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S. A. Thorpe - One of the best experts on this subject based on the ideXlab platform.
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Line Vortices and the Vacillation of Langmuir Circulation
Journal of Physical Oceanography, 2016Co-Authors: Jonathan Malarkey, S. A. ThorpeAbstract:AbstractThree types of breakdown of Langmuir Circulation (Lc) are observed, two of which are represented in large-eddy simulation (LES) models, but the third, vacillation, is not. The stability of Lc can be examined by representing the downwind-aligned vortices by line vortices that are subjected to perturbations. Earlier conclusions relating to stability in homogeneous water of infinite depth are found to be in error because no stationary unperturbed state exists. The motion of vortices is examined and shown to be consistent with an explanation of Lc devised by Csanady. Motion of line vortices in water of limited depth or bounded below by a thermocline is examined. The motion replicates some of the features of vacillation observed by Smith in deep water bounded by a thermocline, including its periodicity and fluctuations in the formation of bubble bands. Vortices describe closed orbits within the Langmuir cells. Particle motions in the vacillating Lc pattern exhibit trapping close to the line vortices or...
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Spreading of floating particles by Langmuir Circulation
Marine pollution bulletin, 2009Co-Authors: S. A. ThorpeAbstract:Particles floating on the sea surface, in particular buoyant algae, are drawn into bands by the converging flow between neighbouring Langmuir cells. Floating bands subsequently amalgamate as a result of Langmuir turbulence. Simple models are developed to describe the rearrangement and dispersion of the floating particles. If the production of floating particles has ceased, the mean separation of the bands of particles increases with time and eventually becomes unrelated to the mean distance between the lines of convergence resulting from Langmuir Circulation. The concentration of particles in the bands, proportional to the width of bands, and the separation of bands with a given concentration of particles, both increase with time. Care is needed in estimating the width of Langmuir cells to distinguish between surface bands of floating material that is continuously being produced and bands made visible by some earlier, but discontinued, generation. An alternative mechanism for the generation of dense bands of floating algae is proposed.
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Bubble Clouds and Langmuir Circulation: Observations and Models
Journal of Physical Oceanography, 2003Co-Authors: S. A. Thorpe, Thomas R. Osborn, David M. Farmer, Svein VagleAbstract:Abstract Concurrent measurements of the rate of dissipation of turbulent kinetic energy and the void fraction and size distribution of near-surface bubbles are described. Relatively high dissipation rates and void fractions are found in bubble bands produced by Langmuir Circulation. The mean dissipation rates observed in the bands are close to those at which the dynamics of algae is significantly affected. The data are used to test basic assumptions underpinning models of subsurface bubbles and associated air–sea gas transfer. A simple model is used to examine the qualitative effect of Langmuir Circulation on the vertical diffusion of bubbles and the representation of Langmuir Circulation in models of gas transfer. The Circulation is particularly effective in vertical bubble transfer when bubbles are injected by breaking waves to depths at which they are carried downward by the Circulation against their tendency to rise. The estimated value of the ratio r of the eddy diffusivity of particles (resembling b...
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Langmuir Circulation and the Dispersion of Oil Spills in Shallow Seas
Spill Science & Technology Bulletin, 2000Co-Authors: S. A. ThorpeAbstract:Abstract Aspects of Langmuir Circulation (Lc) which relate to the dispersion of floating material are reviewed. These include convergence, dispersion by advection (particularly of a plume of floating oil when wind and current are in different directions) and the spread and dispersion by cell instability or breakdown first described by Csanady. There are, however, processes which compete with Lc to diffuse floating material. In shallow tidally mixed seas, where the environmental impact of an oil spill may be greatest, cross-wind dispersion caused by Lc will dominate over that produced by bottom turbulence if the ratio of the wind speed, W, to current, U, is sufficiently large. Observations and rough estimates suggest a transition near W/U=15. A simple model is devised to estimate cross-wind dispersion in shallow unstratified waters when turbulence generated at a flat seabed dominates that produced by Lc, but when the effects of Lc are still evident in aligning filaments of oil, as may commonly be the case in moderate winds in coastal or continental shelf waters.
