152
L. R. M. Maas et al.
Red dots in Fig. 2a indicate mooring locations in these two regions. Instruments on
the moorings provide long-term records of velocity, pressure and hydrographic data.
Previous analysis of MC data concerned long-term velocity and temperature records
(instruments not shown in Fig. 2b). These were dominated by low-frequency variations due to the yearly, southward passage of four to seven MC-wide eddies [40].
These eddies seem to match the width of the Channel in size but are most pronounced
on the Western side and could as well be interpreted as cross-channel propagating
Rossby waves [13]. The velocity records also showed evidence of the persistent presence of highly intermittent internal tides with currents ranging from a few up to 10
cm/s [23].
East of Madagascar, the dominant motion is marked by a western boundary current, the East Madagascar Current (EMC). This along-coastal, southward-directed
mean-flow eventually feeds the Agulhas Currents further south. At 23 ◦ S a steady
train of westward propagating cyclones and anticyclones drive the EMC’s main band
of variability [31].
Here, we focus on data from pressure recording instruments, carried by SBE37SM MicroCATs in the Mozambique Channel and by upward-looking Acoustic
Doppler Current Profilers (ADCPs—RDI Workhorse Long Ranger 75 kHz) off Eastern Madagascar. The positions of these instruments on the mooring lines are indicated in Fig. 2b and c and in Tables 2 and 3.
Tidal amplitudes and phases are obtained from observed pressure time series
with T_TIDE, a Harmonic Analysis package [28]. Using a least-squares method, this
package determines amplitudes and phases for a given number of tidal constituents
whose frequencies are known from celestial mechanics. Here we pay attention to
the restricted set of frequencies listed in Table 1. We exclude the very long period
tides (such as the Solar Semi-Annual, SSA, and monthly, MSM), as well as higher
harmonics, such as M4, and will concentrate on the most prominent diurnal and
semidiurnal frequencies, such as M2.
To eliminate the influence of low-frequency phenomena and associated blowdowns the pressure data were handled in the following way: (1) The mean pressure is subtracted. (2) Resulting pressures are divided by the product of density, í µí¼,
and acceleration of gravity, g, yielding an equivalent surface ‘displacement’ time
series, where a positive surface elevation is presented as a negative displacement.
(3) The resulting displacement series was detrended. (4) A High-Pass (HP) filter
was applied. (5) Blow-down events were eliminated to avoid estimating artificially
large tidal amplitudes. (6) The resulting (gappy) time series was subjected to Harmonic Analysis, using T_TIDE applied to periods of one year length, attributing its
resulting amplitudes and phases to its centre time. (7) To see the slow evolution of
harmonic amplitudes and phases, this harmonic analysis was performed over subsequent one year periods, stepping forward by one day at a time, over a period of 9 to
21 months (a procedure that we call Moving T_TIDE, or MT_TIDE).
In step (2) an ‘equivalent surface displacement’ is defined for the following reason. While we measure pressure, both near the bottom, as well as within the water
column, which varies due to hydrostatic effects (weight of fluid column) and nonhydrostatic effects (vertical accelerations), subtracting the mean pressure, its time-
L. R. M. Maas et al.
Red dots in Fig. 2a indicate mooring locations in these two regions. Instruments on
the moorings provide long-term records of velocity, pressure and hydrographic data.
Previous analysis of MC data concerned long-term velocity and temperature records
(instruments not shown in Fig. 2b). These were dominated by low-frequency variations due to the yearly, southward passage of four to seven MC-wide eddies [40].
These eddies seem to match the width of the Channel in size but are most pronounced
on the Western side and could as well be interpreted as cross-channel propagating
Rossby waves [13]. The velocity records also showed evidence of the persistent presence of highly intermittent internal tides with currents ranging from a few up to 10
cm/s [23].
East of Madagascar, the dominant motion is marked by a western boundary current, the East Madagascar Current (EMC). This along-coastal, southward-directed
mean-flow eventually feeds the Agulhas Currents further south. At 23 ◦ S a steady
train of westward propagating cyclones and anticyclones drive the EMC’s main band
of variability [31].
Here, we focus on data from pressure recording instruments, carried by SBE37SM MicroCATs in the Mozambique Channel and by upward-looking Acoustic
Doppler Current Profilers (ADCPs—RDI Workhorse Long Ranger 75 kHz) off Eastern Madagascar. The positions of these instruments on the mooring lines are indicated in Fig. 2b and c and in Tables 2 and 3.
Tidal amplitudes and phases are obtained from observed pressure time series
with T_TIDE, a Harmonic Analysis package [28]. Using a least-squares method, this
package determines amplitudes and phases for a given number of tidal constituents
whose frequencies are known from celestial mechanics. Here we pay attention to
the restricted set of frequencies listed in Table 1. We exclude the very long period
tides (such as the Solar Semi-Annual, SSA, and monthly, MSM), as well as higher
harmonics, such as M4, and will concentrate on the most prominent diurnal and
semidiurnal frequencies, such as M2.
To eliminate the influence of low-frequency phenomena and associated blowdowns the pressure data were handled in the following way: (1) The mean pressure is subtracted. (2) Resulting pressures are divided by the product of density, í µí¼,
and acceleration of gravity, g, yielding an equivalent surface ‘displacement’ time
series, where a positive surface elevation is presented as a negative displacement.
(3) The resulting displacement series was detrended. (4) A High-Pass (HP) filter
was applied. (5) Blow-down events were eliminated to avoid estimating artificially
large tidal amplitudes. (6) The resulting (gappy) time series was subjected to Harmonic Analysis, using T_TIDE applied to periods of one year length, attributing its
resulting amplitudes and phases to its centre time. (7) To see the slow evolution of
harmonic amplitudes and phases, this harmonic analysis was performed over subsequent one year periods, stepping forward by one day at a time, over a period of 9 to
21 months (a procedure that we call Moving T_TIDE, or MT_TIDE).
In step (2) an ‘equivalent surface displacement’ is defined for the following reason. While we measure pressure, both near the bottom, as well as within the water
column, which varies due to hydrostatic effects (weight of fluid column) and nonhydrostatic effects (vertical accelerations), subtracting the mean pressure, its time-