Dynamics and Variability of Topography-induced Shear Instabilities in Western Boundary Currents

Shear instability is considered to be one of the most important turbulence generation mechanisms in the ocean, in which flow instabilities develop in stratified shear flows and dominate the flow dynamics and mixing. Observations near Green Island, southeast of Taiwan, show that Kelvin-Helmholtz billows are characterized by cold water rolled up with a vertical scale of around fifty meters due to the vertical circulating flows. The observed billows presumably owing to the interaction between the Kuroshio flow and a system of seamount varies between 50-100 m in vertical scale and within O(100 m) in horizontal scale. Computational Fluid Dynamics (CFD) model OpenFOAM with Large Eddy Simulation (LES) closure was applied to investigate the dominant mechanism controlling the scale of billows in stratified shear flow at high Reynolds Number. The Ozmidov length scale defined as the square root of the ratio between the dissipation and the third power of the buoyancy frequency describes the turbulent modulation by stratification, and separates the buoyancy subrange from the inertial subrange. The Ozmidov length scale estimated by the measurement data is around 10 m. With vertical grid resolution smaller than the Ozmidov length scale, the effect of turbulence on flow dynamics dominated by 3D shear instabilities can be explored. The simulation results show the growth of the shear instability behind the seamount and the downstream thickening of the shear layer. The patterns of simulated turbulent dissipation rate are in good agreement with the patterns of observed high echo intensity. The vertical scale of billows increases with current velocities but the horizontal scale of billows decreases with velocities. In addition, the horizontal scale of billows also decreases with the steepness of seamount geometry. Numerical experiments with different current velocities and slopes indicate the criterion of Richardson number and Froude number for the occurrence of instabilities.