Efficiency of mixing in the main thermocline

Estimates of heat flux from direct measurements of vertical velocity-temperature fluctuation correlations have been obtained from vertical profiles through turbulent patches in the main thermocline. These have been compared to more indirect flux estimates derived from dissipation rates of turbulent kinetic energy and temperature variance. Because record lengths are limited by the thickness of observed turbulent patches, uncertainties are larger than would be expected from relatively longer horizontal records. The best estimate of dissipation flux coefficient from these data is about 0.15-0.2, but it is characterized by a large range of sample values. This implies mixing efficiencies (flux Richardson numbers) are about 0.13-0.17. This is within the range of laboratory estimates but is different from measurements in turbulent tidal fronts

Ocean Speed and Turbulence Measurements Using Pitot-Static Tubes on Moorings

A low-power (<10 mW), physically small (15.6 cm long × 3.2 cm diameter), lightweight (600 g Cu; alternatively, 200 g Al), robust, and simply calibrated pitot-static tube to measure mean speed and turbulence dissipation (ε ) is described and evaluated. The measurement of speed is derived from differential pressure via Bernoulli’s principle. The differential pressure sensor employed here has relatively small, but significant, adverse sensitivities to static pressure, temperature, and acceleration, which are characterized in tests in the college’s laboratory. Results from field tests on moorings indicate acceptable agreement in pitot-static speed measurements with independent acoustic Doppler current profiler speeds, characterized as linear fits with slope = 1 (95% confidence), ±0.02 m s⁻¹ bias, and root-mean-square error of residuals (observed minus fitted values) = 0.055 m s⁻ ¹. Direct estimates of ε are derived from fits of velocity spectra to a theoretical turbulence inertial subrange. From near-bottom measurements, these estimates are interpreted as seafloor friction velocities, which yield drag coefficients consistent with expected values. Noise levels for ε, based on 40-min spectral fits, are <10⁻⁹ m² s⁻³. In comparison to the airfoil (or shear) probe, the pitot-static tube provides the full spectrum of velocity, not just the dissipation range of the spectrum. In comparison to acoustic measurements of velocity, the pitot-static tube does not require acoustic scatters in the measurement volume. This makes the sensor a candidate for use in the deep ocean, for example, where acoustic scatterers are weak.Keywords: In situ oceanic observations, Instrumentation/sensorsKeywords: In situ oceanic observations, Instrumentation/sensor

Energy-containing scales of turbulence in the ocean thermocline

From measurements of the energy‐containing scales of turbulence in the ocean thermocline, two new formulations are examined: (1) an inviscid estimate for the viscous dissipation rate of turbulent kinetic energy and (2) a mixing length estimate for the turbulent heat flux. These formulations are tested using coincident measurements of the relevant properties of both energy‐containing and dissipation scales of stratified turbulence in the ocean's main thermocline obtained from a vertical microstructure profiler. It is found that energy‐containing scale estimates of both dissipation rate and heat flux compare favorably with dissipation scale estimates. Since the energy‐containing scales are many times greater than the dissipation scales, the measurement constraints on these new estimates are considerably less strict than for dissipation scale estimates of the same quantities. These observations also suggest that the timescale for viscous decay of turbulent motions is greater than that for diffusive smoothing of scalar fluctuations. It is argued that this is consistent with current estimates of mixing efficiencies

Observations of broadband acoustic backscattering from nonlinear internal waves : assessing the contribution from microstructure

