UC San Diego

We describe and interpret in situ observations of tidally driven turbulence that were obtained in the vicinity of a small channel that transects the crest of the Mendocino Ridge in the north-eastern Pacific. Flows are funneled through the channel and have tidal excursion lengths comparable to the width of the ridge crest. Once per day, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave past the site location, and subsequent devel- opment of a hydraulic jump. During this phase of the tide the strong transports were associated with full depth flows, however, during the weaker beat of the tide transports were shallow and surface-confined, generating negligible turbulence. A regional numerical model of the area finds that the subinertial K1 (diurnal) tidal constituent generates topographically trapped waves which propagate anticycloni- cally around the ridge, and are associated with enhanced near-topographic K1 transports. The interaction of the trapped waves with the M2 (semidiurnal) sur- face tide produces a baroclinic tidal flow that is alternately surface confined and full depth. Consistent with observations, full depth flows are associated with the gen- eration of a large amplitude tidal lee wave on the northward face of the ridge, while surface confined flows are largely nonturbulent. The regional model demonstrates that nearfield dissipation over the entire ridge is diurnally modulated, despite the larger amplitude of the M2 tidal constituent, indicating that the trapped wave modulates near-field dissipation and mixing at this location.

In the final section, we study the interaction of the barotropic tide with a tall, two-dimensional ridge both analytically and numerically at latitudes where the tide is subinertial, and contrast it to when the tide is superinertial. When the tide is subinertial the energy density associated with the response grows with latitude as both the oscillatory along-ridge flow and near ridge isopycnal displacement become large. Nonlinear processes lead to the formation of along-ridge jets, which become faster at high latitudes. Mixing occurs mainly through hydraulic jumps on the flanks of the topography in both cases, though a superinertial tide may additionally generate mixing above topography arising from the convective breaking of radiating waves.