Parametrization of irreversible diapycnal diffusivity in salt-fingering turbulence using DNS

Abstract In this paper the authors investigate the action of ambient turbulence on thermohaline interleaving using both theory and numerical calculations in combination with observations from Meddy Sharon and the Faroe Front. The highly simplified models of ambient turbulence used previously are improved upon by allowing turbulent diffusivities of momentum, heat, and salt to depend on background gradients and to evolve as the instability grows. Previous studies have shown that ambient turbulence, at typical ocean levels, can quench the thermohaline interleaving instability on baroclinic fronts. These findings conflict with the observation that interleaving is common in baroclinic frontal zones despite ambient turbulence. Another challenge to the existing theory comes from numerical experiments showing that the Schmidt number for sheared salt fingers is much smaller than previously assumed. Use of the revised value in an interleaving calculation results in interleaving layers that are both weaker and thinner than those observed. This study aims to resolve those paradoxes. The authors show that, when turbulence has a Prandtl number greater than unity, turbulent momentum fluxes can compensate for the reduced Schmidt number of salt fingering. Thus, ambient turbulence determines the vertical scale of interleaving. In typical oceanic interleaving structures, the observed property gradients are insufficient to predict interleaving growth at an observable level, even when improved turbulence models are used. The deficiency is small, though: gradients sharper by a few tens of percent are sufficient to support instability. The authors suggest that this is due to the efficiency of interleaving in erasing those property gradients. A new class of mechanisms for interleaving, driven by flow-dependent fluctuations in turbulent diffusivities, is identified. The underlying mechanism is similar to the well-known Phillips layering instability; however, because of Coriolis effects, it has a well-defined vertical scale and also a tilt angle opposite to that of finger-driven interleaving.

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Floquet Instability of Gravity-Modulated Salt Fingering in a Porous Medium

Industrial & Engineering Chemistry Research ◽

10.1021/acs.iecr.6b03866 ◽

2017 ◽

Vol 56 (10) ◽

pp. 2851-2864 ◽

Cited By ~ 1

Author(s):

S. Saravanan ◽

S. Keerthana

Keyword(s):

Porous Medium ◽

Salt Fingering

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Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

Journal of Physical Oceanography ◽

10.1175/jpo2821.1 ◽

2005 ◽

Vol 35 (12) ◽

pp. 2425-2443 ◽

Cited By ~ 55

Author(s):

J. A. MacKinnon ◽

M. C. Gregg

Keyword(s):

New England ◽

Internal Waves ◽

Mixed Layer ◽

Wave Interaction ◽

Heat Fluxes ◽

Surface Mixed Layer ◽

Turbulent Dissipation ◽

Surface Heat Fluxes ◽

Bottom Boundary Layers ◽

Diapycnal Diffusivity

Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.

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Enhanced diapycnal mixing by polarity-reversing internal solitary waves in the South China Sea

10.5194/npg-2021-21 ◽

2021 ◽

Author(s):

Yi Gong ◽

Haibin Song ◽

Zhongxiang Zhao ◽

Yongxian Guan ◽

Kun Zhang ◽

...

Keyword(s):

South China Sea ◽

Solitary Wave ◽

Solitary Waves ◽

South China ◽

The South China Sea ◽

Internal Solitary Wave ◽

Internal Solitary Waves ◽

Diapycnal Mixing ◽

Dongsha Atoll ◽

Diapycnal Diffusivity

Abstract. Shoaling internal solitary waves near the Dongsha Atoll in the South China Sea dissipate their energy and thus enhance diapycnal mixing, which have an important impact on the oceanic environment and primary productivity. The enhanced diapycnal mixing is patchy and instantaneous. Evaluating its spatiotemporal distribution requires comprehensive observation data. Fortunately, seismic oceanography meets the requirements, thanks to its high spatial resolution and large spatial range. In this paper, we studied three internal solitary waves in reversing polarity near the Dongsha Atoll, and calculated the spatial distribution of resultant diapycnal diffusivity. Our results show that the average diffusivities along three survey lines are two orders of magnitude larger than the open-ocean value. The average diffusivity in the internal solitary wave with reversing polarity is three times that of the non-polarity-reversal region. The diapycnal diffusivity is higher at the front of one internal solitary wave, and gradually decreases from shallow to deep water in the vertical direction. Our results also indicates that (1) the enhanced diapycnal diffusivity is related to reflection seismic events; (2) convective instability and shear instability may both contribute to the enhanced diapycnal mixing in the polarity-reversing process; and (3) the difference between our and previous diffusivity profiles is about 2–3 orders of magnitude, but their vertical distribution is almost the same.

