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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.

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To evaluate the impact of temporal variation of primary productivity on the recruitment of Japanese sardine (Sardinops melanostictus) in the Sea of Japan, the phenology of sea surface phytoplankton abundance was estimated from 8 day multiple satellite (SeaWiFS, MODIS‐Aqua, MERIS, and VIIRS) derived sea surface chlorophyll (SSChl) a concentrations from January 1998 to December 2015. Because relationships between SSChl a and in situ chlorophyll a concentrations were significantly different among periods based on the satellite combinations used, maximum and minimum SSChl a concentrations of 1 year were relativized as 1 and 0, respectively. Spatio‐temporal variation of relativized SSChl a concentrations was determined by using empirical orthogonal function (EOF) analysis. Scores in the first EOF mode denoted the basin‐scale variations of SSChl a concentrations in the Sea of Japan, and the major peak from the end of February to the end of May displayed the spring bloom. The logarithm of recruitment per spawner (LNRPS) for sardine was positively affected by delays in the start and end dates of the spring phytoplankton bloom. The delay of the date of the lowest sea surface temperature contributed to the delay of the end of the spring bloom during the period 1998–2015 and elevated the LNRPS during the period 1982–2015. Sardine spawns in the southern Sea of Japan from April to May, hence, delays of the spring bloom prolonged its overlap with sardine larval periods, and thus improved the recruitment of Japanese sardine in the Sea of Japan.

We investigated the interdecadal trend of Oyashio velocity and transport during [1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003][2004][2005][2006][2007][2008][2009][2010][2011] based mainly on linear trend analysis of altimetry and in situ temperature-salinity data from a monitoring line (''A-line'') off the southeastern Hokkaido coast. Significant trends of increasing sea level were detected on the continental slope, north of the Kuril-Kamchatka Trench. Sea level anomaly data revealed a localized clockwise circulation centered near the trench, the suggestion being that the strength of the Oyashio on the slope and the offshore return flow had decreased. The Oyashio mainstream seemed to have shifted from a nearshore to an offshore path. Steric heights estimated from the A-line data exhibited an increasing trend north of the trench, where 50-80% of the increase was determined by halosteric components attributable to a trend of decreasing salinity in the subsurface. The trend of decreasing salinity was related to downward displacement of isohaline/isopycnal surfaces. The largest displacement was above the trench. Horizontal pressure gradients associated with southwestward flows on the slope were weakened. The Oyashio transport decreased by 8.9 Sv (10 6 m 3 s 21 ) in 19 years. A mesoscale eddy analysis revealed that clockwise eddies appeared more frequently in recent years near the trench around the A-line and could decrease the Oyashio transport. A baroclinic, long Rossby-wave model also predicted that a large-scale baroclinic response to the wind stress could weaken the Oyashio velocity in the upper layer. Dynamical linkage between the localized eddies and large-scale response remains to be clarified in future work.