Study On The Oceanic Eddy And Its Impact On Internal Tides In The East China Sea

Oceanic eddy plays an inmportant role in transportation of mass,energy in the ocean,which is a connection of large scale currents to small scale turbulence.In this paper,based on the eddy data set released by Chelton and the buoy drifting trajectory from 1993 to 2015,we match the Euler eddy extracted from satellite altimeter and Lagrange eddy extracted from buoy drifting trajectory in the same period.The paired-eddy indicates that the pairing success rate of the Euler eddies and the Lagrange eddies is not uniform in the spatial and time distribution.Between20 and 60 degrees in both north and south latitudes,the eddy pairing success rate reaches 25%,but in the equatorial region,this value is less than 10%.Due to the Coriolis effect in the low latitude area is insignificant,the satellite altimeter observations could not be effective to the Euler eddy;however,the Lagrange eddies identified by the buoy trajectory are abundant,which indicates that the eddy observation by the drifting buoy in the low latitude region could effectively overcome the area limitation of the satellite altimeter observation.Upon further analysis,the Euler eddy radius(closed loop)is generally larger than that of the paired Lagrange eddy.The eddy radius obtained by the two identification methods is roughly equivalent inside the ocean,but between 20 degrees in the north and south of the equator(especially near the equatorial region),high latitude regions,and western boundary current regions,the Euler eddy radius is more than triple the radius of the simultaneous Lagrange eddy closed loop.In addition,when analyzing the global distribution of the ratio of the Euler eddy Rossby number and the corresponding Lagrange eddy Rossby number,it is observed that the closed loop with smaller Lagrange eddy corresponds to a larger average relative vorticity.That is,after the buoy is captured by the meso-scale vortex,it is easier to form a closed loop where the relative vorticity is larger(such as meso-scale eddy centers,mesos-cale eddy edges,etc.).Then we focus our research area on the Kuroshio region in the East China Sea using buoy drifting trajectory and ROMS model.Most of oceanic eddies generated on both sides of the Kuroshio is outside the boundary of the Kuroshio,the life cycle is much shorter than eddies inside the ocean,and the radius is concentrated within200 km.The radius of the eddy is approximately proportional to the life cycle,and the larger the eddy Rossby number,the smaller the corresponding radius and the shorter the life cycle.The eddy is basically located in the upper 50 m water layer,and the radius decreases as the depth increases,showing a bowl-like structure,and the eddy moves downstream along the Kuroshio over time.The spatial-temporal characteristics of the barotropic tides and internal tides(ITs)northeast of Taiwan Island are examined,based on a 1-year mooring current observations from May 2017.The results of harmonic tidal analysis show that the barotropic tides are dominated by semidiurnal tides,which was mainly controlled by M2 tidal components.Moreover,the vertical structures of diurnal and semidiurnal ITs show that seasonal variations of the diurnal IT energy is not significant,whereas the semidiurnal IT shows notable seasonal variation.The semidiurnal IT energy in winter half year is twice that in summer half year.The seasonal variation of semidiurnal IT is mainly modulated by the direction change of the flow field rather than by the topography and stratification.In summer(winter)half year cyclonic(anti-cyclonic)eddies meanly control at this point,so the flow direction is mainly in the southwest(northeast)direction,causing the background flow to flow along(perpendicular)the isobath.When crossing the isobath,the ITs are generated by the interaction of the barotropic tide and the topography,resulting in the increase of the tidal energy in the winter half year.