Shin-ichi Ito

Distribution and Circulation of the Coastal Oyashio Intrusion

Abstract 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...

Cold-Core Anticyclonic Eddies South of the Bussol' Strait in the Northwestern Subarctic Pacific

Abstract 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 ...

Oceanic fronts and jets around Japan: a review

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

Application of a bioenergetics growth model for European anchovy (Engraulis encrasicolus) linked with a lower trophic level ecosystem model

A bioenergetics model is implemented for European anchovy (Engraulis encrasicolus) and applied to the north-eastern Aegean Sea (eastern Mediterranean Sea). The model reproduces the growth of anchovy in a one-way linked configuration with a lower trophic level (LTL) ecosystem model. The LTL model provides densities for three zooplankton functional groups (heterotrophic flagellates, microzooplankton and mesozooplankton) which serve as available energy via consumption for the anchovy model. Our model follows the basic structure of NEMURO.FISH type models (North Pacific Ecosystem Model for Understanding Regional Oceanography for Including Saury and Herring). Several model parameters were specific for the Mediterranean or the Black Sea anchovy and some others were adopted from related species and NEMURO.FISH due to lack of biological information on E. encrasicolus. Simulation results showed that the fastest growth rate occurs during spring and the slowest growth rate from August to December. Zooplankton abundance during autumn was low implying that decreased prey density lead to a reduction in anchovy weight, especially for the age-3 class. Feeding parameters were adjusted to adequately fit the model growth estimates to available weight-at-age data. A detailed sensitivity analyses is conducted to evaluate the importance of the biological processes (consumption, respiration, egestion, specific dynamic action, excretion and egg production) and their parameters to fish growth. The most sensitive parameters were the intercept and exponent slope of the weight-dependent consumption and respiration process equations. Fish weight was fairly sensitive to temperature-dependent parameters.

Regional variability of factors controlling the onset timing and magnitude of spring algal blooms in the northwestern North Pacific

[2]xa0Satellite imagery and oceanographic data collected between 2003 and 2009 were used to examine factors controlling the onset timing and magnitude of spring algal blooms in the northwestern North Pacific. Consistent with the critical depth hypothesis, the spring bloom onsets coincided with the mixed layer depth (MLD) shoaling in the north of the Kuroshio extension and in Oyashio, where complex frontal physical structures and turbulence weakening, respectively, would be responsible for the MLD shoaling. In contrast, in the formation regions of the dense central mode water (D-CMW) and the transition region mode water (TRMW), bloom onsets coincided with possible turbulence weakening but not with MLD shoaling. The peak of chlorophyll a in the formation regions of the D-CMW (0.44u2009±u20090.23 mg m−3) and the TRMW (0.58u2009±u20090.34 mg m−3) were ca. 5 times lower than that in the Oyashio (2.54u2009±u20090.74 mg m−3), despite the fact that nitrate concentration during the prebloom period was high (∼10 µM) and MLDs became shallow enough at the bloom peak in all the three regions. These observations indicated that light conditions and nitrate concentration did not explain the regional variability in the magnitude of spring blooms. The bloom magnitude west of ca. 150°E and in the north Kuroshio extension was increased relative to that in the eastern region, suggesting a chemical property in the water delivered from the Okhotsk Sea that would influence the western bloom. Our results demonstrated that factors controlling the timing and magnitude of spring algal blooms depend on the physicochemical regime in the northwestern North Pacific.

ADCP-Referenced Kuroshio and Oyashio Water Transports for North Pacific Intermediate Water Formation*

Abstract 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...

Absolute Volume Transports of the Oyashio Referred to Moored Current Meter Data Crossing the OICE

During November 2000–June 2002, both direct current measurements from deployment of a line of five moorings and repeated CTD observations were conducted along the Oyashio Intensive observation line off Cape Erimo (OICE). All the moorings were installed above the inshore-side slope of the Kuril-Kamchatka Trench. Before calculating the absolute volume transports, we compared vertical velocity differences of relative geostrophic velocities with those of the measured velocities. Since both the vertical velocity differences concerned with the middle three moorings were in good agreement, the flows above the continental slope are considered to be in thermal wind balance. We therefore used the current meter data of these three moorings, selected among all five moorings, to estimate the absolute volume transports of the Oyashio referred to the current meter data. As a result, we estimated that the southwestward absolute volume transports in 0–1000 db are 0.5–12.8 × 106 m3/sec and the largest transport is obtained in winter, January 2001. The Oyashio absolute transports in January 2001, crossing the OICE between 42°N and 41°15′ N from the surface to near the bottom above the continental slope, is estimated to be at least 31 × 106 m3/sec.

Currents Associated with the Quasi-Stationary Jet Separated from the Kuroshio Extension

AbstractThe 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 th...

Impacts of climate change on growth, migration and recruitment success of Japanese sardine (Sardinops melanostictus) in the western North Pacific

We developed a multi-trophic level ecosystem model by coupling physical, biogeochemical-plankton and fish models. An oceanic general circulation model was coupled with a lower trophic level ecosystem model and a Japanese sardine migration model, and applied to the western North Pacific. To investigate the impact of global warming on the pelagic fish ecosystem, such as Japanese sardine, we conducted numerical experiments of growth and migration of Japanese sardine using physical fields for the present day and future with a global warming scenario simulated by a high-resolution climate model. The model results demonstrated possible impacts of global warming on the growth and migration pattern of Japanese sardine. The growths of fish in the current main spawning region under the global warming scenario were significantly slower than those under the present climate scenario. Fish in this region will be at disadvantage for their recruitment under the global warming condition. Prey conditions in the spawning region were projected not to markedly change under global warming condition while water temperature increased. As a result sardine spawning ground was projected to shift towards more north areas. During the feeding migration period in summer, geographical distribution of juveniles fish was projected to shift northwards by one to two degrees latitude under the global warming condition following the change in the distribution of optimal temperature region for feeding. However, this northwards shift of the optimal temperature for feeding was minimized adjacent to the western North Pacific by the cooler water supply by the intensification of the Oyashio.

Meridional transport of the North Pacific Intermediate Water in the Kuroshio-Oyashio Interfrontal Zone

Meridional transport and origin of the North Pacific Intermediate Water (NPIW) in the Kuroshio-Oyashio interfrontal zone (K/O zone) were examined using hydrographic and velocity data with LADCP (Lowered Acoustic Doppler Current Profiler) along 37°N in the northwestern Pacific. Dominant northward transports were observed west of the Shatsky Rise (coast-157°E) in both upper (26.6–27.0σθ) and lower NPIW (27.0–27.5σθ). In the upper NPIW, northward transports of 4.3 Sv which consist of the dominant Kuroshio (3.2 Sv) and small Oyashio component (1.1 Sv) were observed, suggesting that the upper NPIW in the K/O zone was formed by the mixing between the southward Oyashio water across the Oyashio Front and the Kuroshio water bifurcated from the Kuroshio Extension. In the lower NPIW, 9.4 Sv of northward transports (Kuroshio 5.5 Sv and Oyashio 3.9 Sv) were observed, suggesting that the lower NPIW in the K/O zone was maintained by the northward transport of the NPIW formed near the Kuroshio Extension.