Efficiency of turbulent mixing in the abyssal ocean circulation

Mashayek, Ali; Salehipour, Hesam; Bouffard, Damien; Caulfield, C. P.; Ferrari, Raffaele; Nikurashin, Maxim; Peltier, W. R.; Smyth, William D.

doi:10.1002/2016gl072452

Cited by 106 publications

(160 citation statements)

References 54 publications

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“…Finally, L t could be assumed to be proportional to the height, z , above a solid boundary, as in Monin‐Obukhov theory (Grachev et al, ), although as seen above, neither of the two data sets for which z / L O was available could dependence of Ri f on z / L O be differentiated from dependence on Re B . In any case, it is possible that Ri f is dependent on more than local, instantaneous conditions; for example, besides parameters like Re B , Ri f may also depend on how the given flow was established or is maintained (Mashayek et al, ).…”

Section: Discussionmentioning

confidence: 99%

Mixing Efficiency in the Presence of Stratification: When Is It Constant?

Monismith

Koseff

White

2018

Geophysical Research Letters

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The efficiency of the conversion of mechanical to potential energy, often expressed as the flux Richardson number, Rif, is an important determinant of vertical mixing in the ocean. To examine the dependence of Rif on the buoyancy Reynolds number, ReB, we analyze three sets of data: microstructure profiler data for which mixing is inferred from rates of dissipation of turbulent kinetic energy (ε) and temperature variance (χ) measured in the open ocean, time series of spectrally fit values of ε and covariance‐derived buoyancy fluxes measured in nearshore internal waves, and time series of spectrally fit values of ε and χ measured in an energetic estuarine flow. While profiler data are well represented by Rif ≈ 0.2 for 1 < ReB < 1,000, the covariance data have much larger values of ReB and, consistent with direct numerical simulation results, show that Rif ~ ReB−0.5. The estuarine data have values of ReB that fall between those of the other two data sets but also shows Rif ≈ 0.2 for ReB < 5000. Overall, these data suggest that Rif is in general not constant and may be substantially less than 0.2 when ReB is large, although the value at which the transition from constant to ReB‐dependent mixing may depend on additional parameters that are yet to be determined. Nonetheless, for much of the ocean, ReB < 100 and so Rif is constant there.

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Section: Discussionmentioning

confidence: 99%

Mixing Efficiency in the Presence of Stratification: When Is It Constant?

Monismith

Koseff

White

2018

Geophysical Research Letters

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“…Diapycnal mixing is expected to be small both when the stratification is sufficiently “weak”(compared to the turbulence) so there is little density variation to be homogenized, and when the stratification is sufficiently “strong” to suppress vertical motions and hence mixing. Therefore, it is plausible that there is a range of values of turbulence and stratification within which mixing is more efficient; that is, Γ is larger (De Lavergne et al, ; Mashayek, Salehipour, et al, ). Such relations between mixing and the ocean stratification and turbulence are important when inferring

scriptM

from the internal wave energy conversion, but they are not accounted for with a constant flux coefficient.…”

Section: Irreversible Mixing From Internal Tide Breakingmentioning

confidence: 99%

“…Such relations between mixing and the ocean stratification and turbulence are important when inferring

scriptM

from the internal wave energy conversion, but they are not accounted for with a constant flux coefficient. Here, we use the parameterization described in Mashayek, Salehipour, et al (), which assumes the expected nonmonotonic dependence of Γ on the buoyancy Reynolds number, as shown in Figure f.…”

Section: Irreversible Mixing From Internal Tide Breakingmentioning

confidence: 99%

“…(d) Internal tide local breaking efficiency q ( x , y ), reconstructed from Vic et al (). (e) Flux coefficient Γ( R e b ) at 3,500 m, using (f) the parameterization of Mashayek, Salehipour, et al (). The maximum flux coefficient (Γ * =0.35) occurs at

R e_{b}^{*} = 100

; both values fall within the bound reported in Mashayek, Salehipour, et al ().…”

