Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 78

Fig. 5.18 Left : Schema of adjustment of Ekman transport resulting in up- and downwelling.
Ekman transport ( green arrows ) is detracted to wind direction by theoretical ~90 to the wind
( black arrows ). Due to velocity wake ( light blue area ), Ekman transport is reduced, which causes
convergence/divergence of water masses (change in surface elevation ζ is light gray ) and

downwelling Right : Schema of conditions of OWF-affected ocean system

along cross-section S-N through the OWF. Constraint for downwelling is a positive decline in
surface elevation

x and upwelling o.

' ' ' '

and convergence. The opposite is essential for upwelling. The pycnocline
shows the opposite change of surface elevation

ζ

region or to converge toward some region. The last one is on hand here. Figure 5.18
schematically illustrates the term of conditions. Here, the horizontal velocity and
Ekman transport are reduced within the area of the wake and increased surface
elevation. The area of the velocity wake can be treated as a barricade or and

now downwelling occurs where Ekman transport moves surface waters towards the
region of velocity wake (

coast,

' '

); the water must pile up and sink. On the other side
of the velocity wake, upwelling occurs where Ekman transport moves surface water
away from the wake area (coast); surface water is then replaced by water that wells
up from below. Upwelling and downwelling illustrate mass continuity in the ocean;
that is, the change in distribution of water in the ocean area is accompanied by a
compensating change in water distribution in another area. And those two areas are
the dipole formation of the surface elevation northerly and southerly of OWF. The
formation and dimension of surface elevation

coast

' '

s dipole is a result of the Coriolis
effect and the wind wake due to geostrophic conditions.
Finally, the vertical motion is a mandatory constraint of the wind-driven change
in pressure (barotropic effect), in the velocity field, and so in the Ekman transport
and can be defined as a wind-driven but coastal independent upwelling/
downwelling. The final dynamical situation of the model area around the OWF is
illustrated in Fig. 5.18 . A positive change in surface elevation means a lowered
pycnocline, respectively the opposite for negative surface elevation. The zone
where the upper part of the water column has a lower density is characterized by
an increased sea surface height and a deepened pycnocline. At surface convergence
is incurred that support downwelling, while divergence occurs at low surface

'