Mixing Enhancement Modulated by Unsteady Shear Flow in the Kuroshio above A System of Seamount

Abstract:

The Kuroshio, similar to the Gulf Stream in the North Atlantic, transports warm, tropical water with a variety of important marine organisms northward along the route from Philippine Sea-Luzon Strait to the south of Japan as well as its marginal shelf and inner seas. The Kuroshio affects the temperature, salinity, and nutrient transport through turbulence processes with surrounding waters, incorporating properties of the water‐mass in areas along its route. The field experiment shows that the observed turbulence dissipation rate is up to O(10^-3) W kg^-1 with eddy diffusivity =10^-1 m²s^-1. The analysis of recent field observations along the route of the Kuroshio further indicates that the enhancement of vertical mixing could be modulated by the inflow velocity, background stratification, and sharply varying topography. The effect of rotation is relatively insignificant due to large Rossby number ~ O(10³) with the velocity U₀=0.5- 1.5 ms^-1. In this study, a combined theoretical and numerical approach is implemented to investigate the turbulent generation mechanisms in stratified flow over topography. The computational Fluid Dynamics (CFD) model OpenFOAM with Large Eddy Simulation (LES) turbulent closure was applied to investigate dominant mechanisms that control the spatial and temporal scales of instabilities in the stratified shear flow at high Reynolds Number. Our analysis indicates that high turbulence is mainly populated in the forward inclined, asymmetric, cusp-like bands. The strong flow on the top of the shear zone leads to flow instabilities with greater TKE dissipation rate, which is considerably elevated in the braid mixing zone of instability waves with a period of 5-15 minutes. The frequency of mixing enhancement is proportional to the bifurcation slope, which indicates the significance of non-hydrostatic effects in the upper layer of the shear flow.