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Abstract Finescale properties of Kelvin–Helmholtz (KH)-like shear instabilities on the trailing edge of a nonlinear lee wave generated by the Kuroshio impinging on a seamount were measured using a towed CTD chain, shipboard ADCP, and echosounder. Lee-wave vertical velocity amplitudes vary in phase with the upstream semidiurnal along-stream current. The instabilities are analogous to atmospheric billows induced by a recirculation on the trailing edge of mountain lee waves. A total of 135 KH billows were identified in a 4-day-long time series roughly 300 m downstream of the center of the lee wave. The KH billows have heightsH= 52 ± 11 m, widthsL= 162 ± 72 m, and aspect ratiosH/L= 0.39 ± 0.18. Positive reduced shear squaredS2− 4N2(whereSis the vertical shear magnitude andNis the buoyancy frequency) in the shear-stratified billows suggests actively growing instabilities, with comparable contributions from across- and along-flow vertical shear. Billow cores are convectively unstable (N2< 0). Large turbulent kinetic energy dissipation ratesare inferred from density overturns. Density, shear, and inferred turbulence properties vary with billow aspect ratios. AsH/Lincreases, density gradients smear out. For 122 billows withH/L< 0.6, dissipation rates increase by one order of magnitude with increasingH/L. These observations of ∼1-m vertical and ∼5-m horizontal resolution billow structures and density overturn dissipation rates can provide a reference for future high-Reynolds-number direct numerical simulations.
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Shear Instability and Turbulent Mixing in the Stratified Shear Flow Behind a Topographic Ridge at High Reynolds Number
Observations on the lee of a topographic ridge show that the turbulence kinetic energy (TKE) dissipation rate due to shear instabilities is three orders of magnitude higher than the typical value in the open ocean. Laboratory-scale studies at low Reynolds number suggest that high turbulent dissipation occurs primarily within the core region of shear instabilities. However, field-scale studies indicate that high turbulence is mainly populated along the braids of shear instabilities. In this study, a high-resolution, resolving the Ozmidov-scale, non-hydrostatic model with Large Eddy Simulation (LES) turbulent closure is applied to investigate dominant mechanisms that control the spatial and temporal scales of shear instabilities and resulting mixing in stratified shear flow at high Reynolds number. The simulated density variance dissipation rate is elevated in the cusp-like bands of shear instabilities with a specific period, consistent with the acoustic backscatter taken by shipboard echo sounder. The vertical length scale of each cusp-like band is nearly half of the vertical length scale of the internal lee wave. However, it is consistent with instabilities originating from a shear layer based on linear stability theory. The model results indicate that the length scale and/or the period of shear instabilities are the key parameters to the mixing enhancement that increases with lateral Froude number Fr L , i.e. stronger shear and/or steeper ridge.
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- Award ID(s):
- 2128895
- PAR ID:
- 10355953
- Date Published:
- Journal Name:
- Frontiers in Marine Science
- Volume:
- 9
- ISSN:
- 2296-7745
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3389/fmars.2022.829579
