Increasing trend in Japan Sea Throughflow transport
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Increasing trend in Japan Sea Throughflow transport Shinichiro Kida1 · Katsumi Takayama1 · Yoshi N. Sasaki3 · Hiromi Matsuura2 · Naoki Hirose1 Received: 5 May 2020 / Revised: 18 August 2020 / Accepted: 19 August 2020 © The Author(s) 2020
Abstract A long-term increasing trend in the transport of the Japan Sea Throughflow is observed from sea-level differences across the Tsushima Strait. Tidal gauge observations show sea level at Hakata, Japan, increasing at a higher rate than that at Busan, Korea. Numerical modeling results suggest that this increasing trend is forced by a northward shift in the Kuroshio axis. As the Kuroshio axis moves northward, sea level along the southern coast of Japan increases. The signal then propagates anticyclonically along the coast as topographic Rossby waves and Kelvin waves, raising sea level and, thus, increasing transport through the Tsushima Strait. Keywords Tsushima strait · Japan sea throughflow · Long-term trend · Sea-level rise · Kuroshio
1 Introduction The sea surface temperature of the Japan Sea has risen by about 1.3–1.7 °C over the last 100 years, roughly twice as much as the global ocean (Japan Meteorological Agency, State of the ocean climate 2020). Due to its location close to human activities, changes in the water mass properties of the Japan Sea are of vital importance to local climate and fisheries. One crucial element that controls the water mass properties of the Japan Sea, and, therefore, its circulation, is the Japan Sea Throughflow (JSTF; Kida et al. 2016). The Japan Sea is a semi-enclosed sea with connections to the surrounding seas limited to three major straits, the Tsushima, Tsugaru, and Soya Straits (Fig. 1). The JSTF is the flow through these three straits, which carries warm water from the south while the local atmosphere cools the oceanic water (Hirose et al. 1996). The dynamics that govern the long-term trends in JSTF transport provide the basis for understanding how the circulation and water mass properties of the Japan Sea will respond to climate change.
* Shinichiro Kida [email protected]‑u.ac.jp 1
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Japan
2
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Japan
3
Faculty of Science, Hokkaido University, Sapporo, Japan
Observations show that the Tsushima Strait serves as the entry point for the JSTF, whereas the Tsugaru and Soya Straits serve as exits. Because these three straits are only 50–150 m deep, the JSTF is concentrated near the surface (~ 200 m). The Tsushima Strait is a well-observed strait, with an annual average transport of about 2.5 Sv (e.g., Takikawa et al. 2005; Fukudome et al. 2010), which brings warm subtropical surface water of the Kuroshio from the East China Sea to the Japan Sea. Outflow through the Tsugaru Strait to the Pacific is slightly greater than that through the Soya Strait to the Okhotsk Sea, where observations show the transport of about 1.5 Sv (Onishi and Ohtani 1997) and 1.0 Sv
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