[A-OS17] Ocean circulation and material cycle in coastal seas

[AOS17-05] Semidiurnal Internal Tides in the Bungo Channel: Characteristics, Energetics, and Seasonal variation

*SHUYA WANG¹, XINYU GUO¹, Akihiko Morimoto¹, ANZHOU CAO^2,1, Eisuke Tsutsumi³, Yasumasa Miyazawa⁴, Sergey M. Varlamov⁴ (1.Center for Marine Environmental Studies, Ehime University, 2.Ocean College, Zhejiang University, 3.Faculty of Fisheries, Kagoshima University, 4.Application Laboratory, Japan Agency for Marine-Earth Science and Technology)

Keywords:Internal tide, Bungo Channel, Seasonal variation

Tidal mixing in the Bungo Channel plays an important role in modulating the water exchange between the Seto Inland Sea and the Pacific Ocean. However, the generation, propagation, and dissipation of internal tides in this region are not fully understood. In this study, based on moored observations and model results from the Japan Coastal Ocean Predictability Experiment—Tides (JCOPE-T), semidiurnal internal tides in the Bungo Channel are investigated. Observational results indicate that semidiurnal internal tides induce strong baroclinic currents reaching 0.3 m/s. Their energy shows obvious spring-neap cycles, generally coinciding with the local barotropic tidal forcing. By adopting the empirical orthogonal function method, we find that the observed semidiurnal internal tides are mainly dominated by the first two baroclinic modes. The JCOPE-T results suggest two main generation sites for semidiurnal internal tides in the region: one is located at the Hayasui Strait, and the other is at the shelf break south of the Bungo Channel. The latter makes a major contribution to the observed semidiurnal internal tides. Northward internal tides generated at the shelf break are superposed with those generated at the Hayasui Strait, causing a complex interference pattern in the channel. Semidiurnal internal tides in the Bungo Channel show significant seasonal variations, which are weak in winter and spring but intensified in summer and autumn. This is due to the seasonal variation of stratification in the channel, which is partly modulated by the Kuroshio warm water intrusion in warming seasons. In contrast, semidiurnal internal tides generated at the shelf break are influenced by both the temporal variations in stratification and background currents. As they propagate southward into the Pacific Ocean, refraction by the Kuroshio leads to different spatial patterns of the beam. Results of this study highlight that the Kuroshio modulates internal tides and associated energy dissipation in this region.