How a Small Reef in the Kuroshio Cultivates the Ocean

• Hasegawa, Daisuke

  Fisheries Resources Institute, Japan Fisheries Research and Education Agency (FRA)

• Matsuno, Takeshi

  Research Institute for Applied Mechanics, Kyushu University

• Tsutsumi, Eisuke

  Atmosphere and Ocean Research Institute, The University of Tokyo

• Senjyu, Tomoharu

  Research Institute for Applied Mechanics, Kyushu University

• Endoh, Takahiro

  Research Institute for Applied Mechanics, Kyushu University

• Tanaka, Takahiro

  Fisheries Resources Institute, Japan Fisheries Research and Education Agency (FRA)

• Yoshie, Naoki

  Center for Marine Environmental Studies, Ehime University

• Nakamura, Hirohiko

  Faculty of Fisheries, Kagoshima University

• Nishina, Ayako

  Faculty of Fisheries, Kagoshima University

• Kobari, Toru

  Faculty of Fisheries, Kagoshima University

• Nagai, Takeyoshi

  Department of Ocean Sciences, Tokyo University of Marine Science and Technology

• Guo, Xinyu

  Center for Marine Environmental Studies, Ehime University

Abstract

[Abstract] / Vertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin-Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo-sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10^−1 m^2 s^−1), resulting in strong nitrate fluxes of O(1–10^3 mmol m^−2 day^−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10^−3 m s^−1), as well as an upwelling nitrate flux of O(10^2 mmol m^−2 day^−1) in the entire subsurface layer. / 　 / [Plain Language Summary] / Vertical nitrate fluxes associated with flow-topography interactions around a small reef in the Kuroshio Current are quantified using state-of-the-art oceanographic instruments. When the flow passes over a shallow sill on the flank of the reef, the velocity differences between layers intensify, resulting in a substantial vertical overturning and mixing of the water column. This turbulent mixing causes the observed nitrate flux from deep water to reach a value among the highest observed worldwide. / 　 / [Key Points] / • Flow separations and trains of Kelvin-Helmholtz billows mix the water column around a small reef in the Kuroshio / • Doming of isopycnals/nitraclines suggests strong upwelling in the lee of the reef / • Turbulent nitrate fluxes reach up to O(10^3 mmol m^−2 day^−1)

Journal

• Geophysical Research Letters

  Geophysical Research Letters 48 (7), e2020GL092063-, 2021-03-25

  Wiley

Keywords

• flow separation
• Kelvin Helmholtz instability
• nitrate flux
• the Kuroshio Current
• turbulent mixing
• upwelling

Details

• CRID: 1050017057727339904
• NII Article ID: 120007027029
• NII Book ID: AA00657102
• ISSN: 19448007
• ISSN: 00948276
• HANDLE: 2324/4377879
• Text Lang: en
• Article Type: journal article
• Data Source:
  • IRDB
  • CiNii Articles