Spear hundreds of 300m high wind turbines into ocean environments and it’s only natural that that environment changes. Downwind air currents are cut and buffeted by the blades of turbines up wind. The phenomenon known as ‘wake turbulence’ reduces output from each turbine within a cluster of turbines, because it reduces and interferes with wind speed and direction within that cluster.
However, it’s the effect on surrounding ocean currents caused by interference with local wind speed and direction that now has weather watchers worried.
As any good meteorologist will tell you, the movement of ocean currents (which mixes cold and warm water, thereby altering sea surface temperatures) drives the climate onshore. Changes in sea surface temperatures determine rainfall patterns around the world. Think El Niño and La Nina – both related to shifts in ocean currents and changes in sea surface temperature.
Where those periodic events are entirely natural occurrences, there’s nothing natural about offshore industrial wind power operations, as Pierre Gosselin outlines below.
Offshore Wind Farms Altering Marine Ecosystems: “Sufficiently Potent To Redirect Existing Currents”
No Tricks Zone
Pierre Gosselin
20 August 2023
It’s ironic: Man is changing the environment and climate in order to prevent change.
A team led by Nils Christiansen recently published a research paper about the impacts offshore wind farms have on ocean dynamics, published in the journal Frontiers in Marine Science. Press release here.
“Wind speed deficits spread up to 70 km behind the wind farms”
The turbines extract kinetic energy from the wind field to generate electricity. Downwind of the wind turbines, the so-called atmospheric wakes develop, and are characterized by reduced windspeed, specific pressure conditions and enhanced air turbulence. During stable atmospheric circumstances, the wind speed deficits spread up to 70 km behind the wind farms.
Using high-resolution hydrodynamic computer simulations, the team analyzed the effects on the southern North Sea for the summer of 2013 (May to September). The analysis shows a link between atmospheric wakes and transformation in the momentum-driven exchange between the atmosphere and water. As a consequence, the horizontal currents and the stratification of the water could be affected.
Redirecting existing currents
The wake effects are sufficiently potent to redirect the existing currents, and thus results in shifting mean temperatures and a changed salinity distribution in the wind farm areas.
“While the occurring changes remain within the range of interannual variability, they illustrate similar magnitudes as the presumed mean changes due to climate change or year-to-year variability,” says Nils Christiansen, from the Hereon Institute for Coastal Systems, who was lead author on the study.
Reduced water surface turbulence
Another wake effect is the reduction of shear-driven processes at the sea surface. In other words, the turbulent mixing of the water surface caused by shear wind is reduced dozens of kilometres around the wind farm. Water is usually stratified, thus a layer of warmer water may lie on top of a layer with cold water. Wind farms disturb this natural stratification. Due to reduced mixing, a stabilized stratification of the water is fostered. This phenomenon was particularly noticeable during the decline in summer stratification.
The natural stratification of the water is especially prominent in summer and decreases towards autumn. In the area of the wind farms, however, a stabilized stratification outside the seasonal variation was measured.
“The magnitude of the induced mean changes does not indicate severe local effects, however far-reaching structural changes in the system occur“, says Christiansen.
“Far-reaching structural changes in the system”
“The transformation concerning currents and mixing are likely to affect plankton production and the food web structure. As a result it may influence the function of protected areas. Hence it is important to consider these consequences while developing marine protection concepts“, says Hereon Institute Director Prof. Corinna Schrum.
Moreover, possible feedbacks on air-sea exchange potentially affects regional atmospheric conditions and ecosystem dynamics.
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