Author Posting. © IEEE, 2010. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 35 (2010): 695-709, doi:10.1109/JOE.2010.2047814.In this paper, measurements of high-frequency broadband (160-590 kHz) acoustic backscattering from surface trapped nonlinear internal waves of depression are presented. These waves are ideal for assessing the contribution from oceanic microstructure to scattering as they are intensely turbulent. Almost coincident direct microstructure measurements were performed and zooplankton community structure was characterized using depth-resolved net sampling techniques. The contribution to scattering from microstructure can be difficult to distinguish from the contribution to scattering from zooplankton using a single narrowband frequency as microstructure and zooplankton are often colocated and can have similar scattering levels over a range of frequencies. Yet their spectra are distinct over a sufficiently broad frequency range, allowing broadband backscattering measurements to reduce the ambiguities typically associated with the interpretation of narrowband measurements. In addition, pulse compression signal processing techniques result in very high-resolution images, allowing physical processes that are otherwise hard to resolve to be imaged, such as Kelvin-Helmholtz shear instabilities. In this study, high-resolution acoustic observations of multiple nonlinear internal waves are presented and regions with distinct scattering spectra are identified. Spectra that decrease in level across the available frequency band were highly correlated to regions of intense turbulence and high stratification, and to Kevin-Helmholtz shear instabilities in particular. Spectra that increase in level across the available frequency band were consistent with scattering dominated by small zooplankton. Simple inversions for relevant microstructure parameters are presented. Limitations of, and improvements to, the broadband system and techniques utilized in this study are discussed.This work was supported in part by the Woods Hole Oceanographic Institution and the U.S. Office of Naval Research under Grant N000140210359

Inertial-Convective Subrange Estimates of Thermal Variance Dissipation Rate from Moored Temperature Measurements

A procedure for estimating thermal variance dissipation rate χ[subscript]T by scaling the inertial-convective subrange of temperature gradient spectra from thermistor measurements on a Tropical Atmosphere Ocean (TAO) equatorial mooring, maintained by NOAA’s National Data Buoy Center, is demonstrated. The inertial-convective subrange of wavenumbers/frequencies is contaminated by the vertical motion induced by the pumping of the surface float by surface gravity waves through the local vertical temperature gradient. The uncontaminated signal can be retrieved by removing the part of the measured signal that is coherent with the signal induced by surface gravity waves, which must be measured independently. An estimate of χ[subscript]T is then obtained by fitting corrected spectra to theoretical temperature gradient spectra over the inertial-convective subrange (0.05 < ƒ < 0.5 Hz); this estimate is referred to as χ[subscript]T[superscript]IC. Here χ[subscript]T[superscript]IC was calculated over 120-min intervals and compared with estimates of χ[subscript]T[superscript]o determined by scaling temperature gradient spectra at high wavenumbers (viscous-convective and viscous-diffusive subranges). Large differences up to a factor of 20 and of unknown origin occur infrequently, especially when both background currents and vertical temperature gradients are weak, but the results herein indicate that 75% of the data pairs are within a factor of 3 of each other. Tests on 15-, 30-, 60-, 120-min intervals demonstrate that differences between the two methods are nearly random, unbiased, and less than estimates of natural variability determined from unrelated experiments at the same location. Because the inertial-convective subrange occupies a lower-frequency range than is typically used for turbulence measurements, the potential for more routine measurements of χ[subscript]T exists. The evaluation of degraded signals (resampled from original measurements) indicates that a particularly important component of such a measurement is the independent resolution of the surface wave–induced signal.KEYWORDS: Kinetic energy, Temperature, Surface observatons, Oceanic wave

Ocean Mixing by Kelvin-Helmholtz Instability

Kelvin-Helmholtz (KH) instability, characterized by the distinctive finite-amplitude billows it generates, is an important mechanism in the development of turbulence in the stratified interior of the ocean. In particular, it is often assumed that the onset of turbulence in internal waves begins in this way. Clear recognition of the importance of KH instability to ocean mixing arises from recent observations of the phenomenon in a broad range of oceanic environments. KH instability is a critical link in the chain of events that leads from internal waves to mixing. After 150 years of research, identifying the prevalence of KH instability in the ocean and defining useful parameterizations that quantify its contribution to ocean mixing in numerical models remain first-order problems

Internal hydraulic flows on the continental shelf: High drag states over a small bank

