Nonlinear interaction of inertial and semidiurnal currents in the northeast pacific

• Main Author: Mihaly, Steven Ferenc
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/10099

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.