[A-OS14] Physical Oceanography (General)

[AOS14-02] Observations of internal gravity waves and the implication of morphodynamics on the East China Sea slope

*Ryoichi Horinouchi ¹, Keita Saito¹, Ryosuke Nagasawa¹, Chikara Tsuchiya¹ (1.Japan Coast Guard )

Keywords:Internal gravity wave, Internal solitary wave, Internal tide, Ray theory, Morphodynamics

Internal gravity waves are known to occur predominantly in areas with steep bathymetry such as ridges. Internal gravity waves have been sometimes observed on the East China Sea slope, as shown in Li et al. (2008), which observed internal solitary waves using satellite data, and Lozovatsky (2015), which observed internal tides according to frequency analysis result of horizontal current velocity data. In this report, we introduce our oceanographic data suggesting the occurrence of internal gravity waves. We also report the implication that internal gravity waves on the slope contributed to the development process of distinct bathymetric structures distributed on the East China Sea slope. The observational data were current velocity profiles, water temperature and salinity profiles, and water column data, acquired by the Japan Coast Guard survey vessel Takuyo in October 2022, and by the survey vessel Kohyo in June 2023. The baroclinic component of the meridional current velocity indicated mode-solution-like vertical structure in the vertical cross-section and the phase seemed to be inverted in approxmately 7 hours. This can imply the occurrence of internal tides. In the water column images, internal-solitary-wave-like acoustic anomalies were observed. And this depth range roughly corresponded to that of the peaks in the squared buoyancy frequency profile. In the June 2023 data, water temperature oscillations with a period of about half a day were also observed. The amplitude is several degrees, and the depth range is from about 300-340 m. According to the above observational results, internal gravity waves can frequently occur on the East China Sea slope, although we have not yet achieved detailed observation of internal gravity waves movements due to the limitation of the current observational methods. Additionally, distinct bathymetric and geological structures, such as wavy bedforms and channel-like structures are prominently distributed on the East China Sea slope (Horinouchi et al., 2024; JpGU). We conducted the theoretical consideration in order to verify that internal gravity waves on the slope contribute to the development process of these bathymetric structures, referring to Miramontes et al. (2020), which investigated the morphodynamic process on the Mozambique shelf slope. We applied the ray theory of internal gravity waves on the slope to the October 2022 data. From the calculation result, the wavy bedforms tend to be under the supercritical condition, which means waves reflect offshore, and the channel-like structures tend to be under the near-critical condition, which means waves are trapped along the seafloor. This calculation result was consistent with the previous study, Miramontes et al. (2020). Although this is just an initial consideration, we conclude that the development process of these bathymetric structures can be attributed to internal gravity waves on the slope.