Observational Study On The Internal Tides And Near-inertial Internal Waves In The Northern South China Sea

Internal waves, which play a remarkable role in the deep ocean mixing, material transport and sound propagation processes, have been frequently observed in the northern South China Sea (SCS), which is the largest marginal sea of the western north Pacific. Due to the special geographic location and complex topography, many scales of motion are active in the SCS, including large scale circulation, mesoscale eddies and strong internal tides etc. Besides, energetic near-inertial oscillations (NIO) are generated locally by strong winds, because the SCS has nearly the most frequently typhoons in the world. Therefore, the SCS is thought to be one of the largest internal wave energetic regions in the world’s oceans. However, due to the lack of field observations, little was known about the temporal variation, spatial distribution and contributions to ocean mixing of the internal tide and near-inertial internal wave. In the present study, the temporal variations and spatial distributions of internal tides and NIW, the beam of diurnal and semidiurnal tides from the Luzon Strait, the effects of the barotropic tides in the source region on the internal tides in the northern SCS, the contributions of the internal tide and near-inertial internal wave to deep ocean mixing, as well as the important role of mesoscale warm eddies in accelerating the downward propagation of NIW energy are carefully examined, based on continuous current or temperature profiles observed by 11 full-depth moorings, in addition with satellite remote sensing data and model experiments.On the continental slope, the diurnal tide kinetic energy is found to exhibit apparent seasonal variability-strongest in summer and weakest in autumn and spring, bearing a significant correlation with coherent (K1,O1 and P1) tidal motion, however the variability of semidiurnal tide is just opposite, but its amplitude is smaller. It is further found that the barotropic tide and the diurnal internal tide observed on the generation site also display a similar seasonal variation with diurnal internal tides in northeast SCS, which indicating that the barotropic tidal current on the source region has the potential to account for the seasonal variability of the diurnal tide on the continental slope of the northeastern SCS. For diurnal tide, the higher modal partitions of energy and flux are increasing, however, for semidiurnal tide, the partitions are invariant during the westward propagation. The internal tides are propagating away from the Luzon Strait, the diurnal internal tides mainly propagate toward the southwest, and the semidiurnal internal tides toward the northwest. Vertical energy flux input by semidiurnal internal tides is stronger than its by diurnal tides, and the input energy is stronger in winter, especially in December.The kinetic energy of NIW is stronger in autumn on the whole due to the presence of the strong winds, however, the NIW energy mainly concentrates in the upper ocean (shallower than 300 m). Based on the analysis of the mooring observation, we find that the stronger NIW energy which persists in the deeper ocean has a good corresponding relation to the mesoscale eddies, and the maximum depth can reach 1300 m. Therefore, mesoscale eddies, especially the warm eddies, are the chimneys through which the near-inertial wind work penetrates rapidly into the deep ocean, and has the potential to maintaining mixing in the deep ocean.