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

[摘要] Based on absolute geostrophic velocity, which was calculated using repeatedhydrographic data of 39 cruises from 2000 to 2009 and nitrate concentrationsmeasured in the same areas from 1964 to 2009, we obtained the temporallyaveraged nitrate flux (the product of velocity and nitrate concentration) andnitrate transport (integration of flux over one section) of four sectionsacross the Kuroshio from the East China Sea (sections PN and TK) to an areasouth of Japan (sections ASUKA and 137E). In addition, we examined section OKeast of the Ryukyu Islands in order to understand how the Ryukyu Currentcontributes to the transport of nutrients by the Kuroshio south of Japan. Themean nitrate flux shows a subsurface maximum core with values of 9.6, 10.6,11.2, 10.5, and 5.7 mol m⁻² s⁻¹ at sections PN, TK, ASUKA, 137E,and OK, respectively. The depth of the subsurface maximum core changes amongthese five sections and is approximately 400, 500, 500, 400, and 800 m atsections PN, TK, ASUKA, 137E, and OK, respectively. The mean downstreamnitrate transport is 204.8, 165.8, 879.3, 1230.4, and 338.6 kmol s⁻¹at sections PN, TK, ASUKA, 137E, and OK, respectively. The transport ofnutrients in these sections suggests the presence of the Kuroshio nutrientstream from its upstream to downstream regions. The deep current structure ofthe Ryukyu Current (section OK) contributes to the same order of nitratetransport as does the Kuroshio from the East China Sea (section TK) to thearea south of Japan; however, the former only has one-fifth the volumetransport of the latter. A budget calculation suggests that the downstreamincrease of transported nitrate along the Kuroshio is mainly caused by therecirculation of nitrate into the Kuroshio. This conclusion, however, dependson water depth. In the upper layers (< 26.5σ_θ), thedownstream change of nitrate concentration along the Kuroshio and that fromthe recirculation of nitrate has a significant contribution to the downstreamincrease of nitrate transport along the Kuroshio. In the deep layers(> 26.5σ_θ), the change in nitrate concentration is smalland the Kuroshio recirculation dominates the downstream increase of nitratetransport.

[学科分类] 地球化学与岩石