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Dispersion of Buoyant Material by Langmuir Circulation and a Tidal Current
Marine Pollution Bulletin, 1999Co-Authors: W.a.m. Nimmo Smith, S. A. ThorpeAbstract:An analytical model is developed which quantifies and describes the dispersion of buoyant material caused by wind and wave generated Langmuir Circulation in a tidal flow. It is found that the angle between the orientation of the Langmuir bands, taken to be in the wind direction, and the current controls the lateral (across-current) dispersion. Largest values of the dispersion occur when the wind and current are almost perpendicular and the wind is strong relative to the current. The lateral diffusivity caused by Langmuir Circulation is compared with that produced by a current alone, and it is shown that the dispersive effects of Langmuir Circulation will usually dominate when the wind speed exceeds about 50 times the current.
S. Leibovich - One of the best experts on this subject based on the ideXlab platform.
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Langmuir Circulation and Instability
2013Co-Authors: S. LeibovichAbstract:Langmuir Circulation is a powerful mixing mechanism in natural bodies of water. The Circulation system is driven by wind and waves, and is important in establishing the mixed layer and thermocline in the ocean and lakes, and as such affects the air-sea exchange of momentum, heat, and gases, as well as the global current structure. The method of the formation of the Langmuir Circulation system as an instability caused by interaction between surface waves and current is outlined.
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resonant Langmuir Circulation internal wave interaction part 2 Langmuir Circulation instability
Journal of Fluid Mechanics, 2005Co-Authors: Gregory P Chini, S. LeibovichAbstract:A long-wavelength weakly nonlinear analysis is used to investigate the possibility for resonant energy exchange between low-mode internal waves and counter-rotating roll vortices known as Langmuir Circulation. The analysis is based on a two-layer ocean model in which the Langmuir Circulation is confined to the upper layer and counter-propagating internal waves travel along the sharp thermocline normal to the axes of the vortices. An asymptotically consistent description of the slow-time behaviour is obtained by making a WKBJ approximation to treat the comparatively high-frequency internal-wave reflections identified in Part 1. When the vortices and waves are modelled as linearly neutral modes, the resulting dynamics take the form of nonlinear oscillations. The theory suggests that Langmuir cells may transiently lose stability to standing internal-wave disturbances whose nodes are aligned with the cell downwelling zones. An exact solution of the Langmuir-Circulation-standing-wave interaction is used to gain insight into the nonlinear instability mechanism. As in Part 1, the modification of the linear internal-wave dynamics by the Craik-Leibovich 'vortex force' is found to be crucial to the interaction.
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resonant Langmuir Circulation internal wave interaction part 1 internal wave reflection
Journal of Fluid Mechanics, 2003Co-Authors: Gregory P Chini, S. LeibovichAbstract:Langmuir Circulation is a convective motion commonly observed in the oceanic mixed layer. Internal waves are a prominent feature of stratified regions, particularly the thermocline bounding the mixed layer. Here, the potential for Langmuir-Circulation-internal-wave coupling is investigated using a two-layer ocean model. The density jump across the sharp thermocline confines all rotational motions, including the wind-aligned Langmuir vortices, to the upper ('mixed') layer. Linear analysis indicates: (i) that thermocline compliance enhances the onset of Langmuir Circulation, and (ii) that the 'vortex force' arising from the interaction of surface waves with the wind-driven shear modifies the dynamics of cross-wind propagating internal waves. Weakly nonlinear analysis reveals that resonant cross-wind propagating internal waves can be nonlinearly reflected from stationary Langmuir Circulation, a dynamic reminiscent of the 'Bragg reflection' of surface waves propagating over sand bars. A key feature of the reflection mechanism is the modification of the linear internal-wave dynamics by the vortex force.
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simulations of three dimensional Langmuir Circulation in water of constant density
Journal of Geophysical Research, 1995Co-Authors: Amit Tandon, S. LeibovichAbstract:Simulations of Langmuir Circulation with constant eddy viscosity that reveal considerable levels of spatial and temporal complexity are reported. In field observations, the simulations could be interpreted as evidence of vacillation in space and time of measured features in the Circulation system, and of the presence of defect structures in the windrow patterns in which the local spacing changes by a factor of 2 in a distance short compared to the windrow spacing. The results also suggest that the ratio of downwind jet speed to downwelling speeds (pitch) can be highly variable and is not a robust feature of Langmuir Circulations.
