Characteristics of Langmuir Turbulence in the Ocean Mixed Layer

Abstract

This study uses large-eddy simulation (LES) to investigate the characteristics of Langmuir turbulence through the turbulent kinetic energy (TKE) budget. Based on an analysis of the TKE budget a velocity scale for Langmuir turbulence is proposed. The velocity scale depends on both the friction velocity and the surface Stokes drift associated with the wave field. The scaling leads to unique profiles of nondimensional dissipation rate and velocity component variances when the Stokes drift of the wave field is sufficiently large compared to the surface friction velocity. The existence of such a scaling shows that Langmuir turbulence can be considered as a turbulence regime in its own right, rather than a modification of shear-driven turbulence.Comparisons are made between the LES results and observations, but the lack of information concerning the wave field means these are mainly restricted to comparing profile shapes. The shapes of the LES profiles are consistent with observed profiles. The dissipation length scale for Langmuir turbulence is found to be similar to the dissipation length scale in the shear-driven boundary layer. Beyond this it is not possible to test the proposed scaling directly using available data. Entrainment at the base of the mixed layer is shown to be significantly enhanced over that due to normal shear turbulence.

Wave-induced mixing and transport of buoyant particles: application to the Statfjord A oil spill

“…Including the Stokes shear production, the TKE budget for horizontally homogeneous flow becomes (e.g., Kantha and Clayson, 2004;Grant and Belcher, 2009) …”

“…The interaction of the Stokes drift with the mean Eulerian flow through a vortex force gives rise to an instability known as the second Craik-Leibovich (CL2) mechanism, which causes Langmuir cells to develop (e.g., Craik, 1977;Leibovich, 1983). The effect of Langmuir circulation (LC) on the turbulence in the ocean mixed layer has been studied using large-eddy simulations (e.g., Skyllingstad and Denbo, 1995;McWilliams et al, 1997;Grant and Belcher, 2009), revealing elevated values of turbulent kinetic energy and dissipation. While the effect of wave breaking is restricted to the uppermost meters of the ocean, Langmuir turbulence affects the entire mixed layer and is more important for mixed layer deepening (e.g., Kantha and Clayson, 2004;Kukulka et al, 2010).…”

Wave-turbulence scaling in the ocean mixed layer

“…Here, directly influences many air-sea processes such as the mixing of near-surface water properties (Garrett, 1996;Stevens et al, 2011), gas transfer across the ocean interface (Lorke and Peeters, 2006;Zappa et al, 2007) and the dynamics and evolution of plankton blooms (Denman and Gargett, 1989;Yamazaki et al, 1991). Parameterizing in the OBL has proven to be more difficult than in the atmospheric boundary layer (ABL) due to the presence of surface gravity waves (Agrawal et al, 1992; and Langmuir circulations (McWilliams et al, 1997;Grant and Belcher, 2009) creating enhanced dissipation relative to what is expected from a shear driven boundary layer.…”

“…There has been a desire for more observations of near-surface values of due to the growing prevalence of using large eddy simulations (LES) to model the OBL (McWilliams et al, 1997;Noh et al, 2004;Grant and Belcher, 2009). The key parameter in balancing the sub-grid dynamics is and due to it not being resolved directly it is required to be parameterized.…”

“…The key parameter in balancing the sub-grid dynamics is and due to it not being resolved directly it is required to be parameterized. However, the lack of observations limits the ability to validate proposed parameterizations (see Noh et al, 2004 or Grant andBelcher, 2009). …”