Internal tidal beams and mixing near Monterey Bay

The spatial structure of velocity, density, and mixing in an internal tidal beam generated at a submarine ridge near Monterey Bay was observed using a combination of vessel-mounted acoustic Doppler current profilers, a towed conductivity-temperature-depth instrument (SeaSoar), and microconductivity sensors mounted on SeaSoar. Three <60 km meridional sections from the surface to 400-670 m in depth were occupied a total of 56 times during 16 days with the sampling pattern detuned from the M-2 tide. Averaging over all observations at a given latitude-depth bin produces a phase average of the M-2 internal tide. Observed velocity and displacement variances are scaled to estimate energy density. A beam in energy density originates from a submarine ridge and reflects with diminished amplitude at the surface. These results compare favorably with a numerical tidal model. The upward and downward beams show modestly elevated turbulence, which is patchy along the beam and has mean values about 50% larger than those outside of the beam. Peak values can be almost an order of magnitude larger in the beam. Dissipation increases with increasing shear and stratification similar to the MacKinnon-Gregg parameterization. Intermediate nepheloid layers were found in over half of the meridional sections. Their phasing and direction indicate that they originate at a secondary, weaker internal tidal generation site found in the model but not in the observations presumably due to mesoscale variability affecting stratification at the generation site and during wave propagation. The offshore movement of sediment is a result of westward mean current and internal wave-driven transport.