Flow and mixing in Ascension, a steep, narrow canyon

Abstract

A thin gash in the continental slope northwest of Monterey Bay, Ascension Canyon, is steep, with sides and axis both strongly supercritical to M2 internal tides. A hydrostatic model forced with eight tidal constituents shows no major sources feeding energy into the canyon, but significant energy is exchanged between barotropic and baroclinic flows along the tops of the sides, where slopes are critical. Average turbulent dissipation rates observed near spring tide during April are half as large as a two week average measured during August in Monterey Canyon. Owing to Ascension's weaker stratification, however, its average diapycnal diffusivity, 3.9 × 10-3 m2 s-1, exceeded the 2.5 × 10-3 m2 s-1 found in Monterey. Most of the dissipation occurred near the bottom, apparently associated with an internal bore, and just below the rim, where sustained cross-canyon flow may have been generating lee waves or rotors. The near-bottom mixing decreased sharply around Ascension's one bend, as did vertically integrated baroclinic energy fluxes. Dissipation had a minor effect on energetics, which were controlled by flux divergences and convergences and temporal changes in energy density. In Ascension, the observed dissipation rate near spring tide was 2.1 times that predicted from a simulation using eight tidal constituents averaged over a fortnightly period. The same observation was 1.5 times the average of an M2-only prediction. In Monterey, the previous observed average was 4.9 times the average of an M2-only prediction

Flow dynamics around downwelling submarine canyons

“…Average diapycnal diffusivity in Ascension Canyon (west coast, North America) has been observed as approximately 3.92×10 −3 m 2 s −1 (Gregg et al, 2011). Diffusion in the current model is small (10 −7 m 2 s −1 ); thus mixing has an insignificant impact on nitrate flux calculations in the model simulations.…”

Propagation and dissipation of the internal tide in upper Monterey Canyon

“…[] and Gregg et al . []); the ratio between the change in winch line‐out and the change in depth is 1:1. Here a different method was used to determine the distance of SWIMS3 behind the ship at each measurement time; i.e., the SWIMS3 depth and the line‐out were assumed to form two legs of a right triangle.…”

“…Gregg et al . [] measured turbulence in nearby Ascension Canyon using shear probes on a microstructure profiling instrument and overturns from a depth‐cycling towed body. Understanding internal tide generation, propagation through complex canyon topography, and dissipation is essential for comprehending the importance of canyons in coastal dynamics.…”

Diapycnal Mixing Processes in the Ocean Interior

“…Much of this low-mode energy may dissipate where waves scatter and reflect off continental slopes and shelves. Canyons littered along continental slopes may also focus incoming waves Gregg et al, 2011), and canyon mixing may contribute significantly to basin-wide averages . The details of such processes are poorly constrained by existing observations but likely are sensitive to both the steepness and the three-dimensional roughness of the slope in question.…”