The Short-term Process Of Cross-shelf Transport Off East China Sea In Summer:Satellite Observation And Mechanism Discussion

The cross-shelf transport is the foundation for sustains an enhanced primary productivity on the shelf of East China Sea and is also a key process that affecting the exchange of material and energy in the East China Sea.Therefore,it has important scientific and practical significance for the study of the dynamic process and physical mechanism of cross-shelf transport.In this thesis,the high spatio-temporal resolution data of the new-generation geostationary satellites are used to replace the traditional polar-orbit satellites to conduct a detailed analysis on two kinds of short-term process in the cross-shelf transport of East China Sea:the cross-shelf transport process of Zhejiang coastal unstable front and the short-term process of surface upwelling off Northeastern Taiwan.We extracted and quantitatively analyzed the detailed features of the short-term variation process,and made a preliminary discussion on the dynamical mechanism.After checking the TSS(Total Suspended Solid Concentration)images of the geostationary ocean color satellite GOCI for nearly 3 years(2011-2013),we found that a complete evolutionary process of frontal waves was monitored by GOCI in the period of 2-11 September 2013.Using the rolled up and detached pattern of the frontal waves,the whole evolutionary process was divided into three stages:2-3 September as the development stage,4-5 September as the maturation stage,and 6-11 September as the decay stage.During development stage,the coastal front evolved from a straight line into submesoscale frontal waves with a wavelength of 10-20 km and grew in wave height by 10 km in the offshore direction.At the maturation stage,the submesoscale(10 km)waves rapidly evolved into mesoscale(20^10 km)meanders and eddies with an average wavelength of 33 km.Meanwhile,all the frontal waves were propagating southward along isobaths at speeds of 0.10-0.25 m/s.At the decay stage,the roll-up waves became detached,extended further offshore,and moved northeastward,while the former mother front zone gradually stabilized into a straight line.In addition to the daily variation,the coastal front waves also have an hourly migration.Driven by the local barotropic reciprocating tide current,the Zhejiang coastal front showed a tidal-like oscillatory movement:An onshore motion of the front with a speed of 0.3-0.9 m/s can be seen between 8:00 and 11:00,shifting to an offshore motion at 0.25-0.60 n/s between 12:00 and 14:00.The oscillatory range was approximately 6 km.Due to the lack of observational data on the three-dimensional structure of unstable fronts in the same period,the verification analysis on the generating mechanism of Zhejiang coastal unstable front was carried out from two aspects of barotropic shear instability and the typhoon-induced submesoscale baroclinic instability.The result shows that the above two mechanism both can reasonably explain this unstable process of Zhejiang coastal front.The high-spatiotemporal-resolution JAXA-SST images taken from 2015 to 2017 show that a complete evolutionary surface upwelling process was recorded by Himawari-8 in the period from July 18 to August 3,2016.The process lasted approximately 17 days and can be divided into two stages based on the variations in the upwelling intensity UPI and core area Sc:from July 18 to July 25(the intensification stage)and from July 26 to August 3(the decay stage).During the intensification stage,between July 18 and July 25,the UPI intensified quickly from l.lOC to 2.5OC,and the core area Sc expanded by over six times over eight days,from 950 km2 to 7,000 km2,and then decreased slightly to 5,900 km2.Affected by the advection component and relative vorticity of the local flow field,the center of the upwelling was located quasi-stationary near the 200 m isobath off northeastern Taiwan throughout that stage.After the intensity peaked on July 25,the upwelling switched to the decay stage until August 3.The UPI decreased quickly from 2.5? to 0.4°C,and the core area Sc decreased by nearly 1/5 over nine days,from 5,900 km2 to 970 km2.Due to the advection of the Kuroshio Current,the center of the upwelling started to move northeastward along the near-shore edge of the Kuroshio Current and crossed over the 200 m isobath from the deep area to the shelf area on July 28.The upwelling center moved by a total of approximately 230 km over nine days,and its average moving speed was 0.30 m/s.In order to verify the results obtained from the high-resolution JAXA-SST data and to reveal any statistical features,another multisatellite combined SST dataset produced by the United Kingdom Met Office(UKMO-SST)was used to make a statistical analysis.The result shows that these short-term variations primarily occur in the summer and their durations range from 66 days to 26 days,with an average of 15 ± 5 days.Moving trajectories of upwelling center can be classified into three types:quasi-stationary,cross-shelf northward,and along the Kuroshio Current northeastward.The latter two types reveal to us a new route of water exchange between the upwelled subsurface Kuroshio waters and the ECS shelf waters.In this thesis,we used several short-term processes in 2016 as examples to examine three dynamical mechanisms that were proposed in previous studies:short-term fluctuations in the Kuroshio Current,surface wind curl,and typhoons.The discussion indicated that these three mechanisms can reasonably explain the surface upwelling variations in different individual cases;therefore,short-term processes may be caused by the collective effects of multiple dynamical mechanisms.