Spatiotemporal characteristics and generation mechanisms of submesoscale currents in the northeastern South China Sea revealed by numerical simulations

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 degrees 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. Plain Language Summary Oceanic submesoscale currents play important roles in energy budgets and material transport in the upper ocean. Marked by horizontal and temporal scales of O(1-10 km) and O(1-10 days) at midlatitudes, these currents are poorly studied in the South China Sea. In this study, the spatiotemporal characteristics and generation mechanisms of submesoscales are investigated in the northeastern South China Sea using output from a 1/30 degrees ocean model. Based on the simulated relative vorticity and vertical velocity, the regions southwest of Taiwan and Luzon Strait are identified as two hot spots of submesoscales. In the region southwest of Taiwan, the submesoscales are strong in winter but weak in summer; the submesoscales here are generated primarily through mixed-layer baroclinic instability whose strength is determined by the combination of mixed-layer depth and the deformation rates of mesoscale eddies. For the Luzon Strait, the submesoscales do not have significant seasonality, and their generations are through barotropic instability associated with the current-islands interactions.