Vertical turbulent nitrate fluxes were estimated in the western North Pacific from direct measurements of vertical turbulent mixing and vertically continuous nitrate profiles during the summer of 2008. We made three north–south transects that covered the area from the subarctic to the subtropics including a section along the Emperor Sea Mounts. Subsurface fluxes generally showed an increasing trend with increasing vertical gradient of nitrate from oligotrophic subtropical to non-oligotrophic subarctic waters. Enhanced fluxes [ O (10 −6 ) mmol m −2 s −1 ] due to elevated mixing [vertical diffusivity: O (10 −5 ) m 2 s −1 ] were observed, especially over the Emperor Sea Mounts. It is suggested that the internal tide generated by the topography enhanced the vertical mixing. In other subarctic areas, the fluxes were estimated as O (10 −7 ) mmol m −2 s −1 . The same order of fluxes was also found in the frontal area between the subarctic and subtropical gyres, the Kuroshio–Oyashio Transition Area. Enhancement of fluxes in the frontal area, including the Kuroshio Extension, was also observed at mid-depth regions, and their vertical divergence suggested nitrate transport from North Pacific Intermediate Water to lighter densities. In the frontal areas, the enhancement of turbulence is caused by the surface wind rather than the internal tide. In contrast, in the subtropical regions, subsurface fluxes were estimated as O (10 −8 ) mmol m −2 s −1 owing to the small nitrate gradient even where diffusivity was enhanced. In these regions, enhancement of diffusivity, including that at mid-depths, corresponded to the elevation of the internal-tide dissipation, in addition to that of surface turbulence.

中文翻译：

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来自北太平洋西部亚北极和亚热带环流直接测量的垂直湍流硝酸盐通量

2008 年夏季，通过对垂直湍流混合和垂直连续硝酸盐剖面的直接测量，估计了北太平洋西部的垂直湍流硝酸盐通量。我们制作了三个南北横断面，覆盖了从亚北极到亚热带的区域，包括沿皇帝海山。随着硝酸盐垂直梯度从贫营养亚热带到非贫营养亚北极水域的增加，地下通量总体呈增加趋势。由于混合增加[垂直扩散率：O (10 -5 ) m 2 s -1 ]，观察到增强的通量[ O (10 -6 ) mmol m -2 s -1 ]，尤其是在皇帝海山上。这表明地形产生的内部潮汐增强了垂直混合。在其他亚北极地区，通量估计为O (10 -7 ) mmol m -2 s -1 。在亚北极和亚热带环流之间的锋区，即黑潮-亲潮过渡区，也发现了相同的通量顺序。在中深度区域也观察到包括黑潮延伸部在内的锋区通量的增强，它们的垂直发散表明硝酸盐从北太平洋中水向较轻的密度迁移。在锋区，湍流的增强是由表面风而不是内部潮汐引起的。相比之下，在亚热带地区，即使在扩散率增强的情况下，由于硝酸盐梯度很小，地下通量估计为 O (10 -8 ) mmol m -2 s -1 。在这些区域，扩散率的增强，包括在中间深度，