Periodic Mesoscale Eddies in the South China Sea

Although numerous mesoscale eddies are observed in the South China Sea (SCS), a comprehensive study that focuses on periodic eddies has not yet been reported. Periodic eddies are a type of eddies that occur nearly annually in fixed time frames with similar patterns and trajectories. On the basis of in situ and satellite observations, this paper reports a periodic anticyclonic eddy in the western SCS (the Western SCS Anticyclonic Eddy, WAE) and elucidates the underlying processes responsible for its generation and evolution. The WAE is a seasonal phenomenon modulated by the Asian monsoon and basin-scale circulation. Usually, the WAE generates around 112 degrees E, 14 degrees N in February-March, induced by the cyclonic gyre in the northern SCS associated with the winter monsoon. The WAE occupies the western SCS for 5 months and finally dissipates in July due to the reversal of the monsoon and thus the circulation transition. The WAE shows important interannual variations associated with large-scale climate variation, and the Nino-3.4 index could be a potential predictor for its interannual variation. Finally, we summarize 13 periodic eddies in the SCS, including those unreported by previous studies. The period eddies in the SCS exhibit an inverse "L" shape vortex train structure along the western boundary and around the 17 degrees N section, with an anticyclonic "phase" in summer and a cyclonic "phase" in winter. This study provides new knowledge for periodic eddies in the SCS and will benefit targeted observations of periodic eddies.