Dynamics of Langmuir circulation in oceanic surface layers

Gnanadesikan, Anand

doi:10.1575/1912/5566

Cited by 7 publications

(13 citation statements)

References 60 publications

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“…A particular strength is the fact that there are no parameters which need to be tuned to obtain a fit to the case at hand. The PWP model does reasonably well at predicting the diurnal cycle of temperature [Stramma et al, 1986;Gnanadesikan, 1994] and has been used to study the observed Ekman spiral under conditions where diurnal restratification is important [Price et al, 1987;Gnanadesikan, 1994;Gnanadesikan and Weller, 1995]. It is used as an operational model for prediction of sea surface temperature by the U.S. Navy.…”

Section: Physical Mixing Modelsmentioning

confidence: 99%

Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics

Gnanadesikan¹

1996

J. Geophys. Res.

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As carbon monoxide within the oceanic surface layer is produced by solar radiation, diluted by mixing, consumed by biota, and outgassed to the atmosphere, it exhibits a diurnal cycle. The effect of dilution and mixing on this cycle is examined using a simple model for production and consumption coupled to three different mixed layer models. The magnitude and timing of the peak concentration, the magnitude of the average concentration, and the air-sea flux are considered. The models are run through a range of heating and wind stress and compared to experimental data reported by Kettle [1994]. The key to the dynamics is the relative size of four length scales; Dmix, the depth to which mixing occurs over the consumption time; L, the length scale over which production occurs; L out, the depth to which the mixed layer is ventilated over the consumption time; and Leorap , the depth to which the diurnal production can maintain a concentration in equilibrium with the atmosphere. If Dmi x >> L, the actual model parameterization can be important. If the mixed layer is maintained by turbulent diffusion, D mi x can be substantially less than the mixed layer depth. If the mixed layer is parameterized as a homogeneous slab, D mi x is equivalent to the mixed layer depth. If Dmi x > Lout, production is balanced by consumption rather than outgassing. The ratio between D mi x and Leonap determines whether the ocean is a source or a sink for CO. The main thermocline depth H sets an upper limit for D mi x and hence D mix/L , D mix/Lout, and D mix/Lcomp. The models are run to simulate a single day of observations. The mixing parameterization is shown to be very important, with a model which mixes using smallscale diffusion producing markedly larger surface concentrations than models which homogenize the mixed layer completely and instantaneously. Paper number 96JC00463. 0148-0227/96/96JC-00463509.00 forcing. If it is O(1 d-•) or smaller, the maximum concentrations are seen at the end of the day.In the presence of mixing, however, the picture becomes more complicated. The importance of the optical properties of the water, the timing of the maximum concentration, and the partition between gas exchange and consumption as sink terms can all depend on the mixing. This paper uses the diurnal cycle of carbon monoxide (CO) in order to attack the following four questions:1. When is dilution important in determining the surface concentration and flux of a photochemically produced trace species such as CO?2. How sensitive is the diurnal cycle to the exact parameterization of the optics, consumption, and outgassing, and how does such sensitivity depend on the mixing? 3. How sensitive are the results to different parameterizations of the dilution? One question of particular interest is the difference between slab models, which homogenize the mixed layer instantaneously and completely, and eddy viscosity models which require some time for the mixed layer to become homogenized. 4. When can a tracer such as CO be used to improve understanding of the...

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Section: Physical Mixing Modelsmentioning

confidence: 99%

Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics

Gnanadesikan¹

1996

J. Geophys. Res.

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“…The equations of motion are taken from Gnanadesikan (1994). They consist of transport equations for alongcell vorticity, alongcell velocity, and density :…”

Section: Bubblementioning

confidence: 99%

“…When this is the case, the Eulerian shear is not parallel with the Stokes drift, and so the cells do not line up with the wind exactly. The effects of Coriolis forces on the instability are considered in Gnanadesikan (1994) and Gnanadesikan & Weller (1995). Additionally, the steady-state case for (1 c) with boundary conditions (4a, b ) is one in which the density flux is constant throughout the layer, something which is only approximately true on long (seasonal) timescales rather than on the much shorter ones associated with Langmuir cells.…”