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large scale Langmuir Circulation and double diffusive convection evolution equations and flow transitions
Journal of Fluid Mechanics, 1994Co-Authors: Stephen M. Cox, S. LeibovichAbstract:Two-dimensional Langmuir Circulation in a layer of stably stratified water and the mathematically analogous problem of double-diffusive convection are studied with mixed boundary conditions. When the Biot numbers that occur in the mechanical boundary conditions are small and the destabilizing factors are large enough, the system will be unstable to perturbations of large horizontal length. The instability may be either direct or oscillatory depending on the control parameters. Evolution equations are derived here to account for both cases and for the transition between them. These evolution equations are not limited to small disturbances of the nonconvective basic velocity and temperature fields, provided the spatial variations in the horizontal remain small. The direct bifurcation may be supercritical or subcritical, while in the case of oscillatory motions, stable finite-amplitude travelling waves emerge. At the transition, travelling waves, standing waves, and modulated travelling waves all are stable in sub-regimes.
Sidney Leibovich - One of the best experts on this subject based on the ideXlab platform.
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Resonant Langmuir-Circulation–internal-wave interaction. Part 1. Internal wave reflection
Journal of Fluid Mechanics, 2003Co-Authors: Gregory P Chini, Sidney LeibovichAbstract:Langmuir Circulation is a convective motion commonly observed in the oceanic mixed layer. Internal waves are a prominent feature of stratified regions, particularly the thermocline bounding the mixed layer. Here, the potential for Langmuir-Circulation-internal-wave coupling is investigated using a two-layer ocean model. The density jump across the sharp thermocline confines all rotational motions, including the wind-aligned Langmuir vortices, to the upper ('mixed') layer. Linear analysis indicates: (i) that thermocline compliance enhances the onset of Langmuir Circulation, and (ii) that the 'vortex force' arising from the interaction of surface waves with the wind-driven shear modifies the dynamics of cross-wind propagating internal waves. Weakly nonlinear analysis reveals that resonant cross-wind propagating internal waves can be nonlinearly reflected from stationary Langmuir Circulation, a dynamic reminiscent of the 'Bragg reflection' of surface waves propagating over sand bars. A key feature of the reflection mechanism is the modification of the linear internal-wave dynamics by the vortex force.
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Eulerian and Lagrangian Langmuir Circulation patterns
Physics of Fluids, 2002Co-Authors: Rajesh Bhaskaran, Sidney LeibovichAbstract:An idealized Langmuir Circulation pattern in the oceanic surface layer consists of parallel rolls with a fixed spatial periodicity. Observed surface manifestations of the Circulation patterns, formed as collections of floating material and known as windrows, are roughly parallel to the surface wind direction, but form a network of intersecting lines. Field observations of the windrow intersections, referred to as “Y-junctions,” indicate a preferred orientation in which the two forks of the “Y” coalesce when viewed from upwind (the Y-junctions are said to point downwind). Weakly nonlinear theory yields patterns in the instantaneous Eulerian fields, for example contours of surface convergence, that show joinings and bifurcations at locations that may be termed pattern defects. These take the form of downwind-pointing Y-junctions occurring at negative defects and their obverse at positive defects with no preference for either type. We show that the preferential orientation of the Y-junctions can be explained...
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Large-scale three-dimensional Langmuir Circulation
Physics of Fluids, 1997Co-Authors: Stephen M. Cox, Sidney LeibovichAbstract:A reduced equation set in two space dimensions capable of describing three-dimensional Langmuir Circulation in a layer of unstratified water is derived. The reduced description is valid for Circulations having large horizontal scales compared to vertical scales, and should be useful for pattern formation studies when this condition is met. Windrow patterns on the surface are found from numerical simulations of the reduced equation set, guided by secondary instability theory.
