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The 137°E repeat hydrographic section of the Japan Meteorological Agency across the western North Pacific was initiated in 1967 as part of the Cooperative Study of the Kuroshio and Adjacent Regions and has been continued biannually in winter and summer. The publicly available data from the section have been widely used to reveal seasonal to decadal variations and long-term changes of currents and water masses, biogeochemical and biological properties, and marine pollutants in relation to climate variability such as the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In commemoration of the 50th anniversary in 2016, this review summarizes the history and scientific achievements of the 137°E section during 1967–2016. Through the publication of more than 100 papers over this 50-year span, with the frequency and significance of the publication increasing in time, the 137°E section has demonstrated its importance for future investigations of physical–biogeochemical–biological interactions on various spatiotemporal scales, and thereby its utility in enhancing process understanding to aid projections of the impact of future climate change on ocean resources and ecosystems over the twenty-first century. [ABSTRACT FROM AUTHOR]

From the moored buoy observation at $$33.9^{\circ }$$ N, $$144.9^{\circ }$$ E south of the Kuroshio Extension (KE), we obtained Eulerian time series of dissolved oxygen concentration (DO) at 200, 400, and 600 m depths from June 2012 to March 2013. We observed ventilation by meso- and submesoscale processes that transport water southward across the KE jet. First, the cyclonic mesoscale eddy in June 2012 substantially depressed DO at depths of 400 and 600 m but maintained DO at 200 m, suggesting near-surface lateral transport of high-DO water derived from the north of the KE. Second, subduction of high-DO (>230 $${\upmu }$$ mol kg $$^{-1}$$ ) water to a depth of 600 m was observed from early February to March 2013, associated with a mesoscale/submesoscale meandering of the KE jet. In mid-March 2013, shipboard hydrographic data were collected where the water mass at the mooring site would be advected by the eastward current on the southern flank of the KE. Based on these data, the subduction event was identified as an intrusion of an anomalously thick water mass from approximately 400-900 dbar. Ventilation of the subtropical mode water at a depth of 200 m around a subsurface DO maximum layer was detected as a rapid rise in DO in January 2013. This occurred after a significant seasonal decrease in DO at a rate of $$-0.130 \pm 0.007$$ $${\upmu }$$ mol kg $$^{-1}$$ day $$^{-1}$$ from July to December 2012. [ABSTRACT FROM AUTHOR]

The spatiotemporal variability of North Pacific Tropical Water (NPTW), characterized as a subsurface salinity maximum, is investigated using a half-century-long repeat hydrographic dataset of the Japan Meteorological Agency (JMA) along the 137°E meridian, historical objective analysis of monthly global temperature and salinity dataset, sea surface dynamic height dataset, and atmospheric dataset. The cross-sectional area of NPTW ( S > 34.9) at 137°E shows decreasing trend with freshening after the mid-1970s. Significant decadal-scale variation is also revealed; this resulted from meridional excursion of its northern boundary, fluctuating between 15°N and 25°N. It was found that the northern boundary excursions at 137°E were induced by westward expansion of the NPTW formation region related to excess evaporation and increased Ekman pumping in the formation region. It is also inferred that the larger eddy activity around the northern boundary of the NPTW also possibly influences the northward movement of the NPTW northern boundary at 137°E through enhanced eddy mass transport. [ABSTRACT FROM AUTHOR]

Regeneration of a warm anticyclonic ring as a result of interaction with cold water masses was observed within the western subarctic gyre of the North Pacific. Satellite, profiling float, and shipboard observations revealed that a warm-core ring originated from the Kuroshio Extension, propagating northeastwards, entrained cold and fresh water masses from the coastal area of Hokkaido, which are typically recognized within the ring as water that is colder than 2.5 °C. The potential temperature and planetary contribution of potential vorticity of the cold water in the coastal area of Hokkaido were <2 °C and 15 × 10 ms, respectively, suggesting that it originated from the Sea of Okhotsk. After the intrusion, the warm core of the ring cooled, freshened, and contracted, while the outer and lower parts became occupied by the cold and fresh water; however, even after the cooling, the positive surface elevation and downward depression of the main pycnocline, typical of an anticyclonic ring, were still evident. The ring continued to propagate northeastwards, with the main part of its structure occupied by the cold water, but changed its direction of travel from northwest to west-southwest 8 months after the cold-water event, and was finally absorbed into another warm-core ring. It is suggested that these anticyclonic rings, which transported and mixed warm and cold water masses, play important roles in the cross-gyre exchange of subtropical and subarctic waters in the North Pacific. [ABSTRACT FROM AUTHOR]

