Seasonal variation of the principal tidal constituents in the Bohai Sea

Abstract. The seasonal variation of tides plays a significant role in water level changes in coastal regions. In this study, seasonal variations of four principal tidal constituents, M 2 , S 2 , K 1 , and O 1 , in the Bohai Sea, China, were studied by applying an enhanced harmonic analysis method to two time series: 1-year sea level observations at a mooring station (named E2) located in the western Bohai Sea and 17-year sea level observations at Dalian. At E2, the M 2 amplitude and phase lag have annual frequencies, with large values in summer and small values in winter, while the frequencies of S 2 and K 1 amplitudes are also nearly annual. In contrast, the O 1 amplitude increases constantly from winter to autumn. The maxima of phase lags appear twice in 1 year for S 2 , K 1 , and O 1 , taking place near winter and summer. The seasonal variation trends estimated by the enhanced harmonic analysis at Dalian are different from those at E2, except for the M 2 phase lag. The M 2 and S 2 amplitudes show semi-annual and annual cycles, respectively, which are relatively significant at Dalian. The results of numerical experiments indicate that the seasonality of vertical eddy viscosity induces seasonal variations of the principal tidal constituents at E2. However, the tested mechanisms, including seasonally varying stratification, vertical eddy viscosity, and mean sea level, do not adequately explain the observed seasonal variations of tidal constituents at Dalian.