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Nishikawa Hatsumi

Faculty of Fisheries Sciences　Marine Bioresource and Environmental Science　Marine Environmental ScienceAssociate Professor

Researcher basic information

■ Degree
  • 博士(学術), 三重大学大学院
■ URL
researchmap URLホームページURL■ Various IDs
Researcher number
  • 10783392
J-Global ID■ Research Keywords and Fields
Research Field
  • Natural Science, Atmospheric and hydrospheric sciences

Career

■ Career
Career
  • Apr. 2020 - Mar. 2024
    The University of Tokyo, Atmosphere and Ocean Research Institute Division of Ocean-Earth System Science, 特任研究員
  • Jan. 2016 - Mar. 2020
    Hokkaido University, Institute of Low Temperature Science Pan-Okhotsk Research Center, 博士研究員
Committee Memberships
  • Jun. 2016 - Present
    日本気象学会, 人材育成・男女共同参画委員会, Society
  • Apr. 2017 - Mar. 2024
    日本海洋学会, 若手武者修行運営委員会, Society

Research activity information

■ Papers
  • Spatial structure of rain pool observed on the Kuroshio and Baiu fronts in the East China Sea
    Tetsutaro Takikawa; Yoshihiro Tachibana; Hirohiko Nakamura; Ayako Nishina; Atsuyoshi Manda; Hatsumi Nishikawa; Satoru Kasuga; Yuta Ando; Teruyuki Kato
    Journal of Oceanography, Springer Science and Business Media LLC, 07 May 2025, [Peer-reviewed], [International Magazine]
    Scientific journal, Abstract

    Spatial three-dimensional structure of low-salinity water on the Baiu and Kuroshio fronts is shown using intensive observation results by three ships in the East China Sea. The surface low-salinity pool along the Kuroshio front was formed by rainfall of the Baiu front during the observations. The horizontal distribution of the low-salinity pool with about 130 km long, 40 km wide, and 9–12 m thick along the Kuroshio front is consistent with the precipitation map of the weather radar. The relationship between the surface low-salinity water and precipitation was also quantitatively explained by the conservation of volume. Barrier layer below the surface rain pool had a thickness from several meters to a maximum of 46 m. Subsurface (~ 20-m depth) low-salinity patches were observed below the barrier layer. Based on the order of vertical flow, it is considered that the low-salinity patches moved from the sea surface to the subsurface layer in about 1 day, and their origin is presumed to be rainfall several days earlier. Below the low-salinity patches, we detected temperature inversion layer with warm and saline water, which is thought to have an effect similar to the barrier layer.
  • Decadal-scale reduction in net primary production in the western subarctic North Pacific: impact of lateral transport of dissolved iron from the Sea of Okhotsk
    Takuya Nakanowatari; Tomohiro Nakamura; Humio Mitsudera; Jun Nishioka; Hatsumi Nishikawa; Hiroshi Kuroda; Keisuke Uchimoto
    Environmental Research Letters, 20, 5, 054027, 054027, IOP Publishing, 11 Apr. 2025, [International Magazine]
    Scientific journal, Abstract

    The subarctic North Pacific (SNP) is a high-nutrient low-chlorophyll region in which decoupling of dissolved iron (dFe) and macronutrient is an essential control of primary production. In this study, we evaluate the influence of decadal-scale climate change on the net primary production (NPP) in the SNP by performing a hindcast experiment for 1979–2016 using an ice–ocean coupled model with a simple biogeochemical model with iron cycle. Simulation results show a significant NPP decrease in the subtropical–subarctic gyre boundary (SGB) region since the 1990s; the trend is −48 mgC m−2 d−1/37 years with a magnitude that is 14.3% of the climatological mean NPP. The NPP decrease in the SGB is prominent during the spring, indicating weakening of the spring bloom. Diagnostic analysis of simulation data reveals that the NPP decrease in the SGB can be explained by the decrease in both dFe and light availability. Sensitivity experiments indicate that wind-driven circulation change mainly explains both the reduction in dFe and the light availability through the northward expansion of oligotrophic subtropical water, but the thermohaline change in the Sea of Okhotsk also has a non-negligible effect on the dFe reduction. Results from our numerical model simulations suggest the importance of lateral and vertical advection of dFe on the decadal-scale changes of NPP in the SNP.
  • The Kuroshio large meander and its various impacts: a review
    Hidetaka Hirata; Hatsumi Nishikawa; Norihisa Usui; Toru Miyama; Shusaku Sugimoto; Akira Kusaka; Takashi Seto
    Journal of Oceanography, Springer Science and Business Media LLC, 29 Mar. 2025, [Peer-reviewed], [International Magazine]
    Scientific journal, Abstract

