[A-OS16] Coastal ocean circulation and material cycle

[AOS16-10] Study on the Kyucho intruded from the west shore of the Bungo Channel in the Seto Inland Sea in 2023

*Naoki Yoshie², Aoto Yamashita¹, Eisuke Tsutsumi³, Xinyu Guo¹, Yasumasa Miyazawa⁴, Toru Miyama⁵ (1.Center for Marine Environmental Studies, Ehime University, 2. Institute for the Promotion of Science and Technology, Ehime University, 3.Faculty of Fisheries, Kagoshima University, 4.Japan Agency for Marine-Earth Science and Technology, 5.Application Laboratory, Japan Agency for Marine-Earth Science and Technology)

Keywords:Oceanic water intrusion, Phytoplankton, Nutrient

The Bungo Channel, located in the western part of the Seto Inland Sea, is strongly influenced by the open ocean due to the intrusion of the Kuroshio Current. The phenomenon of the intrusion of the Kuroshio water at the surface layer of the Bungo Channel is called the Kyucho, and it mainly flows along the eastern shore of the Bungo Channel (this region called the Uwakai). At this time, stagnant coastal water due to the aquacultures of fish along the Uwakai coast is exchanged with clean open ocean water, and the red tide that often occurs during the summer in the Uwakai is terminated. There have been few studies on the Kyucho entering from the western shore of the Bungo Channel, and the effects of the recent increase in torrential rains remain unknown. In this study, we succeeded in observing the Kyucho entering from the western shore of the Bungo Channel, and the effects of the torrential rains in Kyushu are described. The field observations using R/V Isana of CMES of Ehime Univ. were conducted from July 26 to 30, 2023, in the central part of the Uwakai. Observations included water temperature, salinity, dissolved oxygen, Chl.a concentration, and turbidity using the RINKO profiler (JFE Advantech inc.), nitrate concentration using the SUNA v2 optical nitrate sensor (Seabird Elec. inc.), and fluorescence spectrophotometry MultiExciter (JFE Advantech inc.) was used to observe community composition of six phytoplankton groups. We compared the results not only with field observations but also with simulations using the numerical model JCOPE-T DA (JAMSTEC, 3 km horizontal resolution, with tidal current, with data assimilated, with river fluxes due to rainfall) to examine the relevance of the data to the surrounding sea area other than the observation area. The observed Kyucho at the neap tide in the mid-summer was supplied with nutrients from the terrestrial region due to the heavy rainfall in northern Miyazaki Prefecture, and these nutrients were quickly consumed by phytoplankton, resulting in low concentrations. The intrusion of this sufficiently oligotrophic water mass and the associated seawater exchange had terminated the harmful algae bloom of Karenia mikimotoi that had been occurring in the Uwakai.