Stratified turbulence in the nearshore coastal ocean: Dynamics and evolution in the presence of internal bores

Walter, Ryan; Squibb, M. E.; Woodson, C. Brock; Koseff, Jeffrey R.; Monismith, Stephen G.

doi:10.1002/2014jc010396

Cited by 58 publications

(70 citation statements)

References 72 publications

(212 reference statements)

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“…Among a number of nondimensional parameters that have been identified as potentially relevant for the determination of γ in shear‐driven turbulence is the buoyancy Reynolds number,

R e_{b} = \frac{ε}{ν N^{2}},

which can be interpreted as a measure of the ratio of the Ozmidov scale

L_{O} = {false(ε / N^{3} false)}^{1 / 2}

and the Kolmogorov scale

η = {false(ν^{3} / ε false)}^{1 / 4}

. Available data suggest that in the “energetic regime” with

R e_{b} > 100

, the flux coefficient follows a power‐law relationship of the form

γ = a R e_{b}^{- \frac{1}{2}} for R e_{b} > 100,

where a is a model parameter that is presently not well constrained [ Shih et al ., ; Walter et al ., ]. Based on their analysis of a large number of data, Bouffard and Boegman [] suggest that the best fit is provided by a = 2, which corresponds to the value originally proposed by Shih et al .…”

Section: Discussionsupporting

confidence: 77%

“…The computation of the mixing rate

E_{p}

from (8) is complicated by the potential variability of the flux coefficient γ . The use of the canonical value

γ = 0.2

to describe BBL mixing is inconsistent with an increasing body of observational evidence suggesting a significant reduction of γ in energetic turbulent flows, e.g., in shallow coastal areas or in the energetic near‐bottom region [e.g., Walter et al ., ; Bluteau et al ., ; Davis and Monismith , ]. Among a number of nondimensional parameters that have been identified as potentially relevant for the determination of γ in shear‐driven turbulence is the buoyancy Reynolds number,

R e_{b} = \frac{ε}{ν N^{2}},

which can be interpreted as a measure of the ratio of the Ozmidov scale

L_{O} = {false(ε / N^{3} false)}^{1 / 2}

and the Kolmogorov scale

η = {false(ν^{3} / ε false)}^{1 / 4}

.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Efficient boundary mixing due to near-inertial waves in a non-tidal basin: Observations from the Baltic Sea

Lappe

Umlauf

2016

J. Geophys. Res. Oceans

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Diapycnal mixing in large nontidal basins is often assumed to be related to the effect of near‐inertial waves. While the role of near‐inertial shear for the generation of shear instabilities in the stratified interior is relatively well understood, much less is known about the impact of near‐inertial motions on boundary mixing processes in nontidal systems, mainly owing to the lack of appropriate observations. Here, an extensive data set is discussed, describing the variability of boundary mixing induced by near‐inertial motions near the sloping topography of one of the main basins of the Baltic Sea. These data reveal the existence of a vigorously turbulent bottom boundary layer of a few meters thickness covering the entire slope region above and below the permanent halocline that constitutes the main obstacle for vertical transport in the Baltic Sea. Near‐bottom turbulence was driven by the near‐inertial shear in the frictional boundary layer rather than by breaking of near‐inertial waves near critical slopes. Large regions of the bottom boundary layer remained strongly stratified, and therefore, different from the traditional view of inefficient boundary mixing, mixing efficiencies reached values typical for the ocean's interior.

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“…Among a number of nondimensional parameters that have been identified as potentially relevant for the determination of γ in shear‐driven turbulence is the buoyancy Reynolds number,

R e_{b} = \frac{ε}{ν N^{2}},

which can be interpreted as a measure of the ratio of the Ozmidov scale

L_{O} = {false(ε / N^{3} false)}^{1 / 2}

and the Kolmogorov scale

η = {false(ν^{3} / ε false)}^{1 / 4}

. Available data suggest that in the “energetic regime” with

R e_{b} > 100

, the flux coefficient follows a power‐law relationship of the form

γ = a R e_{b}^{- \frac{1}{2}} for R e_{b} > 100,

Section: Discussionsupporting

confidence: 77%

“…The computation of the mixing rate

E_{p}

from (8) is complicated by the potential variability of the flux coefficient γ . The use of the canonical value

γ = 0.2

R e_{b} = \frac{ε}{ν N^{2}},

which can be interpreted as a measure of the ratio of the Ozmidov scale

L_{O} = {false(ε / N^{3} false)}^{1 / 2}

and the Kolmogorov scale

η = {false(ν^{3} / ε false)}^{1 / 4}

.…”