    The Kuroshio south of Japan exhibits bimodal variations between large meander (LM) and non-LM paths. The Kuroshio LM drastically changes the distribution of currents and water temperature in the surface and subsurface layers, affecting atmospheric conditions, marine ecosystems, and fisheries resources. The most recent Kuroshio LM event occurred in August 2017 and is still continuing as of March 2025 (LM2017). Through studies of the long-lived LM2017, many new insights into the Kuroshio LM and its impact have been gained in recent years. This study aims to organize and share our current understanding of the Kuroshio LM and its impacts by reviewing past and present studies. Future directions for research on the Kuroshio LM and its impacts are also presented. This review advances the literature by covering not only the oceanic dynamics of the Kuroshio LM, but also its impacts on ocean conditions, atmospheric phenomena, lower ecosystems, and fisheries resources, thereby presenting the first attempt at such a comprehensive review of the Kuroshio LM.
  • Impact of Eighteen Degree Water thickness variation on the thermal and biogeochemical structure in the euphotic layer
    Hatsumi Nishikawa; Eitarou Oka; Shusaku Sugimoto; Fumiaki Kobashi; Masao Ishii; Nicholas R. Bates
    Journal of Oceanography, Springer Science and Business Media LLC, 08 Mar. 2025, [Peer-reviewed], [Lead author, Corresponding author], ［Internationally co-authored］, [International Magazine]
    Scientific journal, Abstract

    It has been recently found that North Pacific Subtropical Mode Water impacts the overlying thermal structure by uplifting the isotherms when it thickens. How the thickness variation of North Atlantic Subtropical Mode Water, also known as Eighteen Degree Water (EDW), affects the overlying thermal and biogeochemical structure through such uplifting effect has been investigated using Argo float data and shipboard observation data at Bermuda Atlantic Time-series Study site. When EDW was thicker, the overlying isotherms were uplifted, leading to a decrease in temperature centered at 50–100 dbar in the warm season; in addition, the oxycline existing around 100 dbar and the nitracline at 100–150 dbar also tended to be uplifted, leading to an increase of apparent oxygen utilization and nitrate concentrations in the lower euphotic layer; furthermore, there is a tendency that chlorophyll-a maximum around 100 dbar shallowed, and primary production integrated in the euphotic layer increased during the spring bloom season. Thus, although the core of thicker EDW tends to have less nitrate as shown by previous studies, thicker EDW tends to increase biological production in the euphotic layer through the enhanced uplifting effect.
  • Subtropical Mode Water south of Japan impacts typhoon intensity
    Eitarou Oka; Shusaku Sugimoto; Fumiaki Kobashi; Hatsumi Nishikawa; Sachie Kanada; Tomoe Nasuno; Ryuichi Kawamura; Masami Nonaka
    Science Advances, 9, 37, American Association for the Advancement of Science (AAAS), 13 Sep. 2023, [Peer-reviewed], [International Magazine]
    Scientific journal, Subtropical Mode Water (STMW), characterized by vertically uniform temperature of ~17°C, is distributed horizontally over 5000 kilometers at the 100- to 500-meter depths in the subtropical North Pacific Ocean. Its formation and spreading fluctuate in relation to the Pacific Decadal Oscillation and the Kuroshio path variation, but the feedback from STMW on the sea surface temperature (SST) and the overlying atmosphere remains unclear. Using Argo profiling float data, we show that STMW south of Japan, whose thickness varies decadally, modulates the overlying thermal structure throughout the year by increasing isotherm uplift with increasing thickness. The STMW-induced decadal temperature change has a magnitude of up to ~1°C and is large in the warm season in the presence of the seasonal thermocline. Furthermore, 50-year observations, together with numerical simulation, show that SST, upper ocean heat content, and typhoon intensification rate have been significantly lower in years with thicker STMW and higher in years with thinner STMW.
  • Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part II—formation and temporal evolution in the Kuroshio recirculation gyre off Shikoku
    Hatsumi Nishikawa; Eitarou Oka; Shusaku Sugimoto
    Journal of Oceanography, Springer Science and Business Media LLC, 12 Apr. 2023, [Peer-reviewed], [Lead author, Corresponding author], [International Magazine]
    Scientific journal, Abstract

