Tide-induced Lagrangian residual velocity and dynamic analysis based on field observations in the inner Xiangshan Bay,China

In the Xiangshan Bay at the east coast of China,coastal marine pollution is conspicuous and severe in recent years.As transport of the pollutants is closely related to the coastal circulation,there is a great practical significance to investigate the circulation in this area.In this work,the surface pattern and vertical profiles of Lagrangian residual velocity(LRV)were studied based on field observation data from the inner Xiangshan Bay.By tracking GPS-GPRS drifters’trajectories,the surface LRV pattern is going out in the central deep trough and flowing inwards near the shoreside.Combined with data from two mooring stations,vertical profiles of LRV is flowing out at surface and flowing in at the bottom,consistent with the gravitational circulation induced by baroclinic effects at the estuary.However,according to the diagnostic analysis,the main mechanism driving the residual current is barotropic rather than baroclinic.The LRV equation is controlled by the tidally-averaged barotropic pressure gradient force,tidal body force and tidally-averaged turbulent stress,while the tidallyaveraged baroclinic pressure gradient force is one order of magnitude less than other forces.Additionally,the tidally mean eddy viscosity coefficient which is used in the expression of tidally-averaged turbulent stress might be not adequate and requires further studies.

A Lagrangian mean theory on coastal sea circulation with inter-tidal transports I.Fundamentals

A brief review is made on the theory of the Lagrangian residual circulation and inter-tidal transports in a convectively weakly nonlinear system. In the review the emphasis is put on the systematical development of the theory and its weakness of convectively weakly nonlinear approximation. The fundamentals of a Lagrangian tidally-averaged theory on circulation with inter-tidal transport processes have been proposed for a general nonlinear coastal/estuarine system. The Lagrangian residual velocity is strictly de- fined, and it has been verified to be able to embody the velocity field of circulation. A new concept of the concentration for inter- tidal transport processes is presented. The concentration describing the inter-tidal transport processes should be a ＂Lagrangian inter-tidal concentration＂ defined and named, but not the Eulcrian tidally-averaged concentration used traditionally. The circulation described here contains a set of infinite temporal-spatial fields of velocity/concentration, each of which corresponds to a specific value of tidal phases varying continuously over one tidal cycle. When the convectively weakly nonlinear condition（ with a smaller order of eddy diffusion and sources） is approximately satisfied, a set of infinite temporal-spatial fields of velocity/concentration can be reduced to a single one.. the mass transport velocity/the Eulerian tidally averaged concentration as exhibited traditionally.

A Lagrangian mean theory on coastal sea circulation with inter-tidal transports Ⅱ. Numerical Experiments

The results of the new concept of coastal sea circulation are demonstrated by numerical simulations for the first time.The numerical experiments in three types of rectangular model seas illustrate the dependence of circulation on tidal phases due to the convectively nonlinear effect which is estimated by a newly defined drift dispersion index.Then,the present theory is applied in the Bohai Sea of China.At the Bohai Straits and the Huanghe River mouth area the circulation direction even reverses owing to different initial tidal phases which shows that the theory copes with nonlinearity well.The calculated M2 tide-induced residual circulation shows that a clockwise gyre exists in the center of an anticlockwise gyre in the central Bohai Sea due to the topographic features.In the Bohai Gulf the tide induced circulation shows a 3D structure with outflow at the surface and the inflow at the bottom which can partly explains the spread of the Huanghe River fresh water out of the Bohai Gulf and the inflow of the sediment from the Huanghe River.