Phase-Locked Internal-Wave Triads Observed in the Izu-Ogasawara Ridge: Implications for Interplay Between Tides and Winds

Abstract

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. Although 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, 3-day velocity and density yo-yo measurements, conducted near the PSI critical latitude of 28.8°N opportunistically after several storm passages, reveal the coexistence of PSI-induced high-mode NIWs and wind-induced low-mode NIWs. Two distinct pairs of upward- and downward-propagating NIWs are identified, each forming a phase-locked triad with locally generated upward-propagating semidiurnal internal tides. In a mid-depth layer, a pair of high-mode upward- and downward-propagating NIWs forms a scale-separated PSI triad with the internal tides, whereas in an overlying layer, a pair of low-mode downward-propagating NIWs and intermediate-mode upward-propagating NIWs forms a distinct non-scale-separated triad. Consequently, turbulent dissipation is enhanced at the boundary between these distinct near-inertial velocity layers. These findings are not inconsistent with the view that wind-induced low-mode NIWs interact with internal tides, promoting the growth of intermediate-mode NIWs and eventually contributing to the enhanced thermocline turbulence, thus suggesting an overlooked pathway to turbulence in the ocean interior.