Mass transport pattern and mechanism in the tide-dominant Bohai Sea

Based on a numerical hydrodynamic model and Lagrangian particle tracking method, the Lagrangian flow network and the Lagrangian residual velocity were derived to exhibit the mass transport pattern in the tide-dominant Bohai Sea. The characteristics of the Lagrangian flow network, represented by degree and hydrodynamic provinces, show distinct regional features in the Central Basin and the three bays. The first-order Lagrangian residual velocity (i.e., the mass transport velocity) can be well maintained over a dozen averaging tidal periods, with its magnitude slightly decreasing with the increasing number of tidal periods. The second-order Lagrangian residual velocity (i.e., the Lagrangian drift velocity) includes the dependence on the initial tidal phase, which is also weakened with the increasing number of tidal periods. The mechanism behind the above two features of the Lagrangian residual velocity is explained theoretically. In the Bohai Sea, with about 15 averaging tidal periods, the Lagrangian residual velocity is approximately independent of the initial tidal phase and close to the mass transport velocity. The streamlines of the Lagrangian residual velocity averaged over 15 tidal periods are parallel to the province boundaries in the Lagrangian flow network, which means that the Lagrangian flow network is determined by the Lagrangian residual current. This may lay the foundation for using Lagrangian flow network to analyze and quantify flow and transport pathways in shallow, tide-dominated coastal regions.