Deep-Ocean Tides in the South-West Indian Ocean . . .
175
density profiles are stably-stratified down to the bottom, thus allowing for internal
tides to propagate.
Mozambique Channel Coherent Internal Tides
Internal tides are ubiquitous [25], and in the Indian Ocean reach vertical amplitudes
of 80 m or more, especially near Mascarene Ridge, close to the Seychelles (Fig. 2),
to the North-East of Madagascar [16, 25]. There, internal tides take the shape of
solitons, having large vertical isopycnal excursions of O (100 m), that find marked
surface expressions [9]. In Mozambique Channel, internal solitons have also been
observed south of our transect, near Sofala bank at 20.5 ◦ S where tides are stronger
than in the MC narrows [8]. The clear patterns of waves in their satellite observations
have been attributed to horizontally-propagating solitons, trapped to the pycnocline,
that were generated either at the shelf edge or by upward-propagating internal wave
beams impinging on the seasonal pycnocline from below. The latter beam itself was
identified as the bottom reflection of a previously downward-propagating internal
wave beam that was also produced at the shelf edge.
Along our Mozambique Channel transect, indications of mainly incoherent tides
were extracted from ADCP and current meter records during the Agulhas Current
Source EXperiment (ACSEX), a precursor of the LOCO projects used in the present
study [23]. That study shows the internal tides to be quite intermittent. Their spatial structure, however, seems to accord with internal tides taking the shape of relatively broad beams, O (100 km) wide, also found in their numerical model. These
beams follow nearly parabolic paths owing to the decrease in buoyancy frequency
with increasing depth and corresponding steepening of internal wave paths, seen in
Fig. 16.
In the present study, BPR-derived harmonic amplitudes and phases are the only
ones that can confidently be attributed to surface tides. The magnitude of the coherent
internal tides, displayed in Fig. 17, were obtained by subtracting these BPR-derived
surface tidal harmonic vectors. As the semidiurnal surface tide is much larger, O
(113 cm), than the coherent internal tide, having ‘equivalent surface displacements’
O (8 cm), only the surface tidal harmonic vector directions are shown in this figure
(black sticks), their magnitudes, A surf , being listed in each panel’s legend. Recall
that the apparently small magnitudes of the coherent internal tidal vectors represent
a pressure perturbation, here expressed as an equivalent surface displacement, that
can also be interpreted as a much larger internal, isopycnal displacement. The latter
will magnify the surface displacement, crudely speaking by a factor í µí»¿
−1 . For í µí»¿ of
O (10
−3
), this brings us back to an estimated internal isopycnal displacement of
O (80 m), as reported above for solitary wave displacements in the region.
One outstanding feature is the coherency of the surface tide along the Channel cross-section (different rows), a feature already addressed in section “Coherent
Surface and Internal Tides”. As mentioned, the coherent internal tides in Figs. 17
and 18 (coloured vectors) are obtained by subtracting at each level the surface tide
175
density profiles are stably-stratified down to the bottom, thus allowing for internal
tides to propagate.
Mozambique Channel Coherent Internal Tides
Internal tides are ubiquitous [25], and in the Indian Ocean reach vertical amplitudes
of 80 m or more, especially near Mascarene Ridge, close to the Seychelles (Fig. 2),
to the North-East of Madagascar [16, 25]. There, internal tides take the shape of
solitons, having large vertical isopycnal excursions of O (100 m), that find marked
surface expressions [9]. In Mozambique Channel, internal solitons have also been
observed south of our transect, near Sofala bank at 20.5 ◦ S where tides are stronger
than in the MC narrows [8]. The clear patterns of waves in their satellite observations
have been attributed to horizontally-propagating solitons, trapped to the pycnocline,
that were generated either at the shelf edge or by upward-propagating internal wave
beams impinging on the seasonal pycnocline from below. The latter beam itself was
identified as the bottom reflection of a previously downward-propagating internal
wave beam that was also produced at the shelf edge.
Along our Mozambique Channel transect, indications of mainly incoherent tides
were extracted from ADCP and current meter records during the Agulhas Current
Source EXperiment (ACSEX), a precursor of the LOCO projects used in the present
study [23]. That study shows the internal tides to be quite intermittent. Their spatial structure, however, seems to accord with internal tides taking the shape of relatively broad beams, O (100 km) wide, also found in their numerical model. These
beams follow nearly parabolic paths owing to the decrease in buoyancy frequency
with increasing depth and corresponding steepening of internal wave paths, seen in
Fig. 16.
In the present study, BPR-derived harmonic amplitudes and phases are the only
ones that can confidently be attributed to surface tides. The magnitude of the coherent
internal tides, displayed in Fig. 17, were obtained by subtracting these BPR-derived
surface tidal harmonic vectors. As the semidiurnal surface tide is much larger, O
(113 cm), than the coherent internal tide, having ‘equivalent surface displacements’
O (8 cm), only the surface tidal harmonic vector directions are shown in this figure
(black sticks), their magnitudes, A surf , being listed in each panel’s legend. Recall
that the apparently small magnitudes of the coherent internal tidal vectors represent
a pressure perturbation, here expressed as an equivalent surface displacement, that
can also be interpreted as a much larger internal, isopycnal displacement. The latter
will magnify the surface displacement, crudely speaking by a factor í µí»¿
−1 . For í µí»¿ of
O (10
−3
), this brings us back to an estimated internal isopycnal displacement of
O (80 m), as reported above for solitary wave displacements in the region.
One outstanding feature is the coherency of the surface tide along the Channel cross-section (different rows), a feature already addressed in section “Coherent
Surface and Internal Tides”. As mentioned, the coherent internal tides in Figs. 17
and 18 (coloured vectors) are obtained by subtracting at each level the surface tide