170
L. R. M. Maas et al.
mentioned above, may however have been artificial, due to the extrapolation applied
to altimetry-derived tidal constants on approach of the shoreline. Coastal proximity
inhibits direct satellite altimeter measurements of surface elevations. BPRs show a
decrease of surface tidal amplitude with increasing distance to the coast (compare
EMC1 to EMC3) of the largest two semidiurnal frequencies (M2 and S2). This is in
line with their presence as a Kelvin wave, trapped along the East Madagascar slope,
which is expected to propagate northwards.
East of Madagascar, surface diurnal tides shown by BPRs in the first row of
Fig. 13 reach amplitudes of O (1–3 cm) and compare well with altimetry derived
(OTIS) harmonic vectors. The harmonic vectors at the other, mid-water column pressure sensors, however, are all much larger than those at the bottom, indicating their
presence as a strong coherent diurnal internal tide that will be further discussed in
section “Deep Versus Mid-depth Pressure-Measurements of Tides”.
Credo of Smoothness
In the open ocean and in coastal regions, tides represent the response to body or
boundary forces, respectively. In the open ocean, tides are due to the gravitational
attraction by sun and moon, modulated by the motion of these celestial bodies,
together with the motion and rotation of the earth [29]. Coastal regions (and inland
seas and lakes) are too small to be able to respond to tidal forces directly. Coastal
tides result due to the tides present at their ocean boundaries, via co-oscillation [10,
14]. Of course, the nature of this response does not only depend on the strength with
which tides are present in the tidal potential or at the sea’s boundary. Tidal response
also depends on geometrical aspects of ocean or coastal basins, which may lead to
tidal amplification when resonating with a basin’s eigenfrequency, or to its suppression due to choking when in anti-resonance [20, 39]. Viewing the ocean or coastal
response to tidal forcing as being determined mainly by the proximity of any of the
tidal frequencies to any of the eigenfrequencies of these fluid basins, a certain similarity in the spatial distribution of tides of nearly similar frequency should not come
as a surprise. This view was epitomized in the catchy phrase ‘credo of smoothness’
[26] and is clearly born out in the tidal altimetry fields, both for the semidiurnal and
diurnal frequency bands, see Figs. 14 and 15.
At each tidal frequency, the response of the ocean to tidal forcing is given by
an amplitude ratio and a phase difference between its locally observed elevation
amplitude and Greenwich phase with those present in the tidal potential. Viewing the
response of the ocean to tidal forcing as that of an oddly-shaped, damped mechanical
resonator, one expects to see grossly similar features for slightly differing tidal frequencies, frictional effects smoothing out any sharp changes. During passage of such
a resonance, 180 degree phase changes may still occur, but not abruptly. Looking at
L. R. M. Maas et al.
mentioned above, may however have been artificial, due to the extrapolation applied
to altimetry-derived tidal constants on approach of the shoreline. Coastal proximity
inhibits direct satellite altimeter measurements of surface elevations. BPRs show a
decrease of surface tidal amplitude with increasing distance to the coast (compare
EMC1 to EMC3) of the largest two semidiurnal frequencies (M2 and S2). This is in
line with their presence as a Kelvin wave, trapped along the East Madagascar slope,
which is expected to propagate northwards.
East of Madagascar, surface diurnal tides shown by BPRs in the first row of
Fig. 13 reach amplitudes of O (1–3 cm) and compare well with altimetry derived
(OTIS) harmonic vectors. The harmonic vectors at the other, mid-water column pressure sensors, however, are all much larger than those at the bottom, indicating their
presence as a strong coherent diurnal internal tide that will be further discussed in
section “Deep Versus Mid-depth Pressure-Measurements of Tides”.
Credo of Smoothness
In the open ocean and in coastal regions, tides represent the response to body or
boundary forces, respectively. In the open ocean, tides are due to the gravitational
attraction by sun and moon, modulated by the motion of these celestial bodies,
together with the motion and rotation of the earth [29]. Coastal regions (and inland
seas and lakes) are too small to be able to respond to tidal forces directly. Coastal
tides result due to the tides present at their ocean boundaries, via co-oscillation [10,
14]. Of course, the nature of this response does not only depend on the strength with
which tides are present in the tidal potential or at the sea’s boundary. Tidal response
also depends on geometrical aspects of ocean or coastal basins, which may lead to
tidal amplification when resonating with a basin’s eigenfrequency, or to its suppression due to choking when in anti-resonance [20, 39]. Viewing the ocean or coastal
response to tidal forcing as being determined mainly by the proximity of any of the
tidal frequencies to any of the eigenfrequencies of these fluid basins, a certain similarity in the spatial distribution of tides of nearly similar frequency should not come
as a surprise. This view was epitomized in the catchy phrase ‘credo of smoothness’
[26] and is clearly born out in the tidal altimetry fields, both for the semidiurnal and
diurnal frequency bands, see Figs. 14 and 15.
At each tidal frequency, the response of the ocean to tidal forcing is given by
an amplitude ratio and a phase difference between its locally observed elevation
amplitude and Greenwich phase with those present in the tidal potential. Viewing the
response of the ocean to tidal forcing as that of an oddly-shaped, damped mechanical
resonator, one expects to see grossly similar features for slightly differing tidal frequencies, frictional effects smoothing out any sharp changes. During passage of such
a resonance, 180 degree phase changes may still occur, but not abruptly. Looking at