Physical modeling of tidal resonance in a submarine canyon

Current observations in submarine canyons poleward of 30 degrees are usually dominated by the semidiurnal (M-2) tidal frequency, which is superinertial at these latitudes. Observations from a submarine canyon at 44 degrees N (the Gully, Nova Scotia) suggest that canyons can be dominated by the subinertial (K-1) tidal frequency if length scales are correct for resonance of the K-1 frequency. A model of the Gully was constructed in a tank on a rotating table and tidal currents generated to determine factors that influence resonance. Resonance curves were fit to measurements from the laboratory canyon for a range of stratifications, background rotation rates, and forcing amplitudes. Dense water was observed upwelling onto the continental shelf on either side of the laboratory canyon and traveled at least one canyon width along the shelf. Friction values measured in the laboratory were much higher than expected, probably due to upwelled water surging onto the shelf on each tidal cycle, similar to a tidal bore. By scaling observations from the laboratory to the ocean and assuming friction in the ocean is also affected by water traveling onto the shelf, a resonance curve for the ocean was created. Because of the broad resonance curve, the diurnal tide remains strong year round at the Gully, even as stratification at the shelf break changes. Dense water surging onto the shelf on tidal frequencies may affect friction and mixing at other nonresonant canyons.