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Floating plastic debris was investigated in the transition region in the North Pacific between 141°E and 165°W to understand its transportation process from Asian coast to central subtropical Pacific. Distribution was influenced primarily by the current system and the generation process of the high concentration area differed between the western and eastern areas. West of 180°, debris largely accumulated around nearshore convergent area and was transported by eddies and quasi-stationary jet from south to the subarctic region. The average was 15% higher than that previously reported in 1989, suggesting an increase in plastic debris in 30 years. East of 180°, debris concentrated in the calm water downstream of the Kuroshio Extension Bifurcation with considerably high concentration (505,032 ± 991,989 pieces km−2), due to the accumulation of small transparent film caused by calm weather conditions, suggesting a further investigation on small plastic (

A continuous survey examined short-term variations in the zooplankton community and physical ocean environment from the northeastern Izu Islands to Boso Peninsula in Japan. High copepod abundance and small upwellings in the surface layer and salinity minimum layer in the subsurface were observed on the north side of coastal fronts in the westernmost transect, moving southward as the Kuroshio Current left the Boso Peninsula. Thus, the salinity minimum layer might be a key factor forming upwelling and the fronts, leading to large abundance of coastal copepods off the northeastern Izu Islands. A community structure analysis of calanoid copepods revealed an intermediate belt assemblage between coastal and offshore (Kuroshio) assemblages. Copepod abundance was remarkably low and Ctenocalanus vanus dominated (nearly 37%) in the intermediate belt zone, indicating that C. vanus has a relatively high tolerance to adverse environments for calanoid copepods. As the Kuroshio Current left the Boso Peninsula, the coastal assemblage expanded in the same direction, and the intermediate belt assemblage off the northeastern Izu Islands disappeared. The largest population of Calanus sinicus was found along the two western transects off the northeastern Izu Islands (>1000 m depth), which was assumed to be transported from Sagami Bay and advanced southwestward while growing from copepodite stages CIII to CV. Larvae of C. sinicus would be an important food for fish larvae in addition to Paracalanus parvus s.l., the numerically dominant species in the coastal assemblage, and C. vanus under the adverse conditions for coastal copepods.

The Western Isoguchi Jet, a quasi-stationary baroclinic jet that separates from the Kuroshio Extension, has been detected for a decade. Here we report the first observations of the vertical structure and temporal variations of the jet via moored current meters (September 2011–September 2012) and intensive hydrographic measurements (September 2010, 2011, and 2012). The jet flows northeastward and supplies subtropical water (northeastward velocity was ∼ 0.16 m s−1 and temperature was ∼ 4.4 ° C at 300 m depth, and they were ∼ 0.05 m s−1 and ∼ 2.9 ° C at 1000 m depth). We newly estimated the volume, heat, and salt transports of the jet to be 13.6–26.5 Sv, 0.0745–0.173 PW (reference potential temperature of 4.0 ° C ), and 5.58–9.91 × 10 6 kg s−1 (reference salinity of 33.5), respectively. Year-to-year changes of the jet axis complicated its horizontal distribution. The jet flowed stably northeastward throughout the year with weak seasonal variability. The vertical structure of the jet is quite different from that of the surrounding flow. These observations are consistent with the mechanism proposed by Mitsudera et al. (2018) and Miyama et al. (2018) to be responsible for the formation and maintenance of the jet.

We investigated the interdecadal trend of Oyashio velocity and transport during 1993–2011 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 (106 m3 s−1) 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.

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.

We investigated the geographical and vertical distribution of the copepodid stages of Eucalanus californicus in the northwestern (NW) Pacific Ocean, and compared life history characteristics against the northeastern (NE) population of the species and the neighboring subarctic sister species, Eucalanus bungii . In the NW Pacific, multi-layered zooplankton samples were collected between 15°N and 49°N. High abundances (>1000 inds. m −2 ) of E. californicus were restricted to an area around 33°N, whereas E. bungii was highly abundant between 36°N and 49°N. The occurrence of E. californicus in the surface layer was associated with seasonal spring ontogenetic vertical migration (OVM), as suggested by previous studies in the NE Pacific. Gonadal analysis of adult females showed that surface reproduction occurs during the spring in the slope water area, which is a region on the continental slope north of the Kuroshio axis, and within the Kuroshio–Oyashio Transition Area (TA). The seasonal OVM strategy of E. californicus in the NW Pacific is characterized by (1) shorter periods of surface reproduction and growth and (2) deep C5 stage dormancy (500–2000 m). The former is an adaptive response to relatively high water temperatures and short bloom periods in the TA and slope water area. The latter strategy of deep dormancy is considered to be an adaptation to avoid drift caused by a local oceanic feature of deeper westward (>1000 m) recirculation currents, thus retaining the population within the high primary production zones of the TA and slope water area.

