Linear Variation of M2 Tide in the East and South China Seas

Understanding variations of tidal harmonic constants is important for ocean tidal model refinement, coastal disaster prevention, and marine resource development. Previous research mainly relied on long-term hourly tide gauge observations. However, the data’s limited number and locations of observation sites pose a challenge to studying tidal change patterns at the sea basin scale. Given the temporal variations in tidal parameters in regions of the China Seas, we developed a novel harmonic analysis method that incorporates an enhanced time-varying model. We applied this method to nearly 3 decades of sea surface altimetry data from the T/P, Jason 1–3, Geosat Follow-On (GFO), Envisat, and ERS-2 satellites. The research results demonstrate that the amplitudes and phase lags of the M2 tidal constituent in the Taiwan Strait exhibit remarkable variations; the amplitude change rate ranges from 6 to 7 mm/year, and the phase-lag change rate ranges from −0.6 to −0.8°/year. In the coastal waters of the China Sea, the variations in M2 tidal harmonic constants are more drastic than those in the open ocean, a phenomenon closely associated with tidal dynamic mechanisms in deep-water environments, the influence of seabed topography, and hydrological conditions. The findings of the research enhance the understanding of ocean dynamics and offer precise data for sea-level change prediction and tidal model refinement studies.