Steady nutrient upwelling around a biological hotspot of the confluence between the quasi-stationary jet and the Oyashio in the western North Pacific

• Published in: Scientific reports 2024-07, Vol.14 (1), p.17578-12, Article 17578
• Main Authors: Yabe, Itsuka, Ito, Shin-ichi, Kakehi, Shigeho, Nagai, Takeyoshi, Nishioka, Jun
• Format: Article
• Language: English
• Subjects: 704/172; 704/829; Cold; Euphotic zone; Humanities and Social Sciences; Hydrography; Intermediate water; multidisciplinary; Nitrates; Nutrients; Ocean currents; Saline water; Salinity; Science; Science (multidisciplinary); Upwelling; Velocity; Water; Water mixing
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-024-68214-z

The quasi-stationary jet, a branch of the Kuroshio Extension, transports warm saline water in the mixed water region of the western North Pacific. Around the subarctic front between the quasi-stationary jet and Oyashio and its downstream area is a biologically productive area including small pelagic fishes. However, how nutrient is supplied to the euphotic zone in this region remains elusive, especially into the quasi-stationary jet. Using high-resolution hydrography sections across the jet, we showed that Oyashio water isopycnally intrudes under the jet around 26.5–26.8 σ θ and forms nutrient-rich intermediate water. Upwelling associated with ageostrophic secondary circulation across the front, caused by confluence, uplifts the intermediate water. A local nitrate maximum was also identified inside the jet by the hydrographic observation. Upwelling has been suggested as a precondition for nutrient supply from nutrient-rich intermediate water to the jet through water mixing which potentially sustains high biological production in the downstream.