Observations of near-inertial oscillations trapped at inclined front on continental shelf of the northwestern South China Sea

Abstract. Interactions of near-inertial oscillations (NIOs) with other oceanic processes have been broadly investigated in recent years. This study presents observed NIOs from a seabed-based platform, which are trapped at an inclined front with strong stratification on the continental shelf of the northern South China Sea (NSCS) during January–February 2021. The current observations reveal that four NIO events occurred induced by wind bursting. Under the same wind forcing, NIO currents in the third event increased from <0.05 m s⁻¹ to about 0.08 m s⁻¹. The mechanism analysis shows that the amplitude of NIOs was modulated by the shoaling depth of the mix-layer induced by the inclined front, and trapped in the upper layer. More important, the energy transferred from front to NIOs provides a new insight into the NIO dynamics. In addition, a remarkable non-linear interaction (fD1) between NIOs and diurnal spring tide occurred at the front zone owing to a strong vertical current shear at the depths of 20–50 m. The underlying physical phenomenon of this observation would be important for energy exchanges in the oceans.