Section: Introductionmentioning

confidence: 99%

“…Although it is by no means settled that mixing in stratified turbulence is generically a function of Re b (see, e.g., Gregg et al, 2018;Ijichi & Hibiya, 2018;Portwood et al, 2019), there is also strong evidence that the important abyssal mixing process mediated by shear-driven instabilities does indeed vary nonmonotonically with Re b (Mashayek, Caulfield, & Peltier, 2017). Previous studies have investigated the sensitivity of the mixing-driven deep overturning circulation to variations in Γ (De Lavergne et al, 2016b;Mashayek, Salehipour, et al, 2017) but without exploring its covariation with other parameters.…”

Section: Introductionmentioning

confidence: 99%

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Sensitivity of Deep Ocean Mixing to Local Internal Tide Breaking and Mixing Efficiency

Cimoli

Caulfield

Johnson

et al. 2019

Geophysical Research Letters

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There have been recent advancements in the quantification of parameters describing the proportion of internal tide energy being dissipated locally and the "efficiency" of diapycnal mixing, that is, the ratio of the diapycnal mixing rate to the kinetic energy dissipation rate. We show that oceanic tidal mixing is nontrivially sensitive to the covariation of these parameters. Varying these parameters one at a time can lead to significant errors in the patterns of diapycnal mixing-driven upwelling and downwelling and to the over and under estimation of mixing in such a way that the net rate of globally integrated deep circulation appears reasonable. However, the local rates of upwelling and downwelling in the deep ocean are significantly different when both parameters are allowed to covary and be spatially variable. These findings have important implications for the representation of oceanic heat, carbon, nutrients, and other tracer budgets in general circulation models.Plain Language Summary Deep ocean basins are filled with dense waters that form at high latitudes and sink to the abyss. The overturning circulation of the ocean, a key regulator of the climate system, is only feasible if such dense waters can resurface. The breaking of internal waves makes such resurfacing possible. In the deep ocean, internal waves are largely generated by the flow of tides over topography. Their breaking mixes dense deep waters with lighter waters above them, bringing them upward. Two key parameters in climate models for modeling such mixing are (I) the ratio of energy in the wave field that is spent near rough topography due to breaking as opposed to what is radiated away and (II) the amount of energy from wave breaking that goes to mixing versus what is wasted through dissipation by viscosity of seawater. Both parameters are considered constant in climate models. In this work, we quantify the roles of variations in each of these two parameters in setting the patterns of deep ocean upwelling of dense waters and argue that the two parameters need to be changed realistically and interdependently to avoid significant inaccuracies in the quantification of the mixing-induced deep branch of ocean circulation.

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Mixing and Formation of Layers by Internal Wave Forcing

et al. 2017

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The energy pathways from propagating internal waves to the scales of irreversible mixing in the ocean are not fully described. In the ocean interior, the triadic resonant instability is an intrinsic destabilization process that may enhance the energy cascade away from topographies. The present study focuses on the integrated impact of mixing processes induced by a propagative normal mode‐1 over long‐term experiments in an idealized setup. The internal wave dynamics and the evolution of the density profile are followed using the light attenuation technique. Diagnostics of the turbulent diffusivity KT and background potential energy BPE are provided. Mixing effects result in a partially mixed layer colocated with the region of maximum shear induced by the forcing normal mode. The maximum measured turbulent diffusivity is 250 times larger than the molecular value, showing that diapycnal mixing is largely enhanced by small‐scale turbulent processes. Intermittency and reversible energy transfers are discussed to bridge the gap between the present diagnostic and the larger values measured in Dossmann et al. (). The mixing efficiency η is assessed by relating the BPE growth to the linearized KE input. One finds a value of Γ=12−19%, larger than the mixing efficiency in the case of breaking interfacial wave. After several hours of forcing, the development of staircases in the density profile is observed. This mechanism has been previously observed in experiments with weak homogeneous turbulence and explained by Phillips (1972) argument. The present experiments suggest that internal wave forcing could also induce the formation of density interfaces in the ocean.

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Efficiency of turbulent mixing in the abyssal ocean circulation

Cited by 106 publications

References 54 publications

Mixing Efficiency in the Presence of Stratification: When Is It Constant?

Mixing Efficiency in the Presence of Stratification: When Is It Constant?

Sensitivity of Deep Ocean Mixing to Local Internal Tide Breaking and Mixing Efficiency

Mixing and Formation of Layers by Internal Wave Forcing

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