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.

Tidally Driven Processes Leading to Near-Field Turbulence in a Channel at the Crest of the Mendocino Escarpment

“…Finally, these observations were obtained in a channel, where three-dimensional bathymetry strongly influences the flow. One such influence is the generation of trapped topographic waves by the subinertial (K 1 ) tidal component, which can become resonant at certain topographies and are responsible for bottom-enhanced K 1 transports close to the ridge (Swart et al (2011);M16b). Such phenomena are beyond the scope of a parameterization such as that proposed by Klymak et al (2010a).…”

Subinertial canyon resonance

“…Another theory for subinertial canyon resonance has been discussed previously by Swart et al . [] and Le Souëf and Allen []. The Swart et al analysis assumes that the canyon is closed at one end and that the stratification can be approximated by a 1.5 density layer model.…”

“…[] and Le Souëf and Allen [] to “the Gully,” a narrow deep canyon near the shelf edge off Nova Scotia. Although the barotropic tide in the region is predominantly semidiurnal, the “diurnal tide was dramatically amplified in the canyon, its velocities increasing toward the seafloor and the canyon head” [ Swart et al ., ]. Using the parameters of Le Souëf and Allen [] for the continuously stratified theory, namely h = 1600 m, L = 35 km, and N = 2.3 × 10 − 3 s − 1 (appropriate for the average stratification observed in the canyon, p. 1341 of LeSouëf and Allen), we have from that T = 1.38 days.…”

“…The horizontal canyon quarter-wave resonance theory was originally applied by Swart et al [2011] and Le Souëf and Allen [2014] to "the Gully," a narrow deep canyon near the shelf edge off Nova Scotia. Although the barotropic tide in the region is predominantly semidiurnal, the "diurnal tide was dramatically Geophysical Research Letters 10.1002/2016GL068258 amplified in the canyon, its velocities increasing toward the seafloor and the canyon head" [Swart et al, 2011].…”

Modeling and Analysis of Internal-Tide Generation and Beamlike Onshore Propagation in the Vicinity of Shelfbreak Canyons

“…Winds, shelf waves, and ambient shelf currents can drive subinertial flows, enhancing the exchange of shelf and open-ocean waters (Allen and Durrieu de Madron 2009). Steep topography of canyons can trap remotely generated internal waves (IWs) (Chiou et al 2011;Hotchkiss and Wunsch 1982;Jachec et al 2006;Kunze et al 2002), cause resonant amplification of tidal flow (Swart et al 2011), and foster local generation of internal tides (Gregg et al 2011;Hotchkiss and Wunsch 1982), offering a few explanations for observed mixing enhancement in shelfbreak canyons (Gregg et al 2011;Lee et al 2009;Shroyer 2012). IWs can also radiate onshore from shelfbreak canyons and, if enhanced relative to noncanyon areas, may increase mixing.…”