Observation of vertically propagating near-inertial internal waves into the Japan Sea proper water

Akie Sakai1, Tomoharu Senjyu2, Kosuke Mori3 and Takahiro Endoh2, (1)Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan, (2)Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan, (3)Okinawa Institute of Science and Technology Graduate University, Onna-son, Japan
The layer below the main thermocline of the Japan Sea is occupied by a water mass with very narrow temperature and salinity ranges, the Japan Sea Proper Water (JSPW). Near-inertial fluctuations in the flows attributed to near-inertial internal waves (NIWs) have been observed in the abyssal Japan Sea.

The main energy source of NIWs in the Japan Sea is considered to be wind over the sea. To investigate how the wind energies are transmitted to the deep layer from the upper ocean, we carried out the 24 hours continuous observations, using a shipboard ADCP (75 kHz) and a microstructure profiler (VMP-500) in the southeastern basin (the Yamato Basin) in October 2018.

Near-inertial flows over 0.5 m s-1were observed in the upper 50 m layer, showing a clockwise change of the flow direction with a near-inertial period. Although the amplitude of the flows abruptly weakened below the thermocline at about 100 m, flows variations with near-inertial period were recognized at 500 m. A downward energy propagation was suggested by the observed upward phase propagation in the flows, which suggests a downward propagation of a wind-excited NIWs. Significant turbulent energy dissipations sporadically occurred below the thermocline, as well as in the upper layer. A correlation between the significant energy dissipations and vertical shear under the main thermocline suggested a mixing due to NIWs in the Japan Sea Proper Water.