Language：Japanese   Publisher：Progress in Oceanography  

The Kuroshio carries a large amount of nutrients from the east of Luzon Island to the south of Japan. However, only nutrients transported into the euphotic layer can be utilized by phytoplankton. We use the results of an eddy-resolving coupled physical-biological model to investigate (1) the horizontal and vertical transport of nitrate into the euphotic layer (0–100 m) in the Kuroshio and Kuroshio Extension and (2) the contribution of different sources of nitrate (coastal, deep layer and open ocean) to the spatial variation of downstream transport within the euphotic layer along the Kuroshio and Kuroshio Extension. As a mean state, the downstream transport of nitrate in the euphotic layer varies as 2.8 kmol s⁻¹ east of Luzon Island, 7.9 kmol s⁻¹ east of Taiwan, 8.9 kmol s⁻¹ near the Tokara Strait, 21.5 kmol s⁻¹ near the Izu-Ogasawara Ridge and 19.6 kmol s⁻¹ around 160°E. Vertical transport from the bottom of the euphotic layer around the Luzon Strait due to the uplifting of the potential density layer is an important contribution to the increase in downstream transport of nitrate by 3.8 kmol s⁻¹. Horizontal transport from Japan coast area contributes 14.3 kmol s⁻¹ of nitrate to the Kuroshio mainstream. An interesting staggered upward and downward distribution of vertical velocity, which can be explained by conservation of the potential vorticity, was found in the Kuroshio Extension and contributed a total nitrate transport of 5.2 kmol s⁻¹ to the euphotic layer. We also calculated the vertical flux of nitrate resulting from vertical mixing based on observations and demonstrated that vertical mixing can provide a nitrate flux into the euphotic layer in the Kuroshio region comparable to that supplied by vertical velocity.

DOI： 10.1016/j.pocean.2025.103544

Scopus

Language：Japanese   Publisher：Progress in Oceanography  

High-frequency current fluctuations in the Kuroshio current were observed using a long-range Acoustic Doppler Current Profiler (ADCP) moored in the Tokara Strait. The fluctuations were separated as deviations from the 60-min running averaged currents. The duration of individual fluctuation was 3–5 min and the active fluctuation events occurred diurnally during the period of strong Kuroshio current, suggesting both influences from the Kuroshio and tidal currents. The statistical analyses revealed that the current fluctuations were confined in the mid-depths (158–262 m) and the westward component was dominant in the eastward-flowing Kuroshio currents. Close examination of a fluctuation event revealed that strong westward and downward currents occurred in a rank-ordered wave train consisted of 3–4 waves with a period ∼ 3 min. As this wave period was shorter than the ambient buoyancy period (8–13 min), the observed wave train was considered to be a packet of solitary waves (nonlinear internal waves; NLIWs). The observed current structure with a maximum amplitude at the mid-depths were similar to that of the second mode (mode-2) internal waves based on the observed stratification. In addition, the wave propagation direction estimated from the signal time lags between the ADCP transducers agreed well with the maximum current direction at mid-depths in the event. This strongly suggests that the observed current fluctuations were convex type mode-2 NLIWs. Although the NLIW generation was unclear, a role of small seamounts east of the mooring site was suggested.

DOI： 10.1016/j.pocean.2025.103536

Scopus

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

The El Niño-Southern Oscillation (ENSO)-related interannual variability of the Kuroshio in the East China Sea (ECS) was revisited based on reanalysis outputs during 1993–2018. Unlike the synchronized variations from 2006 to 2018, the period of 1993–2005 showed regional differences in how the ECS-Kuroshio responded to ENSO events. Specifically, from the upstream region to the midway of the continental slope, the ECS-Kuroshio exhibited distinct six-year interannual modulation during 1993–2005. In contrast, downstream Kuroshio variability primarily followed a four-year cycle, aligning with ENSO variability during the same period. Further analysis suggested that the sea surface height anomaly (SSHA) east of the Kerama Gap, near the midpoint of the Ryukyu Island chain, extended inside the ECS until the southern side of the Tokara Strait along the ECS-Kuroshio path and was well correlated with the Kuroshio in the Tokara Strait during 1993–2005. The cause of this SSHA signal was attributed to forcing by ENSO-related wind stress curl changes in the interior region. There was an obvious difference in the ENSO-related atmospheric circulation before and after 2005. The wind stress curl pattern in the North Pacific during 1993–2005, characterized by a maximum in the Kerama Gap latitude band, shifted northward compared to that during 2006–2018. The relative northward shift of the ENSO-related wind stress curl, which stimulates the long baroclinic Rossby wave propagating westward and arriving east of the Kerama Gap, affect the interannual variabilities of both the upstream and downstream Kuroshio.

DOI： 10.1029/2024JC021242

Scopus

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

In 2015, four typhoons traversed the regions surrounding the Ryukyu Island Chain, resulting in different near-inertial waves (NIWs), the characteristics of which were investigated through in situ observations around Miyakojima Island and numerical simulations covering the East Asian marginal seas. The spatial distribution of typhoon-induced near-inertial motions was significantly correlated with the typhoon tracks and background currents. Typhoons Chan-hom and Goni traversed the observation transect, resulting in different spatial patterns of NIWs owing to their different tracks. Observations on both the western and eastern sides of the Ryukyu Island Chain failed to capture NIWs after typhoon Chan-hom, because they were positioned to the left of the typhoon track where near-inertial motions were enhanced only in the upper 50 m. In contrast, NIWs were negligible on the left side and energetic on the right side of typhoon Goni. Despite being hundreds of kilometers from the observation transect, typhoons Soudelor and Dujuan induced NIWs with higher energy levels than those induced by typhoons Chan-hom and Goni. The energy propagation of NIWs after typhoons Soudelor and Dujuan was significantly influenced by background currents. The western boundary currents, including the Kuroshio and Ryukyu Currents, create negative relative vorticity on their right, which works like a waveguide for the poleward advection of NIWs. However, the Ryukyu Current can be impeded by westward-propagating cyclonic eddies from the North Pacific, which further disrupts the waveguide.

