The Oyashio Nutrient Stream and Its Nutrient Transport to the Mixed Water Region.

The Oyashio, a subarctic western boundary current in the North Pacific, transports cool, fresh, and nutrient‐rich water equatorward. The Oyashio nutrient stream was assessed using long‐term mean hydrographic observation data. Its nitrate transport (about 350 kmol/s) is comparable to that of the Kuroshio but has a much shallower subsurface core at 250‐m depth. In the layer ranging from 26.6 to 27.4 σθ, the Oyashio nitrate transport to the mixed water region is about 110 kmol/s. This corresponds to the nitrate transport of the Oyashio to the North Pacific Intermediate Water in the mixed water region. Along with this process, the subarctic gyre nitrate export to the subtropic gyre reaches 24 kmol/s. This work first quantifies the Oyashio nutrient transport and its intrusion into the mixed water region and suggests its importance in local/basin‐scale nutrient cycles. Plain Language Summary: The Oyashio carries cool, fresh, and nutrient‐rich water and is significant in local/basin‐scale nutrient cycles. This study confirmed the presence of a nitrate flux subsurface maximum in the Oyashio, which fits the concept of nutrient stream. The Oyashio nutrient stream acts as a subsurface giant conduit that continuously transports subarctic water with a high nutrient concentration equatorward. This nutrient transport plays an important part in the formation of the North Pacific Intermediate Water and nutrient export from the subarctic gyre to the subtropical gyre. Key Points: Oyashio nitrate transport into the North Pacific Intermediate Water in the mixed water region is approximately 110 kmol/sEffective nitrate transport from the subarctic gyre to the subtropic gyre along with NPIW formation is approximately 24 kmol/sOyashio is comparable to the Kuroshio in nutrient transport but has a shallower subsurface maximum of nitrate flux at 250‐m depth [ABSTRACT FROM AUTHOR]