Asymmetric-tidal-mixing Induced Residual Flow In The Jiulong River Estuary

Recent studies have recognized that the asymmetry of tidal mixing(ATM)between flood and ebb tides plays an important role in creating residual currents in periodically stratified estuaries.In this study,ATM and its role in the dynamics of the Jiulong River Estuary,a tidally-dominated estuary in Fujian Province,China,were investigated based on the hydrological observation data obtained from 06/May/2015 to 08/May/2015 and a one-year numerical simulation using the Regional Ocean Model System(ROMS).The observation data indicated that the vertical eddy viscosity coefficient changes periodically with stronger tidal mixing during flood than ebb.The comparison between the simulation results of the model and the observation data has showed that the model provides a good simulation of the hydrodynamic process in the Jiulong River Estuary.The south main channel of Jiulong River Estuary is the focus of this study and the results showed that the main channel exhibits tidal asymmetry in vertical mixing between flood and ebb tides.The main channel can be divided into three regimes,namely inner regime,central regime and outer regime,based on the along-estuary distribution of salinity as well as the distribution of ATM.The tidal mixing is stronger during flood than ebb during strong tides but reverses during weak tides in the inner regime,while in the central regime,the tidal mixing is in general stronger during flood than ebb,during both strong and weak tides.The strength of ATM measured with Ka which decreases from the mouth to the head along the estuary,which has a tidally variation of the amplitude of tidal current.The strength of ATM has a positive correlation with the amplitude of tidal current in the inner regime.It is larger with stronger amplitude and is negative with very weak tide.While in the central regime,the relationship is reversed and almost has no negative Ka.The analysis of potential energy anomaly showed that the along-estuary tidal straining can produce typical ATM with producing mixing during flood and reducing mixing during ebb.As along-estuary advection contributes to produce mixing during the ebb,the ATM is reversed in the inner regime during weak tides.Using the decomposition method of along-estuary residual circulation,the results showed that the density driven flow has a transverse structure with landward flow in the middle and seaward flow near banks of the cross-section in the inner regime,while the ATM-induced flow has a two-layers structure with seaward near the surface and landward near the bottom during strong tides,and the vertical pattern is reversed during weak tides.In the central regime,the structure of density-driven flow has the same structure as that in the inner regime of the estuary,but the ATM-induced flow has an opposite structure compared to the density-driven flow with seaward flow in the middle and landward flow on sides during both strong and weak tides.The strength of ATM-induced flow decreases along the main channel from the mouth to the head of the estuary.The magnitude of ATM-induced flow is about 2-3 times that of density-driven flow,consistent with the previous finding that ATM-induced flow is more important than density-driven flow in periodically stratified estuaries.As the strength of ATM-induced flow is positively related to the tidal fluctuation of vertical shear term and is negatively related to tidal-mean vertical mixing,the strength of ATM is generally large during strong tides and could be extraordinary large during the weak tides because of the extremely weak vertical mixingVorticity analysis found that,the driving mechanism of vertical velocity shear are the nonlinear advection and vertical mixing.The contribution of the two are similar in magnitude but opposite in direction.The ATM-induced flow has a positive correlation with the Simpson Number and the magnitude of ATM during both strong and weak tides is stronger with stronger intensity of tidal mixing asymmetry.