This study investigated temporal variations in the vertical structure and water properties of a warm-core ring that migrated into the western subarctic gyre of the North Pacific, based on analyses of temperature and salinity data derived from two profiling floats, together with shipboard and satellite observation data. The floats were initially deployed into cold and fresh Oyashio water in September 2003, and were entrained into a warm-core ring in October 2003, remaining within the ring until detrainment in December 2004. Drastic cooling and freshening of the upper core water of the ring were observed during the above entrainment of the floats with cold and fresh water into the ring, whereas moderate variations in structure and water properties were observed during a quasi-isolated phase from November 2003 to November 2004 when the ring did not experience major interactions with ambient hydrographic features. The upper part of the core water (upper core), with relatively warm/saline water above 26.6 σ θ , was under the influence of the atmosphere in winter via the formation of a deep mixed layer exceeding 300 dB, and had a prominent pycnostad below the seasonal pycnocline from spring to autumn. In contrast, the lower core, with relatively cold and fresh water below 26.6 σ θ , was not ventilated throughout the observation period. Isopycnal surfaces showed a shoaling trend of about 50 dB/year during the quasi-isolated phase, suggesting viscous decay over a timescale of several years. Markedly cold and thick water was also frequently observed within the ring, indicating the intrusion of water from the Sea of Okhotsk.

The Oyashio region in the western North Pacific supports high biological productivity and has been well monitored. We applied the NEMURO (North Pacific Ecosystem Model for Understanding Regional Oceanography) model to simulate the nutrients, phytoplankton, and zooplankton dynamics. Determination of parameters values is very important, yet ad hoc calibration methods are often used. We used the automatic calibration software PEST (model-independent Parameter ESTimation), which has been used previously with NEMURO but in a system without ontogenetic vertical migration of the large zooplankton functional group. Determining the performance of PEST with vertical migration, and obtaining a set of realistic parameter values for the Oyashio, will likely be useful in future applications of NEMURO. Five identical twin simulation experiments were performed with the one-box version of NEMURO. The experiments differed in whether monthly snapshot or averaged state variables were used, in whether state variables were model functional groups or were aggregated (total phytoplankton, small plus large zooplankton), and in whether vertical migration of large zooplankton was included or not. We then applied NEMURO to monthly climatological field data covering 1 year for the Oyashio, and compared model fits and parameter values between PEST-determined estimates and values used in previous applications to the Oyashio region that relied on ad hoc calibration. We substituted the PEST and ad hoc calibrated parameter values into a 3-D version of NEMURO for the western North Pacific, and compared the two sets of spatial maps of chlorophyll-a with satellite-derived data. The identical twin experiments demonstrated that PEST could recover the known model parameter values when vertical migration was included, and that over-fitting can occur as a result of slight differences in the values of the state variables. PEST recovered known parameter values when using monthly snapshots of aggregated state variables, but estimated a different set of parameters with monthly averaged values. Both sets of parameters resulted in good fits of the model to the simulated data. Disaggregating the variables provided to PEST into functional groups did not solve the over-fitting problem, and including vertical migration seemed to amplify the problem. When we used the climatological field data, simulated values with PEST-estimated parameters were closer to these field data than with the previously determined ad hoc set of parameter values. When these same PEST and ad hoc sets of parameter values were substituted into 3-D-NEMURO (without vertical migration), the PEST-estimated parameter values generated spatial maps that were similar to the satellite data for the Kuroshio Extension during January and March and for the subarctic ocean from May to November. With non-linear problems, such as vertical migration, PEST should be used with caution because parameter estimates can be sensitive to how the data are prepared and to the values used for the searching parameters of PEST. We recommend the usage of PEST, or other

The horizontal transport of calanoid copepod Neocalanus flemingeri, N. plumchrus and N. cristatus in the subarctic North Pacific has been investigated by particle tracking experiments using an ocean circulation model. In our physical numerical model, the current and frontal systems in the subarctic Pacific are reproduced realistically, and fine-resolved (not smoothed) and vertically sheared western boundary currents in the model enable us to assess the differences in horizontal transports among Neocalanus species. In the experiments, seasonal vertical migration and life cycle of Neocalanus species is included, and this attempt is a novel approach in examining the transports of the zooplanktons. The maximal depths of the vertical migrations and the lengths of the surface and mesopelagic dwelling duration are the essential factors in characterizing the trajectory of the horizontal transport. Both small and large forms of N. flemingeri (hereafter NF-SF and NF-LF) whose mesopelagic-inhabited depths are the shallowest among the three Neocalanus species are transported to more distant regions from initial positions compared with the others. The longest transport distance is over 5000 km for NF-SF. On the other hand, the transport distance of the deepest inhabitant N. cristatus (hereafter NC) is the shortest. The differences in the transport distance are due to the current speed of the western boundary currents at the inhabited depth at which they spend most of their lifetime (ca. 3/4 of their lifetime). More than 82% of NF-LF that originated from the Okhotsk Sea completes its life cycle in the Okhotsk Sea, and all the animals out to the Pacific are transported to the south of the subarctic front where their survivals are unexpected due to high temperatures. This is consistent with the observed limited distribution of NF-LF in the Okhotsk Sea and its surrounding waters. Geographical genetic variations of Neocalanus species are reported as very low. This implies frequent genetic exchange all over the Pacific and its adjacent seas. Being consistent with the above, the present study suggests that the trans-Pacific transport of Neocalanus species can be accomplished within a few generations.