DOI： 10.1029/2024JC021169

Scopus

Language：Japanese   Publisher：Earth and Space Science  

Mesoscale convective systems (MCSs) that occur in the Baiu frontal zone (BFZ) can cause devastating flash floods during early summer in Japan; however, the environmental conditions necessary for their development require further investigation. High-frequency atmospheric soundings, conducted using multiple marine vessels in the East China Sea on 19 June 2022, captured the detailed environmental conditions pertaining to the development of an MCS within the BFZ. The MCS, which developed rapidly without any remarkable preceding synoptic or mesoscale disturbance in the mid- or upper troposphere, caused intense precipitation exceeding 80 mm/hr. The MCS persisted for approximately 6 hr, and it intensified when the influx of nearly saturated air near the sea surface toward a weak surface front overlapped with the influx of free-tropospheric moist air. The influx of nearly saturated air near the sea surface ensured conditional instability within the lower troposphere. The influx of moist air in the free troposphere contributed to the near-saturation conditions above the boundary layer, a feature inherent to the BFZ, and played an important role in minimizing the reduction in the buoyancy of air parcels. The results of this study indicate that a better forecast of the horizontal distribution of free tropospheric moist air is beneficial for limiting the potential area of genesis of MCS in the BFZ, and a more comprehensive understanding of the vertical variations in moisture transport contributes to an improved forecast skill for MCS in the BFZ.

DOI： 10.1029/2023EA003486

Scopus

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

Submesoscale processes are essential in balancing global oceanic energy cascades and stimulating air–sea communications. However, measurements of submesoscale turbulence are still not acquired easily. Based on long-term (2003–2012) shipboard current observations, we obtained direct evidence of considerable submesoscale energization in the Kuroshio, implied by conspicuous spectral characteristics across multiple scales. The submesoscale motions (peak at ∼26 km) exhibited topographically dependent features and typical contrarotating vortex-pair forms for island wakes, indicating footprints of upstream islands and seamounts. The vortex regimes and stability were analyzed using dimensionless parameters. Furthermore, the submesoscale motions radiated energy in the form of near-inertial waves, which exhibited consistent shear structures, and spread widely across the entire strait. These results highlight the role of topography in modulating dynamics by continuously promoting energy cascades into submesoscales, which is expected to be universal for rough topography in the ocean.

DOI： 10.1029/2022JC019561

Scopus

Language：Japanese   Publisher：Journal of Physical Oceanography  

Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity, and microscale thermal-variance dissipation rate up-and downstream of the ~10-km-wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates ε ~ O(10^-7 –10^-6) W kg^-1 and diapycnal diffu-sivities K ~ O(10^-2) m² s^-1 above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence, which is expected to decay in less than 100 m based on turbulence decay time of N^-1 ~ 100 s and the 0.5 m s^-1 Kuroshio flow speed. Thus, the turbulent wake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortex wake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy, and (iii) reduced-shear, are tested. Average ε vertical profiles are well reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of -1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.

DOI： 10.1175/JPO-D-22-0242.1

Scopus

Language：Japanese   Publisher：Frontiers in Marine Science  

Entering the East China Sea (ECS) through the east of Taiwan, the Kuroshio flows northeastward, with its pathway confined along the continental slope of the Okinawa Trough. Four 75 kHz Acoustic Doppler Current Profilers (ADCPs) were deployed over a 3-year period (June 2020–June 2023), with two placed across the typical path of the Kuroshio in each of the two latitudinal bands, to examine the relationship of current variability between upstream and downstream in the ECS. The along-path and cross-path velocities from the four ADCPs show dominant intraseasonal variability with a period of 8–32 days. Within this frequency band, current velocities exhibit a strong relationship when the upstream precedes the downstream by 6 days. The amplitude of this lagged co-variability tends to be larger from May to August, with an increase in along-path speed variability of up to about 24% at approximately 250 m depth for the onshore sites and an increase in cross-path speed variability of up to about 22% within the 200–300 m layer for the offshore sites. The intraseasonal variability of the sea level anomaly associated with this observed co-variability displays the Kuroshio meanders along its path, propagating northeastward at a speed of about 14.8 km/day. These results provide observational evidence of the Kuroshio meander and its propagation in the ECS, characterized by subsurface intensification during the warm season.

DOI： 10.3389/fmars.2024.1512572

Scopus

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

Using observational and reanalysis data sets, we investigated interannual variability in the Kuroshio in the East China Sea (ECS-Kuroshio). We exhibited that the surface velocity and position of the ECS-Kuroshio were synchronized on a quasi-3-year interannual timescale during 2005–2016. We further demonstrated that: (a) during 2005–2016, wind stress curl variability related to the El Niño–Southern Oscillation (ENSO) played a leading role in the interannual ECS-Kuroshio variability by exciting baroclinic Rossby waves along the subtropical countercurrent (STCC) zone east of Taiwan; (b) mesoscale eddy activities in the STCC zone, especially long-lived (≥150-day lifetime) cyclonic eddies, probably played a secondary role in the interannual ECS-Kuroshio variability. In addition, we showed that the occurrence of the quasi-3-year interannual variability of the ECS-Kuroshio since 2005 was likely linked to the following changes in the ENSO-related atmospheric circulation: (a) The primary ENSO timescale changed from a quasi-5-year period in 1993–2004 to a quasi-3-year one in 2005–2016; (b) Over the central equatorial Pacific along with the eastern tropical Indian Ocean, the sea surface temperature in 2005–2016 varied with a more intense amplitude than in 1993–2004, which resulted in a different western North Pacific atmospheric response to the ENSO in 2005–2016 from that in 1993–2004.

