The circulation and salt stratification over the south branch of the Yangtze estuary

Abstract The hydrodynamics of China’s Yangtze Estuary is very complicated, due to highly irregular bathymetry, large freshwater discharge from the Yangtze River, strong tidal currents, large salinity gradients and strong interaction between the river and sea waters. Oceanographic measurements were made over the lower part of the Yangtze Estuary (the South Branch of the Yangtze Estuary) to examine the circulation structure and intra-tidal variability of salinity. The least-squares fit to main harmonics was made to explore the sub-tidal circulation. The observed ADCP currents were decomposed into the along-channel and cross-channel velocity to examine the spatial and temporal variability of circulation in the region.The observed study in the 2005 wet season and the 2009 dry season in different channels showed that the tidal straining effect is a major forcing mechanism on salinity stratification and sub-tidal circulation of the Yangtze Estuary. The tidal straining induced circulation was found to be equally important as the classical gravitational circulation in the Yangtze Estuary. This played an important role in maintaining semi-diurnal periodic salinity stratification in the region.