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Abstract Horizontal and vertical wavenumbers ( k x , k z ) immediately below the Ozmidov wavenumber ( N 3 / ε ) 1/2 are spectrally distinct from both isotropic turbulence ( k x , k z > 1 cpm) and internal waves as described by the Garrett–Munk (GM) model spectrum ( k z < 0.1 cpm). A towed CTD chain, augmented with concurrent Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling float microstructure measurements and shipboard ADCP surveys, are used to characterize 2D wavenumber ( k x , k z ) spectra of isopycnal slope, vertical strain, and isopycnal salinity gradient on horizontal wavelengths from 50 m to 250 km and vertical wavelengths of 2–48 m. For k z < 0.1 cpm, 2D spectra of isopycnal slope and vertical strain resemble GM. Integrated over the other wavenumber, the isopycnal slope 1D k x spectrum exhibits a roughly +1/3 slope for k x > 3 × 10 −3 cpm, and the vertical strain 1D k z spectrum a −1 slope for k z > 0.1 cpm, consistent with previous 1D measurements, numerical simulations, and anisotropic stratified turbulence theory. Isopycnal salinity gradient 1D k x spectra have a +1 slope for k x > 2 × 10 −3 cpm, consistent with nonlocal stirring. Turbulent diapycnal diffusivities inferred in the (i) internal wave subrange using a vertical strain-based finescale parameterization are consistent with those inferred from finescale horizonal wavenumber spectra of (ii) isopycnal slope and (iii) isopycnal salinity gradients using Batchelor model spectra. This suggests that horizontal submesoscale and vertical finescale subranges participate in bridging the forward cascade between weakly nonlinear internal waves and isotropic turbulence, as hypothesized by anisotropic turbulence theory. 

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.

 

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. 

The oceanic surface mixed layer salinity (MLS) budget of the central and eastern equatorial Indian Ocean during boreal fall and winter is studied using in situ and remote sensing measurements. Budgets on roughly 100 km scale were constructed using data from twoDYNAmics of theMadden–JulianOscillation and twoResearch MooredArray for African‐Asian‐AustralianMonsoonAnalysis and Prediction moorings near 79°E during September 2011 to January 2012. The horizontal advective salinity flux plays a significant role in the seasonal variation of equatorial MLS. In boreal fall, the equatorial and 1.5°S MLS increases due to horizontal advection and turbulent mixing, despite the freshening surface flux associated with MJOs. In boreal winter, with larger sub‐monthly variation and uncertainties, the decreasing of equatorial MLS is accounted by freshening zonal advection and surface flux, abated by salty meridional advection; the 1.5°S MLS is explained by the combination of freshening meridional advection and surface flux, and salty zonal advection. Budgets between 2011 and 2015 are investigated using data products from Tropical Rainfall Measuring Mission, Aquarius, Ocean Surface Current Analyses Real‐time, Objectively Analyzed air‐sea Fluxes, and Argo mixed layers over a wider region. The eastward development of the equatorial salinity tongue in the central to eastern Indian Ocean in boreal fall and the westward retreat in boreal winter are largely determined by the equatorial zonal current. The meridional migration of ITCZ rainfall plays a secondary role. In order to improve model prediction skills of MLS changes in the equatorial Indian Ocean, both zonal and meridional salinity advective fluxes, at a spatial scale of 1° longitude and latitude and a timescale less than days, need to be properly simulated.

 

Shoaling internal solitary waves (ISWs) were observed at three mooring sites on the upper continental slope in the northern South China Sea over a period of 5–11 months at water depths of 600, 430, and 350 m. Their properties exhibit a fortnightly variation because of their origination from internal tides. ISW amplitudes, current speeds, and propagation speeds are greater and wave widths narrower in summer than in winter, consistent with the effect of increased stratification in summer, as confirmed by Dubreil‐Jacotin‐Long (DJL) solutions. As ISWs propagate up the slope, the differential response of current and propagation speeds to bottom topography provides an opportunity for convective breaking of ISWs. Convective breaking occurs mostly between 430 and 600‐m depths and exhibits a marginal convective instability status such that (a) the maximum current speed remains nearly equal to the propagation speed and (b) for large‐amplitude waves the current speed and propagation speed decrease at nearly the same rate between 600 and 430‐m depths. The marginal convective instability occurs because ISWs adjust gradually to the gently sloping bottom and preserve their structural integrity after the onset of breaking. Vertical velocity variances behind the leading ISWs, which serve as a surrogate for the number of trailing waves, increase when ISWs reach the convective breaking limit, suggesting that convective breaking may accelerate the fission process in leading ISWs or that convective breaking is accompanied by an enhanced nonlinear dispersion of waves trailing ISWs generated by internal tides.

