[A-CG47] 海洋と大気の波動・渦・循環の力学

[ACG47-04] Phase-Locked Internal-Wave Triads Observed in the Ogasawara Ridge: Implications for Interplay between Tides and Winds

*伊地知 敬¹、井上 龍一郎²、古島 靖夫² (1.東京大学、2.海洋研究開発機構)

キーワード：内部潮汐、近慣性波、Parametric subharmonic instability

The Ogasawara subregion of the Izu-Ogasawara-Mariana arc system in the western North Pacific is recognized as one of the most notable hotspots for thermocline turbulent mixing. While parametric subharmonic instability (PSI) is generally considered a primary driver of enhanced turbulence by facilitating energy transfer from low-mode semidiurnal internal tides to high-mode near-inertial waves (NIWs), recent observations suggest that NIWs remotely induced by winds also contribute to significant seasonal variations in thermocline turbulence in this region. Here, three-day velocity and density yo-yo measurements, conducted near the PSI critical latitude 28.8°N in the Ogasawara Ridge opportunistically after several storm passages, reveal the coexistence of PSI-induced high-mode NIWs and wind-induced low-mode NIWs, which jointly contribute to enhanced turbulence. Two distinct phase-locked triads are identified, comprising upward- and downward-propagating NIWs and locally generated upward-propagating semidiurnal internal tides. In a mid-depth layer, a pair of high-mode upward- and downward-propagating NIWs form a typical PSI triad with the internal tides, whereas in an overlying layer, a pair of low-mode downward-propagating NIWs and moderately-high-mode upward-propagating NIWs form a distinct triad with the internal tides. Consequently, turbulent dissipation is enhanced at the boundary between these distinct near-inertial velocity layers. These findings suggest that wind-induced low-mode NIWs interact with internal tides, promoting the growth of moderately-high-mode NIWs and eventually contributing to the enhanced thermocline turbulence, thus revealing an overlooked pathway to turbulence in the ocean interior.