Circulation and mixing along the North Passage in the Changjiang River estuary, China

Simultaneous field observations were made of time series of water level, current velocity, salinity and suspended sediment concentration at ten hydrological gauging stations along the North Passage in the Changjiang River estuary on 17 to 24 February (dry season) and 12 to 18 August 2012 (wet season), respectively. Quantitative analyses of those data are attempted to understand circulation and mixing along the Deepwater Navigational Channel in this estuary. Landward subtidal flow only appears during the neap tide. The maximum bottom landward velocity is in the order of 0.05-0.1 m·s− 1 in the dry season and 0.15-0.2 m·s− 1 in the wet season, respectively. In both dry and wet seasons, calculated mixing parameter (M), which is the ratio of the tidal timescale to the vertical mixing timescale, is below the critical value (1.0) on a neap tide but above it on a spring tide within the middle and lower reaches of the Channel. This suggests that tidal variation of mixing may be able to generate the tidal straining circulation during the periodic stratified spring tide. The interaction between significant river shear flow and longitudinal density gradient, a process termed “river effect” in this study, is revealed as the major reason for seasonal variation of stratification. The competition between tidal stirring, gravitational circulation and river effect could produce an increase in tidal mean value of the potential energy anomaly (ϕ) from neap to spring tides. Tidal mean value of the Simpson number (Si) over a neap tide surpasses the critical value (8.4 × 10− 1) within the middle reach of the Channel, suggesting the occurrence of persistent stratification there. Tidal mean Si over a spring tide is in the order of 8.4 × 10− 1-8.8 × 10− 2 within the middle reach, suggesting the occurrence of strain induced periodic stratification (SIPS). Shear prevails within the pycnocline in the middle and lower reaches of the Channel on a neap tide, with the value of squared shear (S2) exceeding 10− 3 s− 2. Calculated gradient Richardson number (Ri) is small at the salt-fresh water interface, indicating the occurrence of Kelvin-Helmholtz instability there.