DOI： 10.1029/2023JC019749

Scopus

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

Linear trends in the Ryukyu Current, a part of the western boundary current in the western North Pacific flowing on the seaside of the Ryukyu Island chain, were investigated using reanalysis data during 1993–2018. The subsurface Ryukyu Current has weakened along its path during the recent decades. Two determinant factors for the weakened subsurface Ryukyu Current are as follows. First, the first baroclinic topographic Rossby wave propagated signals emanating from the Tokara Strait (TK) and the Kerama Gap (KG) southward along the eastern slope of the Ryukyu Island chain, which depressed the onshore side of the isopycnal. The weakened Kuroshio in the TK and weakened overflow through the KG during this period led to isopycnal shoaling along the onshore side east of the Ryukyu Island chain, slowing down the Ryukyu Current. Second, anticyclonic eddies emanated from the interior region near 30°N and 170°W increased during this period. These anticyclonic eddies, translating southwestward, reached east of the Ryukyu Island chain, and finally deepened the offshore side of the isopycnal depth of the Ryukyu Current. The isopycnal across the Ryukyu Current velocity core became less steep, thereby weakening the subsurface Ryukyu Current. Moreover, a positive trend of sea surface height anomaly southeast of Miyakojima, driven by wind stress changes in high latitudes (near the Kuroshio Extension band), strengthened the northward current in the upper layer southeast of Miyakojima.

DOI： 10.1029/2022JC018957

Scopus

Language：Japanese   Publisher：The Japanese Society of Fisheries Oceanography  

<p>While it has been suggested that advections of coastal waters stimulate plankton standing stocks and productivity in the oligotrophic Kuroshio, there is still limited information. Here, we explore the source and process of coastal waters advected to the Kuroshio using particle-tracking experiments with a high-resolution coastal ocean model. Advections of coastal waters to the Kuroshio were stimulated by encountering frequent disturbances such as meandering and eddy near the Kuroshio front. The source of coastal waters advected to the Kuroshio were the largest from southern Kyushu throughout the year and from the East China Sea during winter to spring. Zooplankton biomass fluxes across the Kuroshio front were equivalent to 2 to 44% of those across the Kuroshio path. The estimated fluxes across the Kuroshio front were comparable to zooplankton biomass increase by their growth rates reported in the Kuroshio. These results suggest that the coastal zooplankton community advected to the Kuroshio supports food requirements for the fish community in the Kuroshio.</p>

DOI： 10.34423/jsfo.86.4_153

CiNii Research

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

Topographic Rossby waves (TRWs) are motions triggered by potential vorticity adjustments, which strongly contribute to deep ocean variabilities. This study discerned near-36-day significant along-slope fluctuations, speculated to be TRWs, using deep current observations over the continental slope in the East China Sea (ECS). The TRWs propagated southwestward along the continental slope with wavelengths of ∼95 km in the northern stations and ∼36 km in the southern stations where the slope is steeper. The group velocity was estimated to be ∼15.5 km/day, and the maximum current speed was ∼10 cm/s. It was assumed that the energy source originates from the Kuroshio path meander, which has a similar typical period to the TRWs and can induce surface eddy kinetic energy, along with depth variations of water column in west of the Tokara Strait. Analyses involving ray tracing model of TRWs showed consistency with the observations in wave characteristics, thereby confirming our postulation regarding the TRWs and their origin. Moreover, the energy rays, emanated from the origin, intensively propagated along the slope; and after climbing over the successive slope, eventually stopped when reaching the flat areas. Additionally, the TRWs' hyperbolic intensification and vertical coherent phase features were revealed in model results. The seasonality of the Kuroshio path meander induced depth variation of water column was estimated to be prevalent in February and August. Overall, this study revealed the propagation features and generation mechanism of TRWs over the continental slope in the ECS for the first time.

DOI： 10.1029/2022JC018667

Scopus

Language：Japanese   Publisher：Progress in Oceanography  

Using satellite altimetry and wind stress data, the spatially coherent decadal variabilities in current intensity and current path during 1993–2018 were investigated for a western boundary current system, namely, the Kuroshio from the east of Luzon Island to the south of Japan and the Kuroshio Extension (KE). Analyses based on Hovmöller diagrams showing decadal variabilities along the entire current demonstrated three types of synchronic relationship between different regions: 1) an out-of-phase relationship in current intensity between the Kuroshio south of Japan and the KE during 1993–2018; 2) an out-of-phase relationship in current intensity between the Kuroshio from the east of Taiwan (ETW) to the East China Sea (ECS) and the KE during 2006–2014; 3) an out-of-phase relationship in current position between the Kuroshio paths in the ETW to ECS and the Luzon Strait during 1993–2018. These patterns were dynamically regulated by the Pacific Decadal Oscillation (PDO). The synchronized current-intensity variability in 2006–2014, which governed almost the entire current system, was caused by coherent phenomena comprising two kinds of baroclinic Rossby wave propagations along the KE and the subtropical countercurrent (STCC), and a regional sea surface height (SSH) anomaly advection from the South China Sea (SCS) to the ECS via the Taiwan Strait. However, the synchronized current-path variability between 1993 and 2018 was caused by an SSH anomaly migration advected by the Kuroshio from the western North Equatorial Current zone to the ECS and an SSH variability localized in the SCS. In this study, we paid particular attention to that the current intensity of the Kuroshio in the ETW to ECS had a positive no-lag correlation with the SSH-based PDO index in 1996–2005 while a positive 3–4 years lag correlation with that index during 2006–2014. It was emphasized that this transition was due to the following mechanism: the current-intensity variability in 1996–2005 was mainly caused by the variability of the eddy activity in the western STCC zone, while that variability in 2006–2014 was mainly caused by the amplification of baroclinic Rossby waves propagating along the STCC zone; the shift from the former to the latter was generated by a rapid phase shift with a relatively large amplitude for the wind stress curl anomaly and a timescale change of the eddy kinetic energy over the STCC zone around 2006.

