Spatial variations in the Kuroshio nutrient transport from the East China Sea to south of Japan

Based on absolute geostrophic velocity calculated from repeated hydrographic data of 39 cruises from 2000 to 2009 and nitrate concentrations measured at the same sections from 1964 to 2011, we obtained temporally averaged nitrate flux (the product of velocity and nitrate concentration) and nitrate transport (integration of flux over a section) through 4 sections along the Kuroshio path from the East China Sea (sections PN and TK) to south of Japan (sections ASUKA and 137E). In addition, we examined section OK east of the Ryukyu Islands in order to understand the contribution of Ryukyu Current to the Kuroshio nutrient transport south of Japan. The mean nitrate flux shows a subsurface maximum core with a value of 10, 10, 11, 11, and 6 mol m–2 s–1 at sections PN, TK, ASUKA, 137E, and OK, respectively. The depth of subsurface maximum core changes among five sections and is approximately 400, 500, 500, 400, and 800 m at sections PN, TK, ASUKA, 137E, and OK respectively. The mean downstream nitrate transport is 199.3, 176.3, 909.2, 1385.5, and 341.2 kmol m–1 at sections PN, TK, ASUKA, 137E, and OK respectively. The nutrient transports at these sections suggest the presence of Kuroshio nutrient stream from its upstream region to downstream. The deep current structure of Ryukyu Current (section OK) makes it contribute more nitrate transport than the Kuroshio in the East China Sea (section TK) to the Kuroshio south of Japan. In addition, the positive difference between the downstream nitrate transport through section ASUKA and the sum of nitrate transports through sections TK and OK, as well as the positive difference of downstream nitrate transport between sections 137E and ASUKA, suggest that the Kuroshio recirculation significantly intensifies the downstream (eastward) nitrate transport by the Kuroshio.