<学術雑誌論文>
Flow Intensification Due to the Superposition of Near-Inertial Internal Waves in the Abyssal Yamato and Tsushima Basins of the Japan Sea (East Sea)

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概要 [Abstract]
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.
 
[Plain Language Summary]
The Japan Sea, a semi-closed marginal sea in East Asia, has its own circulation system driven by the deep water formation. We carried out deep flow observations in the sea 4 times using moored acoustic current meters. The current meters recorded strong flow events accompanying clockwise-rotating cyclic flows of 18–20 h periods. The cyclic flow, called the nearinertial flow, is associated with internal waves that are affected by the Earth rotation. The amplitude of near-inertial flow during the events varied vertically, although the direction of the flow was consistent throughout the observational depths. We tried to explain the observed flow intensification with depth by the superposition of downward-propagating internal wave that can be excited by a strong wind and upward-propagating internal wave that bounced off the seabed. In addition, we suggested that the intensification of near-inertial flow during the events promoted vertical mixing in the deep sea which controls pattern and strength of the deep circulation.
 
[Key Points]
• Near-inertial flow events with 3–5 days duration were observed in the abyssal Japan Sea using moored current meters
• Near-inertial flows during the events exhibited a vertically coherent phase and varying amplitude with depth
• Superposition of near-inertial internal waves was suggested as a cause of the deep flow intensification that promotes vertical mixing
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登録日 2021.04.21
更新日 2021.04.21

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