Program

Vessel mounted ADCP measurements were made of tidal currents along the cross-channel lines AD3, AD5 and AD6 within a Curved Channel of the North Passage in the Changjiang River estuary, respectively. These data are analyzed by using a vorticity approach to examine the temporal and spatial variability of circulation and mixing in the vertical-lateral plane, their physical mechanisms as well as the relative importance of each mechanism. Lateral secondary flows, which have ‘two-layer’ structures separated by irregular interfaces, are present along the three cross-channel lines AD3, AD5 and AD6. Calculated semi-Lagrangian residual flows show: (i) along the three cross-channel lines AD3, AD5 and AD6, longitudinal circulating semi-Lagrangian residual flows at the surface layer are seaward and at the bottom layer landward during the neap tide; but longitudinal semi-Lagrangian residual flows at both the surface and bottom layers are seaward during the spring tide. (ii) Along the cross-channel line AD3, lateral circulating semi-Lagrangian residual flows at the surface layer are toward the Northern Dikes and at the bottom layer toward the Southern Dikes during the neap tide, while at the surface layer toward the Northern or Southern Dikes and at the bottom layer toward the Southern or Northern Dikes during the spring tide. (iii) Along the cross-channel lines AD5 and AD6, lateral circulating semi-Lagrangian residual flows at the surface layer are toward the Northern Dikes and at the bottom layer toward the Southern Dikes during both the neap and spring tides. (iv) The magnitudes of longitudinal semi-Lagrangian residual flows range from -0.2 to 0.7 m/s and that of lateral semi-Lagrangian residual flow from -0.15 to 0.2 m/s. (v) Apparent lateral variability of longitudinal semi-Lagrangian residual flow is present. Analyses of the major physical mechanisms responsible for the formation of circulation with the Curved Channel show: (1) Longitudinal baroclinic pressure gradient, longitudinal internal friction induced mixing, and longitudinal bottom friction induced mixing seem to be the primary physical mechanisms responsible for the formation of longitudinal circulation while the lateral redistribution of along-channel momentum the secondary one, the centrifugal force and the Coriolis force can be neglected. (2) Lateral baroclinic pressure gradient and the internal friction induced mixing are the primary physical mechanisms driving lateral secondary flows while the centrifugal force, the Coriolis force, and the bottom friction induced mixing the secondary ones. (3) Lateral circulation can weaken longitudinal one via the lateral redistribution of along-channel momentum.