Summary, Conclusion, and Outlook - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 143

simulations can develop such statistics, but they can be applied to ocean simulation
by manipulating forcing fields due to OWF effect statistics.
The consequences in atmosphere due to operating OWFs significantly affect the
ocean system. The wind wake causes a wake in the ocean velocity field within and
behind the wind farm, which is connected with a reduced Ekman transport causing
divergence and convergence of the water masses. That ends in a change of the
surface elevation and the barotropic pressure field in the form of a dipole structure
having an increase of surface elevation north to the wind wake and a decrease south
to the wind wake. These effects on the ocean surface again cause vertical motion in
order of several meters per day. Vertical motions mean cells around the OWF of
upwelling and downwelling advecting the temperature field, which results in an
excursion of the thermocline around the OWF in the vertical of possible 10 m,
depending on the ocean

s stratification. Respectively, the salinity and density fields

'

are affected.
Analyses of external impacts triggering the OWF effect on the ocean systems
lead to the result that besides the ocean depth, primarily, the wind wake defines the
intensity and dimension of the OWF induced effect on the ocean system:

• Shallower waters intensify the up- and downwelling cells in the vertical, and
hence stronger hydrographic changes are detected at

the depth of the

thermocline.
• A more intense wind wake leads to greater magnitudes of the up- and
downwelling cells and stronger changes in the hydrography. Additionally, the
vertical exclusion of the thermocline increases with the intensity of the wind
wake, as well as the depth of the thermocline towards lower ocean layers.
• A wider wind wake (orthogonal to the wind direction) leads to a greater
horizontal dimension of the velocity wake, which triggers the horizontal dimen-
sion of vertical cells and of the thermocline exclusion. The effect is positively
linked, so a greater wake results in a greater horizontal OWF effect in the ocean.
The wind wake itself, and so the wake in the ocean velocity field, is defined
dominantly by wind speed; by the number of wind turbines of an OWF, respectively
the number of wind turbines within one model grid cell; and by the OWF ' s size,
respectively the number of grid cells comprising the OWF district. Here, the wind
wake follows the relation that the stronger the wind speed within the range of
OWF

s operation mode, the stronger the wind wake; the higher is the amount of
wind turbines within one grid cell, the more intense is the wind wake; and the larger
the OWF district with operating wind turbines is, the lager is the affected area of
extreme wind reduction.
Additionally, the choice of the ocean forcing influences the OWF effect. The use
of a full meteorological forcing for ocean simulations shows that the influence of
temperature and humidity forcing is the OWF effect on the SST field positively
overlaid. But the influence of offshore wind farms is dominantly driven by the 10-m
wind forcing field.
The OWF impact on the ocean system occurs within minutes after switching to
the operational OWF mode. Simulations show that the duration of the OWF effect

'