Seasonal Variability and Nonlinear Interactions of Near-Inertial Waves and Internal Tides Near the Xisha Islands

Abstract

Based on nearly 8 years of moored observations, this study investigates the seasonal variability and dynamical interactions of near-inertial waves (NIWs) and internal tides (ITs) near the Xisha Islands in the northern South China Sea (SCS). Near-inertial waves (NIWs) are most energetic in autumn and winter, coinciding with frequent typhoon activity, whereas ITs follow a semiannual cycle with peaks in summer and winter. Modal analysis reveals that ITs are dominated by low modes, whereas NIWs exhibit variable vertical structures shaped by typhoon forcing and mesoscale eddy activity. A case study in autumn 2011 highlights the critical role of a cyclonic eddy (CE) in modulating NIWs. Energy budget analyses show that the net transfer from the CE to NIWs accounted for nearly 85% of the observed enhancement of near-inertial kinetic energy (NIKE), supporting its downward penetration to 543 m. Importantly, this transfer occurred even though the background flow energy was much stronger, indicating that NIWs can still gain substantial energy input under highly unbalanced flow-wave energy conditions. Typhoon-generated NIWs, characterized by strong shear, further dominated nonlinear interactions with ITs, leading to the intensification of coupled internal waves. These results underscore the combined influence of atmospheric forcing and mesoscale dynamics on internal wave energetics, providing new insights into energy pathways, eddy-wave coupling, and the regulation of mixing and momentum transport in the tropical ocean.