Diapycnal mixing caused through breaking of large-amplitude internal lee waves generated by sub-inertial diurnal tides, which are modulated with a 18.6-year period, is hypothesized to be fundamental to both the intermediate-layer ventilation and the bi-decadal oscillation around the North Pacific Ocean. The first observational evidence of such wave breaking is presented here. The breaking wave observed had ∼200 m height and ∼1 km width, and its associated diapycnal mixing was estimated to be ∼1.5 m2 s^[-1], with a temporal average ∼10^[4] times larger than typical values in the open oceans. Our estimate suggests that a similar mixing process occurs globally, particularly around the Pacific and Antarctic Oceans
