Contribution of Kuroshio Recirculation to Nutrient Transport Along the Kuroshio South of Japan

As the Kuroshio flows from low latitude to midlatitude regions, it carries not only heat but also many dissolved materials. Nutrients are one of the important materials with horizontal nitrate transport reported to be on the order of 100–200 kmol s−1 in the East China Sea (Chen et al., 1994, 1995, 2017; Guo et al., 2012, 2013) and, downstream, higher nitrate transport is reported to be on the order of 800–1300 kmol s−1 in the Kuroshio south of Japan. The joining of the Ryukyu Current into the Kuroshio south of Japan is partly responsible for this increase because the Ryukyu Current carries nitrate with horizontal transport on the order of 300 kmol s−1 (Guo et al., 2013). Another process is the joining of the Shikoku Basin local recirculation (Nagano et al., 2013) (hereafter called the Kuroshio recirculation) into the Kuroshio, which is estimated to make a large contribution to the downstream increase of horizontal nitrate transport within the Kuroshio between the two sections across the Kuroshio south of Japan. The question is whether intensification of the downstream nitrate transport within the Kuroshio by the Kuroshio recirculation is applicable over the entire area of the Kuroshio south of Japan. In the case that the Kuroshio recirculation joins into the Kuroshio main stream, it acts to increase the horizontal nitrate transport within the Kuroshio. However, in the case that the Kuroshio recirculation departs from the Kuroshio main stream, such intensification is unlikely from the viewpoint of water budget. Currently available estimations by Guo et al. (2013) are based on observations limited to the area west of 137°E. To clarify the contribution of the Kuroshio recirculation on downstream nitrate transport within the Kuroshio main stream, we use the results of a biogeochemical model that covers the entire area of the Kuroshio south AbstrAct