Water Mass Transport And Its Mechanism Between The Changjiang River Estuary And The Subei Coastal Water

The Changjiang River Estuary is located at the junction of Yellow Sea and East China Sea.Huge amounts of freshwater,carrying terrestrial sediments and nutrients,is discharged through the Changjiang River Estuary every year,making great impacts on the biogeochemical environment of the adjacent coastal waters.The Subei Coastal Water,which is thought to be the origin of green tide in South Yellow Sea,is in the north of Changjiang River Estuary.Low salinity water exists persistently in the Subei Coastal Water,with high concentration of terrigenous nutrients,which is considered to be an important factor for stimulating the growth of harmful algae.Because the Changjiang River Estuary is adjacent to Subei Coastal Water,the water mass exchange between these two regions should be frequent.The terrigenous substances from the Changjiang River might be the important nutrient sources of the Subei Coastal Water.Therefore,investigating the water mass exchange between the Changjiang River Estuary and the Subei Coastal Water is important to a better understanding on the pollutant transports,green tide prediction,and protection of the marine ecological environment.Based on the observed data,this study found that there existed year-round low-salinity water in the Suebi Coastal Water,with salinity lower than 31.The characteristics of the Subei nearshore water have significant seasonal variations.In winter and spring,the salinity of nearshore water is relatively high and there is an isolated low salinity patch near the Old Yellow River Estuary.In summer,the low-salinity(<30 psu)Subei nearshore water extends northward,as the Changjiang River plume expands northward and contributes remarkably to the freshwater budget of Subei nearshore water.In addition,the measured data showed that sometimes the Subei nearshore water propagated northward even in winter under a northerly wind.Regardless of the wind direction during the survey period,the residual current of Subei nearshore water was always northward during spring tide.During neap tide,the residual current was southward when the northerly wind prevailed,while it turned to be northward when the southerly wind appeared.The freshwater source of Subei nearshore water and the water movement between the Changjiang River Estuary and the Subei Coastal Water were studied based on numerical simulations.The model results showed that the freshwater input from Changjiang River and Subei rivers contributed differently in different regions.The Changjiang River was the dominant freshwater source south of 33.5°N,while Subei rivers were the major freshwater origin north of 34.5°N.In between,both Changjiang and Subei rivers had contributions.In conclusion,the Changjiang River was the main freshwater source and accounted for 60%~70% of the total freshwater content in Subei nearshore water in winter and autumn,and even reached 80% in summer.The numerical tracer and water age results showed that the freshwater from Subei rivers had significant temporal and spatial variations after discharging into the Subei Coastal Water.When the northerly wind was dominated in winter and autumn,the Subei freshwater was mainly stayed around the estuary and its southern area,which had the tendency of flowing southward,with a part of freshwater transporting to the Changjiang River Estuary.The water age of the Subei freshwater was relatively large,which meant the transport time of the Subei freshwater was long.The Subei freshwater was mainly trapped in the nearshore and was hard to transport to the offshore.In spring and summer,the pathway of the Subei freshwater was that it moved to the vicinity of the Old Yellow River Estuary first,extended offshore to about 121°E,and then flowed southeastward,transported into the interior of Yellow Sea.The water movement between the Changjiang River Estuary and Subei Coastal Water had notable seasonal variations.In winter,the water movement was mainly southward around Changjiang River Estuary.There was an anti-cyclonic residual current field near the Subei shoal.The nearshore water extended northward along Subei coast and it turned to be offshore around 33.5°N.The residual current in Haizhou Bay was southward.In spring and summer,when the southerly wind prevailed,the northward movement tendency between the Changjiang River Estuary and the Subei Coastal Water enhanced gradually.The water near the Old Yellow River Estuary was mainly characterized by offshore movement and it turned to transport southeastward in the outer part of the Subei Coastal Water.The momentum analysis results indicated that under the impact of tide only,the northward movement south of Jianggang was determined by the tidal acceleration.The nearshore water between Jianggang and the Old Yellow River Estuary featured an offshore transport due to the tidal potential energy gradient and mean sea level variation.The water in Haizhou Bay moved southward because of the combined effects of tidal potential energy gradient,advection and friction.The dynamic mechanism responsible for the southeastward transport on the edge of the Subei Coastal Water was the momentum advection.Under the interaction of tide and 6 m/s northerly wind,although the northward current was weakened slightly in the south of 33.5°N,the whole transport pattern was not changed.Only the mean sea level variation changed significantly.Under the influence of northerly wind,the mean sea level rose greatly in the south of Jianggang area leading the enhanced southward transport.As the movement of Enteromorpha is determined by its surrounding water transport,the origin and distribution of Enteromorpha in South Yellow Sea were further studied based on a deep understanding of the water mass movement between the Changjiang River Estuary and the Subei Coastal Water.The Enteromorpha distribution and its impact on Shangdong nearshore area were analyzed during 2012 and 2015,combining the satellite remote sensing results and Lagrange particle tracking simulations.The results showed that Enteromorpha distribution had significant inter-annual variability,the distribution and coverage area of which expanded gradually in recent years.The Enteromorpha intruding to the Shandong nearshore area was mostly originated from Subei shoal and the Old Yellow River Estuary.In the Haizhou Bay,when the laver farming scale and nutrient inputs reached a certain size,the Enteromorpha in Haizhou Bay would have a significant impact on Shandong nearshore area.