Spatio-temporal Characteristics And Influencing Factors Of Current In Qinhuangdao Coastal Area

Qinhuangdao coastal area is an important channel for energy and material exchange between Liaodong Bay and Bohai Bay and the middle part of Bohai Sea.It is also an important tourist area,port and sea food breeding base in the northern coastal area of China.Studying the hydrodynamic characteristics of the Qinhuangdao coastal area and the impact of external environmental changes will not only help us to understand the role of the sea area in the circulation of Liaodong Bay and even the Bohai Sea,but also help to evaluate the differences of hydrodynamics between the culture and the area,and to provide theoretical basis and technical support for the comprehensive ecological environment control of Qinhuangdao coastal area.This paper is based on eight sets of continuous seabed-based observation data from May-August 2017,August-September 2015 and September 2013 in the Qinhuangdao coastal area,combined with the data of the numerical wind field in the same period.The current characteristics of Qinhuangdao coastal area in summer and autumn and its response to large-scale aquaculture were studied by means of harmonic analysis and low-pass filtering.The vertical variation characteristics of the residual current in summer and autumn are analyzed in depth.Finally,the effects of large-scale farming activities,sea surface wind field and thermocline on hydrodynamic field in offshore area were analyzed.The main conclusions are as follows:In summer and autumn,the residual flow velocity is weak overall.The current velocity range in summer is between 0.5 and 1.9 cm/s.The spatial difference of residual current velocity in autumn study area is larger than that in summer,and the velocity range is 0.2².5 cm/s,and the horizontal flow field of the entire sea area in autumn shows obvious divergence characteristics.In summer and autumn,the variation trend of standard deviation of U and V components in the whole depth of the station is consistent.The overall variation of the residual current over time is more severe than that in summer.In summer,the standard deviation of the depth near thermocline increases slightly,which also reflects the influence of the warm salt stratification on the vertical variation of the residual current.By comparing the characteristics of residual current between Qinhuangdao and eastern Liaodong Bay in autumn,it is found that the residual current in the eastern part of Liaodong Bay in autumn is stronger than that in Qinhuangdao.In autumn,the bottom of Qinhuangdao coastal area is generally presented as northward current,which may form a weak clockwise circulation system in the Liaodong Bay coastal area together with the south-western current of the bottom of the eastern side of Liaodong Bay.The vertical distribution characteristics of the residual current in Qinhuangdao coastal area in summer and autumn are more complicated,and there is no obvious regularity.The baroclinicity of the summer residual current is larger than that in autumn,and the vertical pressure of the baroclinic factor accounts for more than half of the summer residual flow.The vertical variation of the residual current is mainly positive pressure in autumn.In the summer,the average time of the residual current in Qinhuangdao coastal area contains the vertical variation flow intensity greater than that in autumn,the vertical change flow energy accounts for 64%of the total kinetic energy,and only 38%in autumn;the summer vertical flow vortex kinetic energy accounts for the total eddy energy 72%,the vertical variation of the current time change is more significant,while the vortex energy of the residual pressure in autumn is only 38%,so the vertical variation of the summer current time change is also greater than the autumn.From the vertical mode,the first mode of summer current shows the characteristics of reverse flow above and below the thermocline,which reflects the obvious baroclinicity of summer flow,and the baroclinicity of the northern and central residual currents in this sea area is greater than that of the south.Environmental factors such as aquaculture activities,sea surface wind fields and thermocline in Qinhuangdao have a greater impact on hydrodynamic fields.The vertical average flow velocity of the residual flow at the A7 and A8 stations in the culture area is reduced by 76%and 18%,respectively,compared with the vertical average flow velocity at the A6 station at the edge of the culture zone.For the observed sea area,wind stress is one of the factors causing the change of residual current,but it is not the main factor,and the effect of summer wind stress on the change of residual current is slightly greater than that of autumn.The variation of U component of residual current in autumn is about 5 h behind the wind stress,and the variation of U component of residual current in summer is about 5.8 h.The existence of the thermocline is analyzed for the A3 station,which makes the sea current in the study sea show different seasonal variation characteristics.The results show that the residual current in Qinhuangdao sea area is greater than that in autumn and the direction is the same;the maximum flow velocity of K₁ and M₂ tidal currents in summer is greater than that in autumn.From the vertical distribution characteristics of the residual current,the level of the residual flow of the three stations A1,A3 and A5in summer is located near the thermocline,indicating that the strong temperature salt stratification in summer has a significant influence on the vertical distribution of the residual current.In this paper,the long-term continuous currents measured in the Qinhuangdao coastal area in summer and autumn using multiple sets of seabed-based observation instruments are used to analyze and summarize the current characteristics of the sea in summer and autumn.For the first time,a variety of analysis methods were used to study the vertical variation characteristics of the summer and autumn residual currents in the sea area.Although some preliminary results have been achieved in the research work of this paper,there is still much work to be done in the future.（1）Because of the high frequency of man-made activities in the culture zones,we carried out a half-month current observation in August-September 2015,and the data obtained are not sufficient to study the hydrodynamic status of the entire culture cycle.In the future,the long-term observation of hydrodynamic process inside and outside the mariculture area and before and after aquaculture will be further carried out in order to deeply study the influence of large-scale aquaculture activities on the hydrodynamic status and material transport capacity of the sea area.（2）In the future,the influence of external forcing on hydrodynamic field is deeply studied by combining the measured data analysis with numerical experimental simulation.