Nodal variations and long-term changes in the main tides on the coasts of China

The long-term changes in the main tidal constituents (O-1, K-1, M-2, N-2, and S-2) along the coasts of China and in adjacent seas are investigated based on 17 tide-gauge records covering the period 1954-2012. The observed 18.61 year nodal modulations of the diurnal constituents O-1 and K-1 are in agreement with the equilibrium tidal theory, except in the South China Sea. The observed modulations of the M-2 and N-2 amplitudes are smaller than theoretically predicted at the northern stations and larger at the southern stations. The discrepancies between the theoretically predicted nodal variations and the observations are discussed. The 8.85 year perigean cycle is identifiable in the N-2 parameters at most stations, except those in the South China Sea. The radiational component of S-2 contributes on average 16% of the observed S-2 except in the Gulf of Tonkin, on the south coast, where it accounts for up to 65%. We confirmed the existence of nodal modulation in S-2, which is stronger on the north coast. The semidiurnal tidal parameters show significant secular trends in the Bohai and Yellow Seas, on the north coast, and in the Taiwan Strait. The largest increase is found for M-2 for which the amplitude increases by 4-7 mm/yr in the Yellow Sea. The potential causes for the linear trends in tidal constants are discussed.