Hotspots of deep ocean mixing on the Oregon continental slope

Abstract

[1] Two deep ocean hotspots of turbulent mixing were found over the Oregon continental slope. Thorpe-scale analyses indicate time-averaged turbulent energy dissipation rates of > 10 À7 W/kg and eddy diffusivities of K r $ 10 À2 m 2 /s at both hotspots. However, the structure of turbulence and its generation mechanism at each site appear to be different. At the 2200-m isobath, sustained >100-m high turbulent overturns occur in stratified fluid several hundred meters above the bottom. Turbulence shows a clear 12.4-h periodicity proposed to be driven by flow over a nearby 100-m tall ridge. At the 1300-m isobath, tidally-modulated turbulence of similar intensity is confined within a stratified bottom boundary layer. Along-slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed. Such topography is common to most continental slopes, providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along-isobath. Citation:

Energy Release Through Internal Wave Breaking

“…e f e f e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f If we hypothetically extrapolate further and use previous estimates (Armi, 1978(Armi, , 1979Garrett, 1990Garrett, , 1991 , which is the value needed to maintain the overall density stratification (Munk and Wunsch, 1998). This value supports earlier conjectures (e.g., Munk, 1966;Armi, 1978;Garrett, 1990) at a variety of (tidally) noncritical slopes (e.g., Klymak and Moum, 2003;Hosegood et al, 2004;Bonnin et al, 2006;Nash et al, 2007). temporal) resolution of internal waveturbulence motions toward understanding their intrinsic properties.…”

Estimating Oceanic Turbulence Dissipation from Seismic Images

“…The Henyey-Gregg-Polzin technique uses the vertical shear in the horizontal velocities on 10-m scales (S 10 ) in an empirical relation that estimates the turbulent dissipation rate « and is related to eddy diffusivity as K r 5 0.2 « N 22 (Osborn 1980). The basic relationship, which depends on the fourth power of S 10 and the shearto-strain ratio R v , has been validated by several field programs (Polzin et al 1995) using combined velocity and turbulence profilers and applied several times to XCP data (D'Asaro and Morison 1992; Lee et al 2006;Nash et al 2007). The main problem with application of the scaling is in low-stratification regions , which is not a concern for these relatively shallow casts.…”

The Direct Breaking of Internal Waves at Steep Topography

“…Breaking waves on the oregon continental slope. Driven by a combination of barotropic and baroclinic tides that flow over small-scale bumps (of order 250-1,000 m horizontal scale) that overlie the otherwise steep continental slope, these waves occur on the leading edge of the downslope phase of the tide (modified from Nash et al 2007). The upper panel is turbulence dissipation rate with isopycnals overlain; middle panel is isopycnal strain, measuring how stretched or compressed parts of the water column are by the internal tide; bottom panel is the upslope semidiurnal tidal velocities.…”

“…In this case, Nash et al, 2007;Martini et al, 2011) and the South China Sea continental slope . The turbulence at these locales was less than at…”