Direct evidence of deep water intrusions onto the continental shelf via surging internal tides

Near-bottom diurnal thermocline shoaling and diurnal baroclinic currents were observed from time series data of water temperature, salinity, pressure, and velocity collected from August 2006 to January 2007 in the nearshore zone off the Huntington Beach where the local inertial frequency is higher than the diurnal frequency (poleward of the diurnal critical latitude). During the stratified season, the deep offshore cold (dense) water, when shoaled into shallow water along the bottom, was often trapped for a few hours and dissipated in the nearshore region (water depth of 10-20 m) during the ebb phase of the diurnal surface tide. It appears that perturbations propagating onshore along the shoaled isopycnals can form an upslope surging front and an internal bolus. Our data allow the application of theoretical/lab criteria for internal hydraulic jumps, internal bolus formation, and internal wave breaking and all criteria are fulfilled during a part of the tidal shoaling cycle. Nonlinear advection associated with the internal boluses causes higher (close to omega(-3)) spectral falloff rate of near-bottom temperature with frequency omega than the canonical Garret-Munk spectra (omega(-2)) in the range of 0.1-1.0 cph, implying strong scattering of tidal energy toward smaller scales. We are able to directly calculate the offshore eddy heat flux by cold water intrusion onto the shelf resulting from this process. Similar impacts and implications are expected for the biogeochemical quantities, as well as for the role of subinertial internal waves on turbulent mixing in the immediate proximity of the generating region, i.e., sloping bottom.