Surface mass (scalar) transfer results from large-eddy simulation (LES) of wind-driven shallow water flow with and without full-depth Langmuir circulation (LC) are reported. Our work is motivated by the airborne infrared imagery of Marmorino et al. (2005) suggesting that LC can affect mass transfer through stretching and compression of molecular sublayers below the surface (i. e. below the air-water interface). LC also serves as an effective mechanism for sea surface renewal of low concentration fluid. LES guided by the full-depth LC measurements of Gargett et al. (2004), shows that this large-scale, downwindelongated structure increases surface mass transfer velocity (a measure of mass transfer efficiency) by approximately 60 percent with respect to a similar flow without surface wave effects (i.e. without LC). Statistical analysis of LES variables reveals that full-depth LC dominates near-surface mass transport as well as transport everywhere else in the water column. In the absence of LC, nearsurface small eddies contribute towards the mass transport at the air-water interface. Finally, the LES is used to test the accuracy of surface renewal-based parameterizations (models) in predicting surface transfer velocity in flows with full-depth LC. It is found that the transfer velocity parameterization of Gulliver and Halverson (1989) based on surface renewal time scale given by characteristics of large-scale (full-depth) streamwise vortices performs best compared to parameterizations based on turbulent quantities measured at the surface.

Results are presented from the large-eddy simulations (LES) of a wind-driven flow representative of the shallow coastal ocean under the influences of Langmuir forcing and surface heating and cooling fluxes. Langmuir (wind and surface gravity wave) forcing leads to the generation of Langmuir turbulence consisting of a wide range of Langmuir circulations (LCs) or parallel, counterrotating vortices that are aligned roughly in the direction of the wind. In unstratified, shallow coastal regions, the largest of the LCs reach the bottom of the water column. Full-depth LCs are investigated under surface waves with a significant wave height of 1.2m and a dominant wavelength of 90mand wave period of 8 s, for a wind speed of 7.8ms(-1) in a 15-m-deep coastal shelf region. Both unstable and stable stratification are imposed by constant surface heat fluxes and an adiabatic bottom wall. Simulations are characterized by Rayleigh and Richardson numbers representative of surface buoyancy forcing relative to wind forcing. For the particular combination of Langmuir forcing parameters studied, although surface cooling is able to augment the strength of LC, a significantly high cooling flux of 560Wm(-2) (such that the Rayleigh number is Ra-tau 51000) is required in order for turbulence kinetic energy generation by convection to exceed Langmuir production. Such a transition is expected at a lower heat flux for weaker wind and wave conditions and thus weaker LCs than those studied. Furthermore, a surface heating flux of approximately 281Wm(-2) (such that the Richardson number is Ri(tau) 5-500) is able to inhibit vertical mixing of LC, particularly in the bottom half of the water column, allowing stable stratification to develop.

AbstractExisting large-eddy simulations (LES) of Langmuir supercells (LS) do not include rotational terms. Despite the fact that the actual coastal ocean is certainly affected by rotation, such simulations are found to provide excellent agreement with a wide range of features of LS observed in the shallow coastal ocean. This note explains why it is indeed acceptable to compare results of a nonrotational LES of LS in a laterally unbounded domain with observations of LS made in a rotating fluid with a lateral boundary.

AbstractInteraction between the wind-driven shear current and the Stokes drift velocity induced by surface gravity waves gives rise to Langmuir turbulence in the upper ocean. Langmuir turbulence consists of Langmuir circulation (LC) characterized by a wide range of scales. In unstratified shallow water, the largest scales of Langmuir turbulence engulf the entire water column and thus are referred to as full-depth LC. Large-eddy simulations (LESs) of Langmuir turbulence with full-depth LC in a wind-driven shear current have revealed that vertical mixing due to LC erodes the bottom log-law velocity profile, inducing a profile resembling a wake law. Furthermore, in the interior of the water column, two sources of Reynolds shear stress, turbulent (nonlocal) transport and local Stokes drift shear production, can combine to lead to negative mean velocity shear. Meanwhile, near the surface, Stokes drift shear serves to intensify small-scale eddies leading to enhanced vertical mixing and disruption of the surface...

