Deep-Ocean Tides in the South-West Indian Ocean . . .
161
For the other instruments, tidal variance contributes a similar amount to the total
variance, O(15–25%), but this does not necessarily represent surface or coherent
internal tides. As testified by the much larger relative standard deviations in Table 5
of all variance estimates at EMC2, 4 and 5 when compared to those of the BPRs at
EMC1 and 3, intermittent internal tides overshadow the surface and coherent internal tides. We will come back to this in section “Deep Versus Mid-depth PressureMeasurements of Tides”.
Coherent Surface and Internal Tides
Harmonic amplitudes and phases of surface tides should not depend on the particular
observational time window from which they are determined (provided the observational period is long enough). By definition also the coherent internal tides have
stable amplitudes and phases, albeit differing from those associated with the surface
tide. Amplitude and phase stability can be used as a criterion to distinguish genuine
from ‘false’ tidal signals (i.e. pressure variations due to tidal-current induced depression of instruments), or on a more positive note, can be used to separate the combined
surface and coherent internal tides from incoherent internal tides. The moving version of the harmonic analysis performed by T_TIDE (MT_TIDE) allows us to make
this separation.
Mozambique Channel Harmonic Constants
The results of running MT_TIDE for the Mozambique Channel pressure records
are presented in Figs. 6 and 7 for semidiurnal and diurnal frequencies, respectively.
The fluctuations in, especially amplitude, that occur for pressure records within
the water column will be discussed in section “Deep Versus Mid-depth PressureMeasurements of Tides”, although it should be noticed here that tidal amplitudes are
not necessarily always larger higher up in the water column, compared to the very
stable bottom amplitudes. See for instance in Fig. 6 the M2 amplitude at lmc8. Phases
are very stable at nearly all locations, amplitudes only near the bottom. It is significant that the least stable estimates and the largest M2 tidal amplitudes are found at
the mid-channel mooring lmc5, especially at the pressure sensors that are highest
above the bottom. At the three ‘bottom’ locations (less than 75 m away from the bottom), M2 amplitudes are around 113 cm. Phase differences are generally small. For
M2 the phase centers around 37
◦ . For the second most important component, S2,
the amplitude is about 60 cm, and its phase about 79
◦ . The other two semidiurnal
components, N2 and K2, both reach amplitudes of O (17 cm).
Diurnal components (Fig. 7) also have quite stable amplitudes and phases. Amplitudes of O (2–7 cm) are smaller than those of the semidiurnal tides. The spreading
in phase estimates is often larger than that of the semidiurnal components. Table 6
161
For the other instruments, tidal variance contributes a similar amount to the total
variance, O(15–25%), but this does not necessarily represent surface or coherent
internal tides. As testified by the much larger relative standard deviations in Table 5
of all variance estimates at EMC2, 4 and 5 when compared to those of the BPRs at
EMC1 and 3, intermittent internal tides overshadow the surface and coherent internal tides. We will come back to this in section “Deep Versus Mid-depth PressureMeasurements of Tides”.
Coherent Surface and Internal Tides
Harmonic amplitudes and phases of surface tides should not depend on the particular
observational time window from which they are determined (provided the observational period is long enough). By definition also the coherent internal tides have
stable amplitudes and phases, albeit differing from those associated with the surface
tide. Amplitude and phase stability can be used as a criterion to distinguish genuine
from ‘false’ tidal signals (i.e. pressure variations due to tidal-current induced depression of instruments), or on a more positive note, can be used to separate the combined
surface and coherent internal tides from incoherent internal tides. The moving version of the harmonic analysis performed by T_TIDE (MT_TIDE) allows us to make
this separation.
Mozambique Channel Harmonic Constants
The results of running MT_TIDE for the Mozambique Channel pressure records
are presented in Figs. 6 and 7 for semidiurnal and diurnal frequencies, respectively.
The fluctuations in, especially amplitude, that occur for pressure records within
the water column will be discussed in section “Deep Versus Mid-depth PressureMeasurements of Tides”, although it should be noticed here that tidal amplitudes are
not necessarily always larger higher up in the water column, compared to the very
stable bottom amplitudes. See for instance in Fig. 6 the M2 amplitude at lmc8. Phases
are very stable at nearly all locations, amplitudes only near the bottom. It is significant that the least stable estimates and the largest M2 tidal amplitudes are found at
the mid-channel mooring lmc5, especially at the pressure sensors that are highest
above the bottom. At the three ‘bottom’ locations (less than 75 m away from the bottom), M2 amplitudes are around 113 cm. Phase differences are generally small. For
M2 the phase centers around 37
◦ . For the second most important component, S2,
the amplitude is about 60 cm, and its phase about 79
◦ . The other two semidiurnal
components, N2 and K2, both reach amplitudes of O (17 cm).
Diurnal components (Fig. 7) also have quite stable amplitudes and phases. Amplitudes of O (2–7 cm) are smaller than those of the semidiurnal tides. The spreading
in phase estimates is often larger than that of the semidiurnal components. Table 6