DOI： 10.1016/j.pocean.2022.102808

Scopus

Language：Japanese   Publisher：Communications Earth and Environment  

Although previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. Here, we conducted a series of intensive in-situ turbulence observations using a state-of-the-art tow-yo microstructure profiler in the Kuroshio flowing over the seamounts of the Tokara Strait, south of Kyusyu Japan, in November 2017, June 2018, and November 2019, and employed a high-resolution numerical model to elucidate the turbulence generation mechanisms. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. The observed turbulence could supply nutrients to sunlit layers, promoting phytoplankton primary production and CO<inf>2</inf> uptake.

DOI： 10.1038/s43247-021-00230-7

Scopus

Language：English   Publisher：Geophysical Research Letters  

Linear trends in Kuroshio transport during a recent global warming hiatus (1998–2013) were evaluated using long-term ferryboat ADCP (acoustic Doppler current profiler) data and tidal gauge data in the Tokara Strait south of Japan. The Kuroshio exhibited a remarkable weakening trend of approximately 0.05 Sv year⁻¹ (1 Sv = 10⁶ m³ s⁻¹). The pycnocline in the weakened Kuroshio was relaxed and displayed shoaling at the offshore edge, which was attributed to vertical thermocline displacement rather than to water mass modification. Importantly, Kuroshio transport trends in the Tokara Strait were affected by sea surface height anomalies, which were driven by the combined effects of the clockwise baroclinic-mode coastal trapped wave propagation along the southern coast of Japan and downstream Kuroshio advection in the East China Sea. Both features were induced by wind stress curl changes related to the global warming hiatus over the North Pacific.

DOI： 10.1029/2021GL094169

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Geophysical Research Letters  

Vertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin-Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo-sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10⁻¹ m² s⁻¹), resulting in strong nitrate fluxes of O(1–10³ mmol m⁻² day⁻¹). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10⁻³ m s⁻¹), as well as an upwelling nitrate flux of O(10² mmol m⁻² day⁻¹) in the entire subsurface layer.

DOI： 10.1029/2020GL092063

Scopus

Language：Japanese   Publisher：Journal of Oceanography  

Seasonal variations in vertical mixing intensity in the intermediate layer of the northern Okinawa Trough was deduced using the seasonal variations in salinity in the North Pacific Intermediate Water (NPIW) using two distinct datasets: the World Ocean Atlas 2013 and historical conductivity, temperature, and depth data in the Okinawa Trough. Water mass analysis of the NPIW with a simple advection model showed that the vertical mixing intensity in the intermediate layer is enhanced in the winter at a rate of 20–50% of the annual mean amplitude value. To determine the driving force behind the seasonal variations in vertical mixing, we examined moored current-meter data in the northern Okinawa Trough during 2004–2011 and showed that the near-inertial internal wave energy in the intermediate layer was enhanced around January beneath the Kuroshio on the continental slope; this was due to strong southwestward flows associated with deep cyclonic eddies developing in the winter. We used density and velocity data from a realistic high-resolution ocean general circulation model simulation to hypothesize that the large-amplitude Kuroshio meander and associated deep cyclonic eddies in the winter generate near-inertial internal waves through centrifugal/inertial instability in the area of negative potential vorticity that appears not only beneath the Kuroshio near the western slope but also near the eastern slope of the trough. In addition, the symmetric instability along the offshore Kuroshio front and the wind energy input in the winter are suggested as possible sources of winter-enhanced near-inertial internal wave energy in the intermediate layer.

DOI： 10.1007/s10872-020-00585-z

Scopus

Language：English  

[Abstract] / Vertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin-Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo-sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10^−1 m^2 s^−1), resulting in strong nitrate fluxes of O(1–10^3 mmol m^−2 day^−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10^−3 m s^−1), as well as an upwelling nitrate flux of O(10^2 mmol m^−2 day^−1) in the entire subsurface layer. / 　 / [Plain Language Summary] / Vertical nitrate fluxes associated with flow-topography interactions around a small reef in the Kuroshio Current are quantified using state-of-the-art oceanographic instruments. When the flow passes over a shallow sill on the flank of the reef, the velocity differences between layers intensify, resulting in a substantial vertical overturning and mixing of the water column. This turbulent mixing causes the observed nitrate flux from deep water to reach a value among the highest observed worldwide. / 　 / [Key Points] / • Flow separations and trains of Kelvin-Helmholtz billows mix the water column around a small reef in the Kuroshio / • Doming of isopycnals/nitraclines suggests strong upwelling in the lee of the reef / • Turbulent nitrate fluxes reach up to O(10^3 mmol m^−2 day^−1)

CiNii Research

Language：Japanese   Publisher：Scientific Reports  

The origin, structure, and variability of the Ryukyu Current (RC) have long been debated, mostly due to limited observations. A mooring array, deployed for two years southeast of Miyakojima in the southern portion of the Ryukyu Island chain, has provided, for the first time, data confirming the existence and revealing the characteristics of the RC in that upstream region, including its velocity structure and variability. The observations show a shoreward-intensified current flowing northeastward, with a subsurface core located near the 1,000 m isobath and having a record-long mean speed of up to 19.4 cm s⁻¹ at 500 m depth. Estimated volume transport across the observation section had mean 9.0 Sv (1 Sv = 10⁶ m³ s⁻¹) and standard deviation 8.7 Sv. The RC shows significant barotropic character compared with other similar mid-latitude currents.

