Poleward Propagation of Typhoon-Induced Near-Inertial Waves in the Northern South China Sea

Zhenyu Sun,Xiaohui Xie,Jianyu Hu,Ruhui Huang

doi:10.3389/fmars.2021.713991

Abstract

One of the main responses of the ocean to typhoons is the generation of near-inertial waves (NIWs), whose intrinsic frequency is close to the local inertial frequency. Based on the mooring observations, we carefully investigated the spatial–temporal variations in NIWs in the northern South China Sea (SCS) after Typhoon “Haima,” which passed through the northern SCS from October 20 to 21, 2016, with its track parallel to the mooring array on its northeast side. Moorings in different locations responded differently to this typhoon, with stronger NIWs found on the continental slope to the shelf break and relatively weak NIWs found on the shallow continental shelf. Strong NIWs appeared twice within a short period and showed different characteristics and frequencies. The first NIWs were generated locally with blue-shifted (super-inertial) frequencies. These initial NIWs were dominated by the first three baroclinic modes and rapidly weakened due to their propagation away and local dissipation. However, the second NIWs mainly consisted of higher baroclinic modes with red-shifted (sub-inertial) frequencies. The analysis of the mean background flow revealed that these red-shifted NIWs were excited at low latitudes, and subsequently advected by the poleward background flow to moorings whose latitudes were higher than their critical latitudes. Higher-mode (n &gt; 3) NIWs were more easily carried away by mean background flow, and, at the same time, high-mode NIWs propagated downward to the subsurface layer, leading to the significant elevation of velocity shear therein. Given these findings, the mean background flow may provide an important route to redistributing the energy of the upper ocean imported from typhoons.

Highlights

In traditional internal wave theory, the internal gravity waves with an intrinsic frequency close to the local inertial frequency f (f = 2Ωsinθ; is the angular velocity of the Earth and θ is the latitude (Gill, 1982)) are usually referred to as near-inertial internal waves (NIWs)
Based on the observations of three moorings in the northern South China Sea (SCS), this article analyzes the responses of the upper ocean at the continental slope, shelf break, and continental shelf to Typhoon “Haima” and the poleward propagation of typhooninduced near-inertial waves (NIWs)
After the NIWs observed at Stage-1 had decayed completely due to their propagation away and local dissipation, strong NIWs were again observed at moorings A and B at Stage-2

Summary

Introduction

The characteristics (e.g., wave amplitude, duration, phase propagation, and frequency shift) of these NIWs are largely modulated by the wind field and background ocean conditions (Sun et al, 2015; Alford et al, 2016). The ocean conditions, such as mesoscale eddies (Oey et al, 2008; Chen et al, 2013; Fer et al, 2018), ocean fronts (Rubino et al, 2010; Alford et al, 2013), and background flows (Kunze, 1985; Xie et al, 2015; Jeon et al, 2019) play an important role in shaping the NIWs induced by typhoons. The characteristics of NIWs in this area can be much more complicated (Boyer et al, 2020)

Results

Discussion

Conclusion