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1. What controls the deep cycle? Proxies for equatorial turbulence.

   https://doi.org/10.1175/JPO-D-20-0236.1  

   Smyth, W D; Warner, S J; Moum, J N; Pham, H; Sarkar, S (May 2021, Journal of Physical Oceanography)

   AMS (Ed.)

   Abstract Factors thought to influence deep cycle turbulence in the equatorial Pacific are examined statistically for their predictive capacity using a 13-year moored record that includes microstructure measurements of the turbulent kinetic energy dissipation rate. Wind stress and mean current shear are found to be most predictive of the dissipation rate. Those variables, together with the solar buoyancy flux and the diurnal mixed layer thickness, are combined to make a pair of useful parameterizations. The uncertainty in these predictions is typically 50% greater than the uncertainty in present-day in situ measurements. To illustrate the use of these parameterizations, the record of deep cycle turbulence, measured directly since 2005, is extended back to 1990 based on historical mooring data. The extended record is used to refine our understanding of the seasonal variation of deep cycle turbulence. 

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2. Energetic Stratified Turbulence Generated by Kuroshio–Seamount Interactions in Tokara Strait

   https://doi.org/10.1175/JPO-D-22-0242.1  

   Takahashi, Anne; Lien, Ren-Chieh; Kunze, Eric; Ma, Barry; Nakamura, Hirohiko; Nishina, Ayako; Tsutsumi, Eisuke; Inoue, Ryuichiro; Nagai, Takeyoshi; Endoh, Takahiro (February 2024, Journal of Physical Oceanography)

   Abstract Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity, and microscale thermal-variance dissipation rate up- and downstream of the ∼10-km-wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation ratesand diapycnal diffusivitiesabove the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence, which is expected to decay in less than 100 m based on turbulence decay time ofN⁻¹∼ 100 s and the 0.5 m s⁻¹Kuroshio flow speed. Thus, the turbulent wake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortex wake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy, and (iii) reduced-shear, are tested. Averageεvertical profiles are well reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent ofεand spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase withε. 

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3. A scaling law for the shear-production range of second-order structure functions

   https://doi.org/10.1017/jfm.2016.427  

   Pan, Y.; Chamecki, M. (August 2016, Journal of Fluid Mechanics)

   null (Ed.)

   Dimensional analysis suggests that the dissipation length scale ( $$\ell _{{\it\epsilon}}=u_{\star }^{3}/{\it\epsilon}$$ ) is the appropriate scale for the shear-production range of the second-order streamwise structure function in neutrally stratified turbulent shear flows near solid boundaries, including smooth- and rough-wall boundary layers and shear layers above canopies (e.g. crops, forests and cities). These flows have two major characteristics in common: (i) a single velocity scale, i.e. the friction velocity ( $$u_{\star }$$ ) and (ii) the presence of large eddies that scale with an external length scale much larger than the local integral length scale. No assumptions are made about the local integral scale, which is shown to be proportional to $$\ell _{{\it\epsilon}}$$ for the scaling analysis to be consistent with Kolmogorov’s result for the inertial subrange. Here $${\it\epsilon}$$ is the rate of dissipation of turbulent kinetic energy (TKE) that represents the rate of energy cascade in the inertial subrange. The scaling yields a log-law dependence of the second-order streamwise structure function on ( $$r/\ell _{{\it\epsilon}}$$ ), where $$r$$ is the streamwise spatial separation. This scaling law is confirmed by large-eddy simulation (LES) results in the roughness sublayer above a model canopy, where the imbalance between local production and dissipation of TKE is much greater than in the inertial layer of wall turbulence and the local integral scale is affected by two external length scales. Parameters estimated for the log-law dependence on ( $$r/\ell _{{\it\epsilon}}$$ ) are in reasonable agreement with those reported for the inertial layer of wall turbulence. This leads to two important conclusions. Firstly, the validity of the $$\ell _{{\it\epsilon}}$$ -scaling is extended to shear flows with a much greater imbalance between production and dissipation, indicating possible universality of the shear-production range in flows near solid boundaries. Secondly, from a modelling perspective, $$\ell _{{\it\epsilon}}$$ is the appropriate scale to characterize turbulence in shear flows with multiple externally imposed length scales. 

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4. Shear Instability and Turbulent Mixing in the Stratified Shear Flow Behind a Topographic Ridge at High Reynolds Number

   https://doi.org/10.3389/fmars.2022.829579  

   Chen, Jia-Lin; Yu, Xiao; Chang, Ming-Huei; Jan, Sen; Yang, Yiing Jang; Lien, Ren-Chieh (May 2022, Frontiers in Marine Science)

   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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5. Moored Turbulence Measurements using Pulse-Coherent Doppler Sonar

   https://doi.org/10.1175/jtech-d-21-0005.1  

   Zippel, Seth F.; Farrar, J. Thomas; Zappa, Christopher J.; Miller, Una; Laurent, Louis St.; Ijichi, Takashi; Weller, Robert A.; McRaven, Leah; Nylund, Sven; Le Bel, Deborah (July 2021, Journal of Atmospheric and Oceanic Technology)

   Abstract Upper-ocean turbulence is central to the exchanges of heat, momentum, and gasses across the air/sea interface, and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed-layer depths and sea surface temperature. In part, progress has been limited due to the difficulty of measuring turbulence from fixed moorings which can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring Turbulent Kinetic Energy (TKE) dissipation rates, ϵ , from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-ocean Regional Study (SPURS) to collect two year-long data sets. We find the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate two-week missions for (10 −8 ) ≤ ϵ ≤ (10 −5 ) m 2 s −3 . Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1-10 km. We also find that dissipation estimates from two different moorings at 12.5 m, and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory. 

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