Section: Discussionmentioning

confidence: 99%

Efficient boundary mixing due to near-inertial waves in a non-tidal basin: Observations from the Baltic Sea

Lappe

Umlauf

2016

J. Geophys. Res. Oceans

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“…Within the thermocline, the intensity of diapycnal mixing generated by the observed values of dissipation of turbulent kinetic energy is estimated by using the Osborn relation for every ɛ estimate:

K_{ρ} = Γ \frac{ɛ}{N^{2}},

where Γ is the so called “mixing efficiency.” Γ has been frequently set to 0.2 for shear‐generated mixing (Osborn, ; Thorpe, ), although more recent studies reporting on direct numerical simulations (Shih et al, ) and oceanic field measurements (Walter et al, ) define

Γ = f (I_{A})

.…”

Section: Resultsmentioning

confidence: 99%

Turbulence and Mixing in a Shallow Shelf Sea From Underwater Gliders

2017

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The seasonal thermocline in shallow shelf seas acts as a natural barrier for boundary‐generated turbulence, damping scalar transport to the upper regions of the water column and controlling primary production to a certain extent. To better understand turbulence and mixing conditions within the thermocline, two unique 12 and 17 day data sets with continuous measurements of the dissipation rate of turbulent kinetic energy (ɛ) collected by autonomous underwater gliders under stratified to well‐mixed conditions are presented. A highly intermittent ɛ signal was observed in the stratified thermocline region, which was mainly characterized by quiescent flow (turbulent activity index below 7). The rate of diapycnal mixing remained relatively constant for the majority of the time with peaks of higher fluxes that were responsible for much of the increase in bottom mixed layer temperature. The water column stayed predominantly strongly stratified, with a bulk Richardson number across the thermocline well above 2. A positive relationship between the intensity of turbulence, shear, and stratification was found. The trend between turbulence levels and the bulk Richardson number was relatively weak but suggests that ɛ increases as the bulk Richardson number approaches 1. The results also highlight the interpretation difficulties in both quantifying turbulent thermocline fluxes as well as the responsible mechanisms.

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“…It is worth noting that the present buoyancy Reynolds numbers [Re b 5 «/(nN (Shih et al 2005;Davis and Monismith 2011;Dunckley et al 2012;Bouffard et al 2013;Walter et al 2014). Our observations may thus highlight a specific case of efficient mixing that occurs during internal wave breaking on deep sloping topography, when the breaking is associated with high available potential energy.…”

Section: On the Apparent Mixing Efficiency Of Turbulence Above Slopinmentioning

confidence: 93%

“…These difficulties lie in the fact that turbulence is generally intermittent (the stationary hypothesis needed to compute G does not hold) and that separating reversible and nonreversible density fluctuations is complex, making any estimation of J b difficult (Bouffard et al 2013). This question was addressed in numerical (Slinn andRiley 1996, 1998;Umlauf and Burchard 2011;Mashayek and Peltier 2013) and laboratory studies (Ivey and Nokes 1989) but also from in situ measurements (van Haren et al 1994;Davis and Monismith 2011;Dunckley et al 2012;Walter et al 2014).…”

Section: On the Apparent Mixing Efficiency Of Turbulence Above Slopinmentioning

confidence: 99%

Observations of Small-Scale Secondary Instabilities during the Shoaling of Internal Bores on a Deep-Ocean Slope

Cyr

Haren

2016

Journal of Physical Oceanography

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The Rockall Bank area, located in the northeast Atlantic Ocean, is a region dominated by topographically trapped diurnal tides. These tides generate up-and downslope displacements that can be locally described as swashing motions on the bank. Using high spatial and time resolution of moored temperature sensors, the transition toward the upslope flow (cooling phase) is described as a rapid upslope-propagating bore, likely generated by breaking trapped internal waves. Buoyant anomalies are found during the bore propagation, likely resulting from small-scale instabilities. The imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing (i.e., young) and have high mixing potential.

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Stratified turbulence in the nearshore coastal ocean: Dynamics and evolution in the presence of internal bores

Cited by 58 publications

References 72 publications

Efficient boundary mixing due to near-inertial waves in a non-tidal basin: Observations from the Baltic Sea

Efficient boundary mixing due to near-inertial waves in a non-tidal basin: Observations from the Baltic Sea

Turbulence and Mixing in a Shallow Shelf Sea From Underwater Gliders

Observations of Small-Scale Secondary Instabilities during the Shoaling of Internal Bores on a Deep-Ocean Slope

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