The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 323

isotherms is clear in all cases. This is the main internal tidal wave propagating into
the shallower coastal or shelf water. Superimposed on this main wave are much
shorter period oscillations of the isotherms, typically with a period of 10-30 minutes.
Particularly energetic packets of these waves are evident in Fig. 10.12d . These shorter
internal waves are generated as the main internal tidal wave begins to be influenced
by the shoaling seabed. As these waves propagate, they generate an oscillatory
baroclinic flow field which at the sea surface leads to convergences and divergences.
To help visualise the motion involved, see Section 4.2 and particularly Fig. 4.8 . This
surface flow pattern can, under the right wind conditions, cause alternating patterns
of rough and calm water which can be detected by radar. Images taken by Synthetic
Aperture Radar (SAR) can allow the tracking of internal waves over large distances
(Fu and Holt, 1984 ), and can provide information on the source of the often complex
patterns of internal waves seen over the shelf edge. Figure 10.13a shows an example
off the northeast shelf of New Zealand (Sharples et al., 2001a ), with two main
wavefronts crossing onto and across the shelf separated by about 15 km. Each
wavefront has a train of parallel 1-2 km wavelength internal waves behind it. Under
the right sea conditions these smaller internal waves can be seen using standard
marine x-band radar. The image in Fig. 10.13b shows a snapshot of the radar screen
aboard the RRS Charles Darwin in summer 2005 during the cruise when we collected
the data shown in Fig. 10.12d . Being aware of the passage of the internal waves past
the ship turned out to be very useful, as we were able to keep our turbulent dissipa-
tion measurements going and catch the high mixing rates associated with these
packets of waves.
We can see in Fig. 10.14 the development of a packet of these internal waves in
data from near the Hebridean shelf edge (Small et al., 1999 ). The internal tide there is
observed to propagate on to the shelf with a phase speed c 0.4 m s 1 and a

wavelength 18 km. As it does so, it evolves into a group of waves of shorter period

which follow an initial depression of the isotherms. This development, which is due to
non-linear interactions in the wave motion, is captured in a series of 'snapshot'
sections ( Fig. 10.14 a-d) obtained by towing a vertical string of thermistors through
the advancing wave field at a vessel speed of

∼

2.2 m s 1 which is large compared with ∼

c. Over a period of 6 hours, the initial depression of the isotherms is seen to acquire

a following group of waves with wavelength

∼

∼ 500 metres and a crest-to-trough

displacement of 30 metres.

The relatively energetic orbital velocities associated with these waves are shown in
Fig. 10.15 using data collected by an upward-looking moored ADCP on the Hebri-
dean shelf. The waves generate flows of up to 50 cm s 1 with vertical velocities
∼

∼

10 cm s 1 . Large amplitude waves of this kind exhibit rectified flows O[0.02] m s 1
and can thus contribute significantly to net transport across the slope. When the
barotropic motion was removed, the waves shown in Fig. 10.15 were seen to sustain
an off-shelf bottom layer transport of 5 m 2 s 1 for about 1.5 hours (Inall et al., 2001 ).
The internal tide has been described as only 'quasi-periodic' and most long-time
series show indications of intermittency in the internal tidal behaviour. It is
important, therefore, to remember that while internal tidal motions are clearly