Second-mode semi-diurnal internal tides on the continental slope of the southwestern East/Japansea

Abstract

This paper presents second-mode semi-diurnal internal tides observed on the continental shelf-slope of the southwestern East/Japan Sea. During a spring tidal period in early November 2013, we collected 26-hour-long 2 hourly CTD and Lowered ADCP profiles spanning the ~250m water column. The time series of velocity profiles, dominated by semi-diurnal variation, exhibit a three-layer-like vertical structure with an in-phase relationship between the upper and lower layers and an out-of-phase relationship between the upper and middle layers. The baroclinic semi-diurnal signals extracted using a least-squared harmonic fitting demonstrate that energetic secondmode internal tides occurred at the observation site. In order to understand the physical processes generating the second-mode internal tides and their spatio-temporal variations, we utilized 25-month long numerical simulation outputs from a real-time ocean forecasting system of the East/Japan Sea, which includes realistic oceanic circulation and stratification together with tidal forcing of 16 major components along open boundaries. The analysis results reveal that the deepened mixed layer near the semi-diurnal internal tide generation region during the fall months appears to provide a second-mode favorable stratification. The seasonally-varying distributions of second-mode internal tides in the East/Japan Sea will be discussed with their potential impact on the oceanADCP profiles spanning the ~250m water column. The time series of velocity profiles, dominated by semi-diurnal variation, exhibit a three-layer-like vertical structure with an in-phase relationship between the upper and lower layers and an out-of-phase relationship between the upper and middle layers. The baroclinic semi-diurnal signals extracted using a least-squared harmonic fitting demonstrate that energetic secondmode internal tides occurred at the observation site. In order to understand the physical processes generating the second-mode internal tides and their spatio-temporal variations, we utilized 25-month long numerical simulation outputs from a real-time ocean forecasting system of the East/Japan Sea, which includes realistic oceanic circulation and stratification together with tidal forcing of 16 major components along open boundaries. The analysis results reveal that the deepened mixed layer near the semi-diurnal internal tide generation region during the fall months appears to provide a second-mode favorable stratification. The seasonally-varying distributions of second-mode internal tides in the East/Japan Sea will be discussed with their potential impact on the ocean