Second-mode semidiurnal internal tides on the continental slope of the southwestern East/Japan Sea

Abstract

Energetic second-mode semidiurnal internal tides with a three-layer-like structure were observed from time series of 26-hour-long 2 hourly velocity profiles collected on the continental shelf-slope of the southwestern East/Japan Sea during a spring tidal period in early November 2013. Modal-fitted baroclinic semidiurnal signals using a least squared method demonstrate that the kinetic energy of second-mode semidiurnal internal tides is comparable to the first-mode ones. In order to understand physical processes affecting the enhanced generation of the second-mode internal tides and their spatio-temporal variations, we utilize 30-month long numerical simulation outputs from a real-time ocean forecasting system of the East/Japan Sea. The model includes realistic oceanic circulation and stratification together with tides of 16 major components. The analysis results reveal that the most energetic second mode semidiurnal internal tides generate during fall months when the main therniocline was located near the mid depth (100-150 m) over the continental shelf-slope. It appears that the deepened mixed layer together with the enhanced mid-depth stratification induced by the increased flow through the western channel of the Korea Strait provide a second-mode favorable stratification condition near the internal tide generation region. This study has an implication that the seasonally-varying second-mode semidiurnal internal tides would have an impact on the tide induced ocean mixing in the southwestern East/Japan Sea.