Dynamic investigation of formation process of long-lived anti-cyclonic eddy train in the South China Sea

Long-lived anti-cyclonic eddies have been detected in South China Sea (SCS) circulation from observation. Observation has shown that these eddies may form an eddy train along wind-induced jet across the northern SCS basin. The underlying dynamics of the phenomenon remain largely unclear. We conducted a numerical modeling study to investigate the formation and development of the anti-cyclonic eddy train and the associated dynamic processes. The results show that under the forcing of the southwesterly wind, three anti-cyclonic eddies were generated from the west to the east along the wind induced northeastward jet current. The sizes of eddies were determined by the baroclinic Rossby radius of deformation and the largest eddy can reach 500 m vertically. Wind stress curl and stratification are crucial to the formation process of eddy train. Vorticity balance analysis indicated that the divergence of water induced by wind forcing and wind stress curl provided the initial negative vorticity acceleration for a local anti-cycnlonic eddy near western boundary. The eddy was advected eastward by the background jet current and the negative vorticity input from wind stress curl strengthens the eddy. At the northeastern side of the first eddy, eddy-induced divergence reduced the vorticity and subsequently decreased the downstream vorticity advection. This provided a region of negative acceleration for the accumulation of local negative vorticity at the eastern side of the first eddy. These processes finally generated the second eddy and subsequently the eddy train