Numerical Study To3-D Structure Of Water Temperature,Salinity And Current Field Of The Bohai Sea

In this paper, the finite-volume coastal ocean model(FVCOM) is applied to simulate the3-D structure of water temperature, salinity and current field of Bohai Sea, based on the latest coastline and topography. And by doing several numerical experiments, the formation mechanism of the Asymmetric Dual-Core Cold Bottom Water (ADCCBW) is further explored.Firstly, this paper simulates the tide, tidal current of the Bohai Sea accurately, which demonstrate their main features. The simulation results show that:there are mainly four strong tidal current areas in the Bohai Sea, whereas the weak tidal currents locate in five areas. The differences between the Lagrange and Euler tidal residual current, the values of which are generally very small all over the whole Bohai Sea.Based on the FVCOM ocean model with its ice module, the3-D structure of water temperature, salinity and current field of the Bohai Sea are successfully simulated, taking the effects of the tides, wind stress, solar short-wave radiation into account by using the high spatial and temporal resolution NCEP-CFSR dataset. The model results show that:in winter, the distribution characteristics of water temperature generally show higher values in the east and central areas, and the shape of isotherm in the central area likes a "saddle"; the water temperature is vertically uniform. In spring, the distribution characteristics of the SST are determined by the combined effects of the location and local water depth. In summer, below the thermocline, the sea temperature is relatively higher in the coastal areas, and so is the central Bohai Sea, which is surrounded by a nearly-circular-shaped water belt with lower temperature. For the first time, the elliptical cold water belt in southwest-northeast direction located near the north depression basin is successfully simulated. There exists a "Asymmetric Dual-Core Cold Bottom Water"(ADCCBW) in the temperature structure of the Bozhong Section, and for the first time, the simulated low temperature core in the north basin located over the slope from the central ridge to the depression, which has been verified by the hydrographical investigation data. In autumn, the thermocline intensity become weak, which lead to the small sea temperature difference between the surface and mid-layer.On the basis of satisfied simulation of the sea temperature and salinity, this paper further analysis the characteristics of the current structures of the Bohai Sea. There exists a clockwise circulation in the central Bohai all the year around, while its intensity and sphere of influence have seasonal variations. The principal component is the wind-driven currents in winter and thermohaline current in summer, while is the tide-induced residual current in spring and autumn. There are significant nonlinear interactions between the pure wind-driven currents and the tidal current. For the proportion in the general circulation, the largest and the lowest component are the wind-driven currents and c respectively in winter and the thermohaline currents and tide-induced residual currents respectively in summer.By implement three numerical simulations, the formation mechanism of the Asymmetric Dual-Core Cold Bottom Water (ADCCBW) is further explored. The tidal mixing plays a significant role in the formation of the ADCCBW. In the Bohai Sea, there exist complex and significant nonlinear interactions between the tidal current and the pure wind-driven currents as well as the thermohaline currents, which is a major factor in forming the circulation structures in the summer Bohai. That is why we emphasize here that the tide and tidal current should not be neglected in the numerical simulations on the Bohai Sea. The low temperature water existed in the north and south depression basins of the summer Bohai Sea is mainly composed by the cold water remained from winter time, not the intruded water from the north Yellow Sea. The intruded water from the north Yellow Sea could cooling the water located in the east, south and west of the central ridge, while warming the water located in the north depression basin of the central ridge. And it is not the major factor in forming the circulation structures in the summer Bohai. The deeper water depth in the north depression basin is the principal reason of the temperature lower in north depression basin.