Kinematic Characteristics and Water Mass Transports of Submesoscale Coherent Vortices in the Northeastern South China Sea

As a unique phenomenon occurring in the subsurface ocean, submesoscale coherent vortices (SCVs) are believed to have a pivotal role in the long-distance ocean tracer transports. SCVs have been widely observed in the global oceans; however, most of them are captured accidentally, and their kinematic characteristics and water mass transports have only been studied in a limited number of regions. Here, we use 4-year observations of velocity, temperature, and salinity from five moorings in the northeastern South China Sea (NESCS) to examine dozens of newly discovered SCVs. A total of 34 SCVs were identified during the observational period, including 25 convex lens-like anticyclones and 9 concave lens-like cyclones. The maximum swirl velocity, mean radius, and vertical length scale of the anticyclones (cyclones) are 0.19 +/- 0.07 m s-1 (0.19 +/- 0.07 m s-1), 26.4 +/- 13.9 km (17.0 +/- 5.4 km), and 204 +/- 62 m (188 +/- 53 m), respectively. Vertically, the velocity structure of the observed SCVs conforms to a Gaussian function when the effect of stratification is removed. Water mass analysis suggests that 88% (30/34) of the SCVs carried Kuroshio water, which demonstrates the mechanism proposed by Zhang et al. (2022), that they are formed by Kuroshio-islands interactions in the Luzon Strait. This category of SCVs is therefore named Luzon Strait island wake eddies (Liddies). We further estimate that Liddies can result in an equivalent mean volume transport of 0.20 Sv westward across the Luzon Strait, which suggests that they play a non-negligible role in the subsurface water transports between the NESCS and the northwestern Pacific.