The Analysis Of The Structure And The Originate Of The Ryukyu Current

The Ryukyu Current(RC)is an important western boundary current in the northwest Pacific.However,due to the lack of observation in the upstream,the origins of the RC is still unclear.This paper will figure out the structure,origin and formation mechanism of the Ryukyu Current,and analyze the variability of the origins based on the 23-year HYCOM(Hybrid Coordinate Ocean Model)reanalysis data.The RC has a clearly subsurface maximum structure between 700 and 900 m depth,the maximum velocity here could reach 20 cm/s.In addition,Anticyclonic eddies increase the velocity,and cyclonic eddies reduce it.The volume transport of the RC comes from the Kuroshio eastward branch(KEB)east of Taiwan and part of the North Pacific Subtropical Gyre(p-NPSG).From the surface to 2000 m depth,the KEB(p-NPSG)transport contributes 41.5%(58.5%)to the mean total RC transport.The KEB originally forms the subsurface velocity core of the RC east of Taiwan due to blockage of the subsurface Kuroshio by the Ilan Ridge(sill depth: 700 m).Above 700 m,the Kuroshio can enter the East China Sea(ECS)over the Ilan Ridge,meanwhile,the blocked Kuroshio below 700 m turns to the right and flows along the Ryukyu Islands.With the RC flowing northeastward,the p-NPSG contribution strengthens the subsurface maximum structure of the RC owing to blockage by the Ryukyu Ridge.In the surface layer,the p-NPSG cannot form a stable northeastward current due to frequent disturbance by mesoscale eddies and water exchange through the gaps(with net volume transport into ECS)between the Ryukyu Islands.We divided the variation of the KEB into KEB-up and KEB-down.The variation of the KEB-up related to the mesoscale eddies: Anticyclonic(cyclonic)eddies generally correspond to positive(negative)transport anomaly of the KEB-up,however,as the eddy positions move northward,the possibility of a negative(positive)transport anomaly caused by an anticyclonic(cyclonic)eddies increases.Thevariation of p-NPSG is represented by E2-E3,which occurs 72.2% of the p-NPSG inflow contributing to the RC and influenced by the mesoscale eddies.In the selected area,the transport anomaly of E2-E3 is related to the distance between the eddy center and the line AD.Statistically,when the center of a single anticyclonic eddy is located from AD greater than(less than)190 km,the transport anomaly at the E2-E3 tends to be positive(negative).The reverse applies for single cyclonic eddy events.There is often a dipole structure of mesoscale eddies in the selected area,when the anticyclonic in the northeast(southwest)of the cyclonic,the transport at the E2-E3 tends to be increase(decrease).