Nonlinear Internal Waves on the Washington Continental Shelf

Thesis (Ph.D.)--University of Washington, 2014This dissertation studies nonlinear internal waves (NLIW) on the Washington (WA) continental shelf based on 4 years' records from a moored ADCP/thermistor chain. A strong, variable wave field is documented along with their general characteristics, generation, transport, instabilities and spatial features. Mostly propagating onshore towards the northeast, NLIWs take a variety of forms, including internal solitary waves, solitary wave trains and bores, but nearly all are mode-1 depression waves that arrive semi-diurnally along with the internal tide. The NLIW energy flux is correlated with the internal tide energy flux but not the local barotropic forcing, implying that the observed NLIWs arise primarily from shoaling remotely generated internal tides rather than local generation. Estimated onshore transport by the waves can equal or exceed offshore Ekman transport, suggesting the waves may play an important role in the mass balance on the WA continental shelf. Breaking NLIWs with overturning instabilities are examined and their Thorpe-scale-inferred turbulence documented. Categorized based on the instability mechanisms by which NLIWs induce mixing, waves are identified as shear-instability (Type I) and convective instability (Type II) waves using a Froude number criterion. Composites are constructed by averaging over all waves in each category to examine the mean depth structure of dissipation and their relationship with the background parameters for each category, suggesting highest turbulence at the sheared interface for Type I waves and throughout the wave core for Type II. NLIWs exhibit strong spatial variability at 6 synchronous mooring arrays deployed on the WA continental shelf. Based on their regular arrivals on 3 moorings, the same waves can be detected between mooring pairs and their travel times recorded. Using ray-tracing techniques with impacts from bathymetry only, the wave travel time is estimated as a function of wave amplitude and propagation direction. The comparison between the predicted wave travel time and observation is provided