Observed Near-Inertial Waves in the Northern South China Sea

Bing Yang,Po Hu,Yijun Hou

doi:10.3390/rs13163223

Abstract

Characteristics of near-inertial waves (NIWs) induced by the tropical storm Noul in the South China Sea are analyzed based on in situ observations, remote sensing, and analysis data. Remote sensing sea level anomaly data suggests that the NIWs were influenced by a southwestward moving anticyclonic eddy. The NIWs had comparable spectral density with internal tides, with a horizontal velocity of 0.14–0.21 m/s. The near-inertial kinetic energy had a maximum value of 7.5 J/m3 and propagated downward with vertical group speed of 10 m/day. Downward propagation of near-inertial energy concentrated in smaller wavenumber bands overwhelmed upward propagation energy. The e-folding time of NIWs ranged from 4 to 11 days, and the larger e-folding time resulted from the mesoscale eddies with negative vorticity. Modified by background relative vorticity, the observed NIWs had both red-shifted and blue-shifted frequencies. The upward propagating NIWs had larger vertical phase speeds and wavelengths than downward propagating NIWs. There was energy transfer from the mesoscale field to NIWs with a maximum value of 8.5 × 10−9 m2 s−3 when total shear and relative vorticity of geostrophic currents were commensurate. Our results suggest that mesoscale eddies are a significant factor influencing the generation and propagation of NIWs in the South China Sea.

Highlights

Near-inertial waves (NIWs) are commonplace internal waves of the global ocean and have frequencies around local inertial frequency f = 2 Ω sin φ, where Ω and φ represent the angular velocity of earth rotation and latitude
Prior to the formation of Noul, the sea surface temperature (SST) in central South China Sea (SCS) was generally higher than 30 ◦ C, which facilitated the development of Noul
Based on in situ observations, remote sensing, and reanalysis of data, characteristics of near-inertial waves (NIWs) in the northern SCS induced by tropical storm Noul were analyzed

Summary

Introduction

Near-inertial waves (NIWs) are commonplace internal waves of the global ocean and have frequencies around local inertial frequency f = 2 Ω sin φ, where Ω and φ represent the angular velocity of earth rotation and latitude. The excitation mechanism of NIWs can be rapid-changing wind of sporadic storms [1], localized wave–wave interactions [2], or the interaction between sea floor relief and geostrophic currents [3]. The global power input to NIWs by winds is 0.3–1.5 TW [8,9,10,11,12], which is comparable to the power conversion of surface tides to internal tides [13] and the work done by winds on oceanic geostrophic currents [14].

Methods

Discussion

Conclusion