Analysis of Seasonal Differences in Internal Tidal Mean Flow in the South China Sea during Winter and Summer

Based on the ROMS model combined with Lagrangian mean flow theory, this study calculates the M2 internal tidal Lagrangian mean flow (internal tidal mean flow) across the entire South China Sea (SCS). The results reveal that the magnitude of internal tidal mean flow in the SCS reaches 10-2 m/s. Regions with stronger mean flow velocities are primarily distributed in the Luzon Strait, areas near the Dongsha Islands in the northern SCS, western coastal waters, and eastern-southeastern coastal regions. The flow field generally exhibits a similar basin-wide counterclockwise circulation structure. Further analysis demonstrates that seasonal differences in internal tidal mean flow between winter and summer, particularly in the S1 and southeastern regions, are mainly regulated by horizontal thermohaline fields and vertical stratification intensity. The summer season features greater horizontal temperature-salinity gradients that drive stronger density currents. Concurrently, summer exhibits greater thermocline thickness and intensity compared to winter, with buoyancy frequency (n²) being significantly higher. These enhanced stratification characteristics lead to more pronounced internal tidal mean flow during summer.