[A-OS16] Coastal ocean circulation and material cycle

[AOS16-09] Observed turbulent mixing and its relationship to flow and stratification in the Bungo Channel

*Eisuke Tsutsumi¹, Naoki Yoshie², Xinyu Guo³, Akihiko Morimoto³, Takahiro Endoh⁴, Yasumasa Miyazawa⁵, Toru Miyama⁵ (1.Faculty of Fisheries, Kagoshima University, 2.Institute for the Promotion of Science and technology, Ehime University, 3.Center for Marine Environmental Studies, Ehime University, 4.Research Institute for Applied Mechanics, Kyushu University, 5.Application Laboratory, Japan Agency for Marine-Earth Science and Technology)

Keywords:Turbulent mixing, shear instability, convective instability, topographic wake, tidal fronts, Bungo Channel

In the southern part of the Bungo Channel, Kyucho, a sudden intrusion phenomenon of warm water from the Kuroshio region, is known to occur during the summer season, affecting coastal ecosystems and fisheries. In the northern part of the Bungo Channel, a tidal front forms around the Hoyo Strait, which connects to the Seto Inland Sea, and the circulation around the tidal front is thought to promote nutrient transport and biological production. The physical environment is thought to play an important role in the ecosystem of Bungo Channel, and the importance of vertical mixing as a controlling factor of the physical environment has been demonstrated. However, there are few studies with direct measurements of vertical mixing and its understanding is insufficient. In this study, we investigated the vertical mixing process in the Bungo Channel based on turbulence microstructure measurements, and examined the relationship of turbulence intensity to flow and stratification. Turbulence observations were conducted in May 2022, July 2022, May 2023, and July 2023 onboard RT/V ISANA of the CMES, Ehime University. We used a microstructure profiler, TurboMAP-5 (JFE Advantech, 512 Hz) and an acoustic Doppler current profiler (Teledyne RDI, 600-kHz WH ADCP) to observe turbulence and mean flow. Observations in May and July were made in the tidal front region south of Hoyo Strait and around Oitsukami Island off Uwajima City, respectively, and 86 and 198 profiles of turbulent kinetic energy dissipation rates were collected. The ADCP was suspended from the side of the vessel and accurately measured the current velocity from 4 m to 40 m below the sea surface. In the tidal front region, turbulent energy dissipation rate in the subsurface layer was enhanced, probably due to convective instability. Around Oitsukami Island, turbulent kinetic energy dissipation was enhanced due to shear instability in the topographic wake. The relationship between the dissipation rate and background stratification (buoyancy frequency squared N²) and vertical shear (vertical shear squared Sh²) shows different trends in the two regions, with the MacKinnon-Gregg (2003) type independent of the gradient Richardson number Ri_g for the tidal front region, and a Gregg (1989) type or the type of the stability function of the Mellor-Yamada closure scheme (Galperin et al. 1988) for the topographic wake region, which depends on the gradient Richardson number. This difference in the dependence of the dissipation rate on Ri_g (or N²) due to different unstable processes should be taken into account in the turbulence parameterization.