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Simulations of three‐dimensional Langmuir Circulation in water of constant density
Journal of Geophysical Research, 1995Co-Authors: Amit Tandon, Sidney LeibovichAbstract:Simulations of Langmuir Circulation with constant eddy viscosity that reveal considerable levels of spatial and temporal complexity are reported. In field observations, the simulations could be interpreted as evidence of vacillation in space and time of measured features in the Circulation system, and of the presence of defect structures in the windrow patterns in which the local spacing changes by a factor of 2 in a distance short compared to the windrow spacing. The results also suggest that the ratio of downwind jet speed to downwelling speeds (pitch) can be highly variable and is not a robust feature of Langmuir Circulations.
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Secondary Instabilities of Langmuir Circulations
Journal of Physical Oceanography, 1995Co-Authors: Amit Tandon, Sidney LeibovichAbstract:Abstract Finite-amplitude Langmuir Circulation in the form of rolls parallel to the wind direction is shown to be subject to three-dimensional instability under certain circumstances. Density stratification is not required for instability to manifest. The preferred form of this secondary instability appears to be traveling waves propagating in the direction of the wind. These cause the rolls, and their surface windrows, to deviate from the wind direction by a small angle for which estimates are given. The results of the paper show the value of secondary stability results for the design of numerical experiments to simulate Langmuir Circulation.
Peter P. Sullivan - One of the best experts on this subject based on the ideXlab platform.
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nonlocal transport due to Langmuir Circulation in a coastal ocean
Journal of Geophysical Research, 2012Co-Authors: Tobias Kukulka, Albert J. Plueddemann, Peter P. SullivanAbstract:[1] We present observations and simulations of large-scale velocity structures associated with turbulent boundary layer dynamics of a coastal ocean. Special purpose acoustic Doppler current profiler measurements revealed that such structures were frequently present, in spite of complex coastal environmental conditions. Large eddy simulation results are only consistent with these observations if the Langmuir Circulation (LC) effect due to wave-current interaction is included in the model. Thus, model results indicate that the observed large-scale velocity structures are due to LC. Based on these simulations, we examine the shift of energetics and transport from a local regime for purely shear-driven turbulence to a nonlocal regime for turbulence with LC due to coherent large-scale motions that span the whole water column. Without LC, turbulent kinetic energy (TKE) dissipation rates approximately balance TKE shear production, consistent with solid wall boundary layer turbulence. This stands in contrast to the LC case for which the vertical TKE transport plays a dominant role in the TKE balance. Conditional averages argue that large-scale LC coherent velocity structures extract only a small fraction of energy from the wavefield but receive most of their energy input from the Eulerian shear. The analysis of scalar fields and Lagrangian particles demonstrates that the vertical transport is significantly enhanced with LC but that small-scale mixing may be reduced. In the presence of LC, vertical scalar fluxes may be up gradient, violating a common assumption in oceanic boundary layer turbulence parameterizations.
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the influence of crosswind tidal currents on Langmuir Circulation in a shallow ocean
Journal of Geophysical Research, 2011Co-Authors: Tobias Kukulka, Albert J. Plueddemann, John H. Trowbridge, Peter P. SullivanAbstract:[1] Langmuir Circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. On the coastal shelves the largest-scale LC span the whole water column and thus couple the surface and bottom boundary layers and enhance turbulent mixing. Observations and large eddy simulations (LES) of a shallow coastal ocean demonstrate that these relatively large scale Langmuir cells are strongly influenced by crosswind tidal currents. Two mechanisms by which crosswind tidal shear may distort and disrupt Langmuir cells are proposed. The first mechanism involves cell shearing due to differential advection across the whole cell. For the second mechanism, middepth vertical LC currents advect sheared mean crosswind current, leading to the attraction of upwelling and downwelling regions, so that LC cells are unsustainable when both regions overlap. Scaling arguments indicate that LC cells are more susceptible to crosswind shear distortion for smaller LC surface velocity convergence and greater cell aspect ratio (vertical to horizontal LC scale), which is consistent with the results obtained from the observations and LES. These results imply that scaling of LC characteristics in a coastal ocean differs from that in the open ocean, which has important practical implications for parameterizing enhanced mixing due to LC.
