Observational Study on the Internal Tides in the Northern South China Sea

Abstract:

South China Sea (SCS) is the largest marginal sea of the western north Pacific. Due to the interaction between the abrupt topography at the Luzon Strait (LS) and the barotropical tides, internal tides (ITs) are active in the northern SCS. Therefore, to clarify the temporal variations and spatial distributions of the internal tides in the northern SCS, and to investigate the propagation and evolution of internal tides and its contributions to the deep ocean mixing, a mooring array with up to 11 moorings has been built in the northern SCS since 2010. Based on continuous observations for 5 years, a series of studies are conducted here, to analyze the temporal variability and spatial structure of the internal tides in the SCS, to describe the propagation paths of the internal tides in the SCS, to explore the influence of barotropical tides at LS to the internal tides in the northern SCS, and to quantify the contributions of the internal tides to deep ocean mixing in the northern SCS. A series of research results are as follows:

In the northern SCS, the diurnal tide kinetic energy is found to exhibit apparent seasonal variability—strongest in summer and weakest in autumn and spring. However, the variability of semidiurnal tide is just opposite, but with smaller amplitude. It is further found that the barotropical tide and the diurnal internal tide observed at the generation site also display a similar seasonal variation with diurnal internal tides in northeast SCS, which indicates that the barotropical tidal current at the source region has the potential to account for the seasonal variability of the diurnal tide on the continental slope of the northern SCS. During the westward propagation from LS to the continental slope and shelf in the SCS, the energy proportion of higher mode internal waves are increasing for diurnal tide, but decreasing gradually for semidiurnal tide. The internal tides are propagating away from the Luzon Strait, with the diurnal internal tides mainly propagating toward the southwest, and the semidiurnal internal tides toward the northwest. Vertical energy flux of semidiurnal internal tides is stronger than that of diurnal tides, both being stronger in winter, especially in December.