| Summary: | Rotary spectra of horizontal current velocities from moorings in the northeast Pacific are
found to have a significant spectral peak at precisely the sum of the semidiurnal and local
inertial frequencies. The existence of this spectral peak is, in the absence of any known forcing
at the sum frequency, sufficient to assume some form of nonlinear interaction between internal
wave motions at the inertial and semidiurnal frequencies. The mooring locations studied in this
thesis represent three distinct deep (> 2000 m) oceanic regimes: a topographically rough, a
topographically smooth, and a near-coastal region. The amplitude of the spectral peak at the
sum frequency (termed the 'ʃM2" frequency) is consistent among regions, implying that the
nonlinear interaction is not directly linked to specific characteristics of the regions and may be
ubiquitous to the world ocean. Although the measurements are limited, there is no significant
variation in intensity of the spectral peak with depth.
Weak resonant wave-wave interaction theory is the most readily applicable model for the
nonlinear interaction. Although the validity of the weakness assumption used in deriving the
coupling efficiency is in question, the physical mechanism of the coupling of two internal
gravity waves via a resonant triad is well founded. The resonance condition prescribes
geometric constraints on the propagation and wavelengths of a triad of interacting internal
waves. The propagation and wavelengths of inertial and semidiurnal waves observed in the
northeast Pacific are consistent with these constraints and the formation of resonant triads is
likely. Analysis, based on the likelihood of the global formation of resonant triads between
inertial and semidiurnal waves, indicates that higher latitudes are favored.
An examination of two deep-ocean sites along Juan de Fuca Ridge shows that elevated
energy is not usually coincident in the inertial, semidiurnal and ʃM2 frequency bands. When
the energy in all three bands is coincident, motions in the ʃM2 band can be: 1) nonresonantly
forced requiring the support of the inertial and semidiurnal motions or, 2) a result of the
nonlinear transfer of energy from the inertial and semidiurnal motions. When the energy is not
coincident, the energy in the ʃM2 frequency band can only result from nonlinear exchange of
energy from the inertial and semidiurnal motions, for which a plausible mechanism is triad
resonance. Bispectral analysis indicates that there is nonlinear coupling between the inertial,
semidiurnal and ʃM2 frequency bands at the Endeavour segment site but not at the CoAxial
segment site. This implies that the Endeavour segment site may be a generation region for ʃMz
waves while the CoAxial site is in the path of propagating ʃM2 wave energy from a remote
source region.
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