A well-validated 2D hydrodynamic and salinity transport model is established in the Yangtze River Estuary (YRE). The numerical performances reveal that: (1) the highest salinity generally occurs after 1h of the flood current slack in the YRE. (2) The pattern of salinity front in the South Passage (SP) has a two-peak in the flood season and a onepeak in the dry season respectively. (3) The influence of the TGD on the salinity front position in the SP is not significant due to small runoff change. (4) The large seasonal runoff difference significantly impacts the salinity distribution and salinity front position.
The definition of the estuary front (Geyer and Ralston, 2015) is derived from the oceanic front. The oceanic front is a narrow transitional zone between two or more different hydrological features (Ping et al., 2013; Ren et al., 2015; Morales et al., 2018). The estuary fronts can be classified by water temperature, salinity, density, suspended sediment, flow velocity and chlorophyll (Zhan et al., 2017). The salinity front among estuary fronts, attracts more attention in recent years, which has obvious spatial and temporal distribution (Zhao et al., 2011; Liu and Zhang, 2015). The salinity gradient is a criterion for the intensity of salinity front (Mao et al., 1963). Runoff is the major dominant of the salinity front in estuary, however, the salinity front is mainly affected by storms, rainfall, current and climate change in open sea (Kao and Lagerloef, 2014). The salinity front intensity increases slowly in the South Pacific convergence zone over the last 210 years (Linsley et al., 2010). However, based on the monthly mean salinity data in 50 years from the SODA (Simple Ocean Data Assimilation), the salinity front intensity decreases in the South China Sea (Zhao et al., 2012; Carton and Giese, 2008). Salinity fronts can change sediment transport, so that affect the growth of delta (Mao et al., 1963). Abundant microorganisms gather near the salinity front, which is conducive to the development of fishery. The barrier effect of the salinity front hinders the pollutants transport and has a serious impact on the water environment (Ning et al., 2004; Huang et al., 2011). The investigation of salinity front can help us better understand the hydrodynamic conditions in estuary and enrich the estuary management theory. Estuary fronts have been studied since early 1960s in the Yangtze River Estuary (YRE). The fronts of YRE are divided into three levels: the inner front, the pinnate front and the outer front (ocean front), and the pinnate front is characterized by the salinity of 25 PSU (Hu et al., 1995). The density difference between two sides of the salinity front forms a barrier effect, which has influence on sediment transport and bed evolution. (Chen et al., 2001).