Tidal-induced Lagrangian and Eulerian mean circulation in the Bohai Sea

Long-term transport processes in coastal seas with time scales from weeks to seasons time scale are controlled by residual circulation. In the Bohai Sea, an ultrashallow shelf sea of China, tidal residual is almost the dominant factor to the circulation due to slight stratification and weak wind in summer. In order to establish an adequate hydrodynamic base to the ecosystem dynamics of the Bohai Sea, the differences of tide-induced Lagrangian and Eulerian mean circulation are discussed and calculated in this contribution. The Stokes drift is analyzed theoretically. According to Longuet-Higgins [Deep-Sea Res. 16 (1969) 431], the Lagrangian flow is the sum of the Eulerian flow and the Stokes drift that is induced by the mean kinetic energy and coastal nonlinear interaction. Stokes drift is large in the coastal sea and in areas where the vorticity and/or divergence are large. Vertical mass transports by Stokes drift are also the result of nonlinear interaction of current, water level and topography. Hamburg Shelf Ocean Model (HAMSOM) is applied in the Bohai Sea to simulate the tides and tidal currents. The tide-induced Lagrangian mean circulation and the Eulerian one are calculated at the same time. In the area where the Stokes drift is in the same direction as the Eulerian residual, the Lagrangian one is stronger than the Eulerian one. Where they are pointing in opposite directions, the Lagrangian one is small, like in the southwest of the Bohai Bay, Liaodong Bay and Bohai Strait. The Lagrangian residual current flows into the Bohai Bay along its southern bank causing deposition of Huanghe River sediments. This is in agreement with observations.