Section: Bubblementioning

confidence: 99%

“…Such a formulation will be used later in this paper. Previous work on Langmuir cells by a large number of investigators (Leibovich 1977a Gnanadesikan 1994) has also assumed constant turbulent diffusivity and viscosity. Finally, although the scenario here is idealized, the basic principle of evaluating the degree of instability by defining an averaged Stokes drift shear, Eulerian shear, and stratification is one that can be extended to real cases.…”

Section: Bubblementioning

confidence: 99%

“…The results are compared to direct integrations of the equations of motion made using a finite-difference code. The details of this code are found in Gnanadesikan (1994). The overall approach follows that of Roache (1977).…”

Section: Solving the Equations For Nonlinear Equilibriummentioning

confidence: 99%

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Mixing driven by vertically variable forcing: an application to the case of Langmuir circulation

Gnanadesikan

1996

J. Fluid Mech.

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Two-dimensional mixing driven by an instability mechanism which is concentrated near one of the boundaries is considered, with particular application to Langmuir circulations driven by a wave spectrum. The question of how to define the equivalent of the Rayleigh number is attacked using the energy balance equations and simple truncated models of the instability. Given a particular horizontal wavelength for the disturbance, the strength of the forcing on the cells, and thus the growth rate, is determined by a tradeoff between maximizing the depth-averaged forcing and maximizing the depth of penetration. As a result of this tradeoff, long-wavelength cells grow more slowly, but penetrate more deeply and have a larger equivalent Rayleigh number. At finite amplitude, these long-wavelength cells come to dominate the flow field. The depth of penetration of, and density transport accomplished by, Langmuir cells is considered as a function of the mean stratification and diffusion. An application to oceanic mixed layers is considered assuming the Mellor-Yamada 2½-level turbulence closure model to define the background level of turbulent mixing. For many realistic cases, Langmuir cells are predicted to dominate the vertical transport of momentum and density.

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An experimental Stommel Retention Zone facility with an application to oil droplet dispersion

Gurung

Smith

Zeidi

et al. 2022

Limnology & Ocean Methods

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Stommel Retention Zones (SRZs) associated with wind-driven Langmuir circulation (LC) facilitate the subsurface retention of particles and thus affect processes such as oil spill dispersion, sediment suspension, oil-particle aggregation, and plankton distribution. Since the effect of SRZs on these processes is difficult to study in the field, we present a novel laboratory facility designed to recreate the counterrotating vortex pair flow pattern associated with small-scale SRZs of various strengths in two dimensions (2D). We then inject an oil jet into this facility to study buoyant oil droplet retention in these SRZs. 2D particle image velocimetry and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation are used to characterize the flow. Large-scale oil droplet trajectories in the SRZs are visualized, and oil droplet statistics in the downwelling region and vortex core are measured using high-magnification brightfield imaging and a droplet detection algorithm. Values of maximum velocity (7.7-32.2 mm s À1 ), turbulent kinetic energy (10 À6 to 10 À4 m 2 s À2 ), and turbulent kinetic energy dissipation rate (10 À7 to 10 À5 m 2 s À3 ) in the downwelling region of the facility fall within the range of values observed in the field. Flow field strength strongly influences oil droplet trajectories, the shape and size of the SRZ, and oil droplet statistics over time, with stronger flows retaining larger oil droplets in the downwelling region over longer periods of time. Indeed, droplets of almost 400 μm diameter are observed in the downwelling region after almost 10 min for the strongest flow. The facility may be used to study the behavior of microplastics, sediments, and zooplankton in LC in the future.

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Dynamics of Langmuir circulation in oceanic surface layers

Cited by 7 publications

References 60 publications

Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics

Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics

Mixing driven by vertically variable forcing: an application to the case of Langmuir circulation

An experimental Stommel Retention Zone facility with an application to oil droplet dispersion

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