Interactions between tides and other frequencies in the Indonesian seas |
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Authors: | Robin Robertson |
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Institution: | (1) University of New South Wales @ADFA (Australian Defence Force Academy), Northcott Dr., Canberra, ACT, 2617, Australia |
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Abstract: | Interactions of tidal constituents and the transfer of energy from the tidal frequencies to other frequencies are investigated
using 3-D tidal simulations for the Indonesian seas, focusing on an area of active internal tides. Semidiurnal tides strongly
affect diurnal tides; however, semidiurnal tides are essentially unaffected by diurnal tides. The semidiurnal and diurnal
constituents interact with each other through non-linear interference, both destructive and constructive. Semidiurnal tides
generate harmonics at nearly the diurnal frequency and higher vertical wavenumbers. In Ombai Strait, these harmonics are out
of phase with the diurnal tides and interact destructively with the diurnal tides, effectively negating the diurnal response
in some locations. However, this is not a general response, and interactions differ between locations. Energy is also transferred
from both semidiurnal and diurnal tides to other frequencies across the spectrum, with more energy originating from semidiurnal
tides. These energy transfers are not homogeneous, and the spectral responses differ between the Makassar and Ombai Straits,
with the region east of Ombai showing a more active surface response compared to a more intense benthic response in Makassar.
In deep water away from topography, velocity spectra generally follow the Garrett–Munk (GM) relation. However, in areas of
internal tide generation, spectral density levels exceed GM levels, particularly between 4 and 8 cycles per day (cpd), indicating
increased non-linear interactions and energy transfer through resonant interactions. The model indicates strong surface trapping
of internal tides, with surface velocity spectra having significantly higher energy between 4 and 8 cpd even 100 km away from
the prominent sill generating the internal tides. |
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