Subsurface Anticyclonic Eddy Transited from Kuroshio Shedding Eddy in the Northern South China Sea


Subsurface eddies are a special type of oceanic eddies that display the maximum velocity in the subsurface layer. Based on field observations, a lens-shaped subsurface anticyclonic eddy (SAE) was detected in the northern South China Sea (SCS) in May 2021. The SAE was located between 20 and 200 m, with a shoaling of the seasonal thermocline and deepening of the main thermocline. Eddy track indicated that this SAE originated from the Luzon Strait and was generated in the Kuroshio loop current (KLC) last winter. The evolution of SAE was related to the anomalous water properties inside the eddy and the seasonal change of sea surface heat flux. In winter, the continuous surface cooling and Kuroshio intrusion led to a cold-salty core in the upper part of the anticyclonic eddy, which resulted in a subsurface-intensified structure through geostrophic adjustment. As the season changed from winter to spring, sea surface temperature increased. The lens-shaped structure was formed when the seasonal thermocline appeared near the surface that capped the winter well-mixed water inside the eddy. Statistical analysis shows that nearly half of the KLC shedding eddies (12/25) were generated in winter and persisted to the following May during 1993-2021. Reanalysis data confirmed that these eddies all evolved into lens-shaped structures in late spring. This result indicates that the transition of KLC shedding eddy to SAE is a common phenomenon in the northern SCS, which is potentially important for local air-sea interaction, heat–salt balance and biogeochemical processes.