Observations of currents, hydrography, and turbulence provide unambiguous evidence for hydraulic control of flow over an isolated three-dimensional topographic feature on Oregon’s continental shelf. The flow becomes critical at the crest of the bank, forming a strong supercritical downslope flow in the lower layer. Farther downstream, internal hydraulic jumps form as the bottom flow becomes subcritical. As a consequence, turbulence is greatly enhanced in the bottom boundary layer, in the sheared interface above the swiftly flowing bottom current, and in the internal hydraulic jump. The dissipation rate of turbulent energy is consistent with the mean energy removal rate for a hydraulic jump in an idealized two-layer flow. This enhanced turbulence constitutes a “high drag” state of the flow in which the form drag introduced by the topography exerts significant influences on the flow around it and mixing is increased 10² - 10³ times the background values

Internal solitary waves of elevation advancing on a shoaling shelf

A sequence of three internal solitary waves of elevation were observed propagating shoreward along a near-bottom density interface over Oregon’s continental shelf. These waves are highly turbulent and coincide with enhanced optical backscatter, consistent with increased suspended sediments in the bottom boundary layer. Nonlinear solitary wave solutions are employed to estimate wave speeds and energy. The waves are rank ordered in amplitude, phase speed, and energy, and inversely ordered in width. Wave kinetic energy is roughly twice the potential energy. The observed turbulence is not sufficiently large to dissipate the waves’ energy before the waves reach the shore. Because of high wave velocities at the sea bed, bottom stress is inferred to be an important source of wave energy loss, unlike near-surface solitary waves. The wave solution suggests that the lead wave has a trapped core, implying enhanced cross-shelf transport of fluid and biology

Enhancement of fronts by vertical mixing

Microstructure observations near upwelled fronts indicate considerable variation in the structure of vertical mixing across the frontal region. Observations of cool filaments off northern California indicate that within the cool (dense) core of filaments the raised pycnocline inhibits the penetration to middepths of surface-generated mixing. The microstructure profiles are used to estimate the available wind energy for mixing as a function of pycnocline, or mixed layer depth. A greater portion of energy input at the surface is available for entrainment of dense fluid through the pycnocline and into the surface mixed layer where the pycnocline is shallow. Hence surface-forced mixing may cause a more rapid increase in mixed layer density within the cool filament than outside the filament, resulting in an enhanced horizontal density gradient in the mixed layer. Assuming the flow adjusts towards geostrophy, the enhanced horizontal density gradient at the front could result in an accelerated mixed layer in the direction of the preexisting geostrophic flow. Proportions relating the gain in potential energy to the wind energy vary with pycnocline depth and differ by as much as an order of magnitude from the findings of Denman and Miyake (1973) and Davis et al. (1981). Horizontal variability of pycnocline erosion may not be properly taken into account in some models and should more realistically be parameterized by including dependence on pycnocline depth.Copyrighted by American Geophysical Union

Biases in Thorpe-scale estimates of turbulence dissipation. Part I : Assessments from large-scale overturns in oceanographic data

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2497–2521, doi:10.1175/JPO-D-14-0128.1.Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale LT and the Ozmidov length scale LO. Historic evidence supporting LT ~ LO has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of LT and LO, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that LT/LO increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting LT ~ LO are revisited, and conceptual regimes explaining the relationship between LT/LO and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.B.D.M. and S.K.V. gratefully acknowledge the support of the Office of Naval Research under Grants N00014-12-1-0279, N00014-12-1-0282, and N00014-12-1-0938 (Program Manager: Dr. Terri Paluszkiewicz). S.K.V. also acknowledges support of the National Science Foundation under Grant OCE-1151838. L.S.L. acknowledges support for BBTRE by the National Science Foundation by Contract OCE94-15589 and NATRE and IWISE by the Office of Naval Research by Contracts N00014-92-1323 and N00014-10-10315. J.N.M. was supported through Grant 1256620 from the National Science Foundation and the Office of Naval Research (IWISE Project).2016-04-0