Intrathermocline eddies (ITEs) are a special type of subsurface-intensified eddies in the ocean. They exhibit lens-shaped structures within the thermocline and can significantly modulate the dynamic and biogechemical processes in the subsurface ocean. Based on temperature and salinity observations from 19 profiling floats in autumn 2019, an ITE was directly detected in the southwestern South China Sea. The ITE exhibited a convex lens-like thermohaline structure with low potential vorticity (PV) at its core depth of 30–80 m and its radius was estimated to be 170 km. Correspondingly, the ITE had anticyclonic horizontal velocity with the maximum magnitude reaching 0.54 m s^-1 at 40 m. Isopycnals of the ITE outcropped on its northwestern edge where it met the strong Vietnam eastward jet. Based on eddy-resolving reanalysis data between 1993–2019, we revealed 12 similar ITEs in summer and autumn seasons in this region, demonstrating the common occurrence of such ITEs. Generation mechanisms of the ITEs were further investigated and two pivotal mechanisms were proposed. First, the low-PV water within ITEs is originated from the upper boundary layer in the outcropped region, which is subducted into the thermocline through frontogenesis induced secondary circulation. Further PV budget analysis demonstrates that the PV destruction by frictional forces associated with down-front wind is the dominant mechanism of the low PV in the frontal outcropped region, while the roles of diabatic and advective processes are minor. Second, the upwelling driven by eddy-wind interactions can prompt the doming of upper-layer isopycnal surfaces, also facilitating the formation of ITEs.

Intrathermocline Eddies,Generation Mechanisms,southwestern South China Sea