2:45 PM - 3:00 PM
[AOS21-05] Observation of the occasional intrusion of Kuroshio water (Kyucho) and turbulent mixing off Shitaba Bay, Uwajima, in the summer of 2024
Keywords:Occasional intrusion of Kuroshio water (Kyucho), Tidal mixing, Bungo Channel, Phytoplankton, nutrients
In the Bungo Channel, Kyucho, an occasional intrusion of warm water from the Kuroshio region, occurs in summer, affecting coastal ecosystems and local fisheries. It is known that tidal mixing in the archipelago area along the coast of the Bungo Channel is closely related to the occurrence of Kyusho. In this study, we conducted a field campaign to quantitatively evaluate vertical mixing in the Bungo Channel and to clarify the relationship between the occurrence of Kyucho and associated changes in the physical and biogeochemical water properties.
The field campaign was conducted in the waters off Shitaba Bay, Uwajima, using the research training ship ISANA of the CMES, Ehime University, from August 1 to 8, 2024. A turbulence microstructure profiler TurboMAP-5 was used to measure water temperature, salinity, chlorophyll fluorescence, turbidity, and turbulence kinetic energy dissipation rate. At a reference station, water temperature, salinity, chlorophyll, turbidity, and dissolved oxygen were measured with the RINKO-Profiler, and nitrate concentration was measured with the SUNA-V2.
During the observation period from about August 4 (spring tide) to August 6, the surface water temperature in the observation area increased by about 3℃, indicating the occurrence of Kyucho. The coastal water temperature (You see U-Sea: https://akashio.jp/kaisuion/) and the Himawari-9 satellite sea surface temperature indicate that warm water from the Kuroshio region south of Bungo Channel intruded in the observation area. The northern limit of the intrusion was around the archipelago area around the observation site, and no rapid increase in water temperature was observed in the north. This is likely due to the intrusion was prevented by the considerable vertical mixing with vertical eddy diffusivity of 1 m2 s-1 associated with the island wake in the archipelago area. Chlorophyll fluorescence and nitrate decreased with this warm water intrusion. This is consistent with previous findings for Kyucho, which is oligotrophic and low in phytoplankton. However, the surface salinity also decreased by about 0.5 psu with the Kyucho, which cannot be explained simply by the advection of saline water from the Kuroshio region. A possible reason for this is that the Kuroshio water was transported to offshore Uwajima while being transformed by mixing with coastal water derived from rivers in Kyushu and Shikoku Islands.
The field campaign was conducted in the waters off Shitaba Bay, Uwajima, using the research training ship ISANA of the CMES, Ehime University, from August 1 to 8, 2024. A turbulence microstructure profiler TurboMAP-5 was used to measure water temperature, salinity, chlorophyll fluorescence, turbidity, and turbulence kinetic energy dissipation rate. At a reference station, water temperature, salinity, chlorophyll, turbidity, and dissolved oxygen were measured with the RINKO-Profiler, and nitrate concentration was measured with the SUNA-V2.
During the observation period from about August 4 (spring tide) to August 6, the surface water temperature in the observation area increased by about 3℃, indicating the occurrence of Kyucho. The coastal water temperature (You see U-Sea: https://akashio.jp/kaisuion/) and the Himawari-9 satellite sea surface temperature indicate that warm water from the Kuroshio region south of Bungo Channel intruded in the observation area. The northern limit of the intrusion was around the archipelago area around the observation site, and no rapid increase in water temperature was observed in the north. This is likely due to the intrusion was prevented by the considerable vertical mixing with vertical eddy diffusivity of 1 m2 s-1 associated with the island wake in the archipelago area. Chlorophyll fluorescence and nitrate decreased with this warm water intrusion. This is consistent with previous findings for Kyucho, which is oligotrophic and low in phytoplankton. However, the surface salinity also decreased by about 0.5 psu with the Kyucho, which cannot be explained simply by the advection of saline water from the Kuroshio region. A possible reason for this is that the Kuroshio water was transported to offshore Uwajima while being transformed by mixing with coastal water derived from rivers in Kyushu and Shikoku Islands.