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 analyses suggest 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 within 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 annual-mean volume transport of 0.20 Sv westward across the Luzon Strait, which suggests that they play a nonnegligible role in the subsurface water transports between the NESCS and the northwestern Pacific.

The Kuroshio Loop Current (KLC) is an important form of Kuroshio intrusion into the northeastern South China Sea (NESCS), which has significant influences on dynamical and biogeochemical processes in the NESCS. Recent studies suggested that the KLC is a hot spot of submesoscale processes (submesoscales) with spatiotemporal scales of O(1–10) km and O(1–10) days, but submesoscales’ roles in energy cascade and salt and heat transports remain obscure. Here, we investigate this issue through analyzing outputs from a 1/48° simulation. The kinetic energy exchange rate between submesoscale and larger-scale processes (KER) is overall positive in the KLC region, which suggests the dominance of forward cascade. The magnitude of KER is comparable with the temporal change rate of larger-scale kinetic energy in the upper 200 m. We also find that magnitude and direction of KER are closely associated with strain rate and horizontal divergence of background flows, respectively. In addition, the KLC region shows elevated submesoscale salinity and heat diffusivities with magnitudes reaching O(102) m2s-1. During the KLC period, horizontal mixing by submesoscales can transport 0.90×1013 kg salt and 0.71×1020 J heat westward into the NESCS interior, which are an order of magnitude larger than those caused by the KLC eddy shedding. These results suggest that submesoscales play important roles not only in energy cascade but also in salt and heat transports in the KLC region. Therefore, the roles of submesoscales should be taken into account when studying energy, salt, and heat budgets in the NESCS.

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 analyses suggest 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 within 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 annual-mean volume transport of 0.20 Sv westward across the Luzon Strait, which suggests that they play a nonnegligible role in the subsurface water transports between the NESCS and the northwestern Pacific.