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A Climatological View of the Kuroshio/Oyashio System East of Japan*
ABSTRACT
Time-averaged structure of the Kuroshio/Oyashio system east of Japan was examined using historical hydrographic data. Unlike most of the earlier climatological analyses, the data were averaged along isopycnal rather than pressure surfaces in a 0.5 deg X 0.5 deg grid. As a result, most of the detailed phenomena associated with the narrow western boundary currents were revealed. Water from the Oyashio is seen to overshoot the zero zonally integrated wind-stress-curl line by more than 50, approaching as far south as 36 deg-38 degN at the western boundary. Water from the Kuroshio Extension, by contrast, tends to feed into the Oyashio Front in the interior ocean. This exchange of waters leads to a zero of zonally integrated (western boundary-180 deg) meridional transport at about 44 deg N, reasonably coinciding with the zero of zonally integrated wind stress curl in the western North Pacific. A well-defined double-front structure is seen at depths of the thermocline, but it does not appear to have a strong signature in the surface dynamic topography. Though always accompanied by strong temperature and salinity gradients, water density changes little across the Oyashio Front near the surface. Both the Kuroshio Extension and Oyashio Front have a significant deep component, but below 1000 m the former seems to be dominated by eddy features associated with the Kuroshio Extension recirculation gyre.
1. Introduction
The Kuroshio/Oyashio system (KOS) has captured the interest of oceanographers as well as fishermen for about a century because of the abundant fishery resource in the mixed water region (MWR) east of Japan (cf. Kawai 1972). Since the early 1990s, interest in the KOS has increased further due to the long-lasting negative excursion of the Southern Oscillation index, which among other things has been linked to the Pacific decadal variability (e.g., Nakamura et al. 1997; Miller et al. 1998; Xie et al. 2000). Hypotheses to account for this variability involve changes in the shallow, meridional circulation cells that allow exchange of waters between the main oceanic gyres, namely the subtropical and subpolar cells. The KOS is especially important in these cells because it is a crossroad of their prominent pathways (McCreary and Lu 1994; Lu et al. 1998).
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