Observations and Theories of Langmuir Circulation: A Story of Mixing

Smith, Jerome A.

doi:10.1007/3-540-44512-9_16

Cited by 22 publications

(19 citation statements)

References 34 publications

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“…As another example, we examine in § 5.3 the case of a waves scattered by sinusoidal current in constant depth, and show that there are cases where strong enhancement of the wave amplitude could be obtained within downwave-directed current jets. This is in agreement with previous observations which suggested that wind-wave amplitudes might be enhanced within the downwind-directed current maxima associated with alternating 'wind streaks' or 'Langmuir circulation' (Smith 1983(Smith , 2001, leading to preferential breaking of waves along such current jets. Finally, in § 5.4, we investigate the influence of longshore-type currents over sinusoidal bottom topography on the Bragg scattering of obliquely incident water waves and discuss their effects on the shifting of the first-order resonant frequencies and the enhancement/reduction of reflection.…”

Section: Introductionsupporting

confidence: 93%

A coupled-mode model for the scattering of water waves by shearing currents in variable bathymetry

Belibassakis

2007

J. Fluid Mech.

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A coupled-mode model is presented for wave–current–seabed interaction, with application to the problem of wave scattering by ambient shearing currents in variable bathymetry regions. We consider obliquely incident waves on a horizontally non-homogeneous current in a variable-depth strip, which is characterized by straight and parallel bottom contours. The flow associated with the current is assumed to be directed along the bottom contours and it is considered to be steady and known. In a finite subregion containing the bottom irregularity, we assume that the horizontal current profile is general and smoothly varying. Outside this region, the current is assumed to be uniform (or zero). Based on a variational principle, in conjunction with a rapidly convergent local-mode series expansion of the wave pressure field in the finite subregion containing the current variation and the bottom irregularity, a new coupled-mode system of equations is obtained, governing the scattering of waves in the presence of variable bathymetry and longshore shearing currents. By keeping only the propagating mode in the local-mode series, a new one-equation model is derived, having the property to reduce to the modified mild-slope equation when the current is zero, and to the enhanced mild-shear equation when the bottom is flat. An important aspect of the present model is that it can be further elaborated to treat shearing currents with general, depth-dependent vertical structure, and to include the effects of weak nonlinearity.

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Section: Introductionsupporting

confidence: 93%

A coupled-mode model for the scattering of water waves by shearing currents in variable bathymetry

Belibassakis

2007

J. Fluid Mech.

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“…This criticism is mostly directed at the failure of experimentalists to obtain sufficiently systematic and quantitative data sets necessary for such validation. The measurements of LT that are available in the literature tend to be either indirect or episodic and insufficiently quantitative (e.g., Gargett & Wells, ; Melville et al, ; Smith, ; Weller et al, , among others), thus lacking the systematic quality necessary for conclusive model evaluation. This, in part, motivates the present study and sets an objective to provide a benchmark data set that can be used for validation of wave‐driven aspects of LES.…”

Section: Introductionmentioning

confidence: 99%

The Impact of Nonbreaking Waves on Wind‐Driven Ocean Surface Turbulence

2020

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A comprehensive study on the properties of upper ocean turbulence and its response to a variety of wind and wave forcing conditions was conducted at the Surge Structure Atmosphere INteraction (SUSTAIN) facility, University of Miami, RSMAS. The dimensions of SUSTAIN wind-wave-current tunnel are 18 m long, 6 m wide, and 2 m high, uniquely allowing for freely forming turbulence unconstrained by side walls. The grid of input parameters covered a range of wave steepness (ak: 0-0.27) within each investigated wind speed (U 10 : 2-20 m/s), allowing for the isolation of wave effects on the turbulence driven by both wind and waves. The response of turbulence to each pair of wind-wave parameters was measured in steady-state conditions (after 20+ min of settling time) by a suite of nonintrusive turbulence visualization techniques, located at 10 m fetch. These included 3-D visualization of water tracing dye entrainment by turbulence, underwater particle image velocimetry, and passive and active thermal imagery resolving surface skin temperature and velocity fluctuations. The resulting data set includes both qualitative 3-D view of subsurface turbulent structures and precise quantitative turbulent kinetic energy (TKE) measurements mapped out in response to the grid of input parameters. In lower winds, TKE was found to grow substantially in response to increasing wave steepness, in line with the expected effect of increasing wave forcing. However, in higher winds the effect of increasing wave steepness on TKE was found to be negative. It is hypothesized to be due to the rise of airflow separation, effectively sheltering much of the water surface from turbulence production by wind friction.Plain Language Summary This laboratory study aims to visualize and quantify small-scale turbulence taking place below the ocean surface. Of a particular interest here is the effect of surface wave motion, outside of wave breaking, and how it influences ocean turbulence, which is otherwise forced by wind. Unlike in the field, the laboratory setup allows independent control of wind and wave parameters, thus allowing decomposition of wind-and wave-driven aspects of the turbulent flow. A mixture of well-known and novel nonintrusive turbulence visualization techniques employed in this study gave unprecedented clear view of turbulent response under varying forcing conditions. Two major counteracting wave-dependent processes were found to impact surface turbulence. One is additional turbulence production due to the action of wave induced drift current, another is turbulence dampening due wave induced airflow separation, causing a reduction in wind friction.

