Temporal Variation of Kuroshio Nutrient Stream South of Japan

Abstract

AbstractThe temporal variation of the Kuroshio nutrient stream south of Japan was examined using quarterly hydrographic data at section 137E from 2000 to 2013. During the seasonal cycle, nitrate flux and transport reach the maximum (minimum) in autumn (winter). The Kuroshio recirculation contributes about 60% to the mean eastward nitrate transport and the seasonal variations thereof. Empirical orthogonal function analysis showed two dominant modes in nitrate flux, nitrate concentration, and velocity at section 137E; these were associated with Kuroshio path meander and the Kuroshio Extension decadal oscillation. The Kuroshio large meander induced a significant increase in the eastward nitrate transport, from 609 to 1,723 kmol/s, at the section. Due to the modulation of the Kuroshio Extension decadal oscillation, both the eastward nitrate transport and the Kuroshio recirculation were stronger in an unstable state compared to when in a stable state. The variation in the eastward nitrate transport in these dynamic patterns can be attributed to the strength, location, and spatial expansion of the Kuroshio recirculation south of Japan. Nitrate transport from the south of Japan to the Kuroshio Extension was relatively stable at 400 kmol/s. There was a strong seasonal variation in the surface chlorophyll concentration over the subtropical gyre including Kuroshio recirculation region, whereas variations in the Kuroshio region was controlled by the Kuroshio path shift. The velocity‐weighted nitrate concentration in the dense layer along the Kuroshio path increased throughout the year while that in the light layer decreased in spring and autumn and increased in summer and winter.