[A-OS12] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS12-P08] Observational evidence of nutrient supply from the Oyashio water into the Quasi-Stationary Jet in the Kuroshio-Oyashio mixed water region

*Itsuka Yabe¹, Shin-ichi Ito¹, Shigeho Kakehi², Takeyoshi Nagai³ (1.Atmosphere and Ocean Research Institudte, The University of Tokyo, 2.Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3.Department of Ocean Sciences, Tokyo University of Marine Science and Technology)

Keywords:the Kuroshio-Oyashio mixed water region, Isoguchi Jet, isopycnal mixing, nutrient

The Kuroshio-Oyashio mixed water region (MWR) in the northwestern Pacific is a suitable feeding ground for marine fish species such as the Pacific saury (Cololabis saira). They move to the MWR from their spawning grounds by their own swimming and advection due to the ocean current, e.g., Kuroshio and Kuroshio Extension. Quasi-stationary jet (QSJ) is a northeastward branch of the Kuroshio Extension and carries eggs and larvae to the MWR. The Kuroshio water initially occupies the QSJ, which gradually mixes with the adjacent Oyashio water. Nutrient concentration of the jet increases according to the mixing. Observations were carried out around the QSJ to elucidate the mixing mechanism of the Oyashio water during research cruise KH17-09 in August 2017 by R/V Shinsei-Maru (JAMSTEC). We set multiple sections across the jet to measure the vertical structures of hydrography and current, and their variations. The QSJ meandered along multiple anticyclonic vortices. Current magnitude at the flow axis exceeds 0.3 m s^-1 near the surface, and the maximum value was approximately 0.7 m s^-1 on the most downstream section at 40.1°N, 152.5°E. Northeastward current over 0.1 m s^-1 existed from the surface to about the depth of 600 m. Mixing ratio between the Kuroshio and the Oyashio indicates the intrusion of the Oyashio water along the 26.5–2.68 σ_θ isopycnal. Nitrate concentration was increased in two layers, i.e., upper layer of the intrusion and the deeper layer of the main current axis of the QSJ.