Flow Intensification by the Superposition of Near-inertial Internal Waves in the Abyssal Japan Sea (East Sea)

Near-inertial internal waves (NIWs) in the abyssal Yamato and Tsushima Basins of the Japan Sea were investigated using data from a moored acoustic Doppler current profiler (ADCP) and single-point current meters. In the Yamato Basin, inertial motions attributed to NIWs were dominant in observations made in May and October 2014, showing significant amplitude variations. An upward propagation of an active NIW packet from 2475 to 950 m was observed during the period from May 12–17, 2014. The NIWs in this event exhibited vertically coherent phases and bottom intensified characters for flows below 2475 m. The time series of the Richardson number showed that the vertical shear due to the bottom intensified flow induced unstable conditions that promoted vertical mixing in the deep layer. A simple examination of vertically propagating NIWs suggested that the observed significant NIWs below 2475 m were formed by the superposition of downward-propagating NIWs, which were excited by a strong wind event and the upward-propagating NIWs that bounced off the seabed. Similar vertically coherent structures and bottom intensified characters to those in the Yamato Basin were recognized in NIWs during the observations in May in the Tsushima Basin. This suggests that the flow intensification near the bottom, owing to the superposition of upward and downward propagating NIWs, is a general phenomenon in the deep sea.