Seasonal variations of coastal trapped waves (CTWs)’ propagation in the South China Sea

Coastal trapped waves (CTWs) are sub-inertial waves with energy confined to the continental shelf and play a crucial role in the coastal dynamics. In this study, we focus on analyzing the seasonal characteristics of CTWs in the South China Sea shelf region by using observations from tide gauges and reanalysis data. Our findings reveal significant seasonal variations in both the phase speed and amplitude of CTWs in the South China Sea. It is shown that phase speed tends to be higher during the winter, followed by spring and autumn, and with less distinct propagation characteristics during the summer. In terms of amplitude, apart from seasonal variations, the CTW amplitudes on the narrower Vietnam shelf are relatively smaller. Drawing on classical theories, we analyze the Kelvin wave mode, the first shelf wave mode, and the second shelf wave mode in CTWs, and investigate the effects of winds, background currents, and stratification. Results indicate that the winds have a significant impact on the generation of CTWs, and the greatest influence of winds occurs upstream relative to the direction of CTW propagation. Background currents can have a significant impact on the phase speed of CTWs, and the seasonal variations of CTWs are closely linked to the background circulations. Additionally, we discuss the influence of seawater stratification on the phase speed of CTWs (measured by Burger number, Bu). Under conditions of strong stratification in the summer, the Bu is larger and leads to higher phase speed of CTWs, particularly when the Bu reaches its maximum on the continental slope. This study provides a better understanding of the CTWs dynamics in the South China Sea shelf region. Our findings can be useful for predicting and managing coastal hazards in the region.