The mechanism of freshwater extension from the North Channel to the North Branch in the Changjiang Estuary in dry season

The field data during December 2011 and January 2012 in the Changjiang Estuary indicated that the freshwater in the North Channel (NC) extended to the mouth and lower reaches of North Branch (NB), and decreased the salinity there during the spring and subsequent middle tides, while that was not detected during the neap and subsequent middle tides. The well-validated 3D numerical model was employed to reproduce the observed feature. To investigate the underlying mechanism of this pattern of extension, numerical experiments were conducted and the freshwater flux decomposing method was used to decompose the process of freshwater extension into several mechanisms. During the spring and subsequent middle tides, the freshwater in the NC extends to the NB mainly through its northern outlet (NONC) and neighboring shoals due to the Lagrangian transports. The deepening of NONC has intensified the freshwater extension, and decreases the saltwater intrusion in the NB. On the contrary, the increased northerly wind speed restricts the freshwater extension, and thus increases the saltwater intrusion in the NB.