Observations of vertical turbulent nitrate flux across the Kuroshio

The vertical turbulent nitrate flux was calculated across the Kuroshio based on direct turbulence measurements and nitrate concentrations. Profile data were analyzed relative to the main axis of the Kuroshio front, the location of maximum surface velocity. Large upward nitrate flux on the northern side of the front, O(10−6) mmol N m−2 s−1, was observed from 300 m up to the base of the euphotic layer. Enhanced turbulence and a large vertical nitrate gradient contributed to this flux. On this side, high concentrations of chlorophyll‐a were also observed near the surface, and the large nitrate flux is consistent with a previous estimate of the rate of new production. In contrast, south of the axis, the nitrate flux was O(10–8–10−7) mmol N m−2 s−1 because of the small vertical nitrate gradient, although vertical diffusivity at the euphotic layer base remained relatively high.