The Impact of a Variable Mixing Efficiency on the Abyssal Overturning

Lavergne, Casimir de; Madec, Gurvan; Sommer, Julien Le; Nurser, A. J. George; Garabato, Alberto C. Naveira

doi:10.1175/jpo-d-14-0259.1

Cited by 93 publications

(98 citation statements)

References 93 publications

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“…The different spatial distribution of the internal tide and lee-wave energy input is largely responsible for the sensitivity described in Melet et al (2014), highlighting the previously reported importance of the patchiness of internal wave-driven mixing in the ocean (e.g., Simmons et al 2004a;Jayne 2009;Friedrich et al 2011). Using a hydrographic climatology and a similar parameterization for lee wave-driven mixing, Nikurashin and Ferrari (2013) and De Lavergne et al (2016) also show substantial water mass transformation in the Southern Ocean due to internal lee wave-driven mixing. Trossman et al (2013Trossman et al ( , 2016 implemented an inline wave drag parameterization (for both propagating and nonpropagating lee waves) from the atmospheric community (Garner 2005) into a high-resolution ocean general circulation model (Fig.…”

Section: Internal Lee Waves Theory and Observationsmentioning

confidence: 70%

Climate Process Team on Internal Wave–Driven Ocean Mixing

MacKinnon

Zhao

Whalen

et al. 2017

Bulletin of the American Meteorological Society

279

207

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O cean turbulence influences the transport of heat, freshwater, dissolved gases such as CO 2 , pollutants, and other tracers. It is central to understanding ocean energetics and reducing uncertainties in global circulation and simulations from climate models. The dissipation of turbulent energy in stratified water results in irreversible diapycnal (across density surfaces) mixing. Recent work has shown that the spatial and temporal inhomogeneity in diapycnal mixing may play a critical role in a variety of climate phenomena. Hence, a quantitative understanding of the physics that drive the distribution of diapycnal mixing in the ocean interior is fundamental to understanding the ocean's role in climate.Diapycnal mixing is very difficult to accurately parameterize in numerical ocean models for two reasons. The first one is due to the discrete representation of tracer advection in directions that are not perfectly aligned with isopycnals, which can result in numerically induced mixing from truncation errors that is larger than observed diapycnal mixing (Griffies et al. 2000;Ilıcak et al. 2012). The second reason is related to the intermittency of turbulence, which is generated by complex and chaotic motions that span a large space-time range. Furthermore, this mixing is driven by a wide range of processes with distinct governing physics that create a rich global geography [see MacKinnon et al. (2013c) for a review]. The difficulty is also related to the relatively sparse direct sampling of ocean mixing, whereby sophisticated ship-based measurements are generally required to accurately characterize ocean mixing processes. Nonetheless, we have sufficient evidence from theory, process models, laboratory experiments, and field measurements to conclude that away from ocean boundaries (atmosphere, ice, or the solid ocean bottom), diapycnal mixing is largely related to the breaking of internal gravity waves, which have a complex dynamical underpinning and associated geography. The study summarizes recent advances in our understanding of internal wave-driven turbulent mixing in the ocean interior and introduces new parameterizations for global climate ocean models and their climate impacts.

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Section: Internal Lee Waves Theory and Observationsmentioning

confidence: 70%

Climate Process Team on Internal Wave–Driven Ocean Mixing

MacKinnon

Zhao

Whalen

et al. 2017

Bulletin of the American Meteorological Society

279

207

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“…When considering the case for the oceans,

δ / D = 0.1

in Figure provides corresponding STF‐generated

κ_{ρ}

and R f values of

2 \times 10^{- 6} {normalm}^{2} / s

and 0.95, respectively. This suggests that the contribution from STF to the mixing rate in the oceans is 1 order of magnitude smaller than the measured value of

O (10^{- 5} {normalm}^{2} / s)

for the ocean interior and that the corresponding mixing efficiency is much larger than the empirical value of 0.17 for the real ocean (de Lavergne et al, ; Osborn, ). This implies that mixing in the oceanic interior must be dominated by energies from other sources such as winds and tides.…”

Section: Discussionmentioning

confidence: 91%

“…In a steady state, a portion of the APE is converted to kinetic energy that is mostly dissipated into heat via viscosity, while the remainder is allocated to mixing, which is proportional to the strength of the stratification characterized by the buoyancy frequency N (=

{[- g / ρ_{0} (\partial ρ / \partial z)]}^{1 / 2}

). In terms of energy per unit time, this balance can be expressed as

G_{A P E} = ϵ + κ_{ρ} 〈 N^{2} 〉,

where G APE is the generation rate of the APE, ϵ is the rate of turbulent kinetic energy dissipation (Barkan et al, ; de Lavergne et al, ; Gayen et al, ; Peltier & Caulfield, ; Tailleux, ), the proportionality constant

κ_{ρ}

is known as the mixing rate (i.e., the vertical/diapycnal diffusivity), and

〈 〉

denotes a spatial average over the whole volume. The mixing efficiency can be defined as

R_{f} = κ_{ρ} 〈 N^{2} 〉 / G_{A P E}

, which is the fraction of the APE used for mixing.…”

Section: Introductionmentioning

confidence: 99%

Contribution of Surface Thermal Forcing to Mixing in the Ocean

2018

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A critical ingredient of the meridional overturning circulation (MOC) is vertical mixing, which causes dense waters in the deep sea to rise throughout the stratified interior to the upper ocean. Here, we report a laboratory study aimed at understanding the contributions from surface thermal forcing (STF) to this mixing process. Our study reveals that the ratio of the thermocline thickness to the fluid depth largely determines the mixing rate and the mixing efficiency in an overturning flow driven by STF. By applying this finding to a hypothetical MOC driven purely by STF, we obtain a mixing rate of O( 10−6 normalm2/s) and a corresponding meridional heat flux of O( 10−2 petawatt, PW), which are far smaller than the values found for real oceans. These results provide quantitative support for the notion that STF alone is not sufficient to drive the MOC, which essentially acts as a heat conveyor belt powered by other energy sources.

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“…Moreover, the overturning rate may be sensitive to the uncertainty in q ( x , y ) and to the specific values of Γ * and

R e_{b}^{*}

used in the parameterization of Γ( Re b ), both of which are discussed in the supporting information. The overturning rate is also sensitive to the vertical function of energy dissipation (e.g., De Lavergne et al, , ). Stratification‐dependent decay functions have been proposed (e.g., Polzin, ), but the exponential decay function with an e ‐folding scale of 500 m used here (equation ) has been shown to match well with observations of turbulence in the deep ocean (St Laurent et al, ).…”

Section: Resultsmentioning

confidence: 99%

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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The Impact of a Variable Mixing Efficiency on the Abyssal Overturning

Cited by 93 publications

References 93 publications

Climate Process Team on Internal Wave–Driven Ocean Mixing

Climate Process Team on Internal Wave–Driven Ocean Mixing

Contribution of Surface Thermal Forcing to Mixing in the Ocean

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

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