An ocean large‐eddy simulation of Langmuir circulations and convection in the surface mixed layer

Abstract

Numerical experiments were performed using a three-dimensional large-eddy simulation model of the ocean surface mixed layer that includes the Craik-Leibovich vortex force [Craik 1977; Leibovich 1977] to parameterize the interaction of surface waves with mean currents. Results from the experiments show that the vortex force generates Langmuir circulations that can dominate vertical mixing. The simulated vertical velocity fields show linear, small-scale, coherent structures near the surface that extend downwind across the model domain. In the interior of the mixed layer, scales of motion increase to eddy sizes that are roughly equivalent to the mixed-layer depth. Cases with the vortex force have stronger circulations near the surface in contrast to cases with only heat flux and wind stress, particularly when the heat flux is positive. Calculations of the velocity variance and turbulence dissipation rates for cases with and without the vortex force, surface cooling, and wind stress indicate that wave-current interactions are a dominant mixing process in the upper mixed layer. Heat flux calculations show that the entrainment rate at the mixed-layer base can be up to two times greater when the vortex force is included. In a case with reduced wind stress, turbulence dissipation rates remained high near the surface because of the vortex force interaction with preexisting inertial currents. In deep mixed layers (-250 m) the simulations show that Langmuir circulations can vertically transport water 145 rn during conditions of surface heating. Observations of turbulence dissipation rates and the vertical temperature structure support the model results.

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

“…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).…”

Observations and Theories of Langmuir Circulation: A Story of Mixing

“…These simulations have begun to explore aspects of the nonlinear dynamics (chaos and quasi-periodic behavior), and of finite-amplitude stability, etc. For example, Skyllingstad and Denbo (1995) explore the difference in response with and without the "CL vortex force" term, showing that the CL force does indeed make a difference (the first such on/off comparison).…”

Airflow measurements at a wavy air–water interface using PIV and LIF

“…the Stokes drift, and the surface shear current, leads to the generation of Langmuir circulations, causing significant mixing of the water column (Skyllingstad and Denbo 1995;McWilliams et al 1997;Noh et al 2005;Li et al 2005;Harcourt and D'Asaro 2008; Grant and E 2009;Veron et al 2009;Kukulka et al 2010;Belcher et al 2012;D'Asaro 2014). When surface waves break, the turbulence injected into the water column also significantly enhances surface mixing Melville et al 1998;Veron and Melville 1999;Melville et al 2002;Thorpe et al 2003;Gemmrich and Farmer 2004) and leads to substantial deviations from the classical theories (Agrawal et al 1992;Thorpe 1993;Melville 1994;Anis and Moum 1995;Melville 1996;Terray et al 1996; Veron and Melville 2001) and causes significant energy dissipation (Banner et al 2014;Thomson et al 2016;Schwendeman et al 2014;Zappa et al 2016; Melville 2013, 2015).…”