Observed Near-Inertial Waves in the Northern South China Sea

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.

Effects of different stratification changes on both sides of the typhoon track on near-inertial energy propagation

“…However, after Typhoon Kalmaegi's passage, there is no noticeable difference in vorticity distribution on both sides of the typhoon track. Therefore, the propagation of NIWs will not be hindered by the positive vorticity (Lee and Niiler, 1998;Zhang et al, 2015;Zhang et al, 2019;Yang et al, 2021). A similar phenomenon is observed in Figures7e-7h.…”

Spatiotemporal Characteristics and Volume Transport of Lagrangian Eddies in the Northwest Pacific

“…NIOs are a significant component of ocean dynamics, characterized by oscillations persisting for many days in the upper ocean before decay, with the frequency close to the local Coriolis frequency [70,71]. Previous observations and experiments indicated an intricate interaction between NIOs and mesoscale eddies, which involves a modulation effect of eddies on the frequency, propagation and thermal response of NIOs [72][73][74]. This symbiotic relationship contributes to complexities in the interpretation of SSH anomalies.…”

Spatial and seasonal variations of near-inertial kinetic energy in the upper South China Sea: Role of synoptic atmospheric systems

“…The variability of HYCOM near-inertial currents is found to be broadly consistent with that of the observations. HYCOM simulations have also been used previously to study the influence of mesoscale eddies on the near-inertial waves and the typhoon-induced near-inertial waves (Cao et al, 2021;Yang et al, 2021).…”