Cross-isobath motions significantly impact oceanographic processes and coastal dynamics by driving the exchange of water masses and altering the physical nature of coastal waters. Under the potential vorticity conservation, the shelf current generally flows along the isobaths. However, deviations in current curvature from the isobath curvature trigger intensified cross-isobath motions. This study examines the mechanisms of topography-modulated cross-isobath motions from the perspective of current curvature over the Northern South China Sea (NSCS) shelf. Utilizing high-resolution numerical simulations, we found that to the east of Hainan Island and Taiwan Bank in the NSCS, convex topography disrupts the shelf current curvature, leading to downstream geostrophic cross-isobath motion. The curvature vorticity and cross-isobath velocity exhibit consistent spatial and temporal patterns. In contrast, south of Shanwei, where isobaths are relatively straight and parallel, the curvature vorticity aligns with the isobath curvature, and cross-isobath motion is generated by ageostrophic currents associated with the Ekman dynamic. Diagnostic analysis indicates that curvature vorticity primarily arises from the divergence of flow and is partially converted from shear vorticity. When the curvature of the isobaths surpasses a threshold, the generation of curvature vorticity and the conversion of shear vorticity to curvature vorticity become the primary process driving cross-isobath motion. This understanding significantly enhances our comprehension of the processes governing coastal water exchange.

cross-isobath motions; topography modulation; curvature vorticity; numerical simulation