Characteristics of internal tides in deep water northwest of the Xisha Islands

Internal tides are internal gravity waves with quasi-tidal periodicities, which are ubiquitous in the ocean. Their characteristics in deep water areas between the Hainan Island and Xisha Islands in the northwestern South China Sea (SCS) are investigated based on 5-year moored current observations. The methods used in this study include power spectral analysis, harmonic analysis and dynamical mode decomposition are performed. The results show that the barotropic and baroclinic tidal currents in the study region are both dominated by diurnal oscillations. The vertical isopycnal diurnal displacement can reach 40 m at a depth of 500 similar to 900 m. Diurnal internal tidal currents mainly propagate across the continental slope, approximately 41% of which is phase locked with astronomical tides. The ratio is nearly double of that in the northeast SCS. In the upper ocean, it is found that the variability of internal 01 amplitude is significantly positively correlated with the local sea level height, which suggests a strong modulation by the upper oceanic density field. On the other hand, the Kt internal tid e shows a semi-annual variation with intensification in summer and winter due to the superposition of barotropic forcing from K-1 and P-1 constituents. Furthermore, dynamical mode decomposition shows that the two baroclinic modes together account for more than 70% of O-1 constituent energy, while the contribution of the third mode in the K-1 cannot be neglected, which has 25% of total K-1 energy in someone deployment