A Numerical Study On The Seasonal Variability And Dynamic Mechanism Of Circulation In The Bohai Sea

The Bohai Sea, which is very shallow and semi-closed, is one of the shelf seas in China. In recent years, with changes of global climate, physical environment of the Bohai Sea has undergone tremendous variations. Although there have been so many studies on the Bohai Sea circulation system, people haven’t reached an agreement on its distribution and dynamic mechanism due to a lack of data and the poor technology. Incorporating some data with good quality, a three-dimentional barocline ROMS model was built in the Bohai Sea under the new climate background. Based on the results of basic simulation and contrast experiments, a series of scientific issues have been discussed, such as circulation distribution, water exchange in the Bohai Strait and the dynamic mechanism for their seasonal variability. The main conclusions are as follows:Circulation in the Bohai Sea is much weeker than local tidal current, with magnitude smaller than5cm/s in most regions all year around. Strong flow is found in the Bohai Strait and offshore areas with twisting coastlines, while currents in the central area of Bohai Bay and Laizhou Bay are weak all the time. On average, the magnitude of circulation is larger in summer and spring than that in autumn and winter.It seems that the coherent pattern of the Bohai Sea circulation from traditional viewpoint is not so obvious with the existence of weak currents, on the contrary, some small regional eddies are clear and notable, actually. In terms of depth-averaged currents, there is a clockwise circulation near the mouth of Liaodong Bay, and a small counter-clockwise eddy appears on its east in spring and summer; Along the coastal area of Liaodong Bay, some tiny evident vortex rings exist with little seasonal variation. Moreover, in spring and summer a counter-clockwise current arises in the center of the bay; Currents in the Bohai Bay vaires a lot from season to season. During wintertime, there is a counter-clockwise coastal current in the south and a clockwise eddy in the north, while an opposite situation turns up when summer is coming; Main trend of currents in Laizhou Bay is counter-clockwise and the coastal ones among them become strong in autumn and winter. In addition, all year around an obvious current exists along the east coast of Laizhou Bay, which flows to the Northern Yellow Sea through the southern Bohai Strait; What’s more, there is an evident current directing northeast, which finally reaches the central area of the Bohai Sea. Distribution of circulation in the Bohai Sea is three-dimensional, with a big difference between surface and bottom layer. Bottom circulation is similar to the tidal residual currents in most areas and changes little as time goes by. The surface circulation is controlled by wind in autumn and winter, whereas it is in accord with the depth-averaged currents during summertime.The water exchange through Bohai Strait is not as sample as the well-known pattern,’inflow in the north and outflow in the south’. In fact, areas with inflow and outflow present an alternative distribution. Overall, the inflow area is generally narrow, which usually consists of regions near37.9(?),38.3(?),38.55(?) and the northern part. Strong exchange includes the inflow in north end, the outflow in south end and around38.4(?) area of the strait.The nonlinear interaction between tides, topography and coastline, which produces tidal residual currents, forms the basic pattern of circulation system in the Bohai Sea. In the context of it, circulation is influenced by some factors with seasonal variability, such as wind, rivers, heat flux and freshwater flux input from upper surface, and therefore presents a regular annual varation. In winter, with strong north wind, surface currents are mainly comprised of wind-driven flow. However, the wind effect decreases sharply with depth because of tidal dissipation. At the moment, distribution of temperature and salinity is very simple because of a strong convective mixing, so there are few density currents except for one located on the outer edge of river diluted water. With time going by, wind direction changes and thermal stratification arises, and then density currents are becoming strong, while wind-driven currents are fading. Companied with mutual adjustment between circulation and T-S field, they transform into the summer pattern gradually. The main composition of circulation is density currents, and the velocity is relatively high. Afterwards, the climate background will convert back into winter type, so circulation system will run in a cycle.