| Based on the thermohaline data in August of 1958 and 2000, January of 2001, January and August in the Marine Atlas of Bohai Sea, Yellow Sea, East China Sea (Hydrology, 1992), the normal annual wind field data in Bohai Sea, and the near forty years SSS data observed in the four measurement stations Beihuangcheng, Tanggu, Qinhuangdao, Huludao of Bohai Sea: the thermohaline field's variance characteristics are analysed. On base of the data analysis, three-dimensional tide-induced residual currents, the wind-driven and thermohaline currents in the Bohai Sea are diagnostically computed with an orthogonal curvilinear grid system, by means of a three-dimensional ECOMSED model. The winter and summer thermohaline structure and circulation characteristics in Bohai Sea are analysed, and the circulation variance is discussed preliminarily.The tides and tidal currents in Bohai Sea are well reproduced. The main characteristics of the tides and tidal currents and tide-induced residual currents in Bohai Sea are presented. The model results show that the density-reduced currents are comparatively robust and make an important contribution to the general circulation in Bohai Sea in summer. But in winter, the effect of the wind stress on the Bohai Sea general circulation is dominant.Based on the two CTD data sets from the cruise of the summertime in 2000 and wintertime in 2001 in Bohai Sea and the collected historic observed thermohaline data, we analyse systemically the distributional characteristics of temperature, salinity and density in wintertime and summertime in the Bohai Sea and the results reveal that the current hydrologic characteristics of the Bohai Sea have some obvious variances relative to the previous research results. In summer there are three low-temperaturecenters in the middle-layer and underlayer of the middle parts of the Bohai Strait, Liaodong Bay-mouth and Bohai Bay-mouth, but there is a uniform high-temperature center in the middle part of the Bohai Sea. In winter the isotherms extend westward and northward to the Bohai Sea from the warm core of the Bohai Strait and a cold-water tongue off the Qinhuangdao spreads southeastward out, so the saddle-like isotherm pattern is formed in the middle part of the Bohai Sea. The Bohai Sea salinity in summer and winter rise significantly in the past more than forty years and nowadays the Bohai Sea salinity is higher than that of the north Yellow Sea. The salinity distribution characteristics are the high-salinity in the Bohai Bay and the low-salinity in the Boahi Strait. The density distribution shows that there is a large cyclonic density circulation in Bohai Sea and it is likely to exist some density current gyre in the middle part of the Bohai Sea. In winter the saddle-like pattern of the Bohai Sea temperature and salinity shows that the circulation structure is much complexer than the previous research results.From the numerical modeling results, it can be found that there is a clockwise gyre in the middle part of Bohai Sea, whether in wintertime or in summertime. In summer, the clockwise gyre corresponding to the high-temperature and low-density center in the middle Bohai Sea is mainly dominated by the density circulation. And in winter, it is mainly dominated by the wind stress curl. In the east of Bohai Strait, the seawater has an anticlockwise movement in its main part. In the Bohai Strait, the characteristic of the depth-average general circulation is that the seawater flows into Bohai Sea through the northern strait and exits through the southern part. Seen from the whole circulation form of the Bohai Sea, the inflow can't inset the middle Bohai sea directly. In the Liaodong Bay, the seawater moves in clockwise in winter and in anticlockwise in summer. The seawater movement is complexer than the previous research works. In the mouth of the Bohai Bay, there was a clockwise gyre in summertime of 1958 associated with the low-salinity and low-density center, and the gyre is replaced by an anticlockwise gyre associated with the high-salinity and high...
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