At a Glance:

Many recent studies have examined if offshore wind turbines have significant local environmental effects on the ocean stratification, that is the fact that deeper water tends to be colder, saltier, and more nutrient rich than surface waters. The effects tend to be larger in the atmosphere than in the ocean, because 1) there are compensating effects in the ocean leading to both more and less mixing, and 2) because turbines are designed to interrupt the winds while their support platforms are meant to avoid being dragged away by ocean currents or mixing. These effects are complicated to predict, and so the best studies are those observing the changes. The impacts of Europe’s over 5,000 turbines have been well-studied, but some specificities of the eastern U.S. site will need to be monitored over time.

A Deeper Dive:

There are many processes setting the sea surface temperature, which is one of the most difficult aspects of climate modeling.  The sun warms the ocean and the ocean emits infrared light because it is warm.  That much is not too complicated, but the light can be interrupted by clouds and the infrared light is absorbed by greenhouse gasses in the atmosphere and might be reemitted back downward toward the ocean!  The air-sea temperature difference is important, as whichever is warmer will tend to heat the other by direct contact.  Most importantly, if the air is dry and windy, it will tend to evaporate the ocean which leads to significant cooling of the liquid water left behind (because energy is concentrated into the evaporated water).  In short, it’s very difficult to predict what will happen and it will depend on season, time of day, and weather conditions.  Adding turbines into this complicated mix will have some effects, but it’s not at all clear if it will be warming or cooling overall because some changes will go one way and others will go the other.

In other locations, satellites have been used to quantify the effects of wind farms on temperature or water quality using a before and after approach.  In most measured variables aside from wind speed (which windmills are designed to affect!) the changes tend to be small in comparison to background variability.

Understanding Complexities:

Siedersleben and company (2018: https://dx.doi.org/10.1088/1748-9326/aaea0b) observe and simulate offshore wind farms to examine the effects on the atmosphere in the wake of the farm.  They find a reduction in winds, but also sometimes warmer temperatures and drier air which they attribute to alterations in the mixing of the lower atmosphere.  These effects were not always observed, because they depend on the conditions, at other times no warming or drying was observed. If these warmer temperatures reach the ocean surface, it could change the ocean temperatures somewhat.

By design wind farms will reduce the winds in their wake.  As winds at the ocean surface level tend to mix the oceans, this means that ocean mixing by the winds will be reduced.  Less wind tends to bring up less cold water and less nutrients, which would tend to make the surface warmer and less biologically productive.

However, New England’s coast is amazingly biologically productive when compared to other North Atlantic regions.  This is because of a combination of mixing effects–not just winds but also tides. The base of wind turbines will tend to add to mixing by tides, bringing up colder, more nutrient rich water to the surface.  This would tend to counteract the effects of reduced wind.

On the whole, which will win, less mixing from less wind or more mixing from tides?  As argued in other articles on this site, the expected magnitude of changing winds is small: 2 to 10%.  Similarly, the induced mixing by the platform interrupting the tidal flows is also small, as these platforms are widely spaced, only one per square mile, and designed not to catch the currents or tides too strongly because that would damage the platform.  No studies have fully captured all of the complexity of these interactions in the face of much larger variability from season to season and region to region.  Similarly, while some studies have suggested that platform-increased mixing drives productivity and the platform itself can act as a reef for bottom-dwelling organisms, these effects are regionally specific and are hard to measure as they are much smaller in magnitude than the natural year-to-year and season-to-season variability of fish, bottom dwellers, and productivity. 

Observations of increased mixing and increased biological productivity at offshore wind farms: https://doi.org/10.1016/j.pocean.2017.07.003.

A review of shallow versus deep offshore wind and the implications for ocean mixing: https://doi.org/10.3389/fmars.2022.830927

Tides and mixing of nutrients on George’s Bank: https://doi.org/10.1016/j.jmarsys.2008.04.007