St. Laurent Louis C.

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St. Laurent
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Louis C.
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Now showing 1 - 8 of 8
  • Article
    Vertical kinetic energy and turbulent dissipation in the ocean
    (John Wiley & Sons, 2015-09-21) Thurnherr, Andreas M. ; Kunze, Eric ; Toole, John M. ; St. Laurent, Louis C. ; Richards, Kelvin J. ; Ruiz-Angulo, Angel
    Oceanic internal waves are closely linked to turbulence. Here a relationship between vertical wave number (kz) spectra of fine-scale vertical kinetic energy (VKE) and turbulent dissipation ε is presented using more than 250 joint profiles from five diverse dynamic regimes, spanning latitudes between the equator and 60°. In the majority of the spectra VKE varies as inline image. Scaling VKE with inline image collapses the off-equatorial spectra to within inline image but underestimates the equatorial spectrum. The simple empirical relationship between VKE and ε fits the data better than a common shear-and-strain fine-scale parameterization, which significantly underestimates ε in the two data sets that are least consistent with the Garrett-Munk (GM) model. The new relationship between fine-scale VKE and dissipation rate can be interpreted as an alternative, single-parameter scaling for turbulent dissipation in terms of fine-scale internal wave vertical velocity that requires no reference to the GM model spectrum.
  • Article
    Evaluating salt-fingering theories
    (Sears Foundation for Marine Research, 2008-07) Inoue, R. ; Kunze, Eric ; St. Laurent, Louis C. ; Schmitt, Raymond W. ; Toole, John M.
    The NATRE fine- and microstructure data set is revisited to test salt-finger amplitude theories. Dependences of the mixing efficiency Γ, microscale buoyancy Reynolds number Re and thermal Cox number CxT on 5-m density ratio Rρ and gradient Richardson number Ri are examined. The observed mixing efficiency is too high to be explained by linear fastest-growing fingers but can be reproduced by wavenumbers 0.5-0.9 times lower than the fastest-growing wavenumber. Constraining these fingers with a hybrid wave/finger Froude number or a finger Reynolds number cannot reproduce the observed trends with Rρ or Ri, respectively. This suggests that background shear has no influence on finger amplitudes. Constraining average amplitudes of these lower-wavenumber fingers with finger Richardson number Rif ~ 0.2 reproduces the observed dependence of Re and CxT on density ratio Rρ and Ri at all but the lowest observed density ratio (Rρ = 1.3). Separately relaxing the assumptions of viscous control, dominance of a single mode and tall narrow fingers does not explain the difference between theory and data at low Rρ for a critical Rif ~ 0.2.
  • Article
    Diapycnal mixing in the Antarctic Circumpolar Current
    (American Meteorological Society, 2011-01) Ledwell, James R. ; St. Laurent, Louis C. ; Girton, James B. ; Toole, John M.
    The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 ± 0.2) × 10−5 m2 s−1. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 ± 0.07) × 10−5 m2 s−1. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%–50% above the Garrett–Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.
  • Dataset
    How variable is mixing efficiency in the abyss?
    (Woods Hole Oceanographic Institution, 2020-03-02) Ijichi, Takashi ; St. Laurent, Louis C. ; Polzin, Kurt L. ; Toole, John M.
    This directory contains BBTRE/DoMORE processed data (“all_BBTRE.mat” and “all_DoMORE.mat”) that was used to produce all figures in the above research letter. Each mat file has two structure arrays named “location” and “patch10”. The “location” array includes microstructure profile information used in this study (Table D1). The “patch10” array includes 10-m patch-wise parameter estimates used in this study (Table D2). Note that bulk averaged parameters can be constructed from parameters saved in “patch10” (see the above paper).
  • Article
    How variable is mixing efficiency in the abyss?
    (American Geophysical Union, 2020-03-28) Ijichi, Takashi ; St. Laurent, Louis C. ; Polzin, Kurt L. ; Toole, John M.
    Mixing efficiency is an important turbulent flow property in fluid dynamics, whose variability potentially affects the large‐scale ocean circulation. However, there are several confusing definitions. Here we compare and contrast patch‐wise versus bulk estimates of mixing efficiency in the abyss by revisiting data from previous extensive field surveys in the Brazil Basin. Observed patch‐wise efficiency is highly variable over a wide range of turbulence intensity. Bulk efficiency is dominated by rare extreme turbulence events. In the case where enhanced near‐bottom turbulence is thought to be driven by breaking of small‐scale internal tides, the estimated bulk efficiency is 20%, close to the conventional value of 17%. On the other hand, where enhanced near‐bottom turbulence appears to be convectively driven by hydraulic overflows, bulk efficiency is suggested to be as large as 45%, which has implications for a further significant role of overflow mixing on deep‐water mass transformation.
  • Article
    Turbulence and diapycnal mixing in Drake Passage
    (American Meteorological Society, 2012-12) St. Laurent, Louis C. ; Naveira Garabato, Alberto C. ; Ledwell, James R. ; Thurnherr, Andreas M. ; Toole, John M. ; Watson, Andrew J.
    Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.
  • Article
    Rates and mechanisms of turbulent dissipation and mixing in the Southern Ocean : results from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES)
    (John Wiley & Sons, 2013-06-04) Sheen, Katy L. ; Brearley, J. Alexander ; Naveira Garabato, Alberto C. ; Smeed, David A. ; Waterman, Stephanie N. ; Ledwell, James R. ; Meredith, Michael P. ; St. Laurent, Louis C. ; Thurnherr, Andreas M. ; Toole, John M. ; Watson, Andrew J.
    The spatial distribution of turbulent dissipation rates and internal wavefield characteristics is analyzed across two contrasting regimes of the Antarctic Circumpolar Current (ACC), using microstructure and finestructure data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Mid-depth turbulent dissipation rates are found to increase from inline image in the Southeast Pacific to inline image in the Scotia Sea, typically reaching inline image within a kilometer of the seabed. Enhanced levels of turbulent mixing are associated with strong near-bottom flows, rough topography, and regions where the internal wavefield is found to have enhanced energy, a less-inertial frequency content and a dominance of upward propagating energy. These results strongly suggest that bottom-generated internal waves play a major role in determining the spatial distribution of turbulent dissipation in the ACC. The energy flux associated with the bottom internal wave generation process is calculated using wave radiation theory, and found to vary between 0.8 mW m−2 in the Southeast Pacific and 14 mW m−2 in the Scotia Sea. Typically, 10%–30% of this energy is found to dissipate within 1 km of the seabed. Comparison between turbulent dissipation rates inferred from finestructure parameterizations and microstructure-derived estimates suggests a significant departure from wave-wave interaction physics in the near-field of wave generation sites.
  • Article
    Enhanced diapycnal diffusivity in intrusive regions of the Drake Passage
    (American Meteorological Society, 2016-04-05) Merrifield, Sophia T. ; St. Laurent, Louis C. ; Owens, W. Brechner ; Thurnherr, Andreas M. ; Toole, John M.
    Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate ε estimated by microstructure are up to two orders of magnitude lower than previously published estimates in the upper 1000 m. Turbulence levels in Drake Passage are systematically higher than values upstream, regardless of season. The dissipation of thermal variance χ is enhanced at middepth throughout the surveys, with the highest values found in northern Drake Passage, where water mass variability is the most pronounced. Using the density ratio, evidence for double-diffusive instability is presented. Subject to double-diffusive physics, the estimates of diffusivity using the Osborn–Cox method are larger than ensemble statistics based on ε and the buoyancy frequency.