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the effect of Langmuir Circulation and breaking waves on subsurface turbulence for realistic wind and wave conditions
2010Co-Authors: Albert J. Plueddemann, Tobias Kukulka, John H. Trowbridge, Peter P. SullivanAbstract:Abstract : Langmuir Circulation (LC) and wave breaking are turbulent processes driven by wind and surface waves that are critical in mixing of the ocean surface layer. The effects of LC and breaking waves on upper ocean turbulence are poorly understood, partly because of their mutual entanglement and their interactions with turbulence generated by shear and buoyancy. Utilizing turbulence measurements and a numerical model, we undertake a systematic comparison between observations and simulations of the wave-influenced surface boundary layer. Data sets obtained from the Surface Waves Process Program, the Coastal Mixing and Optics Experiment, and the Coupled Boundary Layers and Air-Sea Transfer experiment provide unique turbulence observations for a wide range of sea and wind conditions. The numerical model is based on a novel large eddy simulation (LES) that incorporates turbulent processes due to LC, buoyancy, shear, and breaking waves, while satisfying the conservation of momentum and energy. Research objectives include quantitative estimates of LC and breaking wave effects in terms of turbulence statistics for a wide range of wind and wave conditions. This work is an important step towards physics-based parameterizations of upper ocean turbulence, which are needed to improve numerical models of weather and climate.
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Significance of Langmuir Circulation in upper ocean mixing: Comparison of observations and simulations
Geophysical Research Letters, 2009Co-Authors: Tobias Kukulka, Albert J. Plueddemann, John H. Trowbridge, Peter P. SullivanAbstract:Received 4 February 2009; revised 27 March 2009; accepted 20 April 2009; published 28 May 2009. [1] Representing upper ocean turbulence accurately in models remains a great challenge for improving weather and climate projections. Langmuir Circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. We present a direct comparison between observations and large eddy simulations, based on the wave-averaged Navier-Stokes equation, of an LC growth event. The evolution of cross-wind velocity variance and spatial scales, as well as mixed layer deepening are only consistent with simulations if LC effects are included in the model. Our results offer a validation of the large eddy simulation approach to understanding LC dynamics, and demonstrate the importance of LC in ocean surface layer mixing. Citation: Kukulka, T., A. J. Plueddemann, J. H. Trowbridge, and P. P. Sullivan (2009), Significance of Langmuir Circulation in upper ocean mixing: Comparison of observations and simulations, Geophys. Res. Lett., 36, L10603, doi:10.1029/ 2009GL037620.
Albert J. Plueddemann - One of the best experts on this subject based on the ideXlab platform.
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nonlocal transport due to Langmuir Circulation in a coastal ocean
Journal of Geophysical Research, 2012Co-Authors: Tobias Kukulka, Albert J. Plueddemann, Peter P. SullivanAbstract:[1] We present observations and simulations of large-scale velocity structures associated with turbulent boundary layer dynamics of a coastal ocean. Special purpose acoustic Doppler current profiler measurements revealed that such structures were frequently present, in spite of complex coastal environmental conditions. Large eddy simulation results are only consistent with these observations if the Langmuir Circulation (LC) effect due to wave-current interaction is included in the model. Thus, model results indicate that the observed large-scale velocity structures are due to LC. Based on these simulations, we examine the shift of energetics and transport from a local regime for purely shear-driven turbulence to a nonlocal regime for turbulence with LC due to coherent large-scale motions that span the whole water column. Without LC, turbulent kinetic energy (TKE) dissipation rates approximately balance TKE shear production, consistent with solid wall boundary layer turbulence. This stands in contrast to the LC case for which the vertical TKE transport plays a dominant role in the TKE balance. Conditional averages argue that large-scale LC coherent velocity structures extract only a small fraction of energy from the wavefield but receive most of their energy input from the Eulerian shear. The analysis of scalar fields and Lagrangian particles demonstrates that the vertical transport is significantly enhanced with LC but that small-scale mixing may be reduced. In the presence of LC, vertical scalar fluxes may be up gradient, violating a common assumption in oceanic boundary layer turbulence parameterizations.