    Since August 2017, the large meander (LM) path of the Kuroshio south of Japan has continued to date for more than five years, with a local Kuroshio recirculation gyre off Shikoku (RGOS). The formation and temporal evolution of Subtropical Mode Water (STMW) in the isolated RGOS were examined by using Argo profiling float and shipboard observation data. Since the late winter of 2018, which was the first winter of the current LM event, multiple STMW layers with different densities overlapping with each other have been observed for more than three years inside the RGOS. The deeper STMW layer of 18.0 °C, which was formed in the late winter of 2017 before the current LM event began, has survived at least until September 2021 while its thickness and horizontal extent has decreased gradually. We may have succeeded in discovering the longest survived STMW ever observed due to a suitable environment for STMW conservation inside the RGOS. On the other hand, the shallower STMW layer(s) of 19.0–19.8 °C, which was originally formed over the deeper STMW layer in the late winter of 2018, has been repeatedly renewed or dissipated in the following years. The oxygen utilization rate at the core of the deeper STMW at a depth of 500 dbar was estimated to be 4.6 μmol kg−1 yr−1, which is believed to be explained solely by oxygen consumption due to remineralization.
  • High moisture confluence in Japan Sea polar air mass convergence zone captured by hourly radiosonde launches from a ship
    Yoshihiro Tachibana; Meiji Honda; Hatsumi Nishikawa; Hiroaki Kawase; Haruna Yamanaka; Daichi Hata; Yuji Kashino
    Scientific Reports, 12, 1, Springer Science and Business Media LLC, 23 Dec. 2022
    Scientific journal, Abstract

