Subsurface eddies in the southern South China Sea detected from in-situ observation in October 2011

Two anticyclonic subsurface eddies (SSEs) are detected from the in-situ hydrography data of the southern South China Sea (SCS) during 15–25 October 2011. Both SSEs have the lens-shaped water bodies below the thermocline. Their maximum swirl speed appears at the depth of lens׳ core, which is also characterized by a dump in the T–S diagram. These eddies do not have an enclosed saline-water or warm-water body in its lens׳ core, which is different from those SSEs reported in other seas. These SSEs should be locally generated by the horizontal shear of the Southeast Vietnam Offshore Current. In the SSE generation site of the southern SCS, there is an upper-layer anticyclonic eddy (AE2) that is right above the SSE (SE2). After leaving its generation site, the eddy loses its energy source and starts to weaken. In this case, the eddy will decay quickly in the upper layer due to the restraint of the thermocline, and finally evolves into a pure subsurface eddy (i.e. SE4).