Near-inertial oscillations observed by one-year abyssal mooring in Main Gap of the Emperor Seamount Chain

To reveal deep current flowing through the deepest, widest gap in the Emperor Seamount Chain, called “Main Gap”, mooring observation was carried out at 39° 20′ N, 169° 56′E (5895 m) from 10 June 2016 to 25 June 2017. Spectral analysis shows that a prominent peak was found near the inertial frequency (9.24×10^-5 rad s^-1) as well as diurnal and semidiurnal tides at 5445-meter and 5855-meter depths. Rotary spectra show that this peak is clockwisely-rotating oscillation. Energy spectrum seems to cascade down from the inertial frequency to the buoyancy frequency (1.93×10^-4 rad s^-1) except for semidiurnal tides at 5445-meter depth. Although energy peak at the vicinity of the inertial frequency is much higher, this cascading down resembles to the Garrett and Munk model spectrum for internal waves in the version of Munk (1981). Band-passed current speed focused on the variation around the inertial frequency became the lowest during the period from July to October 2016, the highest during the period from November 2016 to January 2017, and decreased toward June 2017. This temporal variation resembles the time series of 10-m wind speed above the mooring site according to the ERA5 dataset. Wind speed on the spot may cause the abyssal near-inertial oscillation though the mechanism should be clarified.