Nutrient Stream in the Kuroshio region
Abstract
The Kuroshio, the western boundary current in the western North Pacific, plays major roles in transporting heat and organic/inorganic materials from the subtropical region to the subarctic one. The impact of the Kuroshio on the neiboring ecosystem is doubtedly enormous as well as on climate changes through air-sea interactions, however the Kuroshio is generally recognized as a mere boundary between the oligotrophic Subtropical waters to the south and more productive Slope waters to the north. The three-dimensional structure of the ecosystem on the Kuroshio jet is not so clear in contrast to an accumulation of intensive studies on the productive enhancement at the frontal region. Our intensive observation in Apr. 2009 detected that nutrient (nitrate, silicate and phosphate) and AOU maxima are located along the jet on the isopycnal surface in the range of 24.5-26.5σθ through the CTD and multi-samplings at intervals of 10 miles along the 5 lines transecting the Kuroshio in 137-141○E south of Japan. The nuitrient maximum along the Kuroshio jet is analogous to the characteristic structure well-known as Nutrient Stream found in the Gulf Stream region. Moreover, the characteristic nutrient/AOU structure similar to one observed in Apr. 2009 was detected in other regions by analysing historical hydrographic data obtained in spring extracted from JODC: in both the Kuroshio upstream region in the East China Sea and the downstream region along the Kuroshio Extension. The historical data indicates that the nutrient/AOU maxima along the Kuroshio jet are distributed starionarily in the whole region of the Kuroshio and their absolute values changes significantly year by year. The nutrient/AOU maxima along the Kuroshio jet seem related to three biogeochemical processes as follows. Firstly, the high nutrient/AOU water originated from the upstream and its surrounding regions is transported isopycnally along the Kuroshio to the downstream region. It can be estimated from the histrocal and climatological data. Secondly, supply of high nutrient/AOU water from the deeper (denser) layer to the target subserface layer is more intensive on the jet and its inshore side in comparison with suppy on the offshore subtropical side. The direct measurement of microstructure conducted in the cruise in Apr. 2009 indicated higher dyapicnal mixing on the jet and its inshore side. Thirdly, the strong baroclinicity inclines isobathes on the isopycnal surfaces; the water depth on the inshore side is much shalower than the offshore one. The nutrient consumption is more active in the inshore Slope water due to higher primaly production enhanced by sufficient irradiance as observed. Consequently the nutrient maximum seems to be formed along the Kuroshio jet on the isopycnal surfaces.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS51A1273K
- Keywords:
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- 4568 OCEANOGRAPHY: PHYSICAL / Turbulence;
- diffusion;
- and mixing processes;
- 4576 OCEANOGRAPHY: PHYSICAL / Western boundary currents;
- 4815 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Ecosystems;
- structure;
- dynamics;
- and modeling;
- 4845 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Nutrients and nutrient cycling