Results are presented from large-eddy simulations of an unstably stratified open channel flow, driven by a uniform pressure gradient and with zero surface shear stress and a no-slip lower boundary. The unstable stratification is applied by a constant cooling flux at the surface and an adiabatic bottom wall, with a constant source term present to ensure the temperature reaches a statistically steady state. The structure of the turbulence and the turbulence statistics are analyzed with respect to the Rayleigh number ( Ra τ ) representative of the surface buoyancy relative to shear. The impact of the surface cooling-induced buoyancy on mean and root mean square of velocity and temperature, budgets of turbulent kinetic energy (and components), Reynolds shear stress and vertical turbulent heat flux will be investigated. Additionally, colormaps of velocity fluctuations will aid the visualization of turbulent structures on both vertical and horizontal planes in the flow. Under neutrally stratified conditions the flow is characterized by weak, full-depth, streamwise cells similar to but less coherent than Couette cells in plane Couette flow. Increased Ra τ and thus increased buoyancy effects due to surface cooling lead to full-depth convection cells of significantly greater spanwise size and coherence, thus termed convective supercells. Full-depth convective cell structures of this magnitude are seen for the first time in this open channel domain, and may have important implications for turbulence analysis in a comparable tidally-driven ocean boundary layer. As such, these results motivate further study of the effect of surface cooling on tidal boundary layers simulated via an oscillating pressure gradient. Such large-scale structures may also have an important impact on RANS-based (Reynolds-averaged Navier–Stokes equations-based) modeling of turbulence within tidal, convective flows.

Turbulence in the ocean surface layer is generated by time-varying combinations of destabilizing surface buoyancy flux, wind stress forcing, and wave forcing through a vortex force associated with the surface wave field. Observations of time- and depth-averaged vertical velocity variance of full-depth turbulence in shallow unstratified water columns under destabilizing buoyancy forcing are used to determine when process domination can be assigned over a wide range of mixed forcings. The properties of two turbulence archetypes, one representing full-depth Langmuir circulations and the other representing full-depth convection, are described in detail. It is demonstrated that these archetypes lie in distinct regions of the plane of log(La, Ra), where La and Ra are Langmuir and Rayleigh numbers, respectively, derived from scaling with surface stress velocity u(*) and a time scale characteristic of the growth of Langmuir circulation t(*) [(dU(S)/dx(3))(dU/dx(3))](-1/2), where U and U-S are mean and Stokes velocities, respectively. Situations in which neither process dominates lie between the two end members, with relative dominance given by proximity to one or the other. Cases dominated by direct stress forcing are conspicuous by their absence. In cases of Langmuir domination, surface Stokes velocity u(S0) is linearly related to u(*), making it impossible to differentiate between scaling depth-averaged vertical velocity variance with u(*), w(*L) (u(*)(2)u(S0))(1/3) and any other scaling involving both u(*) and u(S0). A third nondimensional parameter La-H is introduced and used to assess the importance of bottom boundary layer turbulence in a depth-limited system. Questions of time dependence and applicability of results to the open ocean surface boundary layer are considered.

Large-eddy simulations (LES) of wind-driven shallow water flows with Langmuir turbulence have been conducted and scalar transport and surface scalar transfer dynamics analyzed. In these flows, the largest scales of the Langmuir turbulence consist of full-depth Langmuir circulation (LC), parallel downwind-elongated, counter-rotating vortices acting as a secondary structure to the mean flow. Langmuir turbulence is generated by the interaction of the wind-driven shear current with the Stokes drift velocity induced by surface gravity waves. In the absence of resolved surface waves, the Langmuir turbulence-generating mechanism is parameterized via the well-known Craik–Leibovich vortex force (Craik and Leibovich, 1976) appearing in the momentum equation. Simulations do not resolve surface waves, thus the top of the domain is taken as a non-deforming, free-slip, wind shear-driven surface. LES guided by the full-depth LC field measurements of Gargett and Wells (2007) shows that Langmuir turbulence plays a major role in determining scalar transport throughout the entire water column and scalar transfer at the surface. Langmuir turbulence affects scalar transport and its surface transfer through (1) full-depth, large-scale LC and (2) near-surface, small-scale eddies. Two key parameters controlling the extent of these two mechanisms are (1)the ratio λ˜=λ/H, where λ is the dominant wavelength of the surface waves generating the turbulence and H is the mean water column depth and (2)the turbulent Langmuir number, Lat, which is inversely proportional to wave forcing relative to wind forcing. Results from simulations with varying combinations of λ˜ and Lat are analyzed in order to understand the effect of these two parameters on scalar dynamics.

