Effect of Langmuir circulation on upper ocean mixing in the South China Sea

Effect of Langmuir circulation （LC） on upper ocean mixing is investigated by a two-way wave-current coupled model. The model is coupled of the ocean circulation model ROMS （regional ocean modeling system） to the surface wave model SWAN （simulating waves nearshore） via the model-coupling toolkit. The LC already certified its importance by many one-dimensional （1D） research and mechanism analysis work. This work focuses on inducing LC＇s effect in a three-dimensional （3-D） model and applying it to real field modeling. In ROMS, the Mellor-Yamada turbulence closure mixing scheme is modified by including LC＇s effect. The SWAN imports bathymetry, free surface and current information from the ROMS while exports signifi- cant wave parameters to the ROMS for Stokes wave computing every 6 s. This coupled model is applied to the South China Sea （SCS） during September 2008 cruise. The results show that LC increasing turbulence and deepening mixed layer depth （MLD） at order of O （10 m） in most of the areas, especially in the north part of SCS where most of our measurements operated. The coupled model further includes wave break- ing which will brings more energy into water. When LC works together with wave breaking, more energy is transferred into deep layer and accelerates the MLD deepening. In the north part of the SCS, their effects are more obvious. This is consistent with big wind event in the area of the Zhujiang River Delta. The shallow water depth as another reason makes them easy to influence the ocean mixing as well.

Contrasting dynamic characteristics of shear turbulence and Langmuir circulation in the surface mixed layer

Large eddy simulation （LES） is used to investigate contrasting dynamic characteristics of shear turbulence （ST） and Langmuir circulation （LC） in the surface mixed layer （SML）. ST is usually induced by wind forcing in SML. LC can be driven by wave-current interaction that includes the roles of wind, wave and vortex forcing. The LES results show that LC suppresses the horizontal velocity and greatly modifies the downwind velocity profile, but increases the vertical velocity. The strong downweUing jets of LC accelerate and increase the downward transport of energy as compared to ST. The vertical eddy viscosity Km of LC is much larger than that of ST. Strong mixing induced by LC has two locations. They are located in the 26s-36s （Stokes depth scale） and the lower layer of the SML, respectively. Its value and position change periodically with time. In contrast, maximum Km induced by ST is located in the middle depth of the SML. The turbulent kinetic energy （TKE） generated by LC is larger than that by ST. The differences in vertical distributions of TKE and Krn are evident. Therefore, the parameterization of LC cannot be solely based on TKE. For deep SML, the convection of large-scale eddies in LC plays a main role in downward transport of energy and LC can induce stronger velocity shear （S2） near the SML base. In addition, the large-scale eddies and Sz induced by LC is changing all the time, which needs to be fully considered in the parameterization of LC.

Can Langmuir Circulations Solve the Problem of Insufficient Upper-Ocean Mixing?

Insufficient vertical mixing in the upper ocean during summer is a common problem of oceanic circulation and climate models.The turbulence associated with non-breaking waves is widely believed to effectively solve this problem.In many studies,non-breaking surface wave processes are attributed to the effects of Langmuir circulations(LCs).In the present work,the influences of LCs on the upper-ocean thermal structure are examined by using one-and three-dimensional ocean circulation,as well as climate,models.The results indicated that the effect of vertical mixing enhanced by LCs is limited to the upper ocean.The models evaluated,including those considering LC effects alone and the combined effects of LCs and wave breaking,failed to produce a reasonable summertime thermocline,resulting in a large cold bias in the subsurface layer.Therefore,while they can slightly reduce the biases of mixed layer depths and sea surface temperatures in models,LCs are insufficient to solve the problem of insufficient vertical mixing.Moreover,restriction of non-breaking surface wave-induced processes in LCs may be questionable.

The spacing of Langmuir circulation under modest wind

Spacing characteristics of Langmuir circulation （LC） arc computed by large eddy simulation （LES） model under modest wind. LC is an organized vertical motion, evidenced as buoyant materials forming lines nearly parallel to the wind direction. The horizontal distribution of velocity computed by LES shows clear lines formed by LC. These lines grow and parallel to each other for a while, which we call the stable state, before they finally form Y-junctions. We computed spacing between every two parallel lines by averaging them under the stable state. Statistically, spacing results of 154 tests （seven wind speed cases of 22 test runs each） show high correlations between spacing and wind speed, as well as mixed layer depth. The relationship of spacing and wind is important for future LC parameterization of upper-ocean mixing.

Effect of Stokes drift on upper ocean mixing

Stokes drift is the main source of vertical vorticity in the ocean mixed layer. In the ways of Coriolis - Stokes forcing and Langmuir circulations, Stokes drift can substantially affect the whole mixed layer. A modified Mellor-Yamada 2. 5 level turbulence closure model is used to parameterize its effect on upper ocean mixing conventionally. Results show that comparing surface heating with wave breaking, Stokes drift plays the most important role in the entire ocean mixed layer, especially in the subsurface layer. As expected, Stokes drift elevates both the dissipation rate and the turbulence energy in the upper ocean mixing. Also, ilffluence of the surface heating, wave breaking and wind speed on Stokes drift is investigated respectively. Research shows that it is significant and important to assessing the Stokes drift into ocean mixed layer studying. The laboratory observations are supporting numerical experiments quantitatively.

Numerical study on the effects of progressive gravity waves on turbulence

The wind-wave-ocean system, which contains complex interactive processes, is of great importance for the momentum, heat and mass transport in the atmosphere and ocean and at their interface. In this work, we perform wave-coupled phase-resolved numerical simulations to investigate the effect of progressive gravity waves on wind and ocean turbulence. Initially homogeneous turbulence under a finite-amplitude monochromatic surface wave is simulated to reveal how the wave influences the subsurface turbulence. For the interaction between wind-driven waves and shear turbulence in the ocean, new wave-phase-resolved simulation approaches are developed to capture Langmuir cells. Lastly, wind turbulence over one and two progressive waves is simulated to elucidate the dynamics of turbulence coherent structures impacted by surface waves for improved understanding of wind-wave growth mechanism.