DOI： 10.1038/s41598-020-63836-5

Scopus

PubMed

Language：Japanese   Publisher：Acta Oceanologica Sinica  

Between June 2015 and June 2017, two pressure-recording inverted echo sounders (PIESs) and five current and pressure-recording inverted echo sounders (CPIESs) deployed along a section across the Kerama Gap acquired a dataset of ocean bottom pressure records in which there was significant 21-day variability (P<inf>bot21</inf>). The P<inf>bot21</inf>, which was particularly strong from July-December 2016, was coherent with wind stress curl (WSC) on the continental shelf of the East China Sea (ECS) with a squared coherence of 0.65 for a 3-day time lag. A barotropic ocean model demonstrated the generation, propagation, and dissipation of P<inf>bot21</inf>. The modeled results show that the P<inf>bot21</inf> driven by coastal ocean WSC in the ECS propagated toward the Ryukyu Island Chain (RIC), while deep ocean WSC could not induce such variability. On the continental shelf, the P<inf>bot21</inf> was generated nearly synchronously with the WSC from the coastline to the southeast but dissipated within a few days due to the effect of bottom friction. The detection of P<inf>bot21</inf> by the moored array was dependent on the 21-day WSC patterns on the continental shelf. The P<inf>bot21</inf> driven southeast of the Changjiang Estuary by the WSC was detected while the Pb<inf>ot21</inf> generated northeast of the Changjiang Estuary was not.

DOI： 10.1007/s13131-020-1603-3

Scopus

Language：Japanese   Publisher：Journal of Marine Science and Engineering  

Herein, the temporal variability of the deep overflow through the Kerama Gap between the East China Sea and the Philippine Sea is investigated based on observational data combined with reanalysis data obtained during 2004-2011. The observations and model results show a strong bottom-intensified flow intruding into the deep Okinawa Trough. The observed deep overflow shows intraseasonal variations that are enhanced from August to November. The variability in the deep overflow via the Kerama Gap is well-correlated with the density changes near its sill depth in the Philippine Sea. Additionally, some portion of the dense water originates from a region east of Miyakojima, which can be related to the northeastward-flowing Ryukyu Current at intermediate depths. In contrast, three extreme deep overflow events indicate that the arriving mesoscale eddies propagated from the east resulted in an increase in the density near the Kerama Gap sill than that on the Okinawa Trough side. The density difference associated with the baroclinic pressure gradient across the Kerama Gap forced the deep overflow into the Okinawa Trough. The volume transport of the deep overflow computed by integrating the cross-sectional velocity and through hydraulic theory are 0.14 and 0.11 Sv (1 Sv = 10⁶ m³/s), respectively.

DOI： 10.3390/JMSE8060402

Scopus

Language：Japanese   Publisher：Biogeosciences  

The Kuroshio Current has been thought to be biologically unproductive because of its oligotrophic conditions and low plankton standing stocks. Even though vulnerable life stages of major foraging fishes risk being entrapped by frontal eddies and meanders and encountering low food availability, they have life cycle strategies that include growing and recruiting around the Kuroshio Current. Here we report that phytoplankton growth and consumption by microzooplankton are stimulated by turbulent nitrate flux amplified by the Kuroshio Current. Oceanographic observations demonstrate that the Kuroshio Current topographically enhances significant turbulent mixing and nitrate influx to the euphotic zone. Graduated nutrient enrichment experiments show that growth rates of phytoplankton and microheterotroph communities were stimulated within the range of the turbulent nitrate flux. Results of dilution experiments imply significant microzooplankton grazing on phytoplankton. We propose that these rapid and systematic trophodynamics enhance biological productivity in the Kuroshio.

DOI： 10.5194/bg-17-2441-2020

Scopus

Language：Japanese   Publisher：Scientific Reports  

Near-inertial waves (NIWs), which have clockwise (anticlockwise) rotational motion in the Northern (Southern) Hemisphere, exist everywhere in the ocean except at the equator; their frequencies are largely determined by the local inertial frequency, f. It is thought that they supply about 25% of the energy for global ocean mixing through turbulence resulting from their strong current shear and breaking; this contributes mainly to upper-ocean mixing which is related to air-sea interaction, typhoon genesis, marine ecosystem, carbon cycle, and climate change. Observations and numerical simulations have shown that the low-mode NIWs can travel many hundreds of kilometres from a source region toward the equator because the lower inertial frequency at lower latitudes allows their free propagation. Here, using observations and a numerical simulation, we demonstrate poleward propagation of typhoon-induced NIWs by a western boundary current, the Kuroshio. Negative relative vorticity, meaning anticyclonic rotational tendency opposite to the Earth’s spin, existing along the right-hand side of the Kuroshio path, makes the local inertial frequency shift to a lower value, thereby trapping the waves. This negative vorticity region works like a waveguide for NIW propagation, and the strong Kuroshio current advects the waves poleward with a speed ~85% of the local current. This finding emphasizes that background currents such as the Kuroshio and the Gulf Stream play a significant role in redistribution of the NIW energy available for global ocean mixing.

DOI： 10.1038/s41598-019-46364-9

Scopus

PubMed

Language：Japanese   Publisher：Journal of Geophysical Research: Oceans  

Transverse-vertical structure and temporal variability of the Kuroshio current across the Tokara Strait during 2003–2012 measured by a ferryboat acoustic Doppler current profiler with a 2-km horizontal resolution and a two-day interval are presented. The Kuroshio passing through the Tokara Strait exhibits a multicore velocity structure. Its seasonal volume transport variation is biannual for baroclinic components relative to 700 m, peaking in July and December–January. However, the barotropic transport component exhibits an annual cycle with a maximum in December. Empirical orthogonal function analysis of the cross-sectional velocity is performed. The first two empirical orthogonal function modes reveal the north-south shift of the Kuroshio current axis and the change in Kuroshio volume transport, respectively. Temporal variabilities of the leading two modes correspond to those of the Kuroshio Position Index and the sea level difference across the strait, respectively. The third empirical orthogonal function mode, with a relatively smaller horizontal scale, was examined in terms of turbulent mixing. The banded structure captured by this mode is likely induced by flow-topography interaction because islands in the Kuroshio route could cause horizontal and vertical flow separation. Additional analysis based on high-resolution reanalysis data suggested that (1) inertial instability, which is expected in the areas with negative Ertel's potential vorticity, arises to enhance vertical mixing around the islands in the Tokara Strait, and (2) when the Kuroshio directly impinges the islands, flow divergence in the lee of the islands drives upwelling and leads to uplift of isotherms.

