Near-inertial Wave Variability Induced by Parametric Subharmonic Instability in the Subtropical Northwestern Pacific; [북서태평양 아열대 해역 PSI 기인 준관성주기 내부파의 변동성]

초록

Near-inertial waves (NIWs) are primarily driven by wind, but they can also be generated through nonlinear interactions such as Parametric Subharmonic Instability (PSI). Internal tides transfer their energy to inertial oscillations through PSI when half of the internal tide frequency matches the local inertial frequency. This study investigates PSI-induced NIW variability using ADCP mooring observations obtained at 13.6°N, 17°N, and 21°N in the northwestern Pacific from June 2010 to May 2013. Among the three sites, site M2 (13.6°N), located near the critical latitude for PSI of diurnal internal tides, exhibited persistently strong NIWs and enhanced vertical shear despite the absence of typhoon events. In addition, the NIWs at site M2 likely resulted from the PSI of diurnal internal tides because the energy peak observed in the inertial band was located at a frequency higher than the local inertial frequency. The kinetic energy of the NIWs generally followed the semi-annual variation of diurnal tides. However, from April to October 2012, inertial-band kinetic energy weakened substantially and did not follow the expected tidal variation. During the same period, opposite variations in diurnal-band kinetic energy were observed between sites M2 and M3, implying a change in the propagation path of the diurnal internal tides, likely due to weakened equatorward refraction. This study demonstrates that PSI of remotely generated diurnal internal tides is an important source of NIWs, enhanced vertical shear, and turbulent mixing, highlighting the importance of internal wave interactions in ocean circulation and underscoring the need for numerical simulations to account for these interactions. © 2026 Ocean and Polar Research.

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