5:15 PM - 7:15 PM
[AOS21-P07] Internal solitary wave trains observed on the continental shelf off Boso
Keywords:internal waves, soliton, turbulent mixing
On the continental shelf off Boso, coastal waters, including Tone River water, are adjacent to open-ocean waters influenced by the Kuroshio. The mixing of these water masses is considered to play an important role in the coastal nutrient cycle. Previous measurements in this area revealed the spatial distribution of turbulent kinetic energy dissipation, chlorophyll fluorescence and nitrate concentrations on the continental shelf, suggesting that turbulent diapycnal mixing contributes to the nutrient supply required for summer subsurface chlorophyll. In this study, to explore the mechanisms of turbulence production, mooring observations were conducted from July 3 to August 21, 2024, on the continental shelf off Boso. The mooring was equipped with an acoustic Doppler current profiler (ADCP), 10 CTDs, 4 Chlorophyll-turbidity sensors and 12 temperature loggers (10 SBE-56 and 2 DEFI-T).
Intermittent large-amplitude temperature fluctuations of up to 7℃ were observed in the pycnocline (30-55 m). The temperature fluctuations are consistent with ADCP horizontal and vertical velocities, suggesting internal solitary waves (ISWs). The properties of ISWs are estimated during the event from 3:30 to 5:30 PM on July 10, during which more than six ISWs were observed. The duration of each ISW is ~10 mins. The horizontal wavelength is ~400 m. The estimated isopycnal vertical displacement η is up to 38 m, which is comparable to the water depth at the mooring site (100 m). The propagation speed C reaches up to 0.49 m/s, which is faster than linear internal-wave phase speed 0.3 m/s, suggesting strong nonlinearity. The propagation direction θ is southwestward, indicating that the ISWs originated from the open ocean. The origin and generation mechanism of ISWs are not well understood, but they may result from the amplification and/or fission of open-ocean internal tides entering the continental shelf. Turbulent diapycnal diffusivities, estimated by applying the Ellison-scale parameterization (Ivey et al. 2018) to the 1 Hz temperature time-series, suggest that turbulent mixing is intensified by 100-1000 times during the ISW event.
Intermittent large-amplitude temperature fluctuations of up to 7℃ were observed in the pycnocline (30-55 m). The temperature fluctuations are consistent with ADCP horizontal and vertical velocities, suggesting internal solitary waves (ISWs). The properties of ISWs are estimated during the event from 3:30 to 5:30 PM on July 10, during which more than six ISWs were observed. The duration of each ISW is ~10 mins. The horizontal wavelength is ~400 m. The estimated isopycnal vertical displacement η is up to 38 m, which is comparable to the water depth at the mooring site (100 m). The propagation speed C reaches up to 0.49 m/s, which is faster than linear internal-wave phase speed 0.3 m/s, suggesting strong nonlinearity. The propagation direction θ is southwestward, indicating that the ISWs originated from the open ocean. The origin and generation mechanism of ISWs are not well understood, but they may result from the amplification and/or fission of open-ocean internal tides entering the continental shelf. Turbulent diapycnal diffusivities, estimated by applying the Ellison-scale parameterization (Ivey et al. 2018) to the 1 Hz temperature time-series, suggest that turbulent mixing is intensified by 100-1000 times during the ISW event.