Spatiotemporal Variability And Underlying Dynamics Of The Pearl River Plume Front

Large rivers not only discharge huge amounts of fresh water into the estuary and coastal waters,but also export large amounts of sediments and nutrients,which significantly affect the dynamics,sedimentary,and biogeochemical processes in the receiving water body.The river freshwater is mixed with the saltwater of the sea to form the river plume,which has a remarkable three-dimensional structure.In general,the interface at the surface between plume water and the open sea is called the“surface front",while the interface at the seafloor is the “bottom front”.Salinity fronts are often convergent areas,hindering the transport of momentum,nutrients,and sediment to the sea.As one of the largest estuaries in China,the Pearl River Estuary has a prosperous economy and intensive human activities.A large number of nutrients carried by the Pearl River accumulate at the front,resulting in ecological problems such as eutrophication,algal blooms,and hypoxia.The spatial location of the surface front and bottom front jointly affect the spatial distribution of hypoxia and algal blooms.Therefore,improving the understanding of the salinity front of the Pearl River plume is the basis for further investigating ecological issues such as hypoxia and algal blooms.However,previous studies in the Pearl River Estuary mainly focused on the distribution of surface salinity,and a comprehensive understanding of the three-dimensional structure of the Pearl River plume front is still lacking.Therefore,the three-dimensional structure of the Pearl River plume front,in particular the spatiotemporal variation of the bottom front,is the focus of this study.In this paper,a three-dimensional hydrodynamic numerical model of the Pearl River Estuary and adjacent waters was configured based on ROMS.The comparison of model results against observed data showed that the model can simulate the water level and reproduce the structure of the Pearl River plume satisfactorily,which can meet the needs of this study.The Pearl River plume has significant seasonal and spring-neap tide cycle variations.The freshwater transport indicated that the Pearl River plume completely extended to the west side of Guangdong in winter.April and May were the transition months for the diversion of the Pearl River plume,during when the Pearl River plume turned gradually to extend along the east Guangdong coast.In summer,most of the Pearl River plume extended to the east side of Guangdong,with a small part extending along the west Guangdong coast.The plume extending along the west Guangdong coast varied in the spring-neap tide cycle.The freshwater transport along the west Guangdong coast was strengthened during the spring tide but weakened during the neap tide.In order to accurately describe the Pearl River plume front,the salinity gradient method was used.The salinity gradient at the bottom is strong,exceeding 2 psu/km in the Pearl River Estuary.The surface front intensity is relatively weaker,about half of the bottom front intensity.In summer,the Pearl River plume extending to the east side of Guangdong is a surface-advected front,while that extending to the west side of Guangdong behaves as a bottom-trapped front all year round.The salinity gradient profiles on a cross-shelf section showed that the bottom front has a cross-shelf movement during spring-neap tidal cycles,but the surface front does not have this feature.Statistics on the occurrence probability of fronts show that the spatial position of surface fronts varies greatly.In summer,a salinity front exists in the sea off the Pearl River Estuary and the coastal waters of the east side of Guangdong.In spring,there is a probability of ?10%that a surface salinity front appears offshore of the Pearl River Estuary.The surface front only existed along the west coast in winter.The bottom front is in the shape of a strip distributed along the coast,and its spatial location has little seasonal variation.The EOF analysis indicated that the bottom salinity front had two major modes,corresponding to seasonal and spring-neap tidal cycles.Runoff and wind are the main external forces that caused the seasonal variations of the bottom front.In summer(winter),the runoff increases(decreases),and the wind direction turns southerly(northeasterly),the bottom front of the Pearl River plume strengthened(weakened),and moved towards the sea(land).The tides lead to the change of the spring-neap tidal cycles in the Pearl River.During the spring tide,the bottom front moves towards the sea,and the front intensity weakens;during the neap tide,the bottom front moves towards the land,and the front intensity increases.This is due to the differential mixing of tides.The tidal mixing intensity has not only temporal(spring-neap tide cycles)changes but also spatial differences.In addition to variations in the spring-neap tide cycles,the mixing intensity of tide also has spatial differences.The tidal mixing is stronger in the shallower waters but weaker in the deeper waters.This spatial difference in tidal mixing captures the salinity front at the bottom,making its spatial location relatively fixed,which is conducive to the formation of stable buoyant coastal currents.By comparing the flow driven by wind stress and the buoyancy-driven flow,we found that the coastal current along the west Guangdong coast is mainly driven by buoyancy in summer,while it is mainly driven by wind in other seasons.Due to the nonlinear effect between tides and topography,the coastal current on the west side of Guangdong strengthens during spring tide and weakens during neap tide.Sensitive experiments indicated that the background circulation had little effect on the salinity distribution of the Pearl River Estuary and the adjacent ocean.Local wind and tide are more critical factors than the background circulation.Sensitivity experiments based on the typical winds in summer and winter showed that under the northeasterly wind,the Pearl River plume no longer turned to the east side of Guangdong.Under the southwesterly wind,the freshwater transported to the east side of Guangdong was enhanced.Under the northeasterly wind,the westward lateral circulation of the surface of the Pearl River Estuary is strengthened,and the salinity front moves westward,and its intensity increases.Under the southerly wind,the westward lateral circulation in the surface of the Pearl River Estuary weakened,and the salinity front moved slightly eastward,with no obvious change in intensity.At the same time,tidal mixing plays a key role in maintaining the bottom front in the Pearl River Estuary.In the absence of tides,the Pearl River plume front retreated landward and no longer contacted the bottom.