Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 77

Here (in src53), the effect on the surface elevation develops very slowly, compared
to the normal run, and so the gradient is weaker from the beginning on, which again
weakens all changes in the ocean. Without horizontal diffusion, the rise in surface
elevation due to the velocity wake is more locally limited and not spread over the
whole area and is controlled by the horizontal advection.
Summarizing, horizontal exchanges balance the OWF effect and the vertical
structure of the ocean system. Horizontal exchanges control gradients in the
horizontal, which affects vertical changes. Especially, horizontal diffusion
(TS diffusion and Smagorinsky diffusion) influences the final magnitude of OWF
effect, but dominantly, the vertical processes trigger the OWF effect on the ocean
system.

5.2.3 Assessment and Integration of Effect Analysis

Previous documented analyses deal with the OWF effect on the ocean system under
barotropic and baroclinic conditions and in the case of various exchange process
combinations. The manner of the vertical motion is related to changes in the
barotropic pressure due to the change in the surface elevation released by the
flow reduction due to the wind wake. The treatment of the exchange analysis results
in the statement that especially vertical advection with vertical motion triggers the
change in the hydrographic conditions. Partly betoken during prior explanations,
this section illustrates the main physical principle behind the changes that occurred
on the ocean system.
Starting once more from an initial situation, our ocean system is forced by a
constant wind field, which is affected by a wind farm. The wind turbines of the wind
farm detract the atmosphere s energy by transforming wind energy into a mechan-

ical one. That energy detraction means a reduction of wind speed downstream of
wind farm. So a wind wake is formed by OWFs. That wind field acts with the ocean
surface and creates a surface stress.
Under undisturbed conditions, the constant wind field causes an Ekman trans-
port. The Ekman transport is known as the net motion of water as a result of the
balance between the Coriolis and turbulent drag forces. The net sum of the water
column is theoretically ~90 directed to the movement of the wind (in the northern
hemisphere), at least partly for real conditions.
Under operating OWF conditions, the wind wake causes a wake in ocean flow,
and the locally reduced surface stress results in a reduced Ekman transport. This
again causes a convergence of water masses within the wake and to the left of the
wake and a divergence at the right side of the wake (looking into wind direction).
The convergence/divergence of the water masses is associated with an increase/
reduction of the sea level, which in turn induces downwelling/upwelling.
A commonly known connection between up- and downwelling and Ekman
transport is in relation to coasts. An up- and downwelling also occur, besides
coats, in the open ocean where winds cause the surface water to diverge from a

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