Partly standing internal tides in a dendritic submarine canyon observed by an ocean glider

Abstract

An autonomous ocean glider is used to make the first direct measurements of internal tides within Whittard Canyon, a large, dendritic submarine canyon system that incises the Celtic Sea continental slope and a site of high benthic biodiversity. This is the first time a glider has been used for targeted observations of internal tides in a submarine canyon. Vertical isopycnal displacement observations at different stations fit a one-dimensional model of partly standing semidiurnal internal tides – comprised of a major, incident wave propagating up the canyon limbs and a minor wave reflected back down-canyon by steep, supercritical bathymetry near the canyon heads. The up-canyon internal tide energy flux in the primary study limb decreases from 9.2 to 2.0 kW m−1 over 28 km (a dissipation rate of View the MathML source), comparable to elevated energy fluxes and internal tide driven mixing measured in other canyon systems. Within Whittard Canyon, enhanced mixing is inferred from collapsed temperature-salinity curves and weakened dissolved oxygen concentration gradients near the canyon heads. It has previously been hypothesised that internal tides impact benthic fauna through elevated near-bottom current velocities and particle resuspension. In support of this, we infer order 20 cm s−1 near-bottom current velocities in the canyon and observe high concentrations of suspended particulate matter. The glider observations are also used to estimate a 1 °C temperature range and 12 μmol kg−1 dissolved oxygen concentration range, experienced twice a day by organisms on the canyon walls, due to the presence of internal tides. This study highlights how a well-designed glider mission, incorporating a series of tide-resolving stations at key locations, can be used to understand internal tide dynamics in a region of complex topography, a sampling strategy that is applicable to continental shelves and slopes worldwide

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

“…Glider observations in the dendritic Whittard Canyon also fit a 1-D model of a partly standing wave, and indirect techniques suggest that mixing is similarly enhanced towards the head of the canyon [Hall et al, 2017]. The results from Waterhouse et al [2017], Hall et al, [2017], and the current work suggest that the vertical distribution of dissipation within supercritical canyons [Wain et al, 2013;Carter and Gregg, 2002]. However, in the reflective LJC we observed elevated dissipation and periods of weak stratification at mid-depths.…”

“…There, semidiurnal internal tides that propagated freely through the largely subcritical canyon partially reflected from the steep bathymetry at the head of the canyon and led to enhanced dissipation at mid‐depths—but only at locations close to the head of the canyon. Glider observations in the dendritic Whittard Canyon also fit a 1‐D model of a partly standing wave, and indirect techniques suggest that mixing is similarly enhanced towards the head of the canyon [ Hall et al , ]. The results from Waterhouse et al .…”

“…The results from Waterhouse et al . [], Hall et al , [], and the current work suggest that the vertical distribution of dissipation within supercritical canyons differs from that of their subcritical counterparts, and that reflection near supercritical slopes at canyon heads generates mid‐depth dissipation that may efficiently dissipate incident D 2 energy. Previous work in subcritical canyons such as Monterey found enhanced dissipation near the bottom primarily due to bottom‐trapped bores or topographically controlled hydraulic flows [ Wain et al ., ; Carter and Gregg , ].…”

Cold‐water coral assemblages on vertical walls from the Northeast Atlantic

“…Solenosmilia variabilis globally tends to occur in deeper and colder waters than D. pertusum and M. oculata (Davies & Guinotte, ; Roberts, Wheeler, Freiwald, & Cairns, ) while Isididae and Antipathidae also have deeper mean depths than other deep‐sea coral families (Etnoyer & Morgan, ). In the eastern branch of Whittard Canyon, isopycnal displacement caused by internal tides could lead to daily changes of up to 1°C in temperature (Hall, Aslam, & Huvenne, ), and the wider temperature tolerance window or stronger physiological capacity for adjustment to temperature fluctuations of D. pertusum when compared to M. oculata (Naumann, Orejas, & Ferrier‐Pagès, ) could be another reason for observed differences in abundance across branches.…”

The Global Mode‐2 M2 Internal Tide

“…This section presents the mode-1 and mode-2 M 2 internal tides in the eastern North Atlantic ranging 23-53 ∘ N, 5-35 ∘ W. This region is chosen for close examination because internal tides in this region have long attracted much attention of European oceanographers (e.g., Gerkema & van Haren, 2007;Hall et al, 2017;New, 1988;New & Da Silva, 2002;Pingree et al, 1986;van Haren & Gostiaux, 2010;Vic et al, 2018;Vlasenko et al, 2013). Here the satellite results provide a regional two-dimensional field, which may help interpret field measurements in a large-scale context.…”