The available potential energy of overturns as an indicator of mixing in the seasonal thermocline

Dillon, Thomas M.; Park, Melora M.

doi:10.1029/jc092ic05p05345

Cited by 41 publications

(47 citation statements)

References 15 publications

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“…These differences are more pronounced for/Sp r spectra, in which stable fine structure influences are minimized, than for spectra of/59. The typical spectral slope for class 1 patches is steeper than -5/3, with more of the available potential energy [Dillon and Park, 1987] found at length scales O(Lr). In contrast, class 3 patches are characterized by a -5/3 spectral slope, while spectra from class 5 patches are flatter, and less steep than -5/3, at the high wavenumbers (Figures 5 and 6).…”

Section: Methods For Analyzing Complexity In Mixing Patchesmentioning

confidence: 99%

Shannon entropy as an indicator of age for turbulent overturns in the oceanic thermocline

Wijesekera¹,

Dillon²

1997

J. Geophys. Res.

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Abstract. The Shannon entropy is a measure of the degree of intricacy contained in any graphable n-dimensional realization of an observable quantity. We use the Shannon entropy to measure the intricacy of density overturns in the oceanic thermocline and find that the Shannon entropy is related to the Thorpe scale L r [Thorpe, 1977] and the Ozmidov scale L o [Ozmidov, 1965]. We find that (1) small Shannon entropy corresponds to small values of Rot (---Lo/Lr), while large Shannon entropy is associated with large values of RoT; (2) density spectra are typically more steep than inertial subrange spectra when both Shannon entropy and Rot are small, whereas the spectral slope tends to be flatter than inertial subrange spectra when both Shannon entropy and Roy-are large; (3) the Grashof number is very large (O(10 •ø)) when Shannon entropy is small, indicating that these patches are extremely density unstable; (4) spectral bandwidth is much larger for patches with small Shannon entropy than for those with large entropy, indicating that large-scale, or "bulk" Reynolds number is large when entropy is small. We discuss the hypothesis that the degree of intricacy, and hence the Shannon entropy, increases with increasing time in a turbulent overturn and is observed to decreasc only when the resolution limits of the measuring system are exceeded. On the basis of these arguments we suggest that some classes of overturns are created with Thorpe scale larger than the Ozmidov scale. In these overturns the kinetic energy dissipation rate (s) is small during the initial growth of the overturn. Later, a small-scale structure develops, and a more complex, higher-order flow evolves. This behavior is discussed and compared with gridgenerated laboratory turbulence, in which initially small, energetic, rapidly growing boundary layers detached from the grid and advcct downstream, forcing Rot to be largest adjacent to the grid and thereafter decrease as a result of entrainment. IntroductionTurbulent mixing in stratified oceans is an important mechanism for cross-isosurface transport of properties because the turbulent diffusivity is much larger than the molecular diffusivity [Gregg, 1989] turbulence from vertically profiling, moored, or horizontally towed in situ instruments, having sensors in direct contact with the local fluid, are only "snapshots" of some particular state. The evolution in time of that state cannot be directly determined because a given volume of water is observed only once. It cannot be said with certainty that the same volume is ever sampled again. Indeed, if precisely the same volume were to be sampled again. it would be of little value in determining the evolution of the fluid state because the volume of water would have been disturbed by the sensing instrument itself. Hence we cannot directly measure the evolution of flow instabilities in oceanic environments with contact sensors. As might be expected, the lack of simple, direct, incontrovertible observations of the time history of oceanic instabilities engenders s...