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“…It is the interaction of Stokes drift with wind-driven surface shear flow that induces Langmuir circulation (LC) formation [2]. As one of the most significant features of the mixed layer, LC appears as an array of alternating horizontal roll vortices, whose axes are roughly parallel to the wind direction [3][4][5]. LC could enhance the vertical mixing and alter vertical profiles of temperature and velocity [6][7][8][9], making it crucial in upper ocean studies.…”

Section: Introductionmentioning

confidence: 99%

Parameterization of Wave-Induced Mixing Using the Large Eddy Simulation (LES) (I)

2020

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Turbulent motions in the thin ocean surface boundary layer control exchanges of momentum, heat and trace gases between the atmosphere and ocean. However, present parametric equations of turbulent motions that are applied to global climate models result in systematic or substantial errors in the ocean surface boundary layer. Significant mixing caused by surface wave processes is missed in most parametric equations. A Large Eddy Simulation model is applied to investigate the wave-induced mixed layer structure. In the wave-averaged equations, wave effects are calculated as Stokes forces and breaking waves. To examine the effects of wave parameters on mixing, a series of wave conditions with varying wavelengths and heights are used to drive the model, resulting in a variety of Langmuir turbulence and wave breaking outcomes. These experiments suggest that wave-induced mixing is more sensitive to wave heights than to the wavelength. A series of numerical experiments with different wind intensities-induced Stokes drifts are also conducted to investigate wave-induced mixing. As the wind speed increases, the influence depth of Langmuir circulation deepens. Additionally, it is observed that breaking waves could destroy Langmuir cells mainly at the sea surface, rather than at deeper layers.Atmosphere 2020, 11, 207 2 of 13 is also affected by Langmuir turbulence and is primarily generated by surface waves. The depth at which LC the upper ocean is largely determined by the Stokes drift e-folding depth and the mixed layer thickness [10,11]. The injection of turbulent kinetic energy from breaking surface gravity waves provides another significant source of upper-ocean turbulence [12]. Increased turbulence due to breaking waves (BW) decays following reductions in significant wave heights [13,14].The importance of wave-induced mixing is demonstrated in upper-ocean mixed layer studies. However, most turbulence schemes such as the Mellor-Yamada [15] or K-profile [16] parameterizations do not account for wave processes. These turbulence models are handicapped by the failure to include mixed layer physical processes such as BW and LC. This results in ocean mixed layer models producing too shallow mixed layer depths in high-latitude oceans (especially the Southern Ocean) [17][18][19][20][21][22] and highly diffused thermoclines in tropical oceans [8,[23][24][25][26], leading to weaker El Niño in climate models [27]. Furthermore, the principal way in which waves influence the oceanic circulation is through wave-induced vertical mixing, which is currently limited by model resolution, thus necessitating parameterization [28][29][30]. To more realistically reproduce the upper ocean in ocean general circulation models, a number of ocean mixed layer models have been proposed [28][29][30][31][32][33][34][35][36][37][38][39]. For example, a parameterization scheme including wave-induced mixing is proposed to modify current Mellor-Yamada and K-profile parameterization turbulence models [36,40]. The comparison of numerical results with observat...

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Observations and Theories of Langmuir Circulation: A Story of Mixing

Cited by 22 publications

References 34 publications

A coupled-mode model for the scattering of water waves by shearing currents in variable bathymetry

A coupled-mode model for the scattering of water waves by shearing currents in variable bathymetry

The Impact of Nonbreaking Waves on Wind‐Driven Ocean Surface Turbulence

Parameterization of Wave-Induced Mixing Using the Large Eddy Simulation (LES) (I)

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