Hydrodynamics of the Changjiang Estuary and Adjacent Seas

Abstract

In this chapter, we use our previous observational and modeling studies to analyze some basic dynamic features of the Changjiang (Yangtze River) Estuary, including the tidal and subtidal motion, saltwater intrusion, and the extension of a river plume into the ocean. The dynamics of this estuary are strongly regulated by tides on both intertidal and subtidal scales. Harmonic analysis shows that the tidal regime has one dominant semidiurnal constituent. The tidal range has significant seasonal variations, but is typically between 4 and 5 m during the spring tide and ~2 m during the neap tide. Tidal forcing induces significant subtidal circulation, resulting in a net landward flow into the North Branch of the Changjiang Estuary when river discharge is low. This residual transport forms a type of saltwater intrusion known as the saltwater-spilling-over (SSO). This causes high-salinity water in the North Branch to intrude upstream and finally spill over into the South Branch, which is the major channel of the estuary. The SSO has a major impact on the Changjiang Estuary and on the availability of freshwater for nearby cities. In areas near the river mouth, the tide-induced residual transport also results in significant inter-channel water mass exchange. Freshwater output from the Changjiang forms a large-scale plume, which plays a key role in physical processes in the adjacent East China Sea and Yellow Sea. We found that, in the region close to the river mouth, tidal forcing has a marked effect on the extension of the plume into the open ocean. Wind forcing also plays an important role in the dynamics of the Changjiang Estuary. Northerly winds can enhance the SSO and saltwater intrusion around the river mouth. In the plume-affected region, the wind significantly modulates the plume shape, especially farther from the river mouth.