Hu Y., Guo X., Sasai Y., Tsutsumi E., Nishina A., Nakamura H., Hasegawa D., Matsuno T. . Nutrient supply to the euphotic layer by the advection and vertical mixing over the Kuroshio and Kurosh2025 · 記述言語： 日本語 出版者・発行元： 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<sup>−1</sup> east of Luzon Island, 7.9 kmol s<sup>−1</sup> east of Taiwan, 8.9 kmol s<sup>−1</sup> near the Tokara Strait, 21.5 kmol s<sup>−1</sup> near the Izu-Ogasawara Ridge and 19.6 kmol s<sup>−1</sup> 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<sup>−1</sup>. Horizontal transport from Japan coast area contributes 14.3 kmol s<sup>−1</sup> 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<sup>−1</sup> 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
Senjyu T., Endoh T., Tsutsumi E., Matsuno T., Nakamura H., Nishina A. . High-frequency nonlinear internal waves in the Kuroshio observed by acoustic observations . Progress in Oceanogr2025 · 記述言語： 日本語 出版者・発行元： 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
Chen J., Zhu X.H., Zheng H., Nakamura H., Zhao R., Wang M., Park J.H., Nishina A. . Observation of Topographic Rossby Waves Triggered by Kuroshio Path Meander in the East China Sea . J2022 · 記述言語： 日本語 出版者・発行元： 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
Manda A., Tachibana Y., Nakamura H., Takikawa T., Nishina A., Moteki Q., Zhao N., Iizuka S. . Intensive Radiosonde Observations of Environmental Conditions on the Development of a Mesoscale Co2022 · 記述言語： 日本語 出版者・発行元： 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
