Study On The Along-slope Counter-flow And Kuroshio On-shelf Intrusion Off Northeastern Taiwan

Seas off northeastern Taiwan is the major source region of the Kuroshio waters which feeds the Kuroshio branch currents on the East China Sea(ECS)shelf.A step further,the Kuroshio on-shelf intrusion waters and Kuroshio branch currents have great influences on the circulation,nutrient supplement,ecological environment and marine economy in the ECS.Unique phenomena were found coexist with the Kuroshio intrusion here.The flow structures and the variations of these phenomena as well as the rare of continuous in-situ observations make it tough and challenging for oceanographers to understand the detailed flow structure and their mechanisms off northeastern Taiwan.This thesis aims to reveal the Kuroshio intrusion patterns,flow regime and their variations off northeastern Taiwan,and focus our attention on the spatial and temporal features of the along-slope counter-flow off northeastern Taiwan,the relation between the counter-flow and the Kuroshio on-shelf intrusion,and the relevant mechanisms.The innovative results are summarized as follows:(1)The spatiotemporal features of the along-slope counter-flow off northeastern Taiwan was revealed using historical in situ observations,recent velocity data retrieved from ADCP,in conjunction with satellite altimetry data and HYCOM analysis data.The results revealed that the along-slope counter-flow flows in the subsurface layer with core depth of 200 m to 400 m at the in-situ site.The mean maximum along-shelf velocity of the counter-flow was 19.0 cm/s and the mean maximum velocity of the counter-flow was 24.2 cm/s during the time from May 2017 to May 2018.The counter-flow also shows significant seasonal,intra-seasonal and short-term variations.(2)We explicitly proposed that the along-slope counter-flow significantly intrudes onto the ECS shelf near the MHC while flowing southwestward along the continental slope.The counter-flow intrusion process near the MHC carries the Kuroshio subsurface water below the water depth of 150 m onto the ECS shelf,and a rough estimation of the counter-flow on-shelf volume transport near the MHC derived from the HYCOM data is 0.52 Sv.The along-slope counter-flow and the significant counter-flow intrusion was also reproduced in our ROMS model outputs and the topography ? spiral theory was applied to explain the counter-flow intrusion near the MHC.(3)The linear relationship between the vertical migration of the along-slope counter-flow and Kuroshio on-shelf intrusion off northeastern Taiwan was brought to light.The along-slope counter-flow was enhanced(weakened)and uplifted(lowered)while the Kuroshio on-shelf intrusion through the western slope off northeastern Taiwan was deceased(increased),and the Kuroshio on-shelf intrusi on through the eastern slope off northeastern Taiwan was increased(decreased).The vertical migration of the along-slope counter-flow well reflects the stratification variations of the water column near the slope off northeastern Taiwan,and the variation of the water stratification change the cross-shelf potential vorticity distribution which determines the Kuroshio intrusion intensity variations off northeastern Taiwan.(4)At last,considering historical studies,based on the spatiotemporal patterns of the Kuroshio on-shelf intrusion and the along-slope counter-flow off northeastern Taiwan,this thesis provides sketch maps of three major Kurohsio on-shelf intrusion path off northeastern Taiwan.The sketch maps fully considered the quasi-steady along-slope counter-flow in the subsurface layer,revealed connection behind the multi-scale variations of the flow structures off northeastern Taiwan,and provide new perspectives on the spatiotemporal variations of the flow field and Kuroshio on-shelf intrusion off northeastern Taiwan.This thesis makes basic contributions to the understanding of the flow structures and their variations off northeastern Taiwan.A step further,a better understanding of local flow structure in the source region will provide the details of formation and variation of the Kuroshio branch currents on the East China Sea shelf.The results may also help oceanographers to make breakthroughs in the multi-scale spatiotemporal variations of the Kuroshio on-shelf intrusion off northeastern Taiwan and forecasting of the marine environment and marine economy northeast of Taiwan,eastern Zhejiang and in the East China Sea in the future.