Hydrographic data obtained by high-resolution shipboard observations and Argo profiling floats have been analyzed to study the mesoscale structure and circulation of the North Pacific Subtropical Mode Water (STMW). The float data show that in the late winter of 2008, STMW having a temperature of approximately 18.8°°, 17.7°°, and 16.6°°C formed west of 140°°E, at 140°°--150°°E, and east of 150°°E, respectively, in the recirculation gyre south of the Kuroshio Extension. After spring, the newly formed STMW gradually shift southward, decreasing in thickness. Simultaneously, the STMWs of 16.6°° and 17.7°°C are gradually stirred and then mixed in terms of properties. In late fall, they seem to be integrated to form a single group of STMWs having a temperature centered at 17.2°°C. Such STMW circulation in 2008 is much more turbulent than that in 2006, which was investigated in a previous study. The difference between the two years is attributed to the more variable state of the Kuroshio Extension in 2008, associated with stronger eddy activities in the STMW formation region, which enhance the eddy transport of STMW. High-resolution shipboard observations were carried out southeast of Japan at 141°°--147°°E in the early fall of 2008. To the south of the Kuroshio Extension, STMW exists as a sequence of patches with a horizontal scale of 100--200 km, whose thick portions correspond well to the mesoscale deepening of the permanent pycnocline. The western (eastern) hydrographic sections are occupied mostly by the 17.7°°C (16.6°°C) STMW, within which the 16.6°°C (17.7°°C) STMW exists locally, mostly at locations where both the permanent pycnocline depth and the STMW thickness are maximum. This structure implies that the STMW patches are transported away from their respective formation sites, corresponding to a shift in the mesoscale anticyclonic circulations south of the Kuroshio Extension. Furthermore, 20%%--30%% of the observed STMW pycnostads have two or three potential vorticity minima, mostly near temperatures of 16.6°° and 17.7°°C. The authors presume that such a structure formed as a result of the interleaving of the 16.6°° and 17.7°°C STMWs after they are stirred by mesoscale circulations, following which they are vertically mixed to form the 17.2°°C STMW observed in late fall. These results indicate the importance of horizontal processes in destroying the vertically uniform structure of STMW after spring, particularly when the Kuroshio Extension is in a variable state. [ABSTRACT FROM AUTHOR]

Temperature and salinity data from Argo profiling floats in the North Pacific during 2003--08 have been analyzed to study the structure of winter mixed layer north of the Kuroshio Extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water (CMW). In late winter, two zonally elongated bands of deep mixed layer extend at 33°°--39°° and 39°°--43°°N, from the east coast of Japan to 160°°W. These correspond to the formation region of the lighter variety of CMW (L-CMW) and that of the denser variety of CMW (D-CMW) and the recently identified transition region mode water (TRMW), respectively. In the western part of the L-CMW and D-CMW--TRMW formation regions west of 170°°E, the winter mixed layer becomes deeper and lighter to the east (i.e., to the downstream). As a result, the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus unable to be subducted to the permanent pycnocline. In the eastern part of the formation regions between 170°°E and 160°°W, on the other hand, the winter mixed layer becomes shallower and lighter to the east. From these areas, the L-CMW with potential density of 25.7--26.2 kg m−−3 and the D-CMW--TRMW (mostly the former) of 26.1--26.4 kg m−−3 are subducted to the permanent pycnocline, and they are then advected anticyclonically in the subtropical gyre. These results imply that during the analysis period large-scale subduction to the permanent pycnocline occurs in the density range up to 26.4 kg m−−3 in the open North Pacific, whereas the winter mixed layer density reaches the maximum of 26.6 kg m−−3. This is supported by the vertical distribution of apparent oxygen utilization in a hydrographic section in the subtropical gyre. [ABSTRACT FROM AUTHOR]

