[A-OS15] Physical Oceanography (General)

[AOS15-P10] Shear instability and turbulent mixing in the stratified shear flow behind a topographic ridge

*Wei-Zhan Tsai¹, Jia-Lin Chen¹, Ming-Huei Chang², Yiing Jang Yang², Sen Jan² (1.Dep. of Hydraulic & Ocean Engineering, National Cheng Kung University, Taiwan, 2.Institute of Oceanography, National Taiwan University, Taiwan)

Keywords:lee wave breaking, shear instability, flow topography interaction, LES simulation, turbulent mixing and diffusivity

Observations on the lee of a topographic ridge show that the turbulence kinetic energy (TKE) dissipation rate due to shear instabilities is three orders of magnitude higher than the typical value in the open ocean. The evolution of shear instabilities is of crucial importance in understanding the turbulent process and of key interest in developing mixing parameterizations for ocean circulation and scalar transport models (Smyth and Moum, 2012). Conventionally, the vertical diffusivity in the ocean circulation model is parameterized by a stability function of local buoyancy and flow shear. The dependence of the eddy diffusivity on the intensity of flow shear and the ridge geometry is not considered in the existing model parameterization. The estimation of the viscosity coefficient with the combination of internal wave and shear instabilities generated mixing might be one to two order magnitude smaller than the actual eddy diffusivity under flow-topography interaction, incapable to capture the field observation. Our observational and modeling study indicates that the length scale or the period of shear instabilities are the keys to the mixing enhancement. The length scale and/or the period of shear instabilities are the key parameters to the mixing enhancement that increases with lateral Froude number Fr_L, i.e.. stronger shear and/or steeper ridge.