A reflecting, steepening, and breaking internal tide in a submarine canyon

Abstract

Submarine canyons are common features of the coastal ocean. Although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters, the dynamics of canyon mixing processes are poorly understood. Most studies of internal wave dynamics within canyons have focused on a handful of canyons with along‐axis slopes less steep than semidiurnal (D2) internal wave characteristics (subcritical). Here, we present the first tidally resolving observations within a canyon with a steeply sloping axis (supercritical). A process study consisting of two 24 h shipboard stations and a profiling mooring was conducted in the La Jolla Canyon off the coast of La Jolla, CA. Baroclinic energy flux is oriented up‐canyon and decreases from 182 ±18 W m−1 at the canyon mouth to 46±5 W m−1 near the head. The ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating D2 internal waves at each station, indicating partial reflection. Harmonic analysis reveals that variance is dominated by the D2 tide. Moving up‐canyon, the relative importance of D2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (∼50 m in 100 m water depth) and high strain events. These events coincide with enhanced O( 10−7−10−5 m2 s−3) dissipation of turbulent kinetic energy at mid‐depths.

On the Magnitude of Canyon‐Induced Mixing

“…(2021) conducted a comparison of data collected at disparate canyons to consider the mechanisms by which enhanced dissipation occurs and, while there is a range of physical processes leading to turbulent dissipation, many of the canyons analyzed share the same reflection and dissipative processes as the modeling studies of Nazarian and Legg (2017a, 2017b); namely reflection leading to (a) scattering to higher modes and (b) wave focusing. These processes are also consistent with additional idealized simulations of internal wave‐driven mixing in Eel and Veatch Canyons (Nazarian, 2018) as well as a host of observations (Alberty et al., 2017; Aslam et al., 2018; Bruno et al., 2006; Codiga et al., 1999; Gardner, 1989; Hall & Carter, 2011; Hamann et al., 2021; Hotchkiss & Wunsch, 1982; Kunze et al., 2012; Lee et al., 2009; Petruncio et al., 1998; Wain et al., 2013; Waterhouse et al., 2017). While there are other dissipative processes observed in other canyons, such as a flow reversal in Ascension Canyon (Gregg et al., 2011) or standing wave structure in Eel Canyon (Waterhouse et al., 2017), dissipation due to internal tide scattering to higher modes and increased energy density due to wave reflection and focusing is likewise observed in these cases.…”

“…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 (2021) used published observations of the La Jolla Canyon System [117.3 W, 32.9 N] (Alberty et al, 2017;Hamann et al, 2021), Monterey Canyon [121.9 W, 36.8 N] (Wain et al, 2013), Ascension Canyon [122.5 W, 36.9 N] ( Gregg et al, 2011), Eel Canyon [124.7 W, 40.6 N] (Waterhouse et al, 2017), Gaoping Canyon [120.2 E, 22.3 N] ( Lee et al, 2009), and Juan de Fuca Canyon [125.5 W, 48.0 N] (Alford et al, 2014) to compare the incoming internal tide flux and the canyon-integrated energy loss. We have replicated this observation-based estimate and superimposed the flux derived from our plane wave fit and the corresponding energy loss diagnosed from our parameterization for the La Jolla Canyon System, as well as Monterey, Ascension, Eel, and Gaoping Canyons and present the results in Figure 9.…”

“…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).…”

Observations of diapycnal upwelling within a sloping submarine canyon

“…The BLT canyon, like many other canyons 6 , 29 , 30 , shows vigorous along-canyon flow that oscillates between the up- and down-canyon directions with twice-a-day and once-a-day frequencies (Fig. 2 and Extended Data Fig.…”

Numerical modelling of the upwelling and associated hydrodynamics at various scales along the coral reefs at Sodwana Bay, South Africa

“…Wells et al [10] suggested that non-hydrostatic processes, such as breaking of internal waves along the shelf slope, could be a potential driving mechanism of the temperature anomalies at Sodwana. Breaking of internal waves occurs when the slope of the internal wave beam is equal to the topographic slope of the shelf [43][44][45]. The length of an internal wave is dependent on the vertical density gradient and Brünt-Väisälä frequency [43].…”