Internal Tides, Non‐linear Internal Waves, and Mixing Over a Large River‐Dominated Inner Shelf

Abstract Internal waves (IWs) profoundly influence turbulent mixing in stratified coastal seas, yet their dynamics in large river‐dominated inner shelves remain poorly understood. With high‐frequency observations from bottom‐mounted tripods and stationary ship‐based platforms, this study provides the first comprehensive dynamical characterization of internal tides and high‐frequency non‐linear internal waves (NLIWs) in the Changjiang Estuary (CE). In summer, strong freshwater‐induced stratification supported an energetic IW field consisting of multi‐modal internal tides and frequent mode‐1 NLIWs. Modal analysis revealed that the observed “sandwich” velocity structure arose from the superposition of low‐ and high‐order modes with distinct phase velocities. NLIWs occurred two to three times per day, typically propagating southwestward in mode‐1 form, with the highest total kinetic‐energy flux exceeding 300 W m −1 . Their passage enhanced turbulent dissipation (>10 −5 W kg −1 ) and diffusivity (10 −4 –10 −3 m 2 s −1 ) near the pycnocline, driving intermittent but intense turbulence. Froude number analysis suggests that many NLIWs occurred during transitions from supercritical to subcritical flow, reminiscent of lee wave mechanism over steep slope. In contrast, winter observations showed weakened stratification, reduced baroclinic M 2 flux (∼8% of summer), and the absence of NLIWs, while storm events dominated mixing in the upper water column. Collectively, these results highlight strong seasonal modulation of IW processes and underscore the controlling influence of stratification and topography on diapycnal mixing, which is critical for regulating nutrient fluxes and primary productivity across the East China Sea inner shelf.