How a Small Reef in the Kuroshio Cultivates the Ocean

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 m2 s−1), resulting in strong nitrate fluxes of O(1–103 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(102 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(103 mmol m−2 day−1)