Observations of Small-Scale Secondary Instabilities during the Shoaling of Internal Bores on a Deep-Ocean Slope

Abstract

The Rockall Bank area, located in the northeast Atlantic Ocean, is a region dominated by topographically trapped diurnal tides. These tides generate up-and downslope displacements that can be locally described as swashing motions on the bank. Using high spatial and time resolution of moored temperature sensors, the transition toward the upslope flow (cooling phase) is described as a rapid upslope-propagating bore, likely generated by breaking trapped internal waves. Buoyant anomalies are found during the bore propagation, likely resulting from small-scale instabilities. The imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing (i.e., young) and have high mixing potential.

Abyssal plain hills and internal wave turbulence

“…3) and also considering the generally long averaging periods over many (> 1000) profiles. The mixing efficiency value is close to the tidal mean mixing potential observed by Cyr and van Haren (2016), also in layers in which stratification is weak. Internal waves not only induce mixing through their breaking but also allow for rapid restratification, making the mixing rather efficient.…”

“…Aucan et al, 2006;van Haren and Gostiaux, 2012). This mixing is considered to have a high potential (Cyr and van Haren, 2016) as the back and forth sloshing of the carrier wave ensures a rapid re-stratification down to within a metre from the sea floor. Apparently, mixed waters are transported into the interior along isopycnals or perhaps along isobaths by advective flows.…”

On the influence of cold‐water coral mound size on flow hydrodynamics, and vice versa

“…This model, however, falls short in representing the along‐isobath residual circulation, hypothesized by Mohn et al [] to retain fresh organic material in close proximity of the Rockall slopes. Although not under the scope if this study, it is worth noting that the model is also unable to reproduce the energetic bore propagation observed at the base of the Haas mound [ Cyr and van Haren ,].…”

“…Comparison between modeled and observed near‐bottom currents at the base of the Haas mound (from van Haren et al []; see red star in Figure for mooring location) shows that the model captures 56% of the current variance in v and 24% in u , when computed in the 820–900 m depth range, i.e., where velocity measurements are available. The relatively low variance percentage resolved for this depth range, specially for u , is due to the presence the strong nonlinear bores described in Cyr and van Haren [] that the model is unable to reproduce (not shown).…”

“…A recent mooring deployment above the SE Rockall Bank slope confirmed the diurnal nature of near-bottom currents in the area [van Haren et al, 2014;Cyr and van Haren, 2015]. These studies also revealed large vertical excursions of the isopycnals (>100 m) and relatively strong diurnal currents (|u| < 0.25 ms −1 ) associated with a diurnal succession of sharp temperature fronts generating high turbulence levels, with potential positive effects on exchange of particulate and dissolved compounds (e.g., oxygen and nutrients).…”