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the influence of crosswind tidal currents on Langmuir Circulation in a shallow ocean
Journal of Geophysical Research, 2011Co-Authors: Tobias Kukulka, Albert J. Plueddemann, John H. Trowbridge, Peter P. SullivanAbstract:[1] Langmuir Circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. On the coastal shelves the largest-scale LC span the whole water column and thus couple the surface and bottom boundary layers and enhance turbulent mixing. Observations and large eddy simulations (LES) of a shallow coastal ocean demonstrate that these relatively large scale Langmuir cells are strongly influenced by crosswind tidal currents. Two mechanisms by which crosswind tidal shear may distort and disrupt Langmuir cells are proposed. The first mechanism involves cell shearing due to differential advection across the whole cell. For the second mechanism, middepth vertical LC currents advect sheared mean crosswind current, leading to the attraction of upwelling and downwelling regions, so that LC cells are unsustainable when both regions overlap. Scaling arguments indicate that LC cells are more susceptible to crosswind shear distortion for smaller LC surface velocity convergence and greater cell aspect ratio (vertical to horizontal LC scale), which is consistent with the results obtained from the observations and LES. These results imply that scaling of LC characteristics in a coastal ocean differs from that in the open ocean, which has important practical implications for parameterizing enhanced mixing due to LC.
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the effect of Langmuir Circulation and breaking waves on subsurface turbulence for realistic wind and wave conditions
2010Co-Authors: Albert J. Plueddemann, Tobias Kukulka, John H. Trowbridge, Peter P. SullivanAbstract:Abstract : Langmuir Circulation (LC) and wave breaking are turbulent processes driven by wind and surface waves that are critical in mixing of the ocean surface layer. The effects of LC and breaking waves on upper ocean turbulence are poorly understood, partly because of their mutual entanglement and their interactions with turbulence generated by shear and buoyancy. Utilizing turbulence measurements and a numerical model, we undertake a systematic comparison between observations and simulations of the wave-influenced surface boundary layer. Data sets obtained from the Surface Waves Process Program, the Coastal Mixing and Optics Experiment, and the Coupled Boundary Layers and Air-Sea Transfer experiment provide unique turbulence observations for a wide range of sea and wind conditions. The numerical model is based on a novel large eddy simulation (LES) that incorporates turbulent processes due to LC, buoyancy, shear, and breaking waves, while satisfying the conservation of momentum and energy. Research objectives include quantitative estimates of LC and breaking wave effects in terms of turbulence statistics for a wide range of wind and wave conditions. This work is an important step towards physics-based parameterizations of upper ocean turbulence, which are needed to improve numerical models of weather and climate.
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Significance of Langmuir Circulation in upper ocean mixing: Comparison of observations and simulations
Geophysical Research Letters, 2009Co-Authors: Tobias Kukulka, Albert J. Plueddemann, John H. Trowbridge, Peter P. SullivanAbstract:Received 4 February 2009; revised 27 March 2009; accepted 20 April 2009; published 28 May 2009. [1] Representing upper ocean turbulence accurately in models remains a great challenge for improving weather and climate projections. Langmuir Circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. We present a direct comparison between observations and large eddy simulations, based on the wave-averaged Navier-Stokes equation, of an LC growth event. The evolution of cross-wind velocity variance and spatial scales, as well as mixed layer deepening are only consistent with simulations if LC effects are included in the model. Our results offer a validation of the large eddy simulation approach to understanding LC dynamics, and demonstrate the importance of LC in ocean surface layer mixing. Citation: Kukulka, T., A. J. Plueddemann, J. H. Trowbridge, and P. P. Sullivan (2009), Significance of Langmuir Circulation in upper ocean mixing: Comparison of observations and simulations, Geophys. Res. Lett., 36, L10603, doi:10.1029/ 2009GL037620.
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The Effect of Langmuir Circulation on the Surface Boundary Layer.
1996Co-Authors: Robert A. Weller, Albert J. PlueddemannAbstract:Abstract : The objectives of the effort were to improve our understanding of the dynamics of Langmuir Circulation, to examine its role in controlling the vertical fluxes of heat and momentum which determine the mean vertical structure of the upper ocean, and to make progress toward incorporating the effect of Langmuir Circulation in models predicting how the ocean responds to atmospheric forcing.
