Analysis Of Mesoscale Eddies Entering The Continental Shelf Of The China Offshore Area

An Archiving Validation and Interpretation of Satellite Oceanographic(AVISO)satellite altimeter sea level anomaly meso-scale eddy tracking data set during the period from 1993 through 2017 is used to classify the meso-scale eddies in the East China Sea(ECS)according to the path.The results show that 318 cyclones and 276 anticyclones are detected in the ECS during the 25-year period.These meso-scale eddies are classified into the following types according to the path:(1)eddy generated in shallow sea intrudes into deep sea type(148 cases),(2)eddy generated in deep sea intrudes into shallow sea type(35 cases),(3)eddy spreads along the isobath type(180 cases),(4)eddy wanders around type(121 cases),(5)eddy generated in Pacific Ocean intrudes into the shelf area of the ECS type(25 cases)and(6)eddy generated in Pacific Ocean intrudes into the ECS type(85 cases).These eddies have obvious seasonal variability.The number of mesoscale eddies for type 1 in spring and winter is much more than that in summer and autumn.However,the temporal distribution for type 4 is in contrast to that for type 1.The number of eddies for type 3 in spring and summer is much more than that in autumn and winter.Contrary to that for type 3,the number of eddies for type 6 is small in spring and summer.The mean lifetime of eddies for type 3 is longest,about 44 days.The mean amplitude and diameter of eddies for type 5 is largest,about 13.2 cm and 122 km.The mean lifetime,amplitude and diameter of eddies for type 6 is smallest.In addition to the eddies propagation speed from shallow sea to deep sea,the normalized characteristics of the amplitude and radius of other types of mesoscale vortices in the lifetime show a synchronous change trend,and the propagation speed of the other types is slow first and then fast.The trend is that the rotation speed of the eddies generated from the Pacific Ocean then propagating to the ECS is faster than other types of eddies.The positive and negative vortices is distributed on the east and west sides of the Ryukyu Islands.The motion path of the meso-scale eddies is impact by the background field.There are 184 mesoscale eddies entered the continental shelf area of the northern SCS during 24-year period.According to motion trajectories,we classify the mesoscale eddies into four types,i.e.along the isobath type,intrusion of continental shelf type,local wandering type,and shelf internal generation type.The occurrence numbers of these four types were 87,38,23 and 36,respectively.The mean amplitude and radius of the along the isobath type are the largest,about 18 cm and 153 km,respectively.Furthermore,their average lifetime is also longest,about 93 days.The mean amplitude,radius and lifetime are smallest for the shelf internal generation type,about 16 cm,146 km and 74 days,respectively.The direction and velocity of the background flow field will affect the intrusion path of the mesoscale eddies to continental shelf of the northern SCS.The amplitude and radius of the other three types of eddies show a synchronous change except for the sharp changes in the amplitude of the intrusion of continental shelf type.Except for shelf internal generation type,the propagation speeds of the other three types of eddies show a trend of slowing first and then fast.The propagation speed of shelf internal generation type in both two research areas is greater than that in the deep ocean.At the same time,the seasonal distribution of the mesoscale eddies quantity is also related to the direction and velocity of the corresponding background flow field.After intruding over the continental shelf,mesoscale eddies will decay and dissipation because the bottom boundary friction and marine environmental changes.Using the analytical model of ocean mesoscale eddies decay proposed by the predecessors,the analytical model of mesoscale eddies entering the continental shelf of the China offshore area was verified.