Modulation of Internal Tides by Turbulent Mixing in the South China Sea

Bingtian Li,Yibo Yuan,Xianqing Lv,Shiqiu Peng,Libin Du,Xiangqian Meng

doi:10.3389/fmars.2021.772979

Abstract

Modulations of internal tides (ITs) including the baroclinic tidal energy budget, the incoherency, and the nonlinear interactions among different tidal components by turbulent mixing in the South China Sea (SCS) are investigated through numerical simulations. The baroclinic tidal energy budget can hardly be affected by the structure of mixing. Meanwhile, change in the mixing intensity in a reasonable range also cannot obviously modulate the baroclinic tidal energy budget in the SCS. Compared to the baroclinic energy budget, the distributions of conversion and dissipation are more sensitive to the change of mixing. Turbulent mixing also modulates the incoherency of ITs by changing the horizontal density in the ocean. The horizontal variation of density adds incoherence to ITs largely by affecting the internal tidal amplitudes. Furthermore, nonlinear interactions among different components of ITs are generally modulated by the mixing intensity, whereas the variation of the mixing structure can hardly influence the nonlinear interactions. Therefore, the diapycnal diffusivity can set to be horizontally and vertically homogeneous in most of the internal tidal simulations, except for those in which the incoherency of ITs needs to be simulated. However, excessive strong mixing will destroy the stratification. Thus, the optimum range for IT simulations in the SCS is from O (10–5) to O (10–3) m2s–1.

Highlights

Internal tides (ITs) are internal waves generated as surface tides flow over rough topography such as seamounts and ridges (Nash et al, 2006; Xu et al, 2014; Cao et al, 2017)
The ocean model used in this study is the Massachusetts Institute of Technology General Circulation Model (MITgcm), which has been widely used for simulating ITs around the Luzon Strait (LS) and in the South China Sea (SCS) (Buijsman et al, 2012; Wang et al, 2016)
The simulated O1 is largely stronger than observation and makes the difference larger than other components

Summary

Introduction

Internal tides (ITs) are internal waves generated as surface tides flow over rough topography such as seamounts and ridges (Nash et al, 2006; Xu et al, 2014; Cao et al, 2017). ITs play important roles in enhancing mixing (Egbert and Ray, 2000; Niwa and Hibiya, 2001; Tian et al, 2010; Zhao, 2014; Peng et al, 2021). Turbulent mixing provides energy for the upwelling of deep-water circulation, which is an essential process of global overturning circulation (Toggweiler and Samuels, 1995; Webb and Suginohara, 2001). The averaged diapycnal diffusivity, which is necessary to maintain the global meridional overturning circulation, is one order of magnitude larger than those observed in the open ocean (Munk, 1966). Turbulent mixing in regional seas will be much stronger than that in the open ocean.

Methods

Results

Conclusion