Abstract: Based on data from nine mooring observation points in the South China Sea, including the Luzon Strait, Dongsha, Xisha, and Nansha regions, and the high-resolution MITgcm LLC4320 numerical model, this study analyses the seasonal evolution characteristics of submesoscale processes in different regions of the South China Sea and their energy transfer with mesoscale processes. The results show that submesoscale activity is most pronounced in the Luzon Strait region, where complex island topography prevents the emergence of distinct seasonal patterns; in the Dongsha and Xisha regions, submesoscale activity is strongest during winter (December to February) and spring (March to May), with peak values reaching 5×10^-3m²·s^-2 and 3.5×10^-3m²·s^-2, respectively; the Nansha area is stronger during winter (December to February), but is one order of magnitude lower than the Luzon Strait region and exhibits weaker seasonal variations; while the submesoscale activity in the Eastern Vietnam region is stronger during the summer and autumn seasons. Correlation analysis between the root mean square of vorticity and mixed layer depth and mesoscale strain rate indicates that submesoscale processes in the Dongsha and Xisha regions are jointly regulated by mixed layer instability and mesoscale strain rate; whereas in the Luzon Strait, Nansha, and Eastern Vietnam regions, mesoscale strain rate is the primary driver. Additionally, using the local multiscale energy and vorticity analysis (MS-EVA) method, the study characterised the energy transfer between submesoscale processes and large-scale and mesoscale processes in the South China Sea. Results show that in the Luzon Strait region, the large-scale Kuroshio Current provides energy to submesoscale processes through baroclinic and barotropic instability processes, reaching 4×10^-8m³·s^-3 in December. Compared to the Luzon Strait region, energy transfer intensity in other South China Sea regions is one order of magnitude lower. In the Eastern Vietnam, submesoscale kinetic energy compensates for large-scale kinetic energy through reverse-level compensation during the summer and autumn seasons, reaching 5×10^-9m³·s^-3 in July. This study provides observational evidence for understanding the interactions between multiscale processes in the South China Sea and offers important basis for improving subgrid parameterisation in eddy-resolution models.

Key words: South China Sea, submesoscale processes, energy cascades, mooring observations, MITgcm LLC4320