Title: Observations of Near-inertial Waves Induced by Parametric Subharmonic Instability

Journal: Journal of Oceanology and Limnology, 36(3):651-662

Authors: LI B. -T., A. -Z. Cao, and X. -Q. Lv*

Abstract: Near-inertial waves (NIWs), which can be generated by wind or the parametric subharmonic instability (PSI) of internal tides, are common in the South China Sea (SCS). Moored current observations from the northern SCS have revealed that the PSI of semidiurnal (D₂) internal tides is another source of NIWs. The objective of this study was to examine the energy variance in the PSI of D₂tides. The PSI of D₂internal tides generated NIWs and waves with frequencies around the difference frequency of D₂and f. The observed NIWs induced by PSI could be distinguished clearly from those elicited by typhoon Krosa. Shortly after Krosa entered the SCS, NIWs began to intensify on the surface and they propagated downward over subsequent days. The near-inertial currents were damped quickly and they became relatively weak before the waves were reinforced beneath the mixed layer when wind stress was relatively weak. Rotation spectra indicated an energy peak at exactly the difference frequency D₂–f of the NIWs and D₂, indicating nonlinear wave-wave interaction among D₂, f, and D₂–f. Depth-time maps of band-pass filtered velocities of D₂–f showed the waves amplified when the NIWs were reinforced, and they intensified at depths with strong D₂tides. The energies of the NIWs and D₂–f had high correlation with the D₂tides. The PSI transferred energy of low-mode D₂internal tides to high-mode NIWs and D₂–f waves. For the entire observational period, PSI reinforcement was observed only when mesoscale eddies emerged and when D₂was in spring tide, revealing a close connection between mesoscale eddies and NIWs. Mesoscale eddies could increase the energy in the f-band by enhancing the PSI of D₂internal tides. Thus, this represents another mechanism linking the energy of mesoscale eddies to that of NIWs.