Numerical study on the time of water exchange and the variation of pollutants' concentration in Bohai bay under the effect of M2 constituent

In comparison with some other concept and model for water exchange, it is more representative to use the average residence time as the water exchange time to study the exchange capability of sea areas. Based on the hydrodynamic simulation in the old and new topography in Bohai Bay, the paper studies the water exchange capability, its spatial distribution, and the differences between the old and new topography. The results show that the water exchange capability in Bohai Bay is on a low level: under the old topography, the average water exchange time is 300 days, while under the new topography, it increases to 316 days, indicating that the variation in topography weakens the water exchange capability; the spatial distribution structure of water exchange time is well comparable with that of tide-induced Lagrangian residual current, which demonstrates that the tide- induced Lagrangian residual current is an important dynamic factor in controlling water exchange in Bohai Bay; in the middle of western part of Bohai Bay, there exists a counterclockwise eddy, therefore the water exchange capability is on the lowest level with water exchange time more than 400 days. According to the characteristics of mixing among sub-areas, early mixing is related to geographic distance, and its balance is primary affected by of the characteristics of kinetic field. In addition, the comparison between the distribution structure of water exchange time and that of pollutants' concentration shows good consistency: areas with weak exchange capability are the areas of high concentration of pollutants, while areas with strong exchange capability are the areas of low concentration.