Turbulence Driven by Reflected Internal Tides in a Supercritical Submarine Canyon

Abstract

The La Jolla Canyon System (LJCS) is a small, steep, shelf-incising canyon offshore of San Diego, California. Observations conducted in the fall of 2016 capture the dynamics of internal tides and turbulence patterns. Semidiurnal (D2) energy flux was oriented up-canyon; 62% ± 20% of the signal was contained in mode 1 at the offshore mooring. The observed mode-1 D2 tide was partly standing based on the ratio of group speed times energy cgE and energy flux F. Enhanced dissipation occurred near the canyon head at middepths associated with elevated strain arising from the standing wave pattern. Modes 2–5 were progressive, and energy fluxes associated with these modes were oriented down-canyon, suggesting that incident mode-1 waves were back-reflected and scattered. Flux integrated over all modes across a given canyon cross section was always onshore and generally decreased moving shoreward (from 240 ± 15 to 5 ± 0.3 kW), with a 50-kW increase in flux occurring on a section inshore of the canyon’s major bend, possibly due to reflection of incident waves from the supercritical sidewalls of the bend. Flux convergence from canyon mouth to head was balanced by the volume-integrated dissipation observed. By comparing energy budgets from a global compendium of canyons with sufficient observations (six in total), a similar balance was found. One exception was Juan de Fuca Canyon, where such a balance was not found, likely due to its nontidal flows. These results suggest that internal tides incident at the mouth of a canyon system are dissipated therein rather than leaking over the sidewalls or siphoning energy to other wave frequencies.

On the Magnitude of Canyon‐Induced Mixing

“…This length dependence was corroborated by Hamann et al. (2021), Petruncio et al. (1998), and Waterhouse et al.…”

“…In order to test the robustness of our results, we compare energy loss calculated from five sets of observations with our parameterized energy loss following the framework of Hamann et al. (2021). Specifically, Hamann et al.…”

“…Submarine canyons are regions of enhanced levels of internal tide‐driven mixing due to both their geometry and their relative abundance, with over 10% of the continental slope intersected by canyons (Alberty et al., 2017; Aslam et al., 2018; Bosley et al., 2004; Bruno et al., 2006; Carter & Gregg, 2002; Codiga et al., 1999; Gardner, 1989; Gordon & Marshall, 1976; Gregg et al., 2011; Hall & Carter, 2011; Hamann et al., 2021; Hotchkiss & Wunsch, 1982; Kunze et al., 2012; Lee et al., 2009; Nazarian & Legg, 2017a, 2017b; Petruncio et al., 1998; Wain et al., 2013; Waterhouse et al., 2017; Xu & Noble, 2009). Diapycnal mixing within canyons is important for a host of coastal processes (Cacchione et al., 2002; Leichter et al., 2003; McPhee‐Shaw, 2006; Ramos‐Musalem & Allen, 2019), and for the large‐scale circulation of the ocean, as mixing at depth sustains the global overturning circulation (Melet et al., 2016; Munk, 1966).…”

A Vorticity‐Divergence View of Internal Wave Generation by a Fast‐Moving Tropical Cyclone: Insights From Super Typhoon Mangkhut

“…Here, the constants are ρ 0 = 1,024 kg m −3 and C p = 4000 J kg −1 °C −1 . Comparisons between Thorpe‐inferred quantities and microstructure measurements suggest that Equation can yield valid estimates in the thermocline beneath a TC, where temperature gradients are sharp and convective instability is not the main driver of turbulence (Alford et al., 2006; Dillon, 1982; Dunckley et al., 2012; Ferron et al., 1998; Hamann et al., 2021; Mater et al., 2015). More details on the implementation, assumptions, and limitations of the Thorpe scale method can be found in Johnson and Garrett (2004), A. Thompson et al.…”

Diurnal Coastal Trapped Waves and Associated Deep‐Ocean Mixing Near the Head of Suruga Trough

“…Coastal submarine canyons can therefore be regarded as “mixing hotspots that distribute widely throughout the world's oceans, but inadequately resolved in OGCMs.” Strong turbulent mixing in these submarine canyons causes upwelling of deep water and hence creates productive fishing areas by increasing biological productivity (Sanchez et al., 2013; Vetter & Dayton, 1999). Recent studies discussed the turbulent mixing processes in submarine canyons in terms of horizontal standing modes of semidiurnal internal tides (Hamann et al., 2021; Petruncio et al., 1998; Waterhouse et al., 2017; Zhao et al., 2012). For example, in Eel Canyon located on the north California coast, Waterhouse et al.…”