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(b)
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observed by a moored ADCP for a packet of waves travelling on to the Hebridean shelf. The
dashed grey line indicates the vertical displacement of a particle initially at a depth of 25m.
Note the opposing horizontal flows in the top and bottom layers and the large up and down
vertical velocities as each trough passes the mooring. Image courtesy of Mark Inall,
Scottish Association for Marine Science.
is evidence for such 'action at a distance' for waves travelling away from the shelf
(Holligan et al., 1985), recent observations indicate that most of the energy of the
internal tide propagating on to the shelf is dissipated within 2-3 wavelengths of the
semi-diurnal internal tide which is a distance of
50-75 km for the Celtic Sea shelf
(Green et al.,
2008
). In part, this rapid energy loss from the internal waves results
from their interaction with the local barotropic tide on the shelf which effectively
increases the bottom frictional drag and augments dissipation.
The fact that most of the internal tidal energy is dissipated close to the source
implies that there should be intensified internal mixing near the shelf edge. At the shelf
edge of the Celtic Sea, this enhanced mixing is apparent as a band of cooler water
along the shelf edge. The satellite image of sea surface temperature in
Fig. 10.16a
illustrates this, while an early set of observations from a towed, undulating CTD
shown in
Fig. 10.16b
indicates the spreading of the pycnocline isopycnals over the
shelf edge as a result of the internal mixing. It is worth noting here the contrasting
effects of boundary-driven (e.g. from bed friction of surface wind stress) and internal
wave mixing. Boundary stresses produce mixed boundary layers and tend to sharpen
the pycnocline between the surface and bottom layers. Internal mixing operates directly
on the pycnocline that support the internal waves, and so acts to smear out the
pycnocline.
∼
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