Large-eddy Simulation Of The Effects Of Different Stokes Drift Estimation On The Simulation Of Langmuir Circulation In The Upper Ocean

Langmuir circulation in the ocean surface boundary layer(OSBL)controls turbulent transport and plays an important role in the formation and deepening of the mixed layer.It is of great significance for the ocean mixed layer.Langmuir circulation is generated by the interaction of Stokes drift and vortex force induced by sea surface wind,which affects the exchanges of heat,momentum,and chemical species between the atmosphere and the ocean.Therefore,the Langmuir circulation is an important feature in the upper turbulent mixing.A series of studies have demonstrated the dependence of the Langmuir circulation upon the Stokes drift.It becomes important to model the magnitude and the shear of the Stokes drift velocity correctly to include Langmuir circulation in ocean models.It is often impractical to compute the Stokes drift profile with full two dimensional(2-D)wave spectrums which may not be available.So it is customary to replace the full Stokes drift velocity profile by a monochromatic profile.In this paper,three different Stokes drift profile approximations(monochromatic wave profile,exponential integral profile,Phillips spectrum approximation profile)are used to study the influence of different Stokes drift profiles on the Langmuir circulation.Based on ERA5 data,the annual distribution characteristics of global Stokes drift and Langmuir numbers were studied.The results show that the values of surface Stokes drift velocity and Langmuir numbers in the high latitudes are larger,especially in the area of Antarctic Circumpolar Current(ACC),the values of surface Stokes drift velocity reach the maximum and the Langmuir number is relatively small,which can reach below 0.35.The results show that the mixing effect in most of global oceans is due to the combination of shear instability and Langmuir circulation effect.In some areas,the Langmuir number can reach to 0.3,which proves that the role of Langmuir circulation can not be ignored in large-scale ocean numerical simulation.A three dimensional Large Eddy Simulation model(LES)is used for the ideal contrast experiments to study the influence of different Stokes drift vertical profiles on the Langmuir circulation simulation under constant wind speed.The results show that different Stokes drift vertical profiles have an effect on the velocity field structure of the Langmuir circulation.The results from three different experiments which use different Stokes drift profile approximations all shows the pattern of Langmuir circulation that the strong down-welling zone beneath the surface convergence appear parallel to each other,which is associated with the formation of Langmuir cells.The vertical velocity section appears the large vortex structure from the top to the bottom of the mixed layer.The vertical mixing is enhanced and the vertical velocity variance increases.However,there are differences in the simulation results of the three experiments:The stripe spacing of the monochromatic wave profile experiment is wider,while the stripes of the exponential integral profile experiment are relatively dense.The extreme value of vertical velocity and the combined velocity of the monochromatic wave profile experiment result are larger than those of the the exponential integral profile experiment result and the Phillips spectrum approximation profile experiment result.The maximum value and maximum depth of the vertical velocity variance are also different.Finally,based on the ideal contrast experiment,the measurements collected from the CBLAST-Low experiments are used to study the influence of different Stokes drift vertical profiles and the measured Stokes drift profile on the simulation results under the measured wind speed.The results show that the velocity field,the turbulent kinetic energy distribution and the vertical eddy viscosity distribution of the different Stokes drift vertical profile experiments are of great difference.The relative deviation of the vertical turbulent kinetic energy can reach to 14.62%,which is even larger in the time period that Langmuir circulation is dominant.During that time period,the relative deviation maximum of the vertical eddy viscosities can reach to 22.63%.The results show that using different Stokes drift profile approximations does have an impact on the Langmuir circulation simulation.