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The hydrographic structure of a quasi-stationary jet separated from the Kuroshio Extension, a phenomenon that possibly leads to deepening of the winter mixed layer, is revealed via intensive observations using a conductivity–temperature–depth profiler and a ship-mounted acoustic Doppler current profiler (ADCP) carried out in September 2009. Data collected from profiling floats set to drift isopycnal layers and time series of sea surface buoys and satellite altimeters were also analyzed to understand the continuity, water characteristics, and time variation of the jet. Although the flow field was complex due to disturbances such as energetic eddies, the jet transported subtropical water to the downstream (northeastern) observation region in a layer shallower than 400 dbar and its highest velocity was concentrated in the mid- and deep layers (≤0.30 m s−1 at the sea surface and ≥0.05 m s−1 at 800-m depth). The velocity axes of the jet detected from the satellite data corresponded to those detected through analysis of the ADCP data, and the intensity of the jet at the sea surface varies over interannual-to-decadal time scales. Part of the interannual-to-decadal variation in the velocity field of the jet is controlled by the dynamic state of the Kuroshio Extension and is correlated with that of the Kuroshio Extension latitude (linear correlation coefficient r ~ 0.67). The relationship between these variations can be interpreted qualitatively as being responsible for the inertial streamers that separate from the crests of meanders of the Kuroshio Extension to the jet region due to the large steering effect.

The ADCP-referenced geostrophic circulation of the Oyashio and Kuroshio waters in the density range 26.6–27.2 σθ in the Kuroshio–Oyashio interfrontal zone is examined by synoptic observations with a CTD and shipboard ADCP in May 1992 and May 1994 in order to investigate the absolute transports of the source waters for North Pacific Intermediate Water (NPIW) formation. The Kuroshio and Oyashio components are calculated by the ADCP-referenced transport combined with the mixing ratio assuming isopycnal mixing between pure Kuroshio and Oyashio waters. As a result, the Kuroshio transport involved in the NPIW formation is estimated to be 7–8 Sv (Sv ≡ 106 m3 s−1), which is the net component along the Kuroshio Extension (KE) from the separation point near the Cape Nojima to the east side of the first crest of the KE. The Oyashio transport is estimated to be 5–6 Sv, which consists of coastal 1–2 Sv and offshore 3–4 Sv separated by warm core rings, and which subsequently merges with the KE at the east side...

The distribution and circulation of the coastal Oyashio intrusion were examined with a CTD and shipboard ADCP survey conducted in the vicinity of the east coast of Japan in May 1994. The estimate of mixing ratios in the density range of 26.4–27.2 σθ, assuming isopycnal mixing between the pure Oyashio and Kuroshio waters, and the ADCP velocity data showed the following new results: 1) The distribution of the Oyashio water is horizontally and vertically patchy, and some of the Oyashio water patches formed mesoscale eddies with both cyclonic and anticyclonic circulations. 2) The anticyclonic Oyashio eddy had cold and low-salinity cores with low potential vorticity (PV) suggesting that the origin was the Okhotsk Sea. The cyclonic Oyashio eddy with high PV was considered to originate from the western subarctic gyre (WSAG) in the North Pacific. A new image of the intermediate Oyashio water circulation near the Oyashio–Kuroshio confluence is proposed, in which the eddies from the Okhotsk Sea and the WSA...

Summer hydrographic surveys from 1993 to 1997 in the area south of the Kuril Islands in the northwestern subarctic Pacific showed the existence of anticyclonic eddies south of the Bussol’ Strait at almost the same location but with variable sizes and intensities depending on the year. Every eddy had a cold, low salinity and low potential vorticity core, suggesting a strong influence from the Okhotsk Sea water. Two formation processes and annual variations were found with satellite data analyses. One is the case where eddies are locally formed south of the Bussol’ Strait and intensified from summer to fall with the supply of Okhotsk Sea water as observed in 1993. In the other case, Kuroshio warm-core rings that had translated northeastward are arrested near the Bussol’ Strait and amplified with the supply of Okhotsk Sea water as seen from summer to autumn in 1995. In winter, eddies tend to move northeastward with decay. The 1992 eddy moved northeastward then northward in winter and was eventually ...

A multiship CTD survey was conducted in the Kuroshio-Oyash interfrontal zone in the area of 30°–41°N, 140°E–180° from May to June 1992 to examine the distribution, modification process, and formation site of North Pacific Intermediate Water (NPIW). Low salinity Oyashio water and high salinity Kuroshio water merge along the Kuroshio Extension just off the east coast of Japan and together flow eastward. Eastward geostrophic transport and salinity across the Kuroshio strong current indicate that the mixture of the Kuroshio and Oyashia waters along the Kuroshio Extension can produce new NPIW. Transport estimate suggests that the new NPIW bifurcates into the interfrontal zone and into the recirculation of the subtropical gyre. A salinity minimum structure can he also created due to the isopycnal mixing along the Kuroshio Extension between the Oyashio and Kuroshio waters, which have different salinity and vertical velocity structures. The low-salinity Oyashio water seen along the Oyashio branches and t...