UC San Diego
The dynamics of internal tides and mixing in coastal systems
- Author(s): Hamann, Madeleine Marie
- Advisor(s): Alford, Matthew H
- Lucas, Andrew J
- et al.
This thesis examines two phenomena that occur over the continental shelf and generate mixing in stratified coastal systems.
In Chapter 1, we examine the generation, propagation, and dissipation of nonlinear internal waves (NLIW) in sheared background currents on continental shelf offshore of Washington state. At the shelf break, semi-diurnal (M2) energy flux is onshore and the incident M2 internal tide is partially reflected and partially transmitted. NLIW appear at an inshore mooring at the leading edge of the onshore phase of the baroclinic tide, consistent with nonlinear transformation of the shoaling internal tide as their generation mechanism. Of the M2 energy flux observed at the eastern extent of the generation region approximately 30% goes into the NLIW observed inshore. NLIWs are tracked into shallow (30-40 m) water, where a vertically sheared, southward current becomes strong. As train-like waves propagate onshore, wave amplitudes of 25-30 m and energies of 5 MJ decrease to 12 m and 10 kJ, respectively. The observed direction of propagation rotates from 30◦ N of E to ∼ 30◦ S of E in the strongly sheared region. Bore-like waves do not
rotate and do not appear to lose energy within the observed ranges.
Chapters 2-4 are focused on the dynamics of internal tides in the La Jolla Canyon System (LJCS) a steep, shelf-incising submarine canyon off the coast of San Diego, California.
In Chapter 2 we present the results from a short student-led study at the steep head of the canyon system. Baroclinic energy flux is oriented up-canyon and decreases from 182 ± 18 W m−1 at the canyon mouth to 46 ± 5 W m−1 near the head. Variance is dominated by the semi-diurnal (M2) tide which is partially standing. Moving up-canyon, the relative importance of M2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (∼50 m in 100m water depth) and high strain events. These events coincide with enhanced dissipation of turbulent kinetic energy at mid-depths.
In Chapter 3, we present results from spatial surveys of the LJCS made during a more extensive follow-up experiment. M2 energy flux was oriented up-canyon and contained mostly in mode 1. High values of dissipation occurred near the canyon head at mid-depths associated primarily with high strain. Modal analyses suggest that LJCS was reflective to the mode 1 M2 tide. Higher modes were found to be progressive, and smaller energy fluxes associated with them were oriented down-canyon, suggesting that incident low-mode waves were both back-reflected and scattered. Flux integrated over a canyon cross-section was always onshore, but generally decreased moving shoreward (240 kW to 5 kW), with a jump in flux occurring on a section just inshore of the canyon’s major bend due to reflection of incident waves from the steep sidewalls of the meander. Flux convergence from canyon mouth to head was balanced by the volume integrated dissipation observed. By comparing simple energy budgets from all canyons with sufficient observations, a similar balance is found for most canyons suggesting that much of the elevated turbulence in these canyon systems is driven by internal tide dynamics.
In Chapter 4, we use long, coincident time series from the same experiment to examine temporal variability of the internal wave field, internal tide dynamics, and associated mixing in the LJCS. Results from Chapter 3 are confirmed: the M2 internal tide dominates the signal in both velocity and isopycnal displacement and it is partially standing throughout the measurement period; near the canyon mouth, dissipation is bottom-enhanced and occurs during up-canyon flow periods, while near the canyon head dissipation is elevated during high strain events at mid-depths and time-averaged dissipation is elevated throughout the water column.
At the canyon head spring-neap cycles in depth-integrated energy, energy flux, stratifica- tion, and dissipation occur at times throughout the year. Depth-integrated chlorophyll fluorescence also demonstrates spring-neap variability; maxima in [Chl] lag behind maxima in M2 internal tide energy by 2-3 days. Phase offsets between the surface tide and harmonic fits to isopycnal displacement (η) are relatively constant, suggesting that the internal tide incident at the canyon mouth is generated nearby. At the head of the canyon the skill of a harmonic fit to η over a 90-day fit window is 49.7% – very high when compared to most other coastal mooring records that have been similarly assessed and indicating surprisingly high stationarity of the internal tide in the canyon. Long time series within canyon systems are rare, and this result could motivate an investigation into whether the skill of internal tide predications may be useful near canyon systems or steep topographic features more generally, and whether such regular tidal motions foster significant enhanced productivity nearby.