    Some of the heaviest snowfalls in urban areas in the world occur in Japan, particularly in regions that face the Japan Sea. Many heavy snowfalls are produced by a Japan Sea polar air mass convergence zone (JPCZ), which is an atmospheric river-like cloud zone that forms when Siberian cold air flows over the warm Japan Sea. Quantifying how the air–sea interaction strengthens the JPCZ is key to snowfall prediction. However, until our observations with hourly meteorological balloon launches from a training vessel in 2022, no simultaneous air–sea observations targeting the JPCZ had been conducted. Our observations showed that wind direction shifted drastically by about 90 degrees from the surface to an altitude of about 3.5 km within a narrow horizontal range of about 15 km, indicating airflow convergence from the surroundings. Maximum temperature difference between surface air (3 °C) and water was 11 °C near the JPCZ centre with 17 m s−1 wind speed. Large amounts of heat, 718 W m−2, was thus gained from the warm sea. Water vapour was also concentrated by the horizontal convergence, which caused heavy snow, equivalent to 100 cm of snowfall in 7 h. The surrounding sea greatly affects moisture formation within the JPCZ.
  • Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part I-formation and advection over the entire distribution region
    Eitarou Oka; Hatsumi Nishikawa; Shusaku Sugimoto; Bo Qiu; Niklas Schneider
    JOURNAL OF OCEANOGRAPHY, 77, 5, 781, 795, Oct. 2021
    English, Scientific journal
  • Surface water pathways in the subtropical–subarctic frontal zone of the western North Pacific
    Hatsumi Nishikawa; Humio Mitsudera; Takeshi Okunishi; Shin-ichi Ito; Taku Wagawa; Daisuke Hasegawa; Toru Miyama; Hitoshi Kaneko; Ren-Chieh Lien
    Progress in Oceanography, 199, 102691, 102691, Elsevier BV, Oct. 2021
    Scientific journal
  • Tidally modified western boundary current drives interbasin exchange between the Sea of Okhotsk and the North Pacific
    Hung-Wei Shu; Humio Mitsudera; Kaihe Yamazaki; Tomohiro Nakamura; Takao Kawasaki; Takuya Nakanowatari; Hatsumi Nishikawa; Hideharu Sasaki
    SCIENTIFIC REPORTS, 11, 1, Jun. 2021
    English, Scientific journal
  • Temperature profiling measurements by sea turtles improve ocean state estimation in the Kuroshio-Oyashio Confluence region
    Miyazawa, Yasumasa; Kuwano-Yoshida, Akira; Doi, Takeshi; Nishikawa, Hatsumi; Narazaki, Tomoko; Fukuoka, Takuya; Sato, Katsufumi
    OCEAN DYNAMICS, 69, 2, 267, 282, Feb. 2019, [Peer-reviewed]
    English, Scientific journal
  • In Situ Evidence of Low-Level Atmospheric Responses to the Oyashio Front in Early Spring
    KAWAI Yoshimi; NISHIKAWA Hatsumi; OKA Eitarou
    Journal of the Meteorological Society of Japan. Ser. II, 公益社団法人 日本気象学会, 2019
    English,  Previous modeling studies have indicated that the Oyashio front in the subarctic Pacific Ocean significantly affects the atmosphere on meso- to basin-scales, but there were no in situ observations that captured oceanic imprints on the atmosphere in this region as far as the authors know. We present in situ evidence of atmospheric responses to the Oyashio front by using a total of 103 radiosondes launched around the Oyashio front in April 2013 with continuous surface meteorology and ceilometer observations. Composite profiles showed that the low-level atmosphere below 1000 m was statically stable on the cold side of the Oyashio front, but unstable and mixed on the warm side. In the atmosphere on the warm side, relative humidity dropped sharply at an altitude of around 1000 m, an indication that the mean cloud top was at this altitude. While the frequency of cloud base height peaked at 50–100 m in the cold areas, cloud bases were distributed at higher altitudes in the warm areas. These differences in the atmospheric boundary layer and cloud base heights across the front were clearer under conditions of southerly winds compared with those of northerly winds. Above a local sea surface temperature minimum with a width of approximately 400 km, where the ocean mixed layer depth is known to reach a local maximum, a large horizontal air temperature gradient was observed below an altitude of 1000 m. This horizontal gradient corresponded to a sea level pressure (SLP) anomaly of 1.2 hPa, comparable to observations of SLP anomalies in the Kuroshio Extension region. Furthermore, we found that narrow warm ocean streamers moistened the overlying atmosphere, affecting downward longwave radiation. Over the wide streamer located between 146.4°E and 147.0°E on 5 April, near-surface atmospheric properties were largely different between over the western half and the eastern half.
  • Low ocean-floor rises regulate subpolar sea surface temperature by forming baroclinic jets
