Coherent and incoherent features, seasonal behaviors and spatial variations of internal tides in the northern South China Sea

Based on observations of the currents at six moorings from March 2010 to April 2011, the coherent and incoherent features, seasonal behaviors and spatial variations of internal tides (ITs) in the northern South China Sea (SCS) are investigated. Measurements of the currents indicate that both diurnal and semidiurnal ITs contain stronger coherent signals than incoherent ones at all moorings. In the measuring range, coherent internal tidal current variances explain ~ 70% of the semidiurnal motion at most moorings. However, the proportion of coherent signals in the diurnal motion shows a non-monotonically decreasing trend with the westward propagation of diurnal ITs. Coherent signals of diurnal and semidiurnal ITs exhibit different seasonal variability at the six moorings: Diurnal ITs are stronger in winter (December to February) and summer (June to August) than in spring (March to May) and autumn (September to November), whereas stronger semidiurnal ITs always appear in spring and autumn. Combining these results with the temporal variation of barotropic tidal currents at the Luzon Strait (LS), it can be concluded that the seasonal variability of ITs at the six moorings are determined by the barotropic tides at the LS. In addition, this study shows that there are asymmetric features of ITs to the east and west of the LS.