
On June 29, wind-wakes with a deficit ~25% were detected at FS Heincke vessel height on transects T1, T2, T3, and T4 (Supplementary Figures 10, 11), supporting our analysis of the developing dipole.
Hence, the June 27 onboard wind measurements suggest that the wind-wake had a length of ~14 km, and >20 km on June 29. On June 30, the wind-wake deficits were relatively larger (~46%) on T1 and T2 but on a lower absolute level (Supplementary Figures 12, 13)...
* Water Currents
During the survey, maximum ambient currents in a depth of 15 m were in the order of 0.6 m s-1, which is about one (Ludewig, 2015) to two (Christiansen et al., 2022) magnitudes higher than the mean wind-wake-induced changes in the horizontal surface water velocity field.
Therefore, the spatial orientation of the tidal ellipse in relation to the direction of the wind-wake can be expected to enhance or weaken the development of an upwelling/downwelling dipole. Tidal excursions in this region have a magnitude of ~6–9 km in an east–west direction (Floeter et al., 2017), which was at an ~45° angle to the average wind-wake-induced Ekman transport during our study.
Subsequently, the tidal phase can be expected to have an effect on the vertical velocities and the spatial locations of the dipoles. The BSHcmod-derived wind- and tide-driven ambient currents were similar during the comparative measurements of T1 on June 27 and 29, exhibiting a westward water movement with velocities around 0.2–0.3 m s-1 (Figure 11).
However, the tidal phase was different as on June 27 the 280° westward currents persisted for 12 h before we surveyed T1, whereas the ambient current was eastward (100°) during the 12-h period prior to the TRIAXUS measurements of T1 on June 29.
A hydrodynamic modeling analysis would be needed to assess how the different tidal histories contributed to the observed dipole shapes and dynamics.
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* Wind-wake-induced changes in potential energy anomaly of water column
Earlier model studies have demonstrated that OWF-induced disturbances to the wind field modulate tidal mixing front-related upwelling processes (Paskyabi, 2015) and change the upper ocean stratification pattern (Ludewig, 2015).
To differentiate natural and anthropogenic effects, we calculated the potential energy anomaly of the 5–20-m envelope of the water column following the approach of Simpson (1981) by considering changes in the potential energy relative to the mixed condition [Eq. 2 in Simpson (1981)].
The calculation was based on the potential density anomalies [kg m-3] of the transects, gridded with ODV-DIVA (Schlitzer, 2021; Troupin et al., 2012) applying a horizontal and vertical resolution of 250 m and 0.1 m.
All transects surveyed on June 27 and 29 revealed an overall north–south decrease in the potential energy anomaly, following the stratification trend from offshore to the coast (Figure 12).
The trajectories of the transects with established OWF-induced upwelling/downwelling dipoles (June 27 T1, T2, and June 29 T3) were distinct from all others by showing abrupt changes in potential energy anomaly of ~4 kJ m-3 over a short distance of ~2–4 km (Figure 12).
* Characteristic signatures of the observed upwelling/downwelling dipoles
We identified two characteristic hydrographic signatures of OWF-induced dipoles:
a. Distinct changes in mixed layer depth and potential energy anomaly of the 5–20-m water column envelope over a distance <5 km
b. Diagonal excursion of the thermocline of ~10–14 m over a dipole dimension of ~10–12 km
Whether these anthropogenically induced changes in potential energy anomaly and mixed layer depth are significantly different from the corridor of natural variability awaits further investigations. The same applies to the representativity of the observed signatures
* Potential ecological consequences
In a modeling study, Christiansen et al. (2022) identified reduced vertical mixing of the upper water column due to the wind speed deficit in the OWF wake as the predominant process impacting on the pelagic environment of the German Bight.
When the wind direction changes, the enhancement of stratification and shallowing of the SML is affecting varying areas.
By analyzing monthly mean hydrodynamic results, Christiansen et al. (2022) concluded that OWF wind-wake-induced convergence and divergence of water masses lead to the formation of large-scale sea surface elevation dipoles, which generate structural changes in the stratification strength in the German Bight.
However, because of almost constantly changing wind directions, the magnitude of the monthly averages is so low that it can hardly be distinguished from the interannual variability (Christiansen et al., 2022).
A shallowing of the nutrient-depleted summer SML (Topcu et al., 2011) brings the lower regions of the thermocline, and with it high concentrations of nutrients and phytoplankton cells (Richardson et al., 2000; Zhao et al., 2019) upward into more illuminated water depth levels.
As some light for net primary production is available below the thermocline (Floeter et al., 2017), it can be expected that these phytoplankton organisms are viable and immediately increase their production.
Thus, when in a specific situation the wind direction is stable over at least ~10 h, like the ones we encountered on 2016 June 27 and 29, the shallowing of the mixed layer depth by distinct OWF wind-wakes has the potential to generate significant anthropogenic pulses of enhanced primary production at the lower spatial mesoscale (i.e., ~10–35 km).
While Christiansen et al. (2022) confirmed the correlations between the sea-level dipole anomalies and changes in the vertical density and temperature distributions derived by Ludewig (2015), associated changes in the mean vertical velocity field, i.e. upwelling/downwelling dipoles were not detectable.
The cause can be found in the different nature of the two main effects of OWF wind-wakes on the water column:
1) Wherever a wind-wake leads to reduced vertical mixing, the subsequent enhancement of the stratification is not reversed when the wake direction changes because the wind deficit prevails within the entire wake. Hence, the effects of this first process remain detectable in monthly average flow fields.
2) When a wind-wake directionally persists for some time, it generates an upwelling/downwelling dipole. When the wind direction changes, negative and positive vertical water velocities wipe out each other as the downwelling cell is shifted over the upwelling cell or vice versa. Hence, the effects of this second process are vanishing in monthly average flow fields.
The excursions of the thermocline due to this second wind-wake effect of upwelling/downwelling dipoles are substantially larger (~10–14 m, Figures 4, 7) than the shallowing of the mixed layer depth caused by reduced vertical mixing.
Therefore, it can be expected that they generate more intense but ephemeral pulses of primary production with spatial dimensions at the lower meso-/upper submesoscale.
Whether the magnitude of this anthropogenic primary production enhancement is similar to that of tidal mixing fronts awaits further investigations.
At the current developmental stage of OWFs in the German Bight, their cumulative wind-wake-induced upwelling area is smaller than the tidal front region, but the potential of submesoscale features as drivers of biophysical coupling in the German Bight was already highlighted by North et al. (2016) and their location in stratified regions may make a difference.
The further fate of the manmade additional primary production, which fraction is cascading up the trophic chain (Lévy et al., 2018; Wang et al., 2018; Slavik et al., 2019; Twigg et al., 2020; Kaiser et al., 2021), how much will contribute to oxygen minimum zones (Topcu and Brockmann, 2015; Große et al., 2016; Queste et al., 2016), or the impact on fisheries resources (Methratta and Dardick, 2019; Methratta, 2020; van Berkel et al., 2020); add another level of complexity.
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