Characteristics of near-inertial internal waves observed in the southwestern East/Japan Sea

Long-term moored current measurement at the OceanSITES EC1 station, began in 1996 at 37.33N, 131.45E in a water of deeper than 2200 m. The mooring has been turned around 18 times until September 2015. Although the main purpose of the mooring EC1 is to monitor deep flows, it often carried ADCPs to measure upper currents, which provides an opportunity to observe several features of near-inertial waves (NIWs). Previous studies on NIWs using data obtained mainly from 2002 to 2004 includes the interaction between NIWs and mesoscale eddy, reflection of NIWs in the upper thermostad of the eddy and trapping of NIWs near the base of the eddy center. This study investigates characteristics of NIWs based on data obtained from late March 2011 to June 2012. The mooring captured high NIW energy, which persisted for about 10 days with maximum near-inertial currents over 22 cm/s in October 2011. In this period, wind speed was higher than climatological monthly average value and the mooring site was located on the rim of an anticyclonic mesoscale eddy. With a criterion of NIWs amplitude over 0.1 m/s, the observed NIWs penetrate down to 250 m and their phase lines varies from 0.76f to 1.1f (f: local inertial frequency, 19.79 hours). Calculated propagation speed of the NIWs is about 18 m/day, comparable to those reported in other regions. Temporal variation of NIW energy calculated from a simple damped slab model of the mixed layer is highly correlated with the observation, but the amplitude of the calculated NIW energy is about 2 or 3 times smaller than the observed amplitude. Wavenumber-frequency spectra show that both upward- and downward-propagating clockwise NIWs co-exist with the latter being more energetic. The downward-propagating NIWs energy shows the maximum wavelength of about 100 m at the time of the event and decreased wavelength of about 29 m 5 days after the event. Other NIWs characteristics including their seasonal and long-term variations will be presented.