Published in Dissertation, Stanford University, June 1, 2014.
NOTE: At the time of publication, the author Ryan Walter was not yet affiliated with Cal Poly.
The nearshore coastal environment is generally taken to be one of the most productive and ecologically important parts of the ocean. The nearshore is also a complex environment from a physical standpoint, due in large part to the widespread and often irregular occurrence of nonlinear internal waves (NLIWs) that frequently appear more bore-like than wave-like. Indeed, this region of the ocean can be thought of as the “swash zone” for larger-scale internal wave fields on the continental shelf. These nearshore NLIWs and bores have considerable implications for the cross-shelf exchange and transport of nutrients, sediments, contaminants, larvae, and other scalars; turbulent dissipation and diapycnal mixing; and hypoxia development. Despite the ramifications and a growing body of literature on the subject, many questions still remain with respect to the evolution, fate, and impact of NLIWs and internal bores in the nearshore coastal environment. This dissertation addresses these issues in Monterey Bay, CA. Several observational field studies were conducted using a wide variety of instruments and measurement techniques, such as oceanographic mooring arrays and an underwater turbulence flux tower. This work shows that nearshore internal bores produce transient stratification and mixing events, and represent the dominant source of variability, in the nearshore environment of southern Monterey Bay, CA. Specifically, these events control temperature and circulation dynamics, dominate turbulent mixing in the stratified interior, contribute to the majority of the variance in cross-shelf transport, and can create low dissolved oxygen (hypoxic) events. Individual bores also drive substantial changes to local mixing dynamics and the stratified turbulence field, with considerable v differences between the various phases of the bores. Furthermore, the strength and structure of the observed internal bores are dependent on a combination of low-frequency upwelling wind forcing, offshore stratification, and the local bathymetric slope. Additionally, high resolution observations of rank-ordered packets of NLIWs in northern Monterey Bay, CA are presented. These NLIWs are some of the largest internal waves ever observed on the continental shelf. They are surmised to be generated at an upwelling front, modified by the presence of a strong background shear, and shown to be unstable to shear instabilities with large overturns and elevated turbulent mixing in the stratified interior. Finally, this dissertation examines the applicability of similarity scaling of turbulence spectra and cospectra in a shallow tidal flow in Elkhorn Slough, CA. Implications of the various findings, including biological processes, ecosystem health, numerical models, and nearshore hypoxia risk are discussed.
2014 Ryan Kymn Walter.