Long-term decrease in the M2 tide on the west coast of the Kyushu Island and its relation to changes in the tidal amplification of the Bohai, Yellow and East China Seas

2020 Fiscal Year Final Research Report

• Project/Area Number: 18K13613
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17020:Atmospheric and hydrospheric sciences-related
• Research Institution: The University of Tokyo (2019-2020); Kyushu University (2018)
• Principal Investigator: Tsutsumi Eisuke 東京大学, 大気海洋研究所, 特任助教 (70635846)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: M2潮位振幅 / 東シナ海 / 湾共鳴 / 内部潮汐 / 成層 / 黒潮 / 北西太平洋 / ルソン海峡

Outline of Final Research Achievements

Semi-diurnal tide along the west coast of the Kyushu Island, Japan has been decreasing in recent years, especially in the Ariake Sea. The mechanism and cause of the decreasing tide have been unknown. In this study, I conducted tide gauge data analysis and numerical model experiments using a three-dimensional general circulation model, and found that in the Ariake Sea about 90% of the decrease originated from that in the East China Sea, and 10% originated from the combined effect of the weakening of the bay-resonance due to the coastline modification and the strengthening of the resonance due to the weakening of the frictional drag of the seabed within the Ariake Sea. The results of numerical model experiments suggested that stratification changes over the northwestern Pacific region may have affected the changes in tidal amplitude along the east coast of the East China Sea through internal-tide generation and propagation, but the definitive cause of the changes could not be identified.

Free Research Field

海洋物理学

Academic Significance and Societal Importance of the Research Achievements

本研究では東シナ海沿岸の半日周期潮汐について長期変化の実態を示し、特に有明海内部の変化については力学的な解釈を伴った定量的な評価を初めて示した。潮汐の変化について、湾の地形変化などのローカルな要因と縁辺海や海盆スケールの要因を分離して評価することは現代海洋潮汐研究の課題として挙げられており、当該研究分野の進展に寄与する成果だと考えられる。また有明海の環境変化に関する一連の研究において、湾共鳴の力学とその長期変化を明らかにした点、海洋環境を支配する物理的要因である鉛直混合の長期変動の評価につながる海底摩擦に関する結果を示した点は当該研究分野の進展に貢献する成果だと考えている。