Internal solitary waves (ISWs) in the northern South China Sea (SCS) are among the most energetic in the global ocean. It is well established that the dominant ISWs in this region originate primarily from the Luzon Strait (LS), with highly regular, tide-phase-locked occurrence. Here we present in-situ observational evidence of irregularly occurring internal solitary-like waves on the northern SCS shelf, whose occurrence pattern deviates markedly from the predictable LS-originated ISWs.

We demonstrate that this intermittency arises primarily from local generation of solitary-like waves on the shelf, rather than modulation of remotely propagated ISWs. The local generation is driven by the coupling of barotropic tides and along-shelf background currents, which modulates the strength of topographic flow disturbance and the nonlinear-dispersive balance, leading to intermittent wave formation. Using a combination of shipboard observations, numerical simulations, and theoretical diagnostics, we quantify the relative contributions of tidal forcing, shelf current intensity, and stratification to the generation efficiency, and identify the critical parametric thresholds for intermittent local generation. We further map the spatial distribution and temporal variability of locally generated solitary-like waves over the northern SCS shelf.

This study clarifies the dynamical mechanism of shelf-localized internal solitary wave generation, and improves our understanding and predictability of internal wave fields and associated mixing over continental margins.