Abstract
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.
Highlights
Tidal motion is one of the major dynamical processes in the Bohai Sea and has been widely studied (Fang et al, 2004)
The seasonal variations of several semi-diurnal tides are found to be significant in the Bohai Sea (Fang et al, 1986), but the corresponding comprehensive investigations are sparse, which means that the seasonal variation characteristics and mechanisms need to be further studied
When the monthly analysis was performed in segmented harmonic analysis (SHA), the automated constituent selection algorithm in T_TIDE was used to determine the analysed constituents; in addition, the unresolved constituents P1 and K2 were inferred from K1 and S2, respectively, with the inference parameters taken from a yearly harmonic analysis of the 1-year sea level observations at E2 (Kang et al, 1995; Foreman et al, 2009)
Summary
Tidal motion is one of the major dynamical processes in the Bohai Sea and has been widely studied (Fang et al, 2004). There is no primary seasonal cycle in the moon’s orbit, a significant seasonal variation in the principal lunar tidal constituent has been observed and is dominant in coastal and polar regions (Müller et al, 2014). The seasonal variations of several semi-diurnal tides are found to be significant in the Bohai Sea (Fang et al, 1986), but the corresponding comprehensive investigations are sparse, which means that the seasonal variation characteristics and mechanisms need to be further studied. Kang et al (2002) investigated the seasonal variability of the M2 tide in the Yellow Sea and East China Sea. Georgas (2012) observed seasonal episodes of significant tidal damping and modulation in the Hudson River estuary.
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