Oral

IAPSO

P01 - General Topics in Oceanography (physics and boiogeochemistry)

Long-term observations of near-inertial wave variability induced by parametric subharmonic instability in the subtropical Northwestern Pacific near 14��N

Near-inertial waves (NIWs) are primarily driven by wind, but can also be generated from a non-linear interactions such as Parametric Subharmonic Instability (PSI). An internal wave transfers its energy to inertial oscillations through PSI when half of the internal wave's frequency matches the local inertial frequency. The latitude at which the PSI occurs is called the critical latitude, with PSI of the semidiurnal and diurnal tides occurring near 29��N and 14��N, respectively. This study investigates PSI-induced NIW variability using ADCP mooring observations obtained from 14��N, 17��N, and 21��N (designated as M2, M1, and M3, respectively) in the subtropical Northwestern Pacific (135��E) from June 2010 to May 2013. Kinetic energy and velocity shear calculated at the three sites show strong near-inertial peaks, with M2 exhibiting significantly larger energy than the other two sites. In particular, the data from M2 show a double peak in the frequency band corresponding to half of the K1 and O1 tidal frequencies, unlike the other two locations. Mixed-layer inertial oscillations, calculated using ECMWF wind data and a slab model, show no significant differences among the three sites. However, M2 shows consistently larger NIW energy throughout the year, suggesting PSI as the dominant mechanism driving NIW variability at this location. NIWs observed for 3 years at M2 show time-varying characteristics. These waves likely triggered by diurnal internal tides from the Luzon Strait, which are known to change their propagation significantly depending on eddy activity in the Philippine Sea. Thus, changes in the propagation characteristics of the diurnal internal tide appear to modulate the temporal evolution of PSI-induced NIWs at M2.