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Section: Methods For Analyzing Complexity In Mixing Patchesmentioning

confidence: 99%

Shannon entropy as an indicator of age for turbulent overturns in the oceanic thermocline

Wijesekera¹,

Dillon²

1997

J. Geophys. Res.

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“…The Thorpe scale is nearly equal to the Ozmidov scale (Dillon, 1982;Peters and Gregg, 1988;Moum, 1996;Lorke, 1988). (Dillon and Park, 1987;Ferron et al, 1998;Finnigan et al, 2002;Lorke and Wu¨est, 2002). However, since the measured values of the ratio L O =L T vary from 0.65 to 0.95, the universality of this correlation remains a question (Finnigan et al, 2002).…”

Section: Estimation Of the Vertical Eddy Diffusivity And Other Turbulmentioning

confidence: 99%

Upper-ocean vertical mixing in the Antarctic Polar Front Zone

Cisewski

Strass

Prandke³

2005

Deep Sea Research Part II: Topical Studies in Oceanography

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The mixing regime of the upper 180 m of a mesoscale eddy in the vicinity of the Antarctic Polar Front at 47 S and 21 E was investigated during the R.V. Polarstern cruise ANT XVIII/2 within the scope of the iron fertilization experiment EisenEx. On the basis of hydrographic CTD and ADCP profiles we deduced the vertical diffusivity K z from two different parameterizations. Since these parameterizations bear the character of empirical functions, based on theoretical and idealized assumptions, they were inter alia compared with Cox-number and Thorpe-scale related diffusivities deduced from microstructure measurements, which supplied the first direct insights into turbulence of this ocean region. Values of K z in the range of 10 À4 -10 À3 m 2 s À1 appear as a rather robust estimate of vertical diffusivity within the seasonal pycnocline. Values in the mixed layer above are more variable in time and reach 10 À1 m 2 s À1 during periods of strong winds. The results confirm a close agreement between the microstructure-based eddy diffusivities and eddy diffusivities calculated after the parameterization of Pacanowski and

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“…where C L is often taken as a constant close to 1 (Crawford 1986;Dillon and Park 1987;Peters et al, 1995). Given the validity of Eq.…”

Section: Ozmidov Scale Vs Thorpe Scalementioning

confidence: 99%

Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume

Пака

Zhurbas²,

Rudels³

et al. 2013

Ocean Sci.

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Abstract. To examine processes controlling the entrainment of ambient water into the Denmark Strait overflow (DSO) plume / gravity current, measurements of turbulent dissipation rate were carried out by a quasi-free-falling (tethered) microstructure profiler (MSP). The MSP was specifically designed to collect data on dissipation-scale turbulence and fine thermohaline stratification in an ocean layer located as deep as 3500 m. The task was to perform microstructure measurements in the DSO plume in the lower 300 m depth interval including the bottom mixed layer and the interfacial layer below the non-turbulent ambient water. The MSP was attached to a Rosette water sampler rack equipped with a SeaBird CTDO and an RD Instruments lowered acoustic Doppler current profiler (LADCP). At a chosen depth, the MSP was remotely released from the rack to perform measurements in a quasi-free-falling mode.Using the measured vertical profiles of dissipation, the entrainment rate as well as the bottom and interfacial stresses in the DSO plume were estimated at a location 200 km downstream of the sill at depths up to 1771 m. Dissipation-derived estimates of entrainment were found to be much smaller than bulk estimates of entrainment calculated from the downstream change of the mean properties in the plume, suggesting the lateral stirring due to mesoscale eddies rather than diapycnal mixing as the main contributor to entrainment. Dissipation-derived bottom stress estimates are argued to be roughly one third the magnitude of those derived from log velocity profiles. In the interfacial layer, the Ozmidov scale calculated from turbulence dissipation rate and buoyancy frequency was found to be linearly proportional to the overturning scale extracted from conventional CTD data (the Thorpe scale), with a proportionality constant of 0.76, and a correlation coefficient of 0.77.

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The available potential energy of overturns as an indicator of mixing in the seasonal thermocline

Cited by 41 publications

References 15 publications

Shannon entropy as an indicator of age for turbulent overturns in the oceanic thermocline

Shannon entropy as an indicator of age for turbulent overturns in the oceanic thermocline

Upper-ocean vertical mixing in the Antarctic Polar Front Zone

Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume

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