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, M2, S2, K1,
and O1, 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 M2 amplitude and phase lag
have annual frequencies, with large values in summer and small values in
winter, while the frequencies of S2 and K1 amplitudes are also
nearly annual. In contrast, the O1 amplitude increases constantly from
winter to autumn. The maxima of phase lags appear twice in 1 year for
S2, K1, and O1, 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 M2 phase lag. The
M2 and S2 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.
Seasonal variation of the principal tidal constituents in the Bohai Sea
