Study On Formation And Maintenance Mechanism Of Cold Eddy To The Northeast Of Diaoyu Island

The August 2001 survey found a strong upwelling process to the northeast of the Diaoyu Island,this paper first analyzes the cold water surge process based on the collected survey data,and named it as 'the Cold Eddy to the Northeast of Diaoyu Island',and analyzed its characteristics and seasonal variation.It was found that the cold vortex is a seasonal cold eddy,which occurs every summer.Due to the limitations of investigation time and space,this paper uses the ROMS model to simulate the hydrodynamic characteristics of the southern part of the East China Sea,studies the formation and maintenance mechanism of the Cold Eddy to the Northeast of Diaoyu Island,and analyzes its relationship with the current of ECS.The following results were obtained.(1)The Cold Eddy to the Northeast of the Diaoyu Island(CEND)is a mesoscale cold eddy formed by the upwelling of the bottom water.The isotherms of the temperature and salt contours protruded can be reach 30 m,but it cannot be ventilated with the surface.The horizontal scale of CEND is about 150 km.From the hydrological characteristics of the 50 m layer,the salinity of the water mass is about34.70-34.80,which is higher than the salinity of the surrounding by 0.5 or more,and the characteristic temperature is lower than 20 °C.The seawater temperature is 2-3 °C,the upper part of the cold eddy area is occupied by warm water with temperature higher than 28 °C,the lower layer is cold water with temperature lower than 20 °C,the lowest temperature is about 17.50 °C,the middle thermocline is about 30-50 m layer,the thickness of thermocline is about 20 m,with temperature decreasing from28°C to 20°C,the temperature gradient is up to 0.50°C/m.CEND's intensity is different in different years,and the depth of isopycnal surface is different.In comparison,CEND in August 2001 is stronger than that in August 200,the obversition of AUV in July 2017 showed that CEND is much weaker;however,the location of CEND is basically consistent every year in the range of 123.2-124.2 °E,26-27 °N.According to ruselt of ROMS,the entire bottom layer of the eastern part of the ECS is occupied by cold water with a temperature lower than 20 °C.In the region of(122.5°E-124°E,26.2°N-27.5°N),there is a semi-closed cyclone current over 30 m layer.(2)Analysis of the survey data found that CEND is a seasonal cold eddy,which occurs every summer and begins to form in May and June.The eddy can also be observed in October.At this time,the eddy is weak and can only be observed at the bottom.The data of Dec shows that CEND has completely disappeared.Aug is the strongest time of CEND.Compared with the results of the model,we found that the interannual variation is basically consistent with the results of observated data analysis.But the model showed that its intensity is similar in July and August,and then gradually weakened.In October,its structure has basically disappeared.From November to April,the cold vortex of the Diaoyu Islands in the next year It does not exist at all.The temperature distribution of the section that across CEND shows that the bottom layer and surface temperature of the S zone(123.20°-124.50°E)are antiphase.The semi-closed flow ring structure appearing in the model is basically consistent with the variation of CEND in which we investigate the cognition and pattern cognition.The structure appeared in the development of CEND(May and June)and the strong(July and August)period,CEND weakened,the structure also began to decline,and the semi-closed flow ring developed from the surface down.The recession also began to disappear from the surface,and the recession process was much faster than its development.(3)The existence of specific topography in the northeastern of Diaoyu Island is a necessary condition for CEND,here has a basin that 40m-60 m deeper than surroundings,the scope of the basin is about 123 ° E-124 ° E,26 ° N-27 ° N,same with the CEND occurrence area.The topography changes affect the movement of the bottom cold water.In the real and deepen topography,the cold water from northeastern Taiwan entered the basin of the northeastern Diaoyu Islands,where it then gathered to strengthen the forces of the cold water at the bottom,and then gradually surged upwards and expanded westward,eventually forming the CEND.However,after the topographical change,cold water from the northeastern Taiwan with a temperature lower than 17 °C cannot enter the northeastern part of the Diaoyu Islands.Although cold water with a temperature lower than 20 °C still reaches the northeastern part of the Diaoyu Islands,it then flows out of the sea and cannot be retained.In this area,it is impossible to expand the cold water to the west and cannot form the CEND.The cold water area at the bottom of the section aross CEND is reduced,and the cold water range of 17 °C in the S area is significantly reduced;the cyclonic semi-closed flow ring in the upper layer of CEND disappears after changing the topography.(4)Through the diagnostic analysis of JEBAR term,the seasonal variation mechanism of CEND is basically explained.The topography is basically consistent,and the change of CEND can only be caused by the change of local baroclinity.From January to August.The ocean Stratification is becoming stronger and stronger,when it reaches a certain level,causing the Kuroshio Subsurface Water(KSSW)intrusion,the KSSW encounters the topography that deepens sharply,This local topography causes the intruded water to be subjected to positive vortex-tube stretching,giving rise to cyclonic semi-closed current and upwellings in May,after that,the depth of the semi-closed flow ring gradually deepens,and the cold water of the bottom gradually strengthens,the characteristic structure of CEND begins to appear,the CEND began to emerge,after stratification to further strengthen,the CEND also gradually became stronger until July and August,when CEND reached its strongest,the thickness of the semi-closed current was also the deepest,the cold water upwelling intensity was the strongest,the temperature gradient was the largest,and the local ocean stratification was the strongest.Then,the localized stratification weakened,and the semi-closed flow ring of the surface layer began to disappear,the CEND also began weakened,in October,the temperature gradient(0.08 °C / m)has been lower than the temperature gradient(0.12 ° C / m)when it began to form in May,the semi-closed circulation loop structure also disappeared completely,the bottom cold water surge also disappeared,the model ruselts show that the CEND has basically disappeared.(5)The existence of CEND is conducive to water exchange between Kuroshio and continental shelf.During the existence of CEND,the average offshore flow was much stronger than the average total inflow to the shore.the CEND disappears after changing topography,while the summer offshore average flow slightly reduced(before 0.38 Sv,after 0.37 Sv),The total inflow of intrusion also increased from0.21 Sv to 0.40 Sv.The offshore flow changes from much larger than the total inflow intrusion to less than that.The disappearance of CEND relatively reduced the exchange between Kuroshio and continental shelf.(6)The bottom water of CEND comes from the subsurface water of Kuroshio,but the specific level of its source is different in different periods.At the initial development of CEND(May),the bottom water originated from the 100 m of Kuroshio;then the source of the bottom water gradually deepened,and in CEND's strong period(July and August),the bottom water originated from In the water layer of about 100m-130m;at the ending of CEND(after September),the bottom water comes from the water of 130m-200 m layer;the final destination of the bottom water is back to the Kuroshio.