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 equator-ward. The Oyashio nutrient stream was assessed using long-term mean hydrographic observation data. Its nitrate transport (about 350 kmol s-1) 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 (MWR) is about 110 kmol s-1. This corresponds to the nitrate transport of the Oyashio to the North Pacific Intermediate Water in the MWR. Along with this process, the subarctic gyre nitrate export to the subtropic gyre reaches 24 kmol s-1. This work first quantifies the Oyashio nutrient transport and its intrusion into the MWR and suggests its importance in local/basin scale nutrient cycles.

Reference:Long, Y., Zhu, X.-H., & Guo, X. (2019). The Oyashio Nutrient Stream and its nutrient transport to the mixed water region. Geophysical Research Letters, DOI: 10.1029/2018GL081497.

Figure 1. (a) A global view of the northwest Pacific. (b) Nitrate concentration on 26.8 σθ (color shading). The blue dots denote the hydrographic stations used in the velocity calculation, and the thick, black line is the JMA-line. The magenta boxes indicate the region where we used CTD data to obtain the typical T-S relation of Kuroshio and Oyashio. (c) Mean value (thick magenta line along the section) and standard deviation (thick magenta line normal to the section) of nitrate transport per unit length across the section, surface geostrophic velocity (black arrows) and its magnitude (color shading). The black dots and thin line through the dots represent the hydrographic stations and the reference of zero. The white numbers (5, 10, 17, 22) denote the numbering of the station from north.