Resonant amplification of subinertial tides in a submarine canyon

Abstract

[1] Tidal oscillations dominate the flow field in many submarine canyons. Observations have shown that semidiurnal tidal energy in submarine canyons is significantly amplified with respect to adjacent shelves. This amplification is thought to be caused by focusing of propagating internal tides incident from the open ocean, or local in-canyon generation on critical canyon floor slopes. These mechanisms require freely propagating internal tides, with superinertial frequencies. We present results from a moored array in a canyon at 44°N, where the observed velocities reached over 0.8 m s−1. The canyon flow field was highly unusual because it was dominated by the subinertial diurnal tide. This occurred despite the fact that the barotropic tide was predominantly semidiurnal. The diurnal tide was dramatically amplified in the canyon, its velocities increasing toward the seafloor and canyon head. The diurnal oscillations also exhibited marked modulation in time by the background barotropic forcing. Length scales suggest that the diurnal tide should be resonant in the canyon. An analytical framework is used to explain the mechanisms behind the strong diurnal currents observed by the moored array. In the model, along-shelf barotropic flow sets up a double Kelvin wave response in the canyon, generating along-canyon velocities which are subsequently amplified by resonance. The pattern of the model predictions is in excellent agreement with the observed velocity pattern.