The effect of stratification on the vertical structure of the tidal ellipse in the Changjiang River estuary, China

A field study was undertaken to understand the possible effect of stratification on the vertical structure of the M2 semidiurnal tidal ellipses in the Changjiang River estuary. A pycnocline occurs at each station along the North Passage on a neap tide only. Calculated potential energy anomalies within the North Passage are 100-200 J·m−3 larger on a neap tide than on a spring tide. The tidal ellipses degenerate to rectilinear motion at each layer of the water column at each station along the North Passage, while they rotate clockwise at each station over the Hengsha Shoal. The surface-to-bottom inclination angle difference of the tidal ellipse at a seaward station is 40° on a neap tide but a few degrees on a spring tide. The vertical phase shifts of the tidal ellipses range from 20° to 50° at most stations along the North Passage on a neap tide, while they are less than 10° on a spring tide. Rapid changes in the inclination angles and phase shifts of the tidal ellipses occur at the pycnocline. An abrupt reduction in the vertical eddy viscosity at the pycnocline seems to be the major cause for the vertical variability of ellipticity, inclination angle and phase of the tidal ellipses on a strongly stratified neap tide. The surface-to-bottom ellipticity difference appears to have a positive linear relation with the overall Richardson number mainly along the North Passage on a neap tide. A one-dimensional coastal model with vertical turbulence closure scheme seems to be able to reproduce the major features of the vertical structure of the tidal ellipses at a seaward station on a spring tide.