Numerical Study Of The Effects Of Langmuir Circulation And Wave Breaking On Upper Ocean Mixing

Surface waves affect the ocean surface mixed layer in two ways:(1) breaking waves inject turbulent kinetic energy at the ocean surface and create a near-surface layer of high energy dissipation; (2) the interaction of wave-induced Stokes drift current and mean wind-driven current generates counter-rotating vortices known as Langmuir circulations. Although both the turbulence energy flux due to breaking waves and the Stokes turbulence production are confined to the ocean surface within the top few meters, they can affect turbulent mixing processes throughout most of the ocean mixed layer.Although mixed layer models such as turbulence closure model have been used to simulate OML (Kantha and Clayson,2004; Sun et al.2005), these models have not adequately quantified LC and other mixing processes. Like Craig and Banner (1994), a modified Mellor-Yamada 2.5 level turbulence closure model is used to parameterize LC and other mixing processes'effects on upper ocean mixing through changing boundary conditions and original equations. Numerical modeling study including how LC affects OML under different sea state (different wind speed, strength of WB and heat flux). Strong mixing processes by interaction between LC and WB are discussed particularly.In order to describe the relative contributions of LC and WB to turbulence generation in OML, a three dimensional Large Eddy Simulation model (LES) is improved by including the small scale processes and used to investigate the effects of LC and WB on OML. By analyzing the turbulent kinetic energy (TKE) budget, the author examines the relative importance of standard boundary layer sources of TKE (shear and buoyancy production) to wave sources of TKE (LC and WB). This includes examination of the depths wave processes can influence. Furthermore, a number of cases under different wind and wave conditions have been conducted and the author assesses the relative importance of WB and LC in the mixed-layer dynamics as a function of sea state.Finally, the author compares the LES model results with the turbulence measurements collected during the CBLAST-Low (Coupled Boundary layers and Air-sea Interaction-Low Winds) experiments by Woods Hole Oceanography Institute in five numerical experiments: turbulent Langmuir number Lat>2 (shear case), Lat<1 (LC case), increasing wind speed, decreasing wind speed and crazy down wind speed cases.