Nutrient supply system from the Pacific Ocean to coastal seas; Understanding of bottom intrusion in Bungo Channel

2023 Fiscal Year Final Research Report

• Project/Area Number: 20H01972
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 17020:Atmospheric and hydrospheric sciences-related
• Research Institution: Ehime University
• Principal Investigator: Morimoto Akihiko 愛媛大学, 沿岸環境科学研究センター, 教授 (80301323)
• Co-Investigator(Kenkyū-buntansha): 郭 新宇 愛媛大学, 沿岸環境科学研究センター, 教授 (10322273)
• Project Period (FY): 2020-04-01 – 2024-03-31
• Keywords: 栄養塩供給 / 豊後水道 / 外洋起源栄養塩

Outline of Final Research Achievements

This study used mooring and ship observations to clarify the mechanism of “bottom intrusion”, which occur in the Bungo Channel, located between Kyushu and Shikoku. Bottom intrusion is a phenomenon that low-temperature, high-nutrient water masses around the continental shelf slope suddenly enter the bottom layer of the Bungo Channel mainly during the summer. It was revealed that the five bottom intrusions that occurred in 2021 were caused by a large north-south pressure gradient in the bottom layer due to a decrease in density in the Bungo Channel caused by the intrusion of Kuroshio surface water to the Bungo Channel just before the bottom intrusion occurred. In addition, a numerical model revealed that nutrients supplied to the Bungo Channel by bottom intrusion and other events were transported to the center of the Seto Inland Sea, and that approximately 60% of the nutrients in the Seto Inland Sea were supplied from the Pacific Ocean.

Free Research Field

沿岸海洋学

Academic Significance and Societal Importance of the Research Achievements

豊後水道南部の陸棚斜面底層の低温で高栄養な水塊が突発的に豊後水道へと進入する「底入り潮」の発生メカニズムを観測データにより明らかにした。底入り潮は、その発生の直前に起こった表層での黒潮系暖水の流入により豊後水道内の密度が低下することで発生していた。底入り潮により豊後水道へと供給された栄養塩は瀬戸内海の広域へと輸送されており、太平洋から供給された栄養塩は瀬戸内海の栄養塩の約60%を占めている。したがって、瀬戸内海の栄養塩および生物生産は外洋からの栄養塩供給の影響を強く受けていることが分かった。