As the largest marginal sea in the North Pacific Ocean, the South China Sea (SCS), particularly its northern region, is rich in submesoscale processes, which play important roles in vertical heat transport and biogeochemical material exchange in the upper ocean. However,  their generation mechanisms in marginal seas remain poorly understood. Here, we use three high-resolution simulations (1/30°, 1/60°, and 1/90°) to examine wintertime dynamics in the northern South China Sea (NSCS). Beyond confirming that higher resolution captures more abundant fronts and filaments, our results reveal a novel cross-scale mechanism: topographic Rossby waves (TRWs) interact with submesoscale processes (submesoscales) along the continental slope, enhancing vertical buoyancy flux through frontogenesis. A budget analysis shows that horizontal advection associated with TRW–submesoscale coupling, together with surface buoyancy forcing, dominates the production of vertical buoyancy flux, while vertical advection acts as the primary sink. Compared with coarser runs, the 1/90° simulation exhibits stronger upward buoyancy fluxes and significantly higher correlations between TRW characteristics and submesoscale velocities. These findings provide new understanding of how bathymetry and large-scale circulation regulate submesoscale-driven vertical exchange. More broadly, the results highlight a pathway for transferring energy and tracers from mesoscale currents to turbulence, with implications for submesoscale parameterization in climate and Earth system models.