Vertical mixing in the ocean is an important physical process that brings deep nutrients to the upper layer. The Tokara Strait, where the Kuroshio flows out to the North Pacific from the East China Sea, is known for a mixing hotspot, but the associated vertical nutrient transport has not been well quantified yet, especially in the intermediate layer below 200 m depth. This study involved flow, turbulence, and nitrate observations to quantify the vertical turbulent nitrate flux in the Tokara Strait and discuss its impact on the Kuroshio nutrient transport. Temporal variation of current velocity, derived from a 3.5-day mooring observation, captured semi-diurnal internal waves propagating vertically at depths between 200 and 400 m. Vertical shear associated with the semi-diurnal flow was elevated at this depth range. Vertical shear due to the mean and the diurnal flow was also elevated at about 220 m and 400 m depths, respectively. Mean vertical eddy diffusivity was elevated to O (10 –3 –10 –2 m 2 s −1 ) at the density layers of 25.2–26.4 σ θ (200–450 m depth). Associated vertical turbulent nitrate flux reached O (10 mmol m −2 day −1 ), especially at around 25.6 σ θ (250 m depth) and 26.2–26.4 σ θ (400–450 m depth). The enhanced vertical turbulent nitrate flux suggests a large amount of nitrate is injected vertically to the nutrient stream from the layer just above the North Pacific Intermediate Water and is transported throughout the core of the Kuroshio nutrient stream situated at 25.0–26.2 σ θ .

中文翻译：

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托卡拉海峡黑潮中间层硝酸盐垂直湍流通量增强

海洋中的垂直混合是将深层营养物质带到上层的重要物理过程。黑潮从东海流出北太平洋的东卡拉海峡以混合热点而闻名，但相关的营养物质垂直输运尚未得到很好的量化，尤其是在 200 m 深度以下的中间层。这项研究涉及流动、湍流和硝酸盐观测，以量化 Tokara 海峡的垂直湍流硝酸盐通量，并讨论其对黑潮养分输送的影响。来自 3.5 天系泊观测的电流速度的时间变化捕获了在 200 至 400 m 深度垂直传播的半日内波。在这个深度范围内，与半日流相关的垂直剪切力升高。由于平均和昼夜流导致的垂直切变也分别在约 220 m 和 400 m 深度处升高。在 25.2–26.4 σ θ（200–450 m 深度）的密度层，平均垂直涡流扩散率升高到 O（10 –3 –10 –2 m 2 s -1 ）。相关的垂直湍流硝酸盐通量达到 O（10 mmol m -2 天 -1 ），尤其是在 25.6 σ θ（250 m 深度）和 26.2-26.4 σ θ（400-450 m 深度）附近。增强的垂直湍流硝酸盐通量表明大量硝酸盐从北太平洋中层水正上方的层垂直注入营养流，并通过位于 25.0–26.2 σ θ 的黑潮营养流的核心传输。在 25.2–26.4 σ θ（200–450 m 深度）的密度层，平均垂直涡流扩散率升高到 O（10 –3 –10 –2 m 2 s -1 ）。相关的垂直湍流硝酸盐通量达到 O（10 mmol m -2 天 -1 ），尤其是在 25.6 σ θ（250 m 深度）和 26.2-26.4 σ θ（400-450 m 深度）附近。增强的垂直湍流硝酸盐通量表明大量硝酸盐从北太平洋中层水正上方的层垂直注入营养流，并通过位于 25.0–26.2 σ θ 的黑潮营养流的核心传输。在 25.2–26.4 σ θ（200–450 m 深度）的密度层，平均垂直涡流扩散率升高到 O（10 –3 –10 –2 m 2 s -1 ）。相关的垂直湍流硝酸盐通量达到 O（10 mmol m -2 天 -1 ），尤其是在 25.6 σ θ（250 m 深度）和 26.2-26.4 σ θ（400-450 m 深度）附近。增强的垂直湍流硝酸盐通量表明大量硝酸盐从北太平洋中层水正上方的层垂直注入营养流，并通过位于 25.0–26.2 σ θ 的黑潮营养流的核心传输。