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Temporal Variation of Kuroshio Nutrient Stream South of Japan

2018-11-06

Long, Y., X.-H. Zhu, X. Guo and H. Huang. Temporal Variation of Kuroshio Nutrient Stream South of Japan. Journal of Geophysical Research: Oceans, https://doi.org/10.1029/ 2017JC013635.


The 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.

 

Figure 1. (a) Schematic of the study area. (b) The black dots denote the positions of hydrographic stations, and the black lines and bars denote nitrate transport integrated over the section at 1 km wide intervals and its standard deviation (kmol/s), respectively. The red star and the broken red line denote the subsection dividing point and the virtual boundary of the box model used in the nitrate budget calculation, respectively. The positive direction for the curve is eastward. The black dots connected by thin straight lines present zero values. PN, TK, OK, ASUKA, and 137E indicate section names. The Tokara Strait is located at section TK. The colored shading denotes topography. KU = Kuroshio; KR = Kuroshio recirculation; KE = Kuroshio Extension; RC = Ryukyu Current; ECS = East China Sea.