Near-inertial current oscillations off the mid-east coast of Korea

Abstract

Near-inertial current oscillations were observed at depths of 620 and 790 m at a current meter mooring station in water 840 m deep off the mid-east coast of Korea. The oscillatory inertial motions with an amplitude of up to 8 cm/s existed during 73 days between August 27 and November 7 of 1986 with a large temporal variation in speed. Spectral analysis of the data was conducted and the result showed that the most dominant signal in the records was clockwise rotation whose frequency was close to the inertial frequency. The peak frequency of the near-inertial oscillations was slightly greater than the local inertial frequency ( f = 0.0511 cph) by 2–5%. These clockwise oscillations had the mean amplitude of about 2.5 cm/s, reaching an instantaneous maximum of 7.5 cm/s. The mean ratio of the semi-major axis to the semi-minor one was found to be about 1.2 and the mean direction of the major axis was nearly parallel to the isobaths around the mooring station. High near-inertial energy parallel continuously during the first 15 days and then appeared in the form of group wave packets lasting several days during the remaining period. Three prominent events of wave packets were more energetic at the deeper depth. The phase difference between the two horizontal near-inertial current vectors changed largely with time, but it remained relatively constant during each of the three energetic events, changing its sign from one event to another. The horizontal near-inertial current vector rotated clockwise with increasing depth during one event, but counterclockwise during the other two events. The corresponding phase propagation was upward with a phase speed of 2.5 cm/s for the clockwise polarization with depth, but downward with 1.8 and 1.1 cm/s for the counterclockwise rotating events.