Physical structure and evolution of a cyclonic eddy in the Northern South China sea

The spatial-temporal evolution of a cyclonic eddy (CE) in the northern South China Sea (SCS) was analyzed based on observational data from underwater glider and satellite altimetry and outputs from Hybrid Coordinate Ocean Model (HYCOM). The CE was found to originate in the northeastern SCS and propagate along the continental slope, passing by the Xisha Islands. It eventually vanished offshore of the western Vietnam. Detailed subsurface temperature and salinity structures of CE were observed by underwater glider around the western Xisha Islands. The eddy experienced three evolution stages: continuously increase during its growth stage, gradually decrease when moving by the Xisha Islands, and rapidly weakening and merging with another cyclonic eddy to the west of Vietnam. During the first stage, the generation and growth of this CE were revealed to be due to the barotropic (BT) and baroclinic (BC) instabilities, with the deformation of the eddy being conducive to obtaining energy via the instabilities. During the second stage, i.e., after the CE reached the Xisha Islands, the eddy released (drained) energy to (from) the mean flow with negative (positive) BT and BC values in its northern (eastern) part. Overall, the volume-integrated BT and BC over the eddy area contributed to the eddy energy increase. For the decay of this CE near the Xisha Islands, the eddy kinetic energy budget analysis results indicated that it was mainly due to the interior eddy viscous dissipation.