Modelling the seasonal thermal stratification and baroclinic circulation in the Bohai Sea

The seasonal thermal stratification and baroclinic circulation in the Bohai Sea are investigated with a three-dimensional baroclinic primitive equation model. The seasonal evolution of temperature is well simulated. Stratification appears early in April, intensifies with synoptic and neap-spring fluctuations through summer, and reaches its maximum in the middle of July. It is eventually destroyed at the end of September. There are cold water belts between well-mixed and stratified regions separated by a tidal front and coinciding with the isolines for a temperature difference of 1–2°C between surface and bottom. They are maintained by the entrainment of cold water from the stratified side of the frontal zone and through the mixing by tides and winds. The sea surface temperature (SST) distribution shows a local maximum at the head of three bays and in the area off Qinhuangdao. The latter SST maximum is due to the particularly reduced mixing as both winds and tidal currents are weak in this region. Atmospheric forcing and tides play an essential part for the synoptic and neap-spring variations of SST, stratification and tidal fronts. The circulation in the Bohai Sea results from the combined effects of tides, winds and baroclinic forces. Water transport between the Bohai and Yellow Seas is also controlled by the combination of all these forces. Inflow is strong and mainly confined to a narrow passage in the northern Bohai Strait. Outflow is broad and weak in the rest of the strait.