DOI： 10.1029/2018JC014771

Scopus

Language：Japanese   Publisher：Geophysical Research Letters  

The Kuroshio Current carries a large amount of nutrients in dark subsurface layer along the southern coast of Japan. However, due to lack of sufficient multidisciplinary high-resolution observations, it has been unclear whether and how the subsurface nutrients are injected to sunlit layers on the continental shelf. In this study, using a state-of-the-art tow-yo microstructure profiler and nitrate measurements, we show that the Kuroshio flowing on the shelfbreak induces very strong turbulent dissipation rate ∼O(10⁻⁷W·kg⁻¹) and diffusivity ∼O(10⁻³ m²·s⁻¹) over an across frontal scale of 20–30 km. This strong turbulence could provide a diffusive nitrate flux to euphotic zone of >O(1 mmol N·m⁻²·day⁻¹). As the Kuroshio steadily runs on the continental shelves along the path, the nutrient injection found in this study is a very important supply for biological production on the continental margin along the southern coast of Japan.

DOI： 10.1029/2019GL082680

Scopus

Language：Japanese   Publisher：Coastal Oceanography Research Committee, the Oceanographic Society of Japan  

Plankton community structure, standing stocks and productivity were compared between upstream and downstream of theTokara Strait based on Lagrangian oceanographic observations and sampling along the Kuroshio. Surface waters above 200mwere represented by lower temperature and salinity, and nutrients and chlorophyll a concentrations at the subsurface were higherdownstream than upstream. No significant difference was found in the zooplankton community structure, and the predominantcomponents were naked ciliates for microzooplankton and copepods for net zooplankton. Although the protein synthetase activityof the net zooplankton community was high throughout the stations, large metazoans were one cause of the increased biomass andproduction rates downstream. These results suggest that the nutrient supply and phytoplankton increased at the subsurface, andthe net zooplankton biomass and productivity increased downstream of the Tokara Strait.

DOI： 10.32142/engankaiyo.2019.8.009

CiNii Research

Language：Japanese   Publisher：The Japanese Society of Fisheries Oceanography  

<p>This study investigated transport processes of red tilefish <i>Branchiostegus japonicus</i> from the East China Sea to the coastal area of Miyazaki Prefecture by particle-tracking experiments. Particles were released from areas along the shelf break of the East China Sea in autumn, which are the main spawning ground and season, respectively, and the movement patterns and paths were examined using reanalysis data of an ocean data assimilation system. After 45 days from the release, which is the average duration until settlement, 0.01–0.7% of particles released in the East China Sea reached the coastal area of Miyazaki Prefecture. Among them, 90% and 10% of particles passed through the Ohsumi and the Tokara Straits, respectively. The number of particles reaching Miyazaki Prefecture was positively correlated with those passing through the Ohsumi Strait, while mass arrival events were further related to the deceleration of the Ohsumi Branch Current caused by a small meander of the Kuroshio. Proportions of particles by destination at Day 45, including those not reaching Miyazaki Prefecture, were 10.8% to the Pacific, 1.5% to the Sea of Japan, and 88% remained within the East China Sea. Therefore, it is likely that red tilefish has a major reproduction cycle within the East China Sea, while larvae and juveniles were also supplied from there to Japanese coastal areas. Finally, it is suggested that the Ohsumi Strait is the main passage of larval/juvenile transport of red tilefish, from the East China Sea to the coastal areas of Miyazaki prefecture.</p>

DOI： 10.34423/jsfo.83.2_93

CiNii Research

Language：Japanese   Publisher：Coastal Oceanography Research Committee, the Oceanographic Society of Japan  

Under the Grants-in-Aid for Scientific Research in Innovative Areas（MEXT）project "Ocean Mixing Study（OMIX）", weestablished a system to measure the turbulent nitrate flux by attaching a small nitrate sensor on a turbulence profiler. In thispaper, we explain the instrument setup, the data processing method considering measurement errors, and then demonstrate thecapability of the developed observational system.

DOI： 10.32142/engankaiyo.2019.8.008

CiNii Research

Language：Japanese   Publisher：Japan Society of Civil Engineers  

&nbsp;We have measured a time series of current velocity off Cape Shiono-misaki from April 20, 2016 to April 20, 2017 with a moored ADCP in order to investigate the characteristics of variation of the Kuroshio current in relation to ocean current power generator. The Kuroshio has taken a non-large-meander path for the measurement period. It is clarified from the measurement results that the currents at a depth of about 50m are characterized by the properties that the averaged velocity for one year is 100cm/s and the current direction is almost constant in the direction between the east and east-southeast.

DOI： 10.2208/jscejoe.74.I_928

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2017JC012876

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2016JC012329

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2013JC008899

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Civil Engineers  

Suitable places for ocean current power generation in the East China Sea and the southern region of Honshu, Japan were ex-amined using the data of current path of the Kuroshio at sea surface and current velocity of the Kuroshio. Therefore, the Tokara Strait is shown to be one of the suitable places for ocean power generation, using the strong Kuroshio current. Moreover, it is found that the Kuroshio flows with a speed greater than 0. 8m/s above 60m depth north of Kuchino-shima and south of Nakano-shima.

DOI： 10.2208/jscejoe.69.i_109

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

DOI： 10.1002/2013JC009326

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1175/JCLI-D-12-00326.1

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Language：Japanese   Publisher：Journal of Oceanography  

A time series of surface geostrophic velocity is developed using satellite altimetry data during 1992-2010 for a track across the Kuroshio southeast of Kyushu, Japan. The temporal mean geostrophic velocity is estimated by combining the along-track sea level anomaly and shipboard ADCP data. This approximately 6-km resolution dataset is successful in representing the Kuroshio cross-current structure and temporal variation of the Kuroshio current-axis position during 2000-2010. The authors use this dataset to examine the winter Kuroshio path destabilization phenomenon. Its seasonal features are characterized as follows: the velocity shear on the inshore side of the Kuroshio becomes stronger and the Kuroshio path state becomes unstable from the summer to winter. This evidence is consistent with the hypothetical mechanism governing the destabilization phenomenon discussed in a previous study. Furthermore, the interannual amplitude modulation of the seasonality is examined in relation to interannual variations in the winter northerly wind over the northern Okinawa Trough and the Pacific Decadal Oscillation (PDO) index. The destabilization phenomenon appears 15 times in the period 2000-2010. Ten cases are related to local wind effects, and 7 of these are also connected with the PDO index. This is probably because the winter northerly wind over the northern Okinawa Trough is regulated by the PDO signal in interannual time-scales. Only 4 cases are related to the PDO index, but their driving mechanism remains uncertain. © 2012 The Oceanographic Society of Japan and Springer.

