PSI to turbulence during internal wave beam refraction through the upper ocean pycnocline

Gayen, Bishakhdatta; Sarkar, Sutanu

doi:10.1002/2014gl061226

Cited by 7 publications

(6 citation statements)

References 16 publications

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“…Results from other numerical studies of oscillating flows with convective and shear-driven turbulence, e.g. an oscillating stratified boundary layer on a slope (Chalamalla & Sarkar 2015) and internal wave breaking at a pycnocline owing to parametric subharmonic instability (Gayen & Sarkar 2014) have reported similar findings of Γ > 0.2, with the asymptotic values of Γ reaching 0.3-0.4 in most cases. The long-time Γ (figure 13b) lies between 0.24 and 0.27, independent of the slope angle, when β = 0.…”

Section: Flow Over Steeper Slopessupporting

confidence: 56%

Energetics and mixing in buoyancy-driven near-bottom stratified flow

et al. 2019

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Turbulence and mixing in a near-bottom convectively driven flow are examined by numerical simulations of a model problem: a statically unstable disturbance at a slope with inclination $\unicode[STIX]{x1D6FD}$ in a stable background with buoyancy frequency $N$. The influence of slope angle and initial disturbance amplitude are quantified in a parametric study. The flow evolution involves energy exchange between four energy reservoirs, namely the mean and turbulent components of kinetic energy (KE) and available potential energy (APE). In contrast to the zero-slope case where the mean flow is negligible, the presence of a slope leads to a current that oscillates with $\unicode[STIX]{x1D714}=N\sin \unicode[STIX]{x1D6FD}$ and qualitatively changes the subsequent evolution of the initial density disturbance. The frequency, $N\sin \unicode[STIX]{x1D6FD}$, and the initial speed of the current are predicted using linear theory. The energy transfer in the sloping cases is dominated by an oscillatory exchange between mean APE and mean KE with a transfer to turbulence at specific phases. In all simulated cases, the positive buoyancy flux during episodes of convective instability at the zero-velocity phase is the dominant contributor to turbulent kinetic energy (TKE) although the shear production becomes increasingly important with increasing $\unicode[STIX]{x1D6FD}$. Energy that initially resides wholly in mean available potential energy is lost through conversion to turbulence and the subsequent dissipation of TKE and turbulent available potential energy. A key result is that, in contrast to the explosive loss of energy during the initial convective instability in the non-sloping case, the sloping cases exhibit a more gradual energy loss that is sustained over a long time interval. The slope-parallel oscillation introduces a new flow time scale $T=2\unicode[STIX]{x03C0}/(N\sin \unicode[STIX]{x1D6FD})$ and, consequently, the fraction of initial APE that is converted to turbulence during convective instability progressively decreases with increasing $\unicode[STIX]{x1D6FD}$. For moderate slopes with $\unicode[STIX]{x1D6FD}<10^{\circ }$, most of the net energy loss takes place during an initial, short ($Nt\approx 20$) interval with periodic convective overturns. For steeper slopes, most of the energy loss takes place during a later, long ($Nt>100$) interval when both shear and convective instability occur, and the energy loss rate is approximately constant. The mixing efficiency during the initial period dominated by convectively driven turbulence is found to be substantially higher (exceeds 0.5) than the widely used value of 0.2. The mixing efficiency at long time in the present problem of a convective overturn at a boundary varies between 0.24 and 0.3.

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Section: Flow Over Steeper Slopessupporting

confidence: 56%

Energetics and mixing in buoyancy-driven near-bottom stratified flow

et al. 2019

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“…At the beams and in the vicinity of generation, the dominant mechanism is shear instability evidenced by regions with R i g <0.25, although visualizations show that TKE patches from different regions can also get advected by the strong baroclinic velocity. In addition, other instabilities such as parametric subharmonic instability (Gayen & Sarkar, ) can also lead to turbulence, specifically during beam refraction through the pycnocline. TKE is intensified where the IW beam hits the pycnocline.…”

Section: Flow and Turbulencementioning

confidence: 99%

Large Eddy Simulation of Flow and Turbulence at the Steep Topography of Luzon Strait

Jalali

Sarkar

2017

Geophysical Research Letters

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Steep generation sites for topographic internal gravity waves can also be sites of turbulence. The present work uses turbulence‐resolving Large Eddy Simulation (LES) for tidal flow over multiscale, steep bathymetry patterned after a portion of the west ridge of Luzon Strait at 20.6∘N. Oceanic values of the slope criticality, Froude number, excursion number, and aspect ratio are matched. The amplitude and phasing of velocity, overturns, and dissipation are similar to observations made at a deep mooring. However a tidal phase is noted in which there is a discrepancy that could arise from the resonant interaction with waves generated at the east ridge that is not included in the model. Turbulent dissipation originates from several mechanisms including breaking lee waves during flow reversal, downslope jets, critical‐slope boundary layer, high‐mode internal wave beams, off‐slope lee wave breaking and wave breaking in a valley between subridges. The model baroclinic energy budget is closed with negligible residual and the local turbulence loss of 23.5% is large.

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“…Given that the vertical components of k 1 and k 2 can be an order of magnitude larger than that of k, PSI is effective in cascading wave energy toward turbulence. In later 3D LES, Gayen & Sarkar (2014) demonstrated the cascade to turbulence of the subharmonic. Only 30% of the incident wave energy is contained in the main reflected beam, with the remaining carried by subharmonics from PSI (20%), ISWs and harmonics trapped in the pycnocline (15%), and other downgoing waves (35%).…”

Section: Parametric Subharmonic Instability (Psi)mentioning

confidence: 99%

“…Finally, both mechanisms can be present. Gayen & Sarkar (2014) showed mixing starts as convective driven and only at a later time becomes sustained by shear.…”

Section: Baroclinic Energy Budgetmentioning

confidence: 99%

From Topographic Internal Gravity Waves to Turbulence

Sarkar

Scotti²

2017

Annu. Rev. Fluid Mech.

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Internal gravity waves are a key process linking the large-scale mechanical forcing of the oceans to small-scale turbulence and mixing. In this review, we focus on internal waves generated by barotropic tidal flow over topography. We review progress made in the past decade toward understanding the different processes that can lead to turbulence during the generation, propagation, and reflection of internal waves and how these processes affect mixing. We consider different modeling strategies and new tools that have been developed. Simulation results, the wealth of observational material collected during large-scale experiments, and new laboratory data reveal how the cascade of energy from tidal flow to turbulence occurs through a host of nonlinear processes, including intensified boundary flows, wave breaking, wave-wave interactions, and the instability of high-mode internal wave beams. The roles of various nondimensional parameters involving the ocean state, roughness geometry, and tidal forcing are described.

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PSI to turbulence during internal wave beam refraction through the upper ocean pycnocline

Cited by 7 publications

References 16 publications

Energetics and mixing in buoyancy-driven near-bottom stratified flow

Energetics and mixing in buoyancy-driven near-bottom stratified flow

Large Eddy Simulation of Flow and Turbulence at the Steep Topography of Luzon Strait

From Topographic Internal Gravity Waves to Turbulence

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