Observations of Nonlinear Internal Tides and Turbulence in a Steep Submarine Canyon

Submarine canyons connect the open ocean to coastal waters. Their complex topography can focus internal waves and enhance mixing over continental slopes and shelves. To better understand how internal waves dissipate their energy within a canyon smaller and steeper than most observed to date, a process study was conducted off the coast of La Jolla, CA in the La Jolla Canyon (LJC). 54-hour time series from a moored wave-powered profiler and bottom-mounted Acoustic Doppler Current Profiler (ADCP) were collected at the head of the canyon and supplemented with concurrent 25-hr time series of density, velocity, and temperature microstructure profiles from two shipboard stations further offshore. Isopycnal displacements are large (up to 50m in 100m water depth). High strain events occur twice per M2 tidal cycle alternating between two distinct mid-water column depths. KE turbulent dissipation rates are enhanced O(10^-7 to 10^-5 m² s^-3) in these mid-depth regions of high strain. Harmonic analysis reveals that variance is dominated by semi-diurnal (M2) tide. Moving up-canyon, relative importance of M2 decreases and its higher harmonics (2M2 and 3M2) are needed to account for a majority of the observed variance, indicating steepening. 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, and is smaller than that expected for a free wave, indicating reflection. Flux divergence between stations balances the measured dissipation of kinetic energy to within a factor of 2. Despite its size and reflective character, the LJC dissipates incoming internal wave energy significantly, suggesting that it and other small, steep canyons may be under-represented in mixing parameterizations for large-scale ocean models.

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