Turbulent shear flows on shallow continental shelves (here shallow means that the interaction with the solid, no-slip bottom is important) are of great importance because of their role in vertical mixing as well as on the transport of sediment and bioactive material. The presence of a wave-field in these areas can lead to the appearance of Langmuir circulation (LC) which is known to strongly affect the dynamics of a turbulent flow. We investigate with large eddy simulation the influence of a stable stratification on Langmuir Circulation. Results show that LC intensity is reduced and then suppressed by an increasing statification.

Large-eddy simulations (LES) of wind-driven shallow water flows with and without full-depth Langmuir circulation (LC) are described and near-surface dynamics analyzed. LC consists of parallel counter-rotating vortices or cells that are aligned roughly in the direction of the wind and are generated by the interaction of the wind-driven shear with the Stokes drift velocity induced by surface gravity waves. Simulations do not resolve surface waves; thus the top of the domain is taken as a non-deforming, free-slip, wind shear-driven surface. In the absence of resolved surface waves, the LC-generating mechanism is parameterized via the well-known Craik-Leibovich vortex force (Craik and Leibovich 1976 J Fluid Mech. 73 401-26) appearing in the momentum equation. LES guided by the full-depth LC field measurements of Gargett and Wells (2007 J Fluid Mech. 576 27-61) shows that this large-scale, downwind-elongated structure changes surface log-layer dynamics in terms of mean downwind velocity and budgets of turbulent kinetic energy (TKE). For example, in terms of mean velocity, the mixing due to LC leads to a deviation from the classical surface log-law profile that is exhibited by wind-driven flow without LC. Furthermore, LC leads to a deviation from the classical balance between production and dissipation rates of TKE in the log-layer. Two key parameters controlling the extent of surface log-layer disruption caused by LC are the dominant wavelength (lambda) of the surface waves generating LC and the turbulent Langmuir number, La-t, which is inversely proportional to wave forcing relative to wind forcing.

Turbulent shear flows on shallow continental shelves (here shallow means that the interaction with the solid, no-slip bottom is important) are of great importance because of their role in vertical mixing as well as on the transport of sediment and bioactive material. The presence of a wavefield in these areas can lead to the appearance of Langmuir circulation which is known to strongly affect the dynamics of a turbulent flow. To investigate those dynamical effects within a RANS-type modeling framework, we apply a triple decompostion to the LES results of Langmuir circulation in order to further isolate the coherent structures from fluctuating velocity field. The results are compared to the classical double-decomposition. In contrast to the double-decomposition framework, the triple-decompostion more effectively educes the coherent structure field and quantifies the need to take into account the energy exchange between the coherent and random fluctuations as well as the overall impact of the coherent structures on the turbulence dynamics.

The ability to accurately measure the timing of migration is fundamental in testing hypotheses in marine ecology that deal with migration and movement of fish populations. Timing and patterns of movement in larval and juvenile fish have been estimated using life history scans of the chemical signatures encoded in their otoliths. We provide a quantitative approach to analyzing life history scan data using spectral analysis, which retrospectively measures the timing of ingress for individual fish. Saggital otoliths from juvenile Atlantic croaker (Micropogonias undulatus) were sampled using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Spectral analyses on these data estimate the timing of ingress at 68 days on average using strontium and 85 days using barium. Based on the inflection points of their nonlinear mixing curves, these data reveal entry and subsequent movement up-estuary. Moreover, we use these spectrally derived estimates to show that growth rates did not drive ingress timing for our samples. These data thus lend no support to the critical-size hypothesis in this instance.

Decadal- to centennial-scale variability has been identified in the Arctic Oscillation, but less is known about variations on the millennial scale. A record of sea-ice drift from off the Alaskan coast shows a 1,500-year cycle in the phase of the Arctic Oscillation.

We report on disruption of the log layer in the resolved bottom boundary layer in large-eddy simulations (LES) of full-depth Langmuir circulation (LC) in a wind-driven shear current in neutrally-stratified shallow water. LC consists of parallel counter-rotating vortices that are aligned roughly in the direction of the wind and are generated by the interaction of the wind-driven shear with the Stokes drift velocity induced by surface gravity waves. The disruption is analysed in terms of mean velocity, budgets of turbulent kinetic energy (TKE) and budgets of TKE components. For example, in terms of mean velocity, the mixing due to LC induces a large wake region eroding the classical log-law profile within the range 90 < x(3)(+) < 200. The dependence of this disruption on wind and wave forcing conditions is investigated. Results indicate that the amount of disruption is primarily determined by the wavelength of the surface waves generating LC. These results have important implications for turbulence parameterizations for Reynolds-averaged Navier-Stokes simulations of the coastal ocean.