DOI： 10.1007/s10872-012-0101-4

Scopus

CiNii Research

Language：Japanese   Publisher：Japan Society of Civil Engineers  

Characteristics of oceanic water inflow into Kagoshima Bay were examined using water temperature and current velocity data as well as satellite images of thermal infrared sensor. The oceanic water was shown to enter into Kagoshima Bay along the Osumi Peninsula when the Kuroshio warm filament with horizontal scales less than 150km passes through the Tokara Strait. Moreover, it was suggested that the oceanic water moves from the mouth to the central part of the bay with current speeds in range of 0.16 to 0.26m/s at the neap tide or at the transitional tide between spring and neap tides.

DOI： 10.2208/jscejoe.67.i_715

CiNii Research

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

The Okinawa Trough is geographically separated into the northern and southern parts by the Kerama Gap, and the northern part is composed of the northern, central and southern subbasins formed by bights of the continental slope. Previous observations have indicated that a deep countercurrent is present beneath the Kuroshio on the continental slope in the northern Okinawa Trough. However, its persistence over time and its spatial structure over the entire basin have not been clarified to date. The present study examines moored current meter data on the continental slope in the southern and central subbasins of the northern Okinawa Trough for November 2004 to November 2006. Deep flows on the continental slope have a clear seasonality in the southern subbasin; eddy motions due to Kuroshio meanders are organized into a persistent countercurrent below the Kuroshio in the winter-spring period. During this time, high-frequency Kuroshio meanders with periods near 10 d are likely to diminish in the southern subbasin while low-frequency Kuroshio meanders with periods of 1-3 months tend to dominate in the northern subbasin. In addition, a high-resolution ocean general circulation model is used to explore the deep flow field over the entire Okinawa Trough. The model indicates that the deep flow field is stable in the southern Okinawa Trough over the year, whereas it is unstable in the northern Okinawa Trough particularly in the winter-spring period. This results in a persistent countercurrent driven by cyclonic eddies on the continental slope in the northern Okinawa Trough. Copyright 2008 by the American Geophysical Union.

DOI： 10.1029/2007JC004574

Scopus

Language：Japanese   Publisher：Journal of Oceanography  

Two inverted echo sounders were maintained on coastal and offshore sides of the Kuroshio south of Japan from October 1993 to July 2004. Applying the gravest empirical mode method, we obtained a time series of geostrophic transport. Estimated transports generally agree well with geostrophic transports estimated from hydrography. Their agreement with the hydrographic transports is better than that of transports estimated from satellite altimetry data. The geostrophic transport is expressed as the surface transport per unit depth multiplied by the equivalent depth. The geostrophic transport varies mostly with the surface transport and fractionally with the equivalent depth. Seasonal variation of the geostrophic transport has a minimum in March and a maximum in September, with a range of about one fifth of the total transport. © The Oceanographic Society of Japan/TERRAPUB/Springer 2008.

DOI： 10.1007/s10872-008-0030-4

Scopus

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

Recent observations indicate that the Ryukyu Current system, part of the western boundary current of the North Pacific subtropical gyre on the eastern slope of the Ryukyu Islands, Japan, exhibits bottom intensification to the north of Okinawa Island and a nearly barotropic current to the south. We performed numerical experiments of this system using two primitive equation models that incorporate realistic and idealized topography, respectively, to identify the processes that act to maintain the Ryukyu Current system. The dynamics responsible for baroclinic structures of the Ryukyu Current system are examined in detail by diagnosing numerical solutions based on a characteristic equation derived from two-layer planetary geostrophic equations. The diagnoses show that bottom intensification of the Ryukyu Current system is primarily formed along the characteristics due to the first baroclinic mode topographic Rossby wave that emanates from the Kuroshio in the Tokara Strait. This topographic wave depresses the main thermocline along the inshore slope area east of the Ryukyu Islands such that the passage of the wave over the barotropic northward flow along the slope establishes a state, of no motion in the upper layer and bottom intensification of the northward current in the lower, layer. The diagnoses also suggest that the characteristics that result from this topographic wave are expelled in the slope area south of Okinawa Island by the first baroclinic mode planetary Rossby wave that propagates westward from the North Pacific interior region, resulting in bottom intensification only in the northern slope area. Copyright 2007 by the American Geophysical Union.

DOI： 10.1029/2006JC003595

Scopus

Language：Japanese   Publisher：Geophysical Research Letters  

The meander of the Kuroshio path, with periods of 30-90 days, dominates Tokara Strait, Japan. To understand the time-frequency variability of this meander, two Kuroshio indicators were subjected to wavelet analysis for the period 1984-2004. The 30-90 day variance of the position of the Kuroshio in Tokara Strait shows a seasonfixed nature, with larger amplitudes in the period December-July. The amplitude of the variance in this phenomenon is also modulated by interannual variations, with small variance recorded during 1989-1992, large variance during 1993-1998, and a return to small variance from 1999-2003. This interannual variation is positively correlated with that of Kuroshio volume transport in Tokara Strait. The largest variance during February-April precedes the largest volume transport in April-May by about 1 month, suggesting that mesoscale menders are the forcing of the volume transport increase, not the other way around. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2006GL027516

Scopus

Language：Japanese   Publisher：Research Institute for Applied Mechanics, Kyushu University  

Two inverted echo sounder (IES) sites were maintained on coastal and offshore sides of the Kuroshio south of Japan during October 1993 through July 2004. IESs set on the sea floor measure round-trip acoustic travel times repeatedly between the sea floor and the sea-surface. Applying the gravest empirical mode (GEM) method, we obtain the 9-year long time series of the sea-surface dynamic height (SSDH). The estimated tune series are compared with SSDH from hydrographic data. The IES-derived SSDH agrees well with the hydrographic SSDH referred to 750 dbar on the coastal site and 2000 dbar on the offshore site, although the variational range of the IES-derived SSDH is slightly smaller than the hydrographic one. The estimated errors of the IES-derived SSDH are 4.5 cm on the coastal site and 5.2 cm on the offshore site.

