Analysis Of Decadal Variability Of Salinity Field And Its Influence To Circulation In Bohai And Northern Yellow Sea

Bohai Sea and Yellow Sea are shallow seas in the north of China, with unique environmental and dynamic characters. In recent decades, with changes caused by global climate and local environment, particularly by the impact of human activities, physical environment in Bohai Sea and North Yellow Sea has undergone tremendous changes, the average salinity in Bohai Sea increased nearly 2.0 psu, and the structure of salinity also changed, thereby circulation restructuring was triggered. For the purpose of studying the decadal variability of Salinity Field and its influence to circulation, comprehensive observed salinity data of Bohai Sea and northern Yellow Sea from 1950s to present is collected, the 10-year-averaged salinity distribution of the surface, middle layer (10m) and near-bottom layer (20m) in five decades (1950s,1960s,1970s,1980s,1990s) are obtained by space-time interpolation method after scrupulous data quality control.The Analysis indicates that: Since 1950s, the salinity of Bohai Sea and northern Yellow Sea shares the trend of increase in winter and summer, with an annual increase up to 0.04psu/a in winter and 0.03psu/a in summer. The maximaum increase is located in Bohai Bay of 0.14psu/a in winter and 0.12psu/a in summer, the high salinity tongue extended significantly from northern Yellow Sea into Bohai Sea. In recent 50 years, the salinity isoline of 32 psu bends westward for 3 longitudes; In 1960s, it's different in that the salinity of each layer decreased, since there are extensive rainfalls in 1960s, which lead to an increase in discharge of Yellow River. However, the salinity isoline of 32psu in northern Yellow Sea still extends significantly to Bohai Sea; From 1980s, the patterns of salinity spatial distribution in Bohai and northern Yellow Sea have changed a lot, compared with the 1950s-1970s, of which the salinity in Bohai Bay, Liaodong Bay is higher than the other regions with its horizontal salinity gradient decreased, while the high salinity tongue that originally extended into Bohai Sea in winter disappeared.Salinity in Bohai Sea changes primarily depends on freshwater flux (runoff, rainfall, etc.), intrusion of high salinity water from North Yellow Sea and water exchanges between Bohai and Yellow sea. So method combined space-time interpolation and successive correction is used to get salinity distribution in every year from 1958 to 2000 in Bohai and northern Yellow Sea . Analysis indicates that salinity isoline of 32psu, which stands for high salinity water intrusion from Yellow Sea extends northward and eastward; EOF analysis is made to time series of salinity field, and correlation coefficient between the time series of EOF1 and Yellow River runoff at the same time is -64.57% in winter and -60.24% in summer. So variation of salinity in Bohai and northern Yellow Sea have strong negative correlation with the runoff of Yellow River. Correlation analyse is also made to salinity in part of stations around Bohai Sea and freshwater flux, which find that negative correlation is strongest between precipitation and salinity when lagging 30d in most tations, and correlation coefficient between runoff of the Yellow River and salinity depends on location of stations. The negative correlation are strongest when lagging 0d in Tanggu, Huludao, Qinhuangdao, Laohutan, while Bayuquan is 30d and Yantai ,Longkou stations are 90d .The circulation of Bohai Sea and northen Yellow Sea restructured aroused by changes of salinity filed. With the salinity obtained in summer driven model, the corresponding circulation field,exchanges of water and salinity through the Bohai Straits and 37°N section are calculated. Circulation in the central of Bohai Sea and Laizhou Bay changes a lot, there is obvious change in the direction of the flow in Laizhou Bay, position of the clockwise circle in central Bohai southward, and flow of its east part weakened, after 1990s one counterclockwise circle in Liaodong Bay changes to one clockwise circle in southwest and one counterclockwise circle in the middle of estuary. Changes of circulation structure in the bottom layer is smaller, only the speed decreases. Exchanges of water and saltinity through Bohai strait section decreases. So along with circulation structural adjustment in Bohai Sea, water-exchange capacity has been weakened, and the salinity into Bohai Sea also decreases.