7. Discussions and conclusions

The investigation on the Pearl River estuarine circulation is pursued toward better understanding the Pearl River estuarine circulation dynamics. A hydrographic cruise survey for collecting in situ data of marine environmental parameters in the PRE was conducted in the period between May 3, 2014 and May 11, 2014.

The observation shows that the salinity intrusion along the Lantau Channel exists both for the flood and ebb. The intruded salinity should be responsible for the appearance of the high surface salinity water in the channel located inside the estuary, which facilitates the formation of the "sandwich" structure of the horizontal salinity distribution (high salinity in the channel location and low salinity on the east and west sides). However, instead of the "sandwich" type of the horizontal salinity distribution, the lower estuary shows a two-layer structure of the salinity in the horizontal cross-estuary direction, which may be associated with the stronger salinity intrusion in the lower estuary. The observation reveals that there is an exchange flow structure in the longitudinal direction in the PRE with the surface water flowing out of the estuary and bottom water flowing into the estuary, especially along the Lantau Channel. Another important observation is that there is longitudinal convergence or divergence of the cross-estuary velocity close to the channel location for certain tidal conditions. The observation also indicates that the circulation and density structures are highly varied due to the difference in the wind and tidal forcing for different periods.

The numerical simulations of the FVCOM in the PRE and the adjacent water area are conducted with a horizontal spatial resolution as high as 100 m inside the estuary and 20 vertical levels. An atmospheric model, the WRF Model is also implemented to provide with high spatial and temporal resolution atmospheric forcing for the FVCOM. The comparison of model outputs with in situ observations of salinity and velocity profiles and sea level suggests that the modeling in the PRE can well simulate the estuarine circulations. In a model experiment with tide excluding wind forcing, the PRE longitudinal exchange circulation exhibits a strong spring-neap cycle with the maximum circulation in the neap tide and the minimum in the spring tide, revealing that the mixing induced by the tidal current is a dominant factor influencing the circulation. The higher mixing dissipates more kinetic energy of the residual current, resulting in a reduced exchange flow in the PRE.