Abstract: This study examined the vertical structure of mode waters in the North Pacific using Argo data. Thick low-potential vorticity (Q) pycnostads were detected. These contain well-known water masses such as Subtropical Mode Water (STMW), Eastern STMW (ESTMW), Central Mode Water (CMW), and Transition Region Mode Water (TRMW). To examine differences in the vertical structures of these mode waters, a distribution area was defined for each. Near the Q-minimum core in the STMW area, vertical gradients of both potential temperature (θ ) and salinity (S) were shown to be very small, indicating high vertical homogeneity. Conversely, the vertical gradients of both θ and S are relatively large near the core of pycnostads in the CMW, TRMW, and ESTMW areas, indicating that density-compensating stratification of θ and S occurs within these mode waters. Histograms of θ and S anomalies relative to the core properties inside pycnostads in the STMW area show sharp peaks in θ and S at the core, demonstrating high homogeneity throughout the pycnostad of STMW. However, such histograms for CMW show a flat distribution of water properties inside the pycnostads, indicating relative inhomogeneity of this mode water. Histograms in the ESTMW (TRMW) area show sharp peaks at the core, as was the case for STMW, but are negatively (positively) skewed, suggesting relative inhomogeneity only at the colder (warmer) and fresher (saltier) side of pycnostads. Because these waters are to some extent homogeneous with respect to their potential density, their spiciness is high, implying that they mix with surrounding cold (warm) and fresh (saline) water by double diffusive convection. The Turner angle (Tu) was calculated to examine the potential of double diffusive convection. Tu values near the core of pycnostads in the STMW and CMW areas are 50°–60° and 60°–70°, respectively, indicating that salt fingering is weakly possible in both STMW and CMW. In the TRMW (ESTMW) area, pycnostads show Tu values greater than 70° on the upper (lower) side of the core, indicating that the water properties of TRMW (ESTMW) are probably modified by strong salt fingering with warm (cold) and saline (fresh) water on the upper (lower) side of the pycnostads. These results support an idea previously proposed based on an analysis of climatological data: each mode water has its own vertical structure, presumably owing to differences in its formation and/or modification mechanisms. Density compensating θ and S gradients and the resulting salt-finger-type convection are suggested to be important in mode water processes from their formation through their decay. [Copyright &y& Elsevier]

A repeat hydrographic section along 165°E was analyzed to verify a westward extension of the formation region of the North Pacific Ocean Central Mode Water (CMW) suggested by previous synoptic observations, and to investigate the relation between the formation region and thermohaline fronts. The CMW formation region extends at least as far west as 155°E, much farther than recognized in a previous study based on climatology. It is located in two interfrontal regions between the Kuroshio Extension front and the Kuroshio Bifurcation front (KBF), and between the KBF and the subarctic front, where two types of CMW—namely, the lighter variety with potential density of 25.8–26.2 kg m-3 and the denser one of 26.3–26.4 kg m-3—are formed. How this differential formation of CMW is reflected in its gyrewide distribution was examined using one-time sections of the World Ocean Circulation Experiment (WOCE) Hydrographic Program in the North Pacific. The main circulation paths of the two types of CMW diverge east of the date line; the lighter variety is located in the inner part of the eastern subtropical gyre, and the denser variety is located in the outer part. These results demonstrate that the frontal structure around the northern boundary of the subtropical gyre, particularly the existence of KBF, is essential in determining the properties and the gyrewide distribution of CMW. [ABSTRACT FROM AUTHOR]

The two subsurface temperature-density fronts and the accompanying eastward currents in the central and southern part of the North Pacific subtropical gyre are identified using the repeat hydrographic sections in the western part of the gyre and the sections over the whole gyre from the World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP). The northern eastward current corresponds to what has been known as the subtropical countercurrent (STCC). The countercurrent/front is located typically near 24°N and extends from the western boundary approximately to the international date line. The previous suggestion that the STCC is located at the southern edge of North Pacific Subtropical Mode Water (STMW) is confirmed; the front appears as the southern boundary of the lower potential vorticity (PV)/apparent oxygen utilization (AOU) waters on the isopycnals within the STMW layer. The southern eastward current corresponds to what was recognized earlier but has not been documented in detail. The southern countercurrent/front is located near 18°N west of the date line, but shifts to the north east of the date line. Its eastern limit is around 165°W at 25°N. The southern front corresponds to the southern boundary of the lower PV/AOU waters on the isopycnal surfaces of the wide σ[sub θ] range including those of North Pacific Central Mode Water (CMW). While CMW seems to contribute largely to this low PV/AOU layer, the contribution from other waters is still significant. Judging from the associated PV/AOU features in the thermocline, both fronts can be regarded as inherent boundaries in the ventilated thermocline, dividing the regions ventilated to distinctive degrees. Correspondence between the observational features of the countercurrents/fronts and some of the previously proposed theories concerning the STCC are discussed. [ABSTRACT FROM AUTHOR]