Presentation Information
[AOS27-P03]Processes of Kuroshio intermediate-water supply to the bottom layer of the continental shelf off the Tone River estuary
*Eisuke Tsutsumi1, Sachihiko Itoh2, Anne Takahashi3, Daigo Yanagimoto3, Eiji Masunaga4, Shogo Urakawa5 (1.Faculty of Fisheries, Kagoshima University, 2.Graduate School of Frontier Sciences, The University of Tokyo, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Ibaraki University, 5.Meteorological Research Institute, Japan Meteorological Agency)
Keywords:
cross-shelf exchange,internal tidal bore,Kuroshio intermediate water
The continental shelf adjacent to the Japanese archipelago is characterized by high biological productivity. One of the nutrient sources supporting this productivity is riverine water supplied from land. Offshore from the Japanese archipelago, westerly boundary currents flow, including the Kuroshio, Tsugaru Warm Current, and Oyashio. The intermediate layer of the Kuroshio contains nutrient-rich subtropical mode water and serves as a nutrient source for coastal areas. This study focuses on the continental shelf off the Tone River estuary. In September 2023, we conducted a scientific survey aboard the R/V Shinsei Maru. The survey documented the development of a subsurface chlorophyll maximum over the continental shelf. Analyses of water mass and nitrate distributions suggested that subtropical mode water from the Kuroshio flowing offshore may be a source of nutrients supporting the primary production responsible for this chlorophyll maximum. However, the process by which intermediate water is supplied to the continental shelf is unclear. This study examined the processes responsible for this supply by analyzing mooring observations deployed on the continental shelf during the cruise.
From September 18 to November 4, 2023, we measured current profiles and bottom water temperature using a bottom-mounted acoustic Doppler current profiler (ADCP) at Station MB (water depth 103.5 m), located approximately 10 miles offshore of the Tone River estuary. We calculated temperature flux as an indicator of water transport into the bottom layer and analyzed it by separating it into sub-inertial and higher frequency components.
One notable feature of the observational results was the pronounced semidiurnal tidal variation in current velocity and bottom water temperature. From October onward, internal-tide bores were frequently observed in the bottom layer, exhibiting steep variations in current velocity variation and bottom water temperature. Additionally, sub-inertial current velocity variations were detected, although their relationship with the Kuroshio flowing offshore was not clear. Regarding the bottom temperature flux, the sub-inertial component tended to vary on a timescale of about one week. In contrast, the temperature flux associated with fluctuations on shorter timescales than the sub-inertial component, primarily caused by the semi-diurnal tidal currents, showed a tendency to increase after October. The trends of the two types of fluxes were distinctly different. The time-averaged temperature flux associated with the sub-inertial flow was 0.016 °C m s-1 in a northeast-northeast direction. This flux arose from the positive correlation between current velocity and water temperature, suggesting that warm water was being transported downstream by disturbances such as Kuroshio frontal waves. In contrast, the temperature flux associated with tidal variations was approximately 90 degrees different in direction, pointing southeast at 0.047 °C m s-1, exhibiting a magnitude about three times that of the sub-inertial component. The temperature flux induced by the higher-frequency component was generated by the negative correlation between semidiurnal current velocity and water temperature, interpreted as cold water in the offshore intermediate layer being transported toward the shelf by the internal tidal bores.
From September 18 to November 4, 2023, we measured current profiles and bottom water temperature using a bottom-mounted acoustic Doppler current profiler (ADCP) at Station MB (water depth 103.5 m), located approximately 10 miles offshore of the Tone River estuary. We calculated temperature flux as an indicator of water transport into the bottom layer and analyzed it by separating it into sub-inertial and higher frequency components.
One notable feature of the observational results was the pronounced semidiurnal tidal variation in current velocity and bottom water temperature. From October onward, internal-tide bores were frequently observed in the bottom layer, exhibiting steep variations in current velocity variation and bottom water temperature. Additionally, sub-inertial current velocity variations were detected, although their relationship with the Kuroshio flowing offshore was not clear. Regarding the bottom temperature flux, the sub-inertial component tended to vary on a timescale of about one week. In contrast, the temperature flux associated with fluctuations on shorter timescales than the sub-inertial component, primarily caused by the semi-diurnal tidal currents, showed a tendency to increase after October. The trends of the two types of fluxes were distinctly different. The time-averaged temperature flux associated with the sub-inertial flow was 0.016 °C m s-1 in a northeast-northeast direction. This flux arose from the positive correlation between current velocity and water temperature, suggesting that warm water was being transported downstream by disturbances such as Kuroshio frontal waves. In contrast, the temperature flux associated with tidal variations was approximately 90 degrees different in direction, pointing southeast at 0.047 °C m s-1, exhibiting a magnitude about three times that of the sub-inertial component. The temperature flux induced by the higher-frequency component was generated by the negative correlation between semidiurnal current velocity and water temperature, interpreted as cold water in the offshore intermediate layer being transported toward the shelf by the internal tidal bores.