Oceanography MS DefenseNovember 21, 2:00pm - 3:00pm
Mānoa Campus, POST 723
Department of Oceanography
University of Hawai‘i
“An Empirical Investigation of Parametric Subharmonic Instability in the Kauai Channel”
Abstract: For decades it has been believed that three classes of resonant interactions, based on weakly interacting, non-linear wave theory, are the dominant mechanisms for moving energy through frequency-wavenumber space of the internal wave field. Energy is thought to flow from sources (such as the tides and wind-forced inertial waves) toward small spatial scales and dissipation. A number of recent studies have asserted that one of these mechanisms, Parametric Subharmonic Instability (PSI), is very important, especially near certain “critical” latitudes (+/- 28.8 degrees), for transferring energy from large-scale semi-diurnal internal tides to smaller scale internal waves near the diurnal period, which would more easily be dissipated. However, analytical studies and numerical experiments have been highly idealized, and most observational studies lack sufficient frequency resolution to spectrally distinguish between diurnal tidal constituents and potential PSI waves forced by the principal semi-diurnal (M^2) tide.
Observations for this study come from moored Acoustic Doppler Current Profilers (ADCPs) which were deployed from November 2002 to June 2003 in the Kauai Channel as part of the Hawaii Ocean Mixing Experiment (HOME). These velocity time-series are long enough to provide the necessary resolution to determine whether diurnal spectral peaks are dominated by tidal constituents O^1 (0.930 cycles/day, or cpd) and K^1 (1.003 cpd), or by PSI waves with frequencies near M^2/2 (0.966 cpd). The potential of wave motion to drive turbulence and thus dissipate energy is more strongly related to shear of the horizontal currents than to the magnitude of the currents themselves. At this location, energetic semi-diurnal tidal beams are observed, and diurnal band spectral energies in both velocity and shear fields appear to be dominated by locally generated internal tides at O^1 and K^1. The strength of possible energy transfer from the M^2 internal tide to internal waves of approximately M^2/2 frequency via PSI is quantified and bounded by the observed ratio (10^-2) of velocity power spectral densities at M^2/2 and M^2.
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