Near-inertial internal waves (NIWs) in the abyssal Yamato and Tsushima Basins of the Japan Sea (East Sea) were investigated using data from a moored acoustic Doppler current profiler (ADCP) and single-point current meters. The NIW events with duration of 3–5 days were observed intermittently in both basins. In particular, an active NIW event occurred below 2,475 m in the Yamato Basin during May 12–16, 2014. This was followed by the upward propagation of a wave packet from 2,475 to 950 m. The near-inertial flows (1.07f) during the event exhibited a vertically coherent phase and intensification with depth by a factor of 1.5 from 2,475 to 2,635 m. The near-inertial flows (1.05f) in the Tsushima Basin also showed a vertically coherent phase and intensification with depth, although the amplitude of the flow exhibited a middepth maximum rather than monotonic intensification. As a possible explanation of the observed flow characteristics, the superposition of downward-propagating NIWs that can be excited by a strong wind event and upward-propagating NIWs that bounced off the seabed was examined. The time series of the Richardson number based on the observed shear showed sporadic unstable conditions during the initial stage of the NIW event during May 2014 in the Yamato Basin. In addition, relatively small values of Richardson number were observed over the range of 2,475–2,635 m during the period of active NIWs. This suggests the promotion of vertical mixing in the deep sea during significant NIW events.
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