Vertical turbulent nitrate flux from direct measurements in the western subarctic and subtropical gyres of the North Pacific
Vertical turbulent nitrate flux from direct measurements in the western subarctic and subtropical gyres of the North Pacific
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DOI：
10.1007/s10872-020-00576-0
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发表时间：
2021-01
影响因子：
2.3
通讯作者：
H. Kaneko;I. Yasuda;S. Itoh;S. Ito
中科院分区：
文献类型：
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作者：
H. Kaneko;I. Yasuda;S. Itoh;S. Ito
通过对2008年夏季垂直湍流混合和垂直连续硝酸盐剖面的直接测量，估算了北太平洋西部垂直湍流硝酸盐通量。我们做了三个南北的横断面，覆盖了从亚北极到亚热带的区域，包括沿着皇帝海山脉的一段。随着硝酸盐垂直梯度的增大，亚北极水体的地下通量总体呈增加趋势。由于混合的增加[垂直扩散系数：O(10−5)m2s−1]，观测到通量的增强[O(10−6)mmol m−2s−1]，特别是在皇海山上空。地形产生的内潮增强了垂向混合。在其他亚北极地区，通量估计为asO(10−7)mmol m−2s−1。在亚北极环流和副热带环流之间的锋面区黑潮-冈潮过渡区也发现了相同的通量顺序。锋面区通量增强，包括黑潮伸展，在中深度区域也观察到，它们的垂直辐散表明硝酸盐从北太平洋中间水向较轻密度的水输送。在锋面区，湍流的增强是由地面风而不是内部潮汐引起的。相比之下，在亚热带地区，由于硝酸盐梯度小，即使扩散系数增强，地下通量估计为asO(10−8)mmol m−2s−1。在这些区域，扩散系数的增强，包括在中深度的增强，除了表面湍流的增强外，还对应于内潮耗散的提升。
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−2s−1] due to elevated mixing [vertical diffusivity:O(10−5) m2s−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 asO(10−7) mmol m−2s−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 asO(10−8) mmol m−2s−1owing 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.