| Runoff,as a major component of the hydrodynamics in the estuarine region,can significantly affect the flow field and density structure of the estuary and nearshore area when a large amount of runoff enters the ocean,due to the powerful dynamics and large amount of freshwater.The coastal currents and upwelling are the main components of the circulation in the northern South China Sea,and their interaction with runoff has been a focus of research.Due to the fact that the upwelling in the Pearl River Estuary(PRE)area is closer to the estuarine region,its dynamic mechanism becomes more complex.Therefore,the impact of runoff on the three-dimensional circulation structure in the Pearl River Estuary area,particularly on the upwelling,is the focus of this study.In this study,a 3D barocline numerical model covering the Pearl River upstream river network,estuarine region,and shallow shelf area outside the Pearl River Estuary was established based on observed data and the regional ocean model SCHISM(Semi-implicit Cross-scale Hydroscience Integrated System Model).The model comprehensively considers factors such as runoff,wind,tides,sea surface heat exchange,and the circulation in the northern South China Sea,conducts numerical simulations of the hydrodynamic characteristics in the Pearl River Estuary area.Comparison with observed data shows that the model can reasonably simulate the elevation of water,sea temperature,and salinity in the Pearl River Estuary and coastal areas,the established model meets the needs of this study.This study quantifies the contribution and impact of different seasonal runoff from the Pearl River on the horizontal and vertical circulation,both sides of the Pearl River Estuary,through sensitivity experiments by changing the runoff amount.First,by comparing the sensitivity experiment results with and without runoff input,it is found that the Pearl River runoff has a significant impact on the coastal currents in the Pearl River Estuary area,but the impact of summer runoff on the coastal currents on both sides of the estuary is different.It weakens the coastal currents in the western side of the Pearl River Estuary,reducing the surface current velocity by about 35%and the subsurface current velocity by about 52%.It enhances the coastal currents in the eastern side of the Pearl River Estuary,but only concentrated in the range of water depth shallower than 5m,where the surface coastal currents are enhanced by about 30%due to buoyancy effect.The impact of winter runoff on the nearshore circulation is only reflected in the western side of the Pearl River Estuary,and it is mainly an enhancement effect.By introducing the indices of upwelling intensity TPI(The Topographic Position Index)and WTP1(Weight-accumulative Topographic Position Index)and establishing their relationship with runoff amount,the correlation coefficient between summer runoff amount of the Pearl River and WTP1 from 2003 to 2020 is calculated to be 0.63,showing a high correlation,indicating that besides local wind forcing,summer runoff of the Pearl River is also an important factor causing interannual variability of upwelling intensity in the Pearl River Estuary area.The sensitivity experiment results of changing the runoff amount show that runoff increases the upwelling velocity in the eastern side of the Pearl River Estuary by about 50%,and the relationship between runoff amount and upwelling velocity shows a nonlinear relationship.Runoff reduces the upwelling velocity in the western side of the Pearl River Estuary by about 70%,and it does not reach the surface,and there is no cold water phenomenon in the surface seawater.The study suggests that the inhibitory effect of the runoff from the Pearl River on the upwelling in the western side of the Pearl River Estuary is the main cause of the high sea surface temperature in the west and low sea surface temperature in the east. |
