St. Laurent
Louis C.
St. Laurent
Louis C.
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ArticleVertical 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, AngelOceanic 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.
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ArticleModification of turbulent dissipation rates by a deep Southern Ocean eddy(John Wiley & Sons, 2015-05-07) Sheen, Katy L. ; Brearley, J. Alexander ; Naveira Garabato, Alberto C. ; Smeed, David A. ; St. Laurent, Louis C. ; Meredith, Michael P. ; Thurnherr, Andreas M. ; Waterman, Stephanie N.The impact of a mesoscale eddy on the magnitude and spatial distribution of diapycnal ocean mixing is investigated using a set of hydrographic and microstructure measurements collected in the Southern Ocean. These data sampled a baroclinic, middepth eddy formed during the disintegration of a deep boundary current. Turbulent dissipation is suppressed within the eddy but is elevated by up to an order of magnitude along the upper and lower eddy boundaries. A ray tracing approximation is employed as a heuristic device to elucidate how the internal wave field evolves in the ambient velocity and stratification conditions accompanying the eddy. These calculations are consistent with the observations, suggesting reflection of internal wave energy from the eddy center and enhanced breaking through critical layer processes along the eddy boundaries. These results have important implications for understanding where and how internal wave energy is dissipated in the presence of energetic deep geostrophic flows.
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ArticleTransformation and upwelling of bottom water in fracture zone valleys(American Meteorological Society, 2020-03-03) Thurnherr, Andreas M. ; Clément, Louis ; St. Laurent, Louis C. ; Ferrari, Raffaele ; Ijichi, TakashiClosing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.
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ArticleTurbulence observations in a buoyant hydrothermal plume on the East Pacific Rise(The Oceanography Society, 2012-03) Thurnherr, Andreas M. ; St. Laurent, Louis C.Hot vent fluid enters the ocean at high-temperature hydrothermal vents, also known as black smokers. Because of the large temperature difference between the vent fluid and oceanic near-bottom waters, the hydrothermal effluent initially rises as a buoyant plume through the water column. During its rise, the plume engulfs and mixes with background ocean water. This process, called entrainment, gradually reduces the density of the rising plume until it reaches its level of neutral buoyancy, where the plume density equals that of the background water, and it begins to spread along a surface of constant density.
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ArticleTurbulence 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.
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ArticleTurbulent mixing in a deep fracture zone on the Mid-Atlantic Ridge(American Meteorological Society, 2017-07-13) Clément, Louis ; Thurnherr, Andreas M. ; St. Laurent, Louis C.Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04–0.10-Sv (1 Sv ≡ 106 m3 s−1) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy ε increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides, ε varies from O(10−9) to O(10−10) W kg−1 below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of ε. The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves.
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ArticleRates 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.
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ArticleEnhanced 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.
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ArticleTurbulence and diapycnal mixing over the East Pacific Rise crest near 10°N(American Geophysical Union, 2011-08-13) Thurnherr, Andreas M. ; St. Laurent, Louis C.Turbulent mixing plays an important role in the return path of the global overturning circulation of the ocean. Previous measurements indicate that much of the mixing takes place near topography, in particular near seamounts and mid-ocean ridges. Here we report on the first microstructure data set collected over the crest and flanks of a fast-spreading ridge. The data indicate that in spite of weak tidally modulated background turbulence levels (ε ≈ 10−10 W kg−1) diapycnal diffusivity is elevated above 10−4 m2 s−1 below crest depth of the ridge throughout the entire region because of the weak density stratification. Near the peaks and in the narrow deep passages of a chain of seamounts, where large horizontal velocities have been observed, turbulence levels are elevated by up to an order of magnitude above background. We conclude that topographic organization plays an important role in determining spatial patterns of turbulence in this region and that both tidal and subinertial energy contribute to the mixing.