Variability in Diurnal Internal Tides and Near-Inertial Waves in the Southern South China Sea Based on Mooring Observations

Temporal variations in diurnal internal tides (ITs) and near-inertial waves (NIWs) in the southern South China Sea (SCS) are characterized, based on two 13-month moored current observations. Diurnal ITs, dominated by O1 and K1, are found to exhibit spring–neap cycles of about 14 days and significant seasonal variations. The incoherent components explain 54% and 56% of the total energy in the diurnal band, which further complicates its temporal variabilities. As for NIWs, wind energy input serves as the primary energy source and three strong events are observed. Tropical cyclone RAI passed through two moorings during the event 1 period, and triggered a peak near-inertial kinetic energy of 19.55 J m−3 (18.82 J m−3) at two moorings. After generation, the NIWs propagated downward to around 300 m, becoming the most intense event observed at DA2. In contrast, the NIWs response to tropical cyclone NOCK’s passage during event 3 was relatively weaker. The near-inertial KE generated by NOCK was confined to depths shallower than 150 m, with the average near-inertial KE being only 85% (52%) of that during event 1 for two moorings, despite the near-inertial energy input from NOCK being nearly 400% that of RAI. The modulation of background vorticity is considered the primary factor resulting in the difference in intensity of two NIW events. The penetrating depth of NIWs under the modulation of anticyclonic eddies was more than twice that under the cyclonic eddies. Furthermore, the strongest NIWs during event 2 that were observed below 350 m at mooring 2 (183% stronger than average) were also related to a strong anticyclonic eddy.