DOI： 10.15017/26812

CiNii Research

Language：Japanese   Publisher：Journal of Oceanography  

We describe an operational ocean data assimilation system for the Kuroshio and its validation using a nine-year reanalysis (historical run from 1993 to 2001) dataset of upper-ocean state estimation in the North Pacific. The horizontal structure of volume transport of the Ryukyu Current System (RCS) is shown from the reanalysis: The RCS is connected to the flow of the subtropical gyre, and its volume transport gradually increases from south-east of Okinawa (5-10 Sv) to the east of Amami-Ohshima Island (20 Sv). Comparing the reanalysis with independent observations on the southeast slope of the Amami-Ohshima Island indicates that the root mean square differences (RMSDs) are 0.076 (0.037) m/s in the period of December 1998 to November 1999 (November 1999 to November 2000) respectively. The reanalysis field has a bias (3.1 Sv) of the volume transport of the RCS and the RMSD (3.5 Sv) which is larger than the observed variability (2.81 Sv). Surface velocity and the Kuroshio axis south of Japan are also examined. Comparison of the reanalysis and ADCP data gave maximum RMSD of 0.749 (0.271) m/s in the strong (weak) current regions, respectively. The annual mean value of the axis error is 19 km in 1998. The RMSD of the error is at most 50 km, in 294 cases in the observation period, which is smaller than the observed root mean square variability of the axis (64 km). Copyright © The Oceanographic Society of Japan.

DOI： 10.1023/B:JOCE.0000038336.87717.b7

Scopus

Language：Japanese   Publisher：Journal of Oceanography  

To better understand the mechanism underlying the variation of the Kuroshio south of central Japan, we have examined the variability of current structure in its upstream region, southeast of Amami-Ohshima Island in the northern Ryukyu Islands. By combined use of ship-mounted Acoustic Doppler Current Profiler (ADCP) and the TOPEX/POSEIDON satellite altimeter data on Path 214, the sea surface absolute geostrophic currents were estimated every ten days from January 1998 to July 2002. The 4.5-year mean surface current was found to flow northeastward north of 26.8°N with a maximum speed of 14 cm s^-1 over the shelf slope at 3000 m depth. The moored current-meter observations at three or four mooring stations from Dec. 1998 to Oct. 2002 suggested the existence of a northeastward undercurrent with a maximum core velocity of 23 cm s^-1 at 600 m depth over the shelf slope at 1600 m depth. The mean volume transport in the top 1500 m between 27.9°N and 26.7°N is estimated to be 16 × 10⁶ m³s^-1 northeastward, including the subsurface core current related component of 4 × 10⁶ m³s^-1. © The Oceanographic Society of Japan.

DOI： 10.1023/B:JOCE.0000038341.27622.73

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Language：Japanese   Publisher：Nature  

Observations of changes in the properties of ocean waters have been restricted to surface or intermediate-depth waters, because the detection of change in bottom water is extremely difficult owing to the small magnitude of the expected signals. Nevertheless, temporal changes in the properties of such deep waters across an ocean basin are of particular interest, as they can be used to constrain the transport of water at the bottom of the ocean and to detect changes in the global thermohaline circulation. Here we present a comparison of a trans-Pacific survey completed in 1985 (refs 4, 5) and its repetition in 1999 (ref. 6). We find that the deepest waters of the North Pacific Ocean have warmed significantly across the entire width of the ocean basin. Our observations imply that changes in water properties are now detectable in water masses that have long been insulated from heat exchange with the atmosphere.

DOI： 10.1038/nature02337

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CiNii Research

Language：Japanese   Publisher：Journal of Geophysical Research Oceans  

Horizontal patterns and meander motions of the Kuroshio paths in the northern Okinawa Trough between the continental slope and the Tokara Strait are investigated using surface drifter buoy trajectory data, NOAA sea surface temperature (SST) measurements, and shipboard acoustic Doppler current profiler (ADCP) current observations. Temporal variations are also examined by spectral analyses of 1-year moored velocity/temperature records along the continental slope near 28.8°-30.5°N and of the Kuroshio position time series in the Tokara Strait. Drifter buoy trajectories show that the Kuroshio paths in the northern Okinawa Trough are quasibimodal in character consisting of the northern paths and southern ones, which are associated with anticyclonic and cyclonic Kuroshio circulations, respectively. The Kuroshio position time series show that the northern paths tend to be persistent and intermittently undergo transition to southern paths at periods of 1-3 months. Moored current variations in the slope area and the Kuroshio path variations in the Tokara Strait are highly coherent near a period of 34 days due to the meander motions resulting from the transitions between the northern and southern paths. Successive NOAA SST images and shipboard ADCP current fields show that the transition from the northern path to the southern one is associated with a spatially growing cyclonic eddy, which is initially generated from a downstream-propagating frontal meander with wavelength of about 200 km. When the cyclonic eddy grows into the scale of the northern Okinawa Trough (about 200-km E-W, 250-km N-S), the Kuroshio path changes from the northern path to the southern one. Copyright 2003 by the American Geophysical Union.

DOI： 10.1029/2002jc001450

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Language：Japanese  

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Language：Japanese  

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