Direct breaking of large-amplitude internal waves in the Urup Strait

• We made observations in the Urup Strait where diurnal tide is dominant and subinertial.
• We observed generation of large-amplitude internal waves and strong mixing.
• We show the structure and generation pattern of LAIWs using the model data.
• LAIWs have intrinsic frequencies relevant to the shape of topography.
• The amplification is caused by the topographically trapped waves with hydraulically supercritical velocity at the sill-top.

Observations and model data have been analyzed for the Urup Strait, one of the Kuril Straits, focusing on processes driving the extremely strong vertical mixing. Observations over 1 day on the Pacific side of the sill of the Urup Strait show the generation of a large-amplitude internal wave (LAIW), characterized by the depression of potential density surfaces caused by a strong off-sill flow. When the LAIW developed, the potential density surface of 26.7 σθ was depressed downward by more than 300 m, and vertical mixing was markedly elevated within the 400-m-thick layer 26.6–26.7 σθ, with a layer-mean (maximum) energy dissipation rate and vertical diffusivity of 1.2 × 10−6 (7.9 × 10−6 W kg−1) and 4.0 × 10−1 (2.8 m2 s−1), respectively. A three-dimensional model simulation of the Kuril Straits driven by the diurnal barotropic tide reproduces well the observed features relevant to the generation of LAIWs in and around the Urup Strait. The structure and generation pattern of LAIWs in the Urup Strait are consistent with those of the arrested lee waves observed in the Hawaiian ridge and Luzon Strait, although the depth at the top of the sill is far shallower in the Urup Strait. It is suggested that barotropic tidal energy with diurnal frequency is converted into LAIWs through the excitation of topographically trapped waves with hydraulically supercritical velocity near the sill top, and it is these waves that are responsible for the extremely strong mixing on the sill-flanks of the shallow Kuril Straits.