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The inference of salt fingering from towed microstructure observations

Journal of Geophysical Research Atmospheres ◽

10.1029/jc092ic02p01963 ◽

1987 ◽

Vol 92 (C2) ◽

pp. 1963 ◽

Cited By ~ 9

Author(s):

Greg Holloway ◽

Ann Gargett

Keyword(s):

Salt Fingering

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Mixing in a Moderately Sheared Salt-Fingering Layer

Journal of Physical Oceanography ◽

10.1175/2010jpo4611.1 ◽

2011 ◽

Vol 41 (7) ◽

pp. 1364-1384 ◽

Cited By ~ 15

Author(s):

W. D. Smyth ◽

S. Kimura

Keyword(s):

Turbulent Mixing ◽

Linear Growth ◽

Density Ratio ◽

Effective Diffusivity ◽

Shear Instability ◽

Bulk Richardson Number ◽

Additional Energy ◽

Salt Fingers ◽

Salt Fingering ◽

Secondary Instabilities

Abstract Mixing due to sheared salt fingers is studied by means of direct numerical simulations (DNS) of a double-diffusively unstable shear layer. The focus is on the “moderate shear” case, where shear is strong enough to produce Kelvin–Helmholtz (KH) instability but not strong enough to produce the subharmonic pairing instability. This flow supports both KH and salt-sheet instabilities, and the objective is to see how the two mechanisms work together to flux heat, salt, and momentum across the layer. For observed values of the bulk Richardson number Ri and the density ratio Rρ, the linear growth rates of KH and salt-sheet instabilities are similar. These mechanisms, as well as their associated secondary instabilities, lead the flow to a fully turbulent state. Depending on the values of Ri and Rρ, the resulting turbulence may be driven mainly by shear or mainly by salt fingering. Turbulent mixing causes the profiles of temperature, salinity, and velocity to spread; however, in salt-sheet-dominated cases, the net density (or buoyancy) layer thins over time. This could be a factor in the maintenance of the staircase and is also an argument in favor of an eventual role for Holmboe instability. Fluxes are scaled using both laboratory scalings for a thin layer and an effective diffusivity. Fluxes are generally stronger in salt-sheet-dominated cases. Shear instability disrupts salt-sheet fluxes while adding little flux of its own. Shear therefore reduces mixing, despite providing an additional energy source. The dissipation ratio Γ is near 0.2 for shear-dominated cases but is much larger when salt sheets are dominant, supporting the use of Γ in the diagnosis of observed mixing phenomena. The profiler approximation Γz, however, appears to significantly overestimate the true dissipation ratio.

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Observations of Turbulent Mixing During a Nonstratification Period in the Yellow Sea

Marine Technology Society Journal ◽

10.4031/mtsj.55.2.16 ◽

2021 ◽

Vol 55 (2) ◽

pp. 185-197

Author(s):

Yunli Nie ◽

Xin Luan ◽

Hua Yang ◽

Xu Chen ◽

Dalei Song ◽

...

Keyword(s):

Water Column ◽

Turbulent Mixing ◽

Yellow Sea ◽

Dissipation Rate ◽

Distribution Functions ◽

Cumulative Distribution ◽

The Yellow Sea ◽

Distribution Model ◽

Entire Water Column ◽

Diapycnal Diffusivity

Abstract Microstructure profiling measurements collected at the continental shelf of the Yellow Sea (35°38'N, 121°20'E) from December 4 to 5, 2019, were analyzed by focusing on the characteristics of turbulent mixing in the Yellow Sea and its associated influencing factors. The vertical thermohaline structure of the water column was nonstratified during the observation period, resulting in the vertically and temporally consistent distribution of turbulence dissipation and diapycnal diffusivity. The average (in time and space) dissipation rate and diapycnal diffusivity were 2.95 × 10−8 W kg−1 and 1.86 × 10−4 m2 s−1, respectively. In the vertical distribution, intense mixing occurred near the sea surface and within the bottom layers. The temporal variation in dissipation exhibits a diurnal variation that was strongly affected by surface buoyancy flux and wind energy, and a high amount of dissipation was observed at night, with an average dissipation rate of 2.45 × 10−8 W kg−1, which was almost one order of magnitude higher than that in the daytime (3.55 × 10−9 W kg−1). The cumulative distribution functions of the dissipation rate and diapycnal diffusivity across the entire water column during the measurement period could be parameterized by a lognormal distribution model. Further analysis shows that the dissipation rate was positively related to wind speed and rotational barotropic tidal velocity. Compared with the rotating tidal current, wind-driven turbulence was able to penetrate the surface, thereby causing layer mixing throughout the entire water column (R = 0.71), and is a dominant driver of elevated turbulent mixing during wintertime.

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