The Kuroshio Subsurface Water Intrusion Onto The East China Sea Continental Shelf As Traced By Dissolved Inorganic Iodine Species

Based on the field observations of the dissolved inorganic iodine species(iodate and iodide)combined with hydrographic data(temperature,salinity and density),the intrusion pattern(intrusion location,pathway and extent)of the Kuroshio Subsurface Water(KSSW)onto the East China Sea(ECS)continental shelf and its temporal variation(seasonal and interannual variation)were determined.Furthermore,the Optimum Multiparameter(OMP)method was used to quantify the relative contributions of the KSSW and other water masses in the ECS based on their measured temperature,salinity and dissolved inorganic iodine species,and the quantitative interannual variation of the KSSW intrusion is further discussed.Sample collection was conducted across the ECS and Kuroshio east of Taiwan during cruises in May to June 2014(spring),October to November 2014(autumn),June 2015(spring),August to September 2015(summer),December 2015 to January 2016(winter)and June 2015(spring).Discrete seawater samples were collected for the dissolved inorganic iodine species analyses,and the temperature,salinity and density were measured with a conductivity-temperature-depth(CTD)recorder.Our study shows that a unique iodate-rich(>0.37 ?M)and iodide-poor(<0.10 ?M)water occurred at the bottom of the ECS shelf during all six oceanographic cruises,and this water always exhibits cold,saline and dense characteristics.The characteristics of this water are similar to those of the KSSW and obviously distinct from other water masses in the ECS.The evidence from both the dissolved inorganic iodine species and hydrographic data suggest that the KSSW mainly intrudes onto the ECS shelf from northeast of Taiwan(25.5 °N)via upwelling.However,in addition to the main intrusion center northeast of Taiwan,the dissolved inorganic iodine species concentrations indicate that another weaker intrusion from the KSSW occurs at ~28 °N along the shelf edge.This phenomenon can not be observed via hydrographic data alone;in addition,the dissolved inorganic iodine species have good linear relationships with the hydrographic data during the KSSW intrusion.Thus,the dissolved inorganic iodine species can be used as complementary and effective tracers of the KSSW intrusion.Based on the characteristics of the dissolved inorganic iodine species and hydrographic data,we determined the thickness of the KSSW intrusion(depth from the upper boundary of the KSSW intrusion to the seabed)at each station,which was used to draw the intrusion pattern of the KSSW on the ECS shelf.In spring of 2014 to 2016 and summer of 2015,the KSSW intrusion northeast of Taiwan flowed northwestward then bifurcated into a nearshore branch and an offshore branch at 27.5-28.5 °N.The nearshore branch flowed approximately northward and finally appeared in the subsurface layer within the 50-m isobath off the Zhejiang coast(north of 29 °N);the offshore branch flowed along the 100-m isobath and reached north of 29 °N.The KSSW intrusion northeast of Taiwan was obviously weaker in autumn and winter than that in spring and summer.In autumn and winter,the nearshore branch was not observed,and the offshore branch reached only ~28 °N.The KSSW intrusion in the ECS shows significant seasonal variation.When comparing the KSSW intrusion in spring of 2014,2015 and 2016,the KSSW intrusion northeast of Taiwan in spring of 2015 intruded northernmost off the Zhejiang coast in the nearshore area,but was weakest in the southern region(south of 28 °N),this may because the center of the KSSW intrusion northeast of Taiwan was shifted seaward.The KSSW intrusion in the ECS also has the feature of interannual variation.However,only the above qualitative research of the KSSW intrusion based on the dissolved inorganic iodine species cannot determine the intrusion intensity variation of the KSSW in these three years.Because dissolved inorganic iodine species can behave as quasi-conservative tracers and indicate the movement of seawater(especially the KSSW)in the ECS,we modified the OMP analysis by adding the DIIS as additional parameters.The dissolved inorganic iodine species were added first into the OMP analysis,which enabled us to quantitatively estimate the water composition in the extremely dynamic and productive ECS and discuss the intrusion intensity variation of the KSSW in spring of 2014,2015 and 2016.The OMP results indicate that seawater in the study region of the ECS mainly consisted of Changjiang Diluted Water(CDW),Taiwan Strait Warm water(TSWW),Kuroshio Surface Water(KSW)and KSSW.In spring of 2014,2015 and 2016,the KSSW dominated in the deep water,intruded into the ECS shelf from the northeast of Taiwan(where the KSSW contribution can be >80%)and could reached north of 29 °N off the Zhejiang coast(where the KSSW contribution can be >50%).When comparing the KSSW intrusion in spring of 2014,2015 and 2016,the average KSSW contribution of the bottom KSSW area in spring of 2016(78.9%)was greater than in the previous two years(67.9% and 64.5%).In each transect(especially north of 28 °N),the KSSW intrusion in spring of 2016 also contained the greatest average KSSW contribution and the maximum coverage area.Thus,according to the OMP results,the KSSW intrusion was strongest in spring of 2016,and may be comparable in spring of 2014 and 2015.This interannual variability of the KSSW intrusion is likely related to the El Ni?oSouthern Oscillation(ENSO)rather than the Pacific Decadal Oscillation(PDO).