 

Microstructure measurements in Drake Passage and on the flanks of Kerguelen Plateau find turbulent dissipation rates ε on average factors of 2–3 smaller than linear lee-wave generation predictions, as well as a factor of 3 smaller than the predictions of a well-established parameterization based on finescale shear and strain. Here, the possibility that these discrepancies are a result of conservation of wave action E/ ω_L= E/| kU| is explored. Conservation of wave action will transfer a fraction of the lee-wave radiation back to the mean flow if the waves encounter weakening currents U, where the intrinsic or Lagrangian frequency ω_L= | kU| ↓ | f| and k the along-stream horizontal wavenumber, where kU ≡ k ⋅ V. The dissipative fraction of power that is lost to turbulence depends on the Doppler shift of the intrinsic frequency between generation and breaking, hence on the topographic height spectrum and bandwidth N/ f. The partition between dissipation and loss to the mean flow is quantified for typical topographic height spectral shapes and N/ f ratios found in the abyssal ocean under the assumption that blocking is local in wavenumber. Although some fraction of lee-wave generation is always dissipated in a rotating fluid, lee waves are not as large a sink for balanced energy or as large a source for turbulence as previously suggested. The dissipative fraction is 0.44–0.56 for topographic spectral slopes and buoyancy frequencies typical of the deep Southern Ocean, insensitive to flow speed U and topographic splitting. Lee waves are also an important mechanism for redistributing balanced energy within their generating bottom current.

 

Given the increasing attention in forecasting weather and climate on the subseasonal time scale in recent years, National Oceanic and Atmospheric Administration (NOAA) announced to support Climate Process Teams (CPTs) which aim to improve the Madden‐Julian Oscillation (MJO) prediction by NOAA’s global forecasting models. Our team supported by this CPT program focuses primarily on the improvement of upper ocean mixing parameterization and air‐sea fluxes in the NOAA Climate Forecast System (CFS). Major improvement includes the increase of the vertical resolution in the upper ocean and the implementation of General Ocean Turbulence Model (GOTM) in CFS. In addition to existing mixing schemes in GOTM, a newly developed scheme based on observations in the tropical ocean, with further modifications, has been included. A better performance of ocean component is demonstrated through one‐dimensional ocean model and ocean general circulation model simulations validated by the comparison with in‐situ observations. These include a large sea surface temperature (SST) diurnal cycle during the MJO suppressed phase, intraseasonal SST variations associated with the MJO, ocean response to atmospheric cold pools, and deep cycle turbulence. Impact of the high‐vertical resolution of ocean component on CFS simulation of MJO‐associated ocean temperature variations is evident. Also, the magnitude of SST changes caused by high‐resolution ocean component is sufficient to influence the skill of MJO prediction by CFS.

 

The formation of a recirculating subsurface core in an internal solitary wave (ISW) of depression, shoaling over realistic bathymetry, is explored through fully nonlinear and nonhydrostatic two-dimensional simulations. The computational approach is based on a high-resolution/accuracy deformed spectral multidomain penalty-method flow solver, which employs the recorded bathymetry, background current, and stratification profile in the South China Sea. The flow solver is initialized using a solution of the fully nonlinear Dubreil–Jacotin–Long equation. During shoaling, convective breaking precedes core formation as the rear steepens and the trough decelerates, allowing heavier fluid to plunge forward, forming a trapped core. This core-formation mechanism is attributed to a stretching of a near-surface background vorticity layer. Since the sign of the vorticity is opposite to that generated by the propagating wave, only subsurface recirculating cores can form. The onset of convective breaking is visualized, and the sensitivity of the core properties to changes in the initial wave, near-surface background shear, and bottom slope is quantified. The magnitude of the near-surface vorticity determines the size of the convective-breaking region, and the rapid increase of local bathymetric slope accelerates core formation. If the amplitude of the initial wave is increased, the subsequent convective-breaking region increases in size. The simulations are guided by field data and capture the development of the recirculating subsurface core. The analyzed parameter space constitutes a baseline for future three-dimensional simulations focused on characterizing the turbulent flow engulfed within the convectively unstable ISW.