Roles of Shear and Convection in Driving Mixing in the Ocean

Ivey, Gregory; Bluteau, Cynthia; Gayen, Bishakhdatta; Jones, Nicole L.; Sohail, Taimoor

doi:10.1029/2020gl089455

Cited by 12 publications

(13 citation statements)

References 61 publications

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“…The breaking of ISWs by plunging is anticipated to be dominated by convective instability, whilst ISW breaking by collapsing is anticipated to be shear dominated. Higher mixing efficiencies are anticipated for flows dominated by convection-driven mixing than shear-driven mixing (Ivey et al 2021), however, overall rates of mixing were much higher for the collapsing wave than the plunging wave, with approximately 14 % and 4 % of initial available energy going to irreversible mixing, respectively (despite comparable dissipation). To what extent the mixing is affected by variation of stratification, rather than by changing breaker type would be an interesting area of future study.…”

Section: Relevance At the Ocean Scalementioning

confidence: 96%

Stratification effects on shoaling internal solitary waves

et al. 2021

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This combined numerical/laboratory study investigates the effect of stratification form on the shoaling characteristics of internal solitary waves propagating over a smooth, linear topographic slope. Three stratification types are investigated, namely (i) thin tanh (homogeneous upper and lower layers separated by a thin pycnocline), (ii) surface stratification (linearly stratified layer overlaying a homogeneous lower layer) and (iii) broad tanh (continuous density gradient throughout the water column). It is found that the form of stratification affects the breaking type associated with the shoaling wave. In the thin tanh stratification, good agreement is seen with past studies. Waves over the shallowest slopes undergo fission. Over steeper slopes, the breaking type changes from surging, through collapsing to plunging with increasing wave steepness $A_w/L_w$ for a given topographic slope, where $A_w$ and $L_w$ are incident wave amplitude and wavelength, respectively. In the surface stratification regime, the breaking classification differs from the thin tanh stratification. Plunging dynamics is inhibited by the density gradient throughout the upper layer, instead collapsing-type breakers form for the equivalent location in parameter space in the thin tanh stratification. In the broad tanh profile regime, plunging dynamics is likewise inhibited and the near-bottom density gradient prevents the collapsing dynamics. Instead, all waves either fission or form surging breakers. As wave steepness in the broad tanh stratification increases, the bolus formed by surging exhibits evidence of Kelvin–Helmholtz instabilities on its upper boundary. In both two- and three-dimensional simulations, billow size grows with increasing wave steepness, dynamics not previously observed in the literature.

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Section: Relevance At the Ocean Scalementioning

confidence: 96%

Stratification effects on shoaling internal solitary waves

et al. 2021

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“…We acknowledge that while our specific choice for the parametrization of (as per ) is rooted in physical reasoning and seems plausible based on a wide range of oceanic datasets (as discussed in MCA21), it is one of the existing paradigms. Alternative viewpoints, for example, have recently been expressed by Garanaik and Venayagamoorthy (2019) and Ivey, Bluteau, Gayen, Jones, and Sohail (2021) and can straightforwardly be implemented in our recipe.…”

Section: On the Sensitivity Of Our Results To Specific Choices Within...mentioning

confidence: 99%

A physical–statistical recipe for representation of small-scale oceanic turbulent mixing in climate models

Mashayek

Cael

Cimoli

et al. 2022

Flow

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It is well established that small-scale cross-density (diapycnal) turbulent mixing induced by breaking of overturns in the interior of the ocean plays a significant role in sustaining the deep ocean circulation and in regulating tracer budgets such as those of heat, carbon and nutrients. There has been significant progress in the fluid mechanical understanding of the physics of breaking internal waves. Connection of the microphysics of such turbulence to the larger scale dynamics, however, is significantly underdeveloped. We offer a hybrid theoretical–statistical approach, informed by observations, to make such a link. By doing so, we define a bulk flux coefficient, $\varGamma _B$ , which represents the partitioning of energy available to an ‘ocean box’ (such as a grid cell of a coarse resolution climate model), from winds, tides, and other sources, into mixing and dissipation. Here, $\varGamma _B$ depends on both the statistical distribution of turbulent patches and the flux coefficient associated with individual patches, $\varGamma _i$ . We rely on recent parametrizations of $\varGamma _i$ and the seeming universal characteristics of statistics of turbulent patches to infer $\varGamma _B$ , which is the essential quantity for representation of turbulent diffusivity in climate models. By applying our approach to climatology and global tidal estimates, we show that, on a basin scale, energetic mixing zones exhibit moderately efficient mixing that induces significant vertical density fluxes, while quiet zones (with small background turbulence levels), although highly efficient in mixing, exhibit minimal vertical fluxes. The transition between the less energetic to more energetic zones marks regions of intense upwelling and downwelling of deep waters. We suggest that such upwelling and downwelling may be stronger than previously estimated, which in turn has direct implications for the closure of the deep branch of the ocean meridional overturning circulation as well as for the associated tracer budgets.

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“…More recently, Gargett's ratio raised to the 4/3 power has been referred to as the isotropy parameter (Thorpe 2007; Ivey, Winters & Koseff 2008) and as the buoyancy Reynolds number (Ivey et al. 2021; Mashayek et al. 2022).…”

Section: Nonlinear Solutionsmentioning

confidence: 99%

“…The use of the ratio of Ozmidov to Kolmogorov length scales L O /L K to characterize the bulk effect of stratification on turbulence can be traced to the isotropy parameter of Gargett, Osborn & Nasmyth (1984). More recently, Gargett's ratio raised to the 4/3 power has been referred to as the isotropy parameter I (Thorpe 2007;Ivey, Winters & Koseff 2008) and as the buoyancy Reynolds number Re b (Ivey et al 2021;Mashayek et al 2022). To avoid confusion, we refer to Gargett's ratio raised to the 4/3 power as the Ozmidov-Kolmogorov length-scale ratio.…”

Section: Barotropic Tide Dissipationmentioning

confidence: 99%

Low-Reynolds-number oscillating boundary layers on adiabatic slopes

2022

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We investigate the instabilities and transition mechanisms of Boussinesq stratified boundary layers on sloping boundaries when subjected to oscillatory body forcing parallel to the slope. We examine idealized forms of boundary layers on hydraulically smooth abyssal slopes in tranquil mid- to low-latitude regions, where low-wavenumber internal tides gently heave isopycnals up and down adiabatic slopes in the absence of mean flows, high-wavenumber internal tides, shelf breaks, resonant tide–bathymetry interactions (critical slopes) and other phenomena associated with turbulence ‘hot spots’. In non-rotating low-Reynolds-number flow, increased stratification on the downslope phase has a relaminarizing effect, while on the upslope phase we find transition-to-turbulence pathways arise from shear production triggered by gravitational instabilities. When rotation is significant (low slope Burger numbers) we find that boundary layer turbulence is sustained throughout the oscillation period, resembling stratified Stokes–Ekman layer turbulence. Simulation results suggest that oscillating boundary layers on smooth slopes at low Reynolds number ( $\textit {Re}\leqslant 840$ ), unity Prandtl number and slope Burger numbers greater than unity do not cause significant irreversible turbulent buoyancy flux (mixing), and that flat-bottom dissipation rate models derived from the tide amplitude are accurate within an order of magnitude.

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Roles of Shear and Convection in Driving Mixing in the Ocean

Cited by 12 publications

References 61 publications

Stratification effects on shoaling internal solitary waves

Stratification effects on shoaling internal solitary waves

A physical–statistical recipe for representation of small-scale oceanic turbulent mixing in climate models

Low-Reynolds-number oscillating boundary layers on adiabatic slopes

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