Near-inertial internal waves (NIWs) are a major energy source for turbulent mixing in the ocean interior, yet their interactions with strongly baroclinic western boundary current fronts remain poorly understood. Here, we investigate the influence of the Kuroshio Extension (KE) front on subsurface turbulent mixing using Argo observations and high-resolution shipboard measurements. Turbulent kinetic energy dissipation rates estimated independently from Argo and shipboard observations consistently reveal a pronounced cross-front asymmetry. Although turbulent mixing is slightly stronger north of the front above approximately 300 m, dissipation is substantially enhanced on the southern flank between 300 and 800 m, accompanied by intensified vertical turbulent heat fluxs. Surface near-inertial wind energy input estimated from slab-model calculations and Global Drifter Program observations is greater on the northern side of the KE front, indicating that local wind forcing alone cannot explain the observed subsurface mixing pattern. High-resolution shipboard observations reveal coherent bands of enhanced near-inertial shear confined within the frontal zone. Combined with ray-tracing analysis based on frontal wave dispersion theory, these results demonstrate that downward-propagating NIWs are trapped within slantwise critical layers generated by strong frontal baroclinicity and geostrophic shear. Wave trapping enhances vertical shear, promotes wave breaking, and leads to intensified turbulent dissipation on the southern side of the front. Our results provide observational evidence that frontally trapped NIWs generate a large-scale asymmetry in subsurface turbulent mixing across the KE front, highlighting the important role of front–NIW interactions in regulating interior mixing and diapycnal heat transport in western boundary current systems.

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