Although both geostrophic‐balanced mesoscale eddies and unbalanced small‐scale processes have been well studied in the northeastern South China Sea (NE‐SCS), less attention has been devoted to the submesoscales in between (i.e., O(1–10 km)), which is recognized as an important conduit connecting the balanced and unbalanced motions. Based on the output from a 1/30° OGCM simulation, spatiotemporal characteristics and generation mechanisms of submesoscales in the NE‐SCS are investigated in this study. Through examining the submesoscale relative vorticity and vertical velocity, the regions southwest of Taiwan (ST) and Luzon Strait (LS) are identified as two hot spots of submesoscales in the NE‐SCS. Seasonally, the submesoscales in region ST are much stronger in winter than summer, while those in LS do not show a significant seasonality. Statistical analysis suggests that the strength of submesoscales in regions ST and LS is highly correlated with the product of mixed‐layer depth and mesoscale strain rate (MSR) and with the MSR itself, respectively. By conducting theoretical scaling and energetics analysis, the authors find that the mixed‐layer instability whose strength is determined by the combination of mixed‐layer depth and MSR and the barotropic instability associated with current‐islands interactions are the dominant generation mechanisms of submesoscales in the above two regions, respectively. Further examinations of the submesoscale energy budget indicate that, to keep a balanced state, the generated submesoscales have to be dissipated by a forward energy cascade, highlighting the important role of submesoscales in the energy balance of the NE‐SCS circulation.

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Based on long-term mooring-array and satellite observations, three-dimensional structure and interannual variability of the Kuroshio Loop Current (KLC) in the northeastern South China Sea (SCS) were investigated. The 3-yr moored data between 2014 and 2017 revealed that the KLC mainly occurred in winter and it exhibited significant interannual variability with moderate, weak, and strong strengths in the winters of 2014/15, 2015/16, and 2016/17, respectively. Spatially, the KLC structure was initially confined to the upper 500 m near the Luzon Strait, but it became more barotropic, with kinetic energy transferring from the baroclinic mode to the barotropic mode when it extended into the SCS interior. Through analyzing the historical altimeter data between 1993 and 2019, it is found that the KLC event in 2016/17 winter is the strongest one since 1993. Moored-data-based energetics analysis suggested that the growth of this KLC event was primarily fed by the strong wind work associated with the strengthened northeast monsoon in that La Niña–year winter. By examining all of the historical KLC events, it is found that the strength of KLC is significantly modulated by El Niño–Southern Oscillation, being stronger in La Niña and weaker in El Niño years. This interannual modulation could be explained by the strengthened (weakened) northeast monsoon associated with the anomalous atmospheric cyclone (anticyclone) in the western North Pacific during La Niña (El Niño) years, which inputs more (less) energy and negative vorticity southwest of Taiwan that is favorable (unfavorable) for the development of KLC.

Interannual modulation of eddy kinetic energy (EKE) in the northeastern South China Sea (NE‐SCS) is investigated based on outputs of an eddy‐resolving oceanic general circulation model between 1980 and 2014. The EKE displays distinct interannual modulations with periods between 1.5 and 7 years. The maximum peak‐to‐trough amplitude of the interannual modulation occurred during period 2004–2005, which was about 1.5‐fold the time‐mean EKE level. Further analysis suggested that interannual variability of EKE in the NE‐SCS is primarily modulated by the Luzon Strait transport (LST). During high‐EKE years, the LST increases corresponding to a strengthened Kuroshio intrusion. The strengthened Kuroshio intrusion enhances the baroclinic instability of current in the NE‐SCS and thus leads to a strong EKE. The reverse is true during low‐EKE years when LST is smaller. Influences of ENSO and Pacific mesoscale eddies on the interannual modulation of LST are also discussed in this study.

Although submesoscale coherent vortices (SCVs) have been observed in different parts of the world's oceans, most of them were captured by accident by a limited number of hydrographic profiles. Here, using concurrent velocity and temperature/salinity measurements from a submesoscale‐resolving mooring array (2 km), two oppositely rotating SCVs are for the first time reported in detail in the northeastern South China Sea (NESCS). For the anticyclonic (cyclonic) SCV, its core lays at 210 m (180 m) with a mean Rossby number of −0.76 (0.65); its maximum swirl velocity, radius, vertical scale, and Burger number are estimated to be 0.43 m s−1, 31 ± 5 km, 235 m, and 1.5 (0.23 m s−1, 16 ± 3 km, 100 m, and 2.5), respectively. Corresponding to the SCVs' submesoscale nature, their momentum is governed by gradient‐wind rather than geostrophic balance. Consequently, kinetic energy of the anticyclonic (cyclonic) SCV would be on average underestimated (overestimated) by 47% (68%) if the traditional geostrophic relation was used to diagnose the velocity. Further analysis shows that thermohaline properties within the anticyclonic and cyclonic SCVs are close to the Kuroshio water and the local NESCS water, respectively. By combing water mass tracing and high‐resolution simulations, we suggested that the anticyclonic (cyclonic) SCV was generated in the southern Luzon Strait (southwest of Taiwan) through current‐topography interaction. We further proposed that the observed SCVs here should be a common phenomenon in the NESCS, which may provide a novel route for the tracer exchange between the NESCS and the western Pacific.