Recognition And Tracking Of Mesoscale Eddies In The South China Sea And Their Characteristics Analysis

Mesoscale eddies are ubiquitous in the ocean.They carry huge energy and play a key role in the transportation of energy and substance in the ocean.The circulation structure of the South China Sea is complex and eddy occurs frequently.Therefore,carrying out the identification and tracking of mesoscale eddies in the South China Sea is of great significance for comprehensively in-depth research on the ecological environment,dynamics and climate change in the South China Sea.Based on the altimeter data product of Copernicus Marine Service,this paper uses the sea level anomaly(SLA)closed contour method to detect the mesoscale eddies in the South China Sea from 1998 to 2018,and traces them through the similarity method.Then,the physical characteristics,spatio-temporal distribution characteristics,motion changes and the influence of relevant mechanisms of the eddy are studied and analyzed in detail.Finally,the study is summarized and the following conclusions are drawn:(1)The SLA closed contour detection method has good consistency with other detection methods,which tends to obtain larger eddy fluid shapes.Tracking based on individual eddy will ignore the merging and splitting events of eddies.(2)The mesoscale eddies of the South China Sea are mainly distributed in sea areas with a water depth greater than 200 m,and the seas southwest of Taiwan are the most active areas of eddies.The number of cyclonic eddies is slightly more than that of anticyclonic eddies,there are an average of 20 cyclonic eddies and 19 anticyclonic eddies per day,and more cyclonic eddies in winter and more anticyclonic eddies in summer.They usually alternate in space.Small eddies are prone to appear around large eddies.(3)The topography of the seabed restricts the spatial scale of the eddies,and larger eddies can be produced in the deep sea.The average radius of all eddies is 82.1 km,and the average amplitude is 3.2 cm.Cyclonic eddies and anticyclonic eddies show similar distribution characteristics in radius and amplitude.The radius and amplitude of anticyclonic eddies are slightly larger,and there are very few eddies with large radius(amplitude).Furthermore,the radius and the maximum rotation speed of the eddy are highly correlated with its amplitude,with the increase(decrease)of the amplitude,the maximum rotation speed band of the eddy is closer to the center(edge)of the eddy.The outer shape of the eddy changes greatly during its movement,while the inner part of the maximum speed belt is relatively more stable.(4)For eddies over 30 days: the west of the Luzon Strait,the central-eastern of the sea basin,and the south-central of Vietnam are high-frequency regions where eddies are generated,while the seas of central and southern Vietnam and the vicinity of islands are high-frequency regions that disappear.A total of 1,169 cyclonic eddy trajectories and 1,005 anticyclonic eddy trajectories have been tracked,and their lifespans are generally short,with an average lifespan of 51 days.Moreover,no rule of time change in the number of its production and extinction has been found,but there is not much difference between the number of production and extinction in each year.In the South China Sea,there are mainly three channels for eddies propagation.About 85% of eddies move westward,but they have no polar or equatorial bias,and their average moving speed is about 10 cm/s.(5)The seas west of the Luzon Strait and the seas east of south-central Vietnam have high eddy kinetic energy,and the two regions have significant seasonal variations,and the eddy kinetic energy of these two regions has significant seasonal variation,which showing different patterns with the influence of monsoon,Kuroshio invasion and submarine topography.Most of the eddies in the South China Sea propagate in the form of nonlinear,and the rotation effect of eddies is significant,which is beneficial to the transport of matter and energy.