[A-CG33] Extratropical oceans and atmosphere

[ACG33-12] Increasing trend in the Japan Sea Throughflow Transport

★Invited Papers

*Shinichiro Kida¹, Katsumi Takayama¹, Yoshi N Sasaki², Hiromi Matsuura³, Naoki Hirose¹ (1.Research Institute for Applied Mechanics, Kyushu University, 2.Earth and Planetary Dynamics, Faculty of Science, Hokkaido University, 3.Interdisciplinary Graduate School of Engineering Sciences, Kyushu University)

Keywords:Tsushima Strait, Strait exchange flows, Kuroshio

The mechanism responsible for the increase in the Japan Sea Throughflow during the past two decades is investigated using observational data and numerical model outputs. The Japan Sea is a semi-enclosed sea located in the western Northern Pacific, connected to the Pacific through three narrow straits, Tsushima, Tsugaru, and Soya Straits. The Japan Sea throughflow is the flow through these straits with an inflow from the south (the Tsushima Strait) and outflow to the north (the Tsugaru and Soya Straits). This throughflow bifurcates from the Kuroshio and is considered an important heat source for the Japan Sea.
Observations show that the Japan Sea throughflow transport has an increasing trend from 1997 to 2017. This increase appears to be induced by the northward shift in the Kuroshio axis. Tidal gauge observations show an increase in sea surface height (SSH) gradient across the Tsushima Strait with the Japanese coast (Fukuoka) increasing at a higher rate than the Korean coast (Busan). This sea-level increase along the Japanese coast originates from the southern coast of Japan, and numerical models and satellite observations suggest that this signal is connected to the SSH rise caused by the northern shift in the Kuroshio axis. As the Kuroshio moves north, it excites topographic Rossby waves and Kelvin waves, which then propagate to the Tsushima Strait in the clockwise direction. An increase in the sea level gradient thus occurred across the Tsushima Strait and increased the throughflow transport.