Deep-Ocean Tides in the South-West Indian Ocean . . .
179
Incoherent Internal Tides
So far we have discussed the surface tides (based on BPRs, section “Bottom-Pressure
Versus Altimetry Derived Tides”) and the coherent internal tides (which are longperiod averages from MT_TIDE). For this, in MC we first subtracted the surface
tide (section “Mozambique Channel Coherent Internal Tides”), while in EM the surface tide is much smaller than the coherent internal tide so that the long-period
average immediately give an estimate for the coherent internal tides (section “East
Madagascar Coherent Internal Tides”). Figures 6, 7, 8, 9, 10 and 11, however, also
show large variations of tidal amplitudes and phases around their averages. These
represent incoherent internal tides. These can, crudely speaking, be of comparable
magnitude as the coherent internal tide. The large, mid-water column tidal amplitudes in e.g. Fig. 8 display gradual as well as sudden changes. The sudden jumps
might be related to large-scale (blow-down) events, despite the applied filters. The
data gaps that have been replacing these events, that enter or leave the analysed oneyear time interval, may still have a disruptive effect. We notice that such jumps do not
occur for the bottom instrument. The mid water-column pressure records at lmc5a
are remarkable in showing correlating drops in M2, S2, K2 and P1 amplitudes, taking place from February to July 2013. This drop below the average levels given in
Table 6 suggests that internal tides at instrument levels more than 425 m above the
bottom have a significant presence during these periods. Notice that such internal tide
variations also showed up in the mid water column pressure records of Figs. 6 and 7.
N2, M2 and S2 amplitudes, as well as K2 phases, suggest the internal tides at lmc5a
to be often anti-correlated with those at lmc6, their values fluctuating around those
representative of the surface tide. This change in internal tide amplitude on relatively
nearby moorings suggests that internal tidal beams change path due to variations in
mean density and flow fields.
Conclusions
New bottom pressure records for the South-West Indian Ocean have been presented.
These can be reliably interpreted as providing measurements of the surface tides
and compare in general well with the satellite altimetry derived tides. As such, these
BPRs might be useful in future world-wide assimilation of deep pressure data in
numerical models of ocean tides when combined with altimetric data. According to
Ray’s [34] separation criteria, the moorings may not be separated well enough (by
about 100 km) to be treated as independent, so that MC and EM transects may deliver
two independent data points only. At the same time, spatial variations in BPRs in the
vicinity of the coast may be realistic, and may prompt the development of a better
resolution in near-coastal altimetry measurements.
Pressure measurements within the water column regularly suffer from blow-down
of moorings by eddy, buoyancy, tidal and wind-driven flows. Blow-down leads to
179
Incoherent Internal Tides
So far we have discussed the surface tides (based on BPRs, section “Bottom-Pressure
Versus Altimetry Derived Tides”) and the coherent internal tides (which are longperiod averages from MT_TIDE). For this, in MC we first subtracted the surface
tide (section “Mozambique Channel Coherent Internal Tides”), while in EM the surface tide is much smaller than the coherent internal tide so that the long-period
average immediately give an estimate for the coherent internal tides (section “East
Madagascar Coherent Internal Tides”). Figures 6, 7, 8, 9, 10 and 11, however, also
show large variations of tidal amplitudes and phases around their averages. These
represent incoherent internal tides. These can, crudely speaking, be of comparable
magnitude as the coherent internal tide. The large, mid-water column tidal amplitudes in e.g. Fig. 8 display gradual as well as sudden changes. The sudden jumps
might be related to large-scale (blow-down) events, despite the applied filters. The
data gaps that have been replacing these events, that enter or leave the analysed oneyear time interval, may still have a disruptive effect. We notice that such jumps do not
occur for the bottom instrument. The mid water-column pressure records at lmc5a
are remarkable in showing correlating drops in M2, S2, K2 and P1 amplitudes, taking place from February to July 2013. This drop below the average levels given in
Table 6 suggests that internal tides at instrument levels more than 425 m above the
bottom have a significant presence during these periods. Notice that such internal tide
variations also showed up in the mid water column pressure records of Figs. 6 and 7.
N2, M2 and S2 amplitudes, as well as K2 phases, suggest the internal tides at lmc5a
to be often anti-correlated with those at lmc6, their values fluctuating around those
representative of the surface tide. This change in internal tide amplitude on relatively
nearby moorings suggests that internal tidal beams change path due to variations in
mean density and flow fields.
Conclusions
New bottom pressure records for the South-West Indian Ocean have been presented.
These can be reliably interpreted as providing measurements of the surface tides
and compare in general well with the satellite altimetry derived tides. As such, these
BPRs might be useful in future world-wide assimilation of deep pressure data in
numerical models of ocean tides when combined with altimetric data. According to
Ray’s [34] separation criteria, the moorings may not be separated well enough (by
about 100 km) to be treated as independent, so that MC and EM transects may deliver
two independent data points only. At the same time, spatial variations in BPRs in the
vicinity of the coast may be realistic, and may prompt the development of a better
resolution in near-coastal altimetry measurements.
Pressure measurements within the water column regularly suffer from blow-down
of moorings by eddy, buoyancy, tidal and wind-driven flows. Blow-down leads to