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Scholarly Works (5 results)

  • Article
  • Peer Reviewed
UC San Diego Previously Published Works (2014)
Depth-time records of isopycnal vertical strain have been collected from intensive CTD profiling programs on the research platform (R/P) Floating Instrument Platform (FLIP). The associated vertical wavenumber frequency spectrum of strain, when viewed in an isopycnal-following frame, displays a clear spectral gap at low vertical wavenumber, separating the quasigeostrophic (vortical) strain field and the superinertial internal wave continuum. This gap enables both model and linear-filter-based methods for separating the submesoscale and internal wave strain fields. These fields are examined independently in six field programs spanning the period 1983-2002. Vortical and internal wave strain variances are often comparable in the upper thermocline, of order 0.2. However, vortical strain tends to decrease with increasing depth (decreasing buoyancy frequency as N2 = -g/ρ(dρ/dz) ~(N2)1/2, while internal wave strain variance increases as ~(N2)1/2, exceeding vortical variance by a factor of 5-10 at depths below 500 m. In contrast to strain, the low-frequency spectral gap in the shear spectrum is largely obscured by Doppler-smeared near-inertial motions. The vertical wavenumber spectrum of anticyclonic shear exceeds the cyclonic shear and strain spectra at all scales greater than 10 m. The frequency spectrum of anticyclonic shear exceeds that of both cyclonic shear and strain to frequencies of 0.5 cph, emphasizing the importance of lateral Doppler shifting of near-inertial shear. The limited Doppler shifting of the vortical strain field implies surprisingly small submesoscale aspect ratios: kH/kz ~ 0.001, Burger numbers Br = kH N/kzf ~ 0.1. Submesoscale potential vorticity is dominated by vertical straining rather than the vertical component of relative vorticity. The inferred rms fluctuation of fluid vorticity is far less for the vortical field than for the internal wavefield. © 2014 American Meteorological Society.
    Cover page: Vortical and Internal Wave Shear and Strain
    • Article
    International Symposium on Stratified Flows, Volume 8 (2016)
      Cover page: Tidal mixing, scattering, and reflection on the East Tasman slope
      • Thesis
      • Peer Reviewed
      UC San Diego Electronic Theses and Dissertations (2015)

      We describe and interpret in situ observations of tidally driven turbulence that were obtained in the vicinity of a small channel that transects the crest of the Mendocino Ridge in the north-eastern Pacific. Flows are funneled through the channel and have tidal excursion lengths comparable to the width of the ridge crest. Once per day, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave past the site location, and subsequent devel- opment of a hydraulic jump. During this phase of the tide the strong transports were associated with full depth flows, however, during the weaker beat of the tide transports were shallow and surface-confined, generating negligible turbulence. A regional numerical model of the area finds that the subinertial K1 (diurnal) tidal constituent generates topographically trapped waves which propagate anticycloni- cally around the ridge, and are associated with enhanced near-topographic K1 transports. The interaction of the trapped waves with the M2 (semidiurnal) sur- face tide produces a baroclinic tidal flow that is alternately surface confined and full depth. Consistent with observations, full depth flows are associated with the gen- eration of a large amplitude tidal lee wave on the northward face of the ridge, while surface confined flows are largely nonturbulent. The regional model demonstrates that nearfield dissipation over the entire ridge is diurnally modulated, despite the larger amplitude of the M2 tidal constituent, indicating that the trapped wave modulates near-field dissipation and mixing at this location.

      In the final section, we study the interaction of the barotropic tide with a tall, two-dimensional ridge both analytically and numerically at latitudes where the tide is subinertial, and contrast it to when the tide is superinertial. When the tide is subinertial the energy density associated with the response grows with latitude as both the oscillatory along-ridge flow and near ridge isopycnal displacement become large. Nonlinear processes lead to the formation of along-ridge jets, which become faster at high latitudes. Mixing occurs mainly through hydraulic jumps on the flanks of the topography in both cases, though a superinertial tide may additionally generate mixing above topography arising from the convective breaking of radiating waves.

      • 2 supplemental videos
      Cover page: Tidal interactions with topography: the effects of latitude and tidal constitution on nearfield mixing
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform density mixed layer. Vertical profiles of bio-optical measurements from biogeochemical Argo floats and elephant seal tags in the Southern Ocean are used to assess biological structure in the upper ocean. Within the hydrographically defined mixed layer, the profiles show significant vertical variance in chlorophyll-a (Chl-a) fluorescence and particle optical backscatter. Biological structure is assessed by fitting Chl-a fluorescence and particle backscatter profiles to functional forms (i.e., Gaussian, sigmoid, exponential, and their combinations). In the Southern Ocean, which characteristically has deep mixed layers, only 40% of nighttime bio-optical profiles were characterized by a sigmoid, indicating a well-mixed surface layer. Of the remaining 60% that showed structure, ∼40% had a deep fluorescence maximum below 20-m depth that correlated with particle backscatter. Furthermore, a significant fraction of these deep fluorescence maxima were found within the mixed layer (20–80%, depending on mixed-layer depth definition and season). Results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification. We hypothesize that periods of quiescence between synoptic storms, which we estimate to be ∼3–5 days (depending on season), allow bio-optical gradients to develop within mixed layers that remain homogeneous in density.
        Cover page: When Mixed Layers Are Not Mixed. Storm‐Driven Mixing and Bio‐optical Vertical Gradients in Mixed Layers of the Southern Ocean
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2017)
        Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.
          Cover page: Climate Process Team on Internal Wave-Driven Ocean Mixing.
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