The lateral transport of organic carbon in large grazing copepods (Neocalanus cristatus, Neocalanus flemingeri, Neocalanus plumchrus and Eucalanus bungii) from the Oyashio area to the mixed water region (MWR) by the coastal Oyashio intrusion was estimated using the data of VMPS (vertical multiple plankton sampler) and 1500 dbar-referred geostrophic transport from the CTD (conductivity temperature depth sensor) data of five cruises during June 2001 to April 2002 on a repeat observation section OICE (Oyashio Intensive observation line off Cape Erimo), which extends southeastward from Hokkaido Island, Japan. The transport to MWR by the coastal Oyashio intrusion was estimated to be 5.3 × 1011 g C for the four species. Data from profiling floats also indicated that the copepods were advected from OICE to MWR by the coastal Oyashio intrusion within about 2 months. This transport is considered to be one of the significant sources of organic carbon in MWR as it is larger than the amount of large zooplankton consumed by Pacific saury (Cololabis saira) in MWR, one of the dominant copepod predators in this region.

Intensive hydrographic observations obtained with an underwater glider revealed the vertical structure with high horizontal resolution, as well as the temporal variation, of the Subpolar Front and the Tsushima Warm Current in the Japan Sea. Even when sea surface current velocities were large (>0.50 m s−1), the glider was able to completed the observations along the planned straight line. The glider detected changes in the frontal structures and the water mass distributions on spatial scales smaller than O(10 km) and on timescales smaller than O(1 month). We showed that the assimilation of glider data by an ocean dynamic model increased velocity magnitudes in the simulated flow field.

Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contamination in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dynamics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from

An observation line along the TOPEX/POSEIDON (T/P) ground track 060 was set to estimate the Oyashio transport. We call this line the OICE (Oyashio Intensive observation line off-Cape Erimo) along which we have been conducting repeated hydrographic observations and maintaining mooring systems. T/P derived sea surface height anomaly (SSHA) was compared with velocity and transport on OICE. Although the decorrelation scale of SSHA was estimated at about 80–110 km in the Oyashio region, the SSHA also contains horizontal, small-scale noise, which was eliminated using a Gaussian filter. In the comparison between the SSHA difference across two selected points and the subsurface velocity measured by a moored Acoustic Doppler Current Profiler (ADCP), the highest correlation (0.92) appeared when the smoothing scale was set at 30 km with the two points as near as possible. For the transport in the Oyashio region, the geostrophic transport between 39°30′ N and 42°N was compared with the SSHA difference across the same two points. In this case the highest correlations (0.79, 0.88 and 0.93) occurred when the smoothing scale was set at 38, 6 and 9 km for reference levels of 1000, 2000 and 3000 db, respectively. The annual mean transport was estimated as 9.46 Sv in the 3000 db reference case. The Oyashio transport time series was derived from the T/P SSHA data, and the transports are smaller than that estimated from the Sverdrup balance in 1994–1996 and larger than that in 1997–2000. This difference is consistent with baroclinic response to wind stress field.

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.

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...

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.

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...

In order to understand the actual formation process of the North Pacific Intermediate Water (NPIW), structure of subsurface intrusions of the Oyashio water and the mixing of the Oyashio and the Kuroshio waters in and around the Kuroshio Extension (KE) were examined on the basis of a synoptic CTD observation carried out in May-June 1992. The fresh Oyashio water in the south of Hokkaido was transported into KE region through the Mixed Water Region (MWR) in the form of subsurface intrusions along two main paths. The one was along the east coast of northern Japan through the First Branch of the Oyashio (FBO) and the other along the eastern face of a warm streamer which connected KE with a warm core ring through the Second Branch of the Oyashio (SBO). The fresh Oyashio water extended southward through FBO strongly mixed with the saline NPIW transported by the Kuroshio in the south of Japan (old NPIW) in and around the warm streamer. On the other hand, the one through SBO well preserved its original properties and extended eastward beyond 150°E along KE with a form of rather narrow band. The intrusion ejected Oyashio water lens with a diameter of 50–60 km southward across KE axis and split northward into the MWR involved in the interaction of KE and a warm core ring, which were supposed to be primary processes of new NPIW formation.