Model Study On The Three-dimensional Baroclinic Tide And Tidal Current In The Bohai Sea

The characteristics of the three-dimensional baroclinic tide and tidal current in the Bohai Sea is numerically studied in detailed by using the Princeton Ocean Model(POM). Based on the validated simulation of astronomical tides, the shallow water tides and energy dissipation in the Bohai Sea are further investigated, which has important theoretical significance and application value for deeply understanding the main characteristics of the tide and tidal current in Bohai Sea.In this study, Arakawa C orthogonal rectangular grids are used in the horizontal, and the grid resolution of Bohai Sea is set to 1/30° by 1/30°, so the longitudinal and latitudinal grids are 116 and139, respectively. Meanwhile, the vertical direction is divided into 10 sigma layers. Firstly, the baroclinic tides and tidal currents are simulated, and the harmonic constants of four major astronomical tides agreed well with the observations on the 11 tidal guages. The computational error is either positive or negative meaning that the system error does not exist. Furthermore, the root-mean-square error of the amplitude and the phase-lag of four major astronomical tides is calculated. The results show that: the root-mean-square error of the amplitude and the phase-lag of M2 tide are 11.82 cm and 10.43° respectively, S2 tide are 5.23 cm and 12.74° respectively, K1 tide are 6.40 cm and 9.63° respectively, O1 tide are 4.27 cm and 9.64° respectively. It is also found that the simulated results are in agreement well with the observations from the co-tidal charts of four tides. Two main possible reasons for the error are:(1) the new water depth data is used in our study, which maybe not match the earlier observations well;(2) Observation sites are almost near the shore, the orthogonal grid will exist the blind spot near the shoreline, the result of interpolation may also cause error. In addition, two tidal amphidromic points of M2 tide are far from the shore relatively, which may be one of the reasons of the root-mean-square error of M2 tide is slightly larger.The model results also show that, most areas of the Bohai Sea show irregular semidiurnal tidal type, the Bohai Strait and northeast sea area of Longkou show regular semidiurnal tidal type, and part sea area around amphidromic points of semidiurnal tides show diurnal tidal type. The distribution of maximum possible tidal range in Bohai Sea show that, coastal tidal range is relatively large, about 3m, while the middle part of the Bohai Sea is relatively small, about 2m, which is agreed with the distribution of the amplitude of M2 tide, because M2 tide is dominant in Bohai Sea and amplitude determines the tidal range directly. Compared with the tidal range distribution given in marine hydrological Atlas, the calculation results are also accurate, which further verify the reliability of the results of Bohai Sea tide.It is also showed that from the model results, most areas of the Bohai Sea show semidiurnal tidal current type, only the southeast area of Bohai Sea Strait and Temple Island Archipelago nearby show irregular diurnal tidal current, and only the small part of sea area which is spread by the Yangma island of Yantai show regular diurnal tidal current. The distribution of maximum possible tidal current show that, Laotieshan channel and the nearby sea area, Bohai Bay and the old Yellow River estuary are all strong tidal current area, the maximum possible tidal current can be more than 150cm/s, and most area can be more than 100cm/s. After comparison, the maximum possible tidal current of Liaodong Bay is relatively large, following is the Bohai Bay, and Laizhou Bay is smaller. The above results agreed well with other scholars’ and marine hydrology atlas, which further show the reliability of the calculation results of Bohai Sea tidal current.It is examined of the basic characteristics of three shallow water tides, namely M4, MS4 and M6 in Bohai Sea based on the validation of the model results, and it is found that both M4 and MS4 tides propagate in the similar way, which is characterized by five tidal amphidromic systems in Bohai Sea, and four of them rotate anticlockwise while one of them rotates clockwise. The model results are reasonably consistent with the previous studies. According to the relational formula expression between the shallow water tide and its source tides, the amplitude and phase-lag of MS4 tide are further estimated and compared with that obtained by means of the harmonic analysis, and the results were also reasonably consistent. The M6 tide is also analyzed and the model result shows seven amphidromic points in this sea area, among which four rotate anticlockwise and three rotate clockwise. The model results also show that the amplitudes of three shallow water tides are large in shallow coastal waters, which is closely related to the generation mechanism of shallow water tides.Finally, it is calculated of the tidal energy flux of four tides in Bohai Sea, the common characteristics of distribution are that, magnitude order of the tidal energy flux in Liaodong Bay, top of Laizhou Bay and the southeast of the Bohai Sea Strait is rather small, almost less than 0.1kw/m, which can almost be ignored compared with the central area of Bohai Sea, Bohai Bay and the entrance of the Bohai Sea Strait, most more than 10kw/m. This has a great relationship with characteristics of four tidal current, rectilinear current and rotary current have a direct effect on the distribution. At the same time, the distribution of the bottom boundary dissipation show that, the bottom boundary dissipation value of M2 is about one order of magnitude greater than S2 and K1, two order of magnitude greater than O1. M2 and S2 are dissipated larger in Liaodong Bay and Bohai Bay, M2 is about 10-1w/m2,S2 is about 10-2w/m2, which is particularly evident in the Old Yellow River Estuary, while relatively small in Laizhou Bay; K1 and O1 are dissipated larger in the middle of the Bohai Sea, Bohai Sea Strait and Laotieshan sea area, K1 is about 10-2w/m2, O1 is about 10-3w/m2, while relatively small in three bays. Finally, it is found that the dissipation of tidal energy is positively correlated with the magnitude of tidal current.