14 Internal Solitary Waves System in the Mozambique Channel
275
(see Fig. 14.6). It is this dynamical feature (reversed with respect to mode 1 ISWs)
that produces the characteristic dark bands ahead of the bright bands in their direction
of travel.
Research studies on mode 2 ISWs generated by IW rays hitting a pycnocline
have been recently presented using numerical models and laboratory experiments
(Grisouard et al. 2011 and Mercier et al. 2012, respectively). These studies were
focused on the local generation mechanism (see Sect. 14.2), and have also addressed
the generation of higher mode waves by IW rays. Grisouard et al. (2011) used a (nonlinear and non-hydrostatic) numerical model and concluded that higher mode ISWs
(particularly mode 2) may also be generated by an IW beam hitting a pycnocline at
an angle. In their studies, high mode waves are favoured by a strongly stratified pycnocline or, equivalently, by short-wavelength IT beams. Laboratory measurements of
ISWs generated by an internal wave beam were presented by Mercier et al. (2012),
and are in agreement with the results of Grisouard et al. (2011). Following these
numerical and laboratory studies, we will now show satellite observations of mode
2 ISWs, which are consistent with local generation by an IW beam impinging on the
ocean pycnocline from below.
14.3.2 SAR Observations of Mode 2 ISWs
Figure 14.7 shows a typical example of a SAR image where two wave trains can be
seen to propagate towards the shore (approximately to the west-northwest, and in a
near perpendicular direction to the local isobaths) in the southern end of MC (near
parallel 24
◦ S). The left panel shows an ERS-2 SAR image dated 24 September 2001
(acquired at 7:39 UTC), which is centred approximately at 23.75
◦ S and 35.75
◦ E.
Note that land and coast lines are also marked in gray areas outlined by a black
contour, respectively, and that bathymetry levels for 100, 200, 500 and 1,000 m are
also shown. The black filled circle on the image indicates the predicted position
where an IT beam would impinge on the thermocline from below (assumed at an
average depth of 50 m), and the dashed line indicates its trajectory coming from
deeper bathymetry to the east (passing through the centre and taken perpendicular
to the wave crests). The right hand side of Fig. 14.7 shows a vertical profile, which
is aligned and partially coincident with the black dashed line on the left, together
with a simulated IT beam of semi-diurnal frequency based on local stratification.
The arrow indicates the direction of energy propagation, and the inset shows the
geographic location of the observation (the star in the inset denotes the geographic
location of the image centre).
The SAR image shown in Fig. 14.7 reveals that the ISWs in the deeper wave
packet are characterized by the usual rank-ordered bright and dark bands of radar
intensity, which decrease in amplitude from the front to the rear. However, while
these signatures look like many other typical SAR signatures of ISWs, it can be seen
that in this case the contrast pattern is reversed in relation to the general pattern (of
mode 1 ISWs). Here, dark bands precede bright bands in the propagation direction,
275
(see Fig. 14.6). It is this dynamical feature (reversed with respect to mode 1 ISWs)
that produces the characteristic dark bands ahead of the bright bands in their direction
of travel.
Research studies on mode 2 ISWs generated by IW rays hitting a pycnocline
have been recently presented using numerical models and laboratory experiments
(Grisouard et al. 2011 and Mercier et al. 2012, respectively). These studies were
focused on the local generation mechanism (see Sect. 14.2), and have also addressed
the generation of higher mode waves by IW rays. Grisouard et al. (2011) used a (nonlinear and non-hydrostatic) numerical model and concluded that higher mode ISWs
(particularly mode 2) may also be generated by an IW beam hitting a pycnocline at
an angle. In their studies, high mode waves are favoured by a strongly stratified pycnocline or, equivalently, by short-wavelength IT beams. Laboratory measurements of
ISWs generated by an internal wave beam were presented by Mercier et al. (2012),
and are in agreement with the results of Grisouard et al. (2011). Following these
numerical and laboratory studies, we will now show satellite observations of mode
2 ISWs, which are consistent with local generation by an IW beam impinging on the
ocean pycnocline from below.
14.3.2 SAR Observations of Mode 2 ISWs
Figure 14.7 shows a typical example of a SAR image where two wave trains can be
seen to propagate towards the shore (approximately to the west-northwest, and in a
near perpendicular direction to the local isobaths) in the southern end of MC (near
parallel 24
◦ S). The left panel shows an ERS-2 SAR image dated 24 September 2001
(acquired at 7:39 UTC), which is centred approximately at 23.75
◦ S and 35.75
◦ E.
Note that land and coast lines are also marked in gray areas outlined by a black
contour, respectively, and that bathymetry levels for 100, 200, 500 and 1,000 m are
also shown. The black filled circle on the image indicates the predicted position
where an IT beam would impinge on the thermocline from below (assumed at an
average depth of 50 m), and the dashed line indicates its trajectory coming from
deeper bathymetry to the east (passing through the centre and taken perpendicular
to the wave crests). The right hand side of Fig. 14.7 shows a vertical profile, which
is aligned and partially coincident with the black dashed line on the left, together
with a simulated IT beam of semi-diurnal frequency based on local stratification.
The arrow indicates the direction of energy propagation, and the inset shows the
geographic location of the observation (the star in the inset denotes the geographic
location of the image centre).
The SAR image shown in Fig. 14.7 reveals that the ISWs in the deeper wave
packet are characterized by the usual rank-ordered bright and dark bands of radar
intensity, which decrease in amplitude from the front to the rear. However, while
these signatures look like many other typical SAR signatures of ISWs, it can be seen
that in this case the contrast pattern is reversed in relation to the general pattern (of
mode 1 ISWs). Here, dark bands precede bright bands in the propagation direction,