Abstract: Recent field observations and theoretical analysis revealed the processes with the lateral scale of 0.1-10 km and time scale of O(1 day) in the mixed layer, which are termed sub-mesoscale processes. Physics at these scales are distinguished by big Rossby number (Ro) and small Richardson number (Ri). They are able to effectively extract energy from geostrophic shear through sub-mesoscale instabilities, and then forward cascade the energy to dissipation at smaller scales. Moreover, sub-mesoscale processes can play a crucial role in the transport of upper-ocean mass and energy, mesoscale variability, air-sea interaction, and upper-ocean restratification, among others. Base on the Regional Ocean Modeling System (ROMS) high-resolution (1-km) numerical experiment, we preliminarily discuss sub-mesoscale processes in the northern South China Sea (NSCS) in this paper. The results suggest that mesoscale eddies and fronts were ubiquitous in the NSCS with significant sub-mesoscale process at their edges. Analytical investigation of stability and energy in one sub-mesoscale eddy as a case study indicates that the generation of negative potential vorticity (q) resulting in symmetric instability (SI) was primarily attributable to the strong lateral buoyancy gradients adjacent to frontal vortex filament. This instability is mainly produced by the frontogenesis at the southern edge of the sub-mesoscale eddy. Moreover, the extracted energy by the SI from geostrophic shear tends to forward cascade to the turbulence at small scale, with a maximum energy extraction of 4×10^-7W•kg^-1 at about 20-m depth.

Key words: northern South China Sea, sub-mesoscale physics, numerical modelling, frontogenesis