Rates and mechanisms of turbulent dissipation and mixing in the Southern Ocean: Results from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES)

139

Simultaneous full-depth microstructure measurements of turbulence and finestructure measurements of velocity and density are analyzed to investigate the relationship between turbulence and the internal wave field in the Antarctic Circumpolar Current. These data reveal a systematic near-bottom overprediction of the turbulent kinetic energy dissipation rate by finescale parameterization methods in select locations. Sites of near-bottom overprediction are typically characterized by large near-bottom flow speeds and elevated topographic roughness. Further, lower-than-average shear-to-strain ratios indicative of a less near-inertial wave field, rotary spectra suggesting a predominance of upward internal wave energy propagation, and enhanced narrowband variance at vertical wavelengths on the order of 100 m are found at these locations. Finally, finescale overprediction is typically associated with elevated Froude numbers based on the near-bottom shear of the background flow, and a background flow with a systematic backing tendency. Agreement of microstructure-and finestructure-based estimates within the expected uncertainty of the parameterization away from these special sites, the reproducibility of the overprediction signal across various parameterization implementations, and an absence of indications of atypical instrument noise at sites of parameterization overprediction, all suggest that physics not encapsulated by the parameterization play a role in the fate of bottom-generated waves at these locations. Several plausible underpinning mechanisms based on the limited available evidence are discussed that offer guidance for future studies.

Section: A Near-bottom Finescale Parameterization Overprediction Andsupporting

confidence: 90%

Section: ) Relation To Vertical Scales Consideredmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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Suppression of Internal Wave Breaking in the Antarctic Circumpolar Current near Topography

Waterman

Polzin

Garabato

et al. 2014

139

“…For the case where the planetary vorticity gradient is nonzero, we use β = 2 × 10 −11 m −1 s −1 which corresponds to a non dimensional β (which measures the relative importance of the planetary vorticity gradient relative to the mean shear) of β * = β b/Λ ∼ = 0.47. In midlatitudes, a typical ocean jet has U ∼ 0.5 m s −1 at the surface (e.g., Sheen et al 2013), whereas the barotropic jet used here has a value more typical of the a depth-mean over the main thermocline.…”

Section: Numerical Model Descriptionmentioning

confidence: 99%

A Geometric Interpretation of Eddy Reynolds Stresses in Barotropic Ocean Jets

Tamarin-Brodsky

Maddison

Heifetz

et al. 2016

Barotropic eddy fluxes are analysed through a geometric decomposition of the eddy stress tensor. Specifically, the geometry of the eddy variance ellipse, a two-dimensional visualization of the stress tensor describing the mean eddy shape and tilt, is used to elucidate eddy propagation and eddy feedback on the mean flow. Linear shear and jet profiles are analysed and theoretical results are compared against fully nonlinear simulations. For flows with zero planetary vorticity gradient, analytic solutions for the eddy ellipse tilt and anisotropy are obtained that provide a direct relationship between the eddy tilt and the phase difference of a normal mode solution. This allows a straightforward interpretation of the eddy-mean flow interaction in terms of classical stability theory: the initially unstable jet gives rise to eddies which are tilted "against the shear" and extract energy from the mean flow; once the jet stabilises, eddies become tilted "with the shear" and return their energy to the mean flow. For a nonzero planetary vorticity gradient, ray-tracing theory is used to predict ellipse geometry and its impact on eddy propagation within a jet. An analytic solution for the eddy tilt is found for a Rossby wave on a constant background shear. The ray tracing results broadly agree with the eddy tilt diagnosed from a fully nonlinear simulation.

“…A high-resolution model clearly showed that the link between surface-intensified eddying geostrophic flows, bottom-enhanced velocities, topographic internal gravity waves, and energy dissipation is of primary importance for the energetic of the Antarctic Circumpolar Current (Nikurashin et al 2013). Observations of turbulent mixing also support this scenario in the Southern Ocean (Heywood et al 2002;Naveira Garabato et al 2004;Sloyan 2005;Sheen et al 2013).…”

mentioning

confidence: 96%

Variability of the Turbulent Kinetic Energy Dissipation along the A25 Greenland–Portugal Transect Repeated from 2002 to 2012

Ferron

Kokoszka

Mercier

et al. 2016

The variability of the turbulent kinetic energy dissipation due to internal waves is quantified using a finescale parameterization applied to the A25 Greenland-Portugal transect repeated every two years from 2002 to 2012. The internal wave velocity shear and strain are estimated for each cruise at 91 stations from full depth vertical profiles of density and velocity. . The interannual variability in the dissipation rates is remarkably small over the whole transect. A few strong dissipation rate events exceeding the uncertainty of the finescale parameterization occur at depth between the Azores-Biscay Rise and Eriador Seamount. This region is also marked by mesoscale eddying flows resulting in enhanced surface energy level and enhanced bottom velocities. Estimates of the vertical energy fluxes into the internal tide and into topographic internal waves suggest that the latter are responsible for the strong dissipation events. At Eriador Seamount, both topographic internal waves and the internal tide contribute with the same order of magnitude to the dissipation rate while around the Reykjanes Ridge the internal tide provides the bulk of the dissipation rate.