Characteristics and Driving Mechanisms of Mixing and Stratification in the North Passage of the Changjiang Estuary, China

Based on the measured hydrological and sediment data of spring and neap tides in the dry and flood seasons of 2016 in the North Passage of the Changjiang Estuary, the gradient Richardson number, Simpson number, and potential energy anomalies were calculated to analyze the tidal cycle, spring-neap variation, and seasonal variation of mixing and stratification. Taking into account the vertical difference of horizontal density gradient, the contribution terms of time derivative of potential energy anomalies was derived, including depth-mean straining, advection, nonmean straining, and tidal stirring. The driving mechanisms of mixing and stratification was investigated by calculating and comparing these four contribution terms. The results show that the seasonal variation of freshwater discharge mainly affected the distance of saltwater intrusion, the saline wedge moved upstream in dry season, while the tidal dynamics had a significant impact on mixing and stratification. In the reaches with saline wedge tip migration, the water column was periodically stratified, and the process was dominated by advection term. In the main reaches of saltwater intrusion, during spring tide the water column was periodically stratified, which was stratified during flood tidal current; it was well mixed during ebb tidal current, and the stratification during flood tidal current was promoted by advection and depth-mean straining. The mixing during ebb tidal current was promoted by tidal stirring and advection; during neap tide, the water column was permanently stratified. The stratification was enhanced during flood tidal current and weakened during ebb tidal current, and the stratification was mainly maintained by depth-mean straining.