    Mitsudera, H.; Miyama, T.; Nishigaki, H.; Nakanowatari, T.; Nishikawa, H.; Nakamura, T.; Wagawa, T.; Furue, R.; Fujii, Y.; Ito, S.
    Nature Communications, 9, 2018, [Peer-reviewed]
    Scientific journal
  • Evidence for SST-Forced Anomalous Winds Revealed from Simultaneous Radiosonde Launches from Three Ships across the Kuroshio Extension Front
    Hatsumi Nishikawa; Yoshihiro Tachibana; Yoshimi Kawai; Mayumi K. Yoshioka; Hisashi Nakamura
    MONTHLY WEATHER REVIEW, 144, 10, 3553, 3567, Oct. 2016, [Peer-reviewed]
    English, Scientific journal
  • Synchrony of trend shifts in Sahel boreal summer rainfall and global oceanic evaporation, 1950-2012
    Alima Diawara; Yoshihiro Tachibana; Kazuhiro Oshima; Hatsumi Nishikawa; Yuta Ando
    HYDROLOGY AND EARTH SYSTEM SCIENCES, 20, 9, 3789, 3798, Sep. 2016, [Peer-reviewed]
    English, Scientific journal
  • Influence of the Kuroshio on Mesoscale Convective Systems in the Baiu Frontal Zone over the East China Sea
    Kazutoshi Sato; Atsuyoshi Manda; Qoosaku Moteki; Kensuke K. Komatsu; Koto Ogata; Hatsumi Nishikawa; Miki Oshika; Yuriko Otomi; Shiori Kunoki; Hisao Kanehara; Takashi Aoshima; Kenichi Shimizu; Jun Uchida; Masako Shimoda; Mitsuharu Yagi; Shoshiro Minobe; Yoshihiro Tachibana
    MONTHLY WEATHER REVIEW, 144, 3, 1017, 1033, Mar. 2016, [Peer-reviewed]
    English, Scientific journal
  • Radiosonde observational evidence of the impact of an extremely cold SST spot on a mesoscale anticyclone
    Hatsumi Nishikawa; Yoshihiro Tachibana; Yusuke Udagawa
    JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119, 15, 9183, 9195, Aug. 2014, [Peer-reviewed]
    English, Scientific journal
  • Transient and local weakening of surface winds observed above the Kuroshio front in the winter East China Sea
    Kenki Kasamo; Atsuhiko Isobe; Shoshiro Minobe; Atsuyoshi Manda; Hirohiko Nakamura; Koto Ogata; Hatsumi Nishikawa; Yoshihiro Tachibana; Shin’Ichiro Kako
    Journal of Geophysical Research, 119, 3, 1277, 1291, Wiley-Blackwell, 16 Feb. 2014, [Peer-reviewed]
    English, Scientific journal
  • Possibility of the calculation of divergence of ship-based meteorology observation by continuous radiosonde launches
    西川はつみ; 立花義裕; 池田尚仁; 伊藤匡史; 宮本守; 緒方香都; 大鹿美希; 大富裕里子; 仲里慧子; 中村亨; 前川陽一; 内田誠
    海と空, 89, 1, 25, 32, 海洋気象学会, 31 Jul. 2013
    Japanese
  • A Baiu frontal zone composed of two rain bands observed over East China Sea to the west of Kyushu island-a case study on June 10, 2011-
    久野木梓織; 佐藤和敏; 黒瀧あゆみ; 関真理子; 児玉安正; 小松謙介; 緒方香都; 西川はつみ; 大鹿美希; 大富裕理子; 立花義裕; 三井拓; 茂木耕作; 川合義美; 万田敦昌
    海と空, 89, 1, 9, 18, 海洋気象学会, 31 Jul. 2013
    Japanese
■ Other Activities and Achievements
■ Lectures, oral presentations, etc.
■ Research Themes
  • Heavy rainfall and heavy snowfall associated with rapid warming of the ocean front around Japan
    Grants-in-Aid for Scientific Research
    01 Apr. 2024 - 31 Mar. 2029
    本田 明治; 高橋 直也; 平田 英隆; 西川 はつみ; 川瀬 宏明; 柳瀬 亘; 山田 広幸; 飯塚 聡; 柏野 祐二; 立花 義裕
    Japan Society for the Promotion of Science, Grant-in-Aid for Transformative Research Areas (A), Niigata University, 24H02222
  • Supervision and promotion of Habitable Japan
    Grants-in-Aid for Scientific Research
    01 Apr. 2024 - 31 Mar. 2029
    岡 英太郎; 杉本 周作; 本田 明治; 小坂 優; 遠山 勝也; 岩本 洋子; 榎本 剛; 碓氷 典久; 佐藤 友徳; 時長 宏樹; 西川 はつみ; 望月 崇; 相木 秀則; 野中 正見
    Japan Society for the Promotion of Science, Grant-in-Aid for Transformative Research Areas (A), The University of Tokyo, 24H02220
  • Response of biogeochemical cycle and lower trophic ecosystem to the environmental change in the Southern Ocean, East Antarctica
    Grants-in-Aid for Scientific Research
    Apr. 2023 - Mar. 2028
    原田 尚美; 藤木 徹一; 木元 克典; 川口 慎介; 栗栖 美菜子; 相田 真希; 塩崎 拓平; 山本 彬友
    「しらせ」を用いた第66次南極地域観測隊の本隊の活動（2024年12月5日から2025年4月6日）ならびに別働隊として海鷹丸航海（2025年1月11日から2月7日）に参加した。海鷹丸航海では、南大洋の定在性海洋渦の西サブリナ渦東端に時系列係留系システムを投入・設置した。１年間係留し、第67次南極地域観測隊の2026年1月の海鷹丸航海で回収する予定である。係留系には、海洋酸性化の状況把握のためのpH/pCO2センサーの搭載を見送った以外は、イベントベースドビジョンセンサ（EVS）やセジメントトラップを計画通り搭載した。生物地球化学（BGC）フロートについては、海鷹丸航海で64°S、120°E付近に投入し、第66次南極地域観測行動中に「しらせ」にて61°S、117°E付近に投入した。両フロートとも現在も順調に観測中である。本研究で投入したBGCフロートには植物プランクトンの光合成活性を測定するFRRFセンサーが搭載されている点が他のフロートとの違いである。植物プランクトンの活性を直接測定することで現場の生産能力や栄養塩濃度の推定が可能で、植物プランクトンの活性を示すFv/Fmの鉛直プロファイル結果を既存のフロートデータと比較しながら近年の海氷変動と海洋構造に変化があるか解析する予定である。また、EVSを用いたプランクトン、生物起源粒子の凝集体である沈降粒子の観測手法について原理検証を行なった論文を発表した。また、亜南極における植物プランクトンの種組成分布について論文で公表した。モデルについては、極域のフィヨルドを想定し、氷河融解水と低次生態系の生産の関係の理解を深めるために、海洋低次栄養段階生態系（窒素、リン、ケイ素の各種栄養塩と基礎生産者の植物プランクトン、二次生産者の動物プランクトンを含む生態系）モデルを開発し、物理海洋モデルを組み合わせた結果を論文にて公表した。
    Japan Society for the Promotion of Science, Grant-in-Aid for Specially Promoted Research, The University of Tokyo, 23H05411
  • Overturning circulation that connects the surface and intermediate layers in the North Pacific: A new perspective on its three-dimensional structure and variability
    Grants-in-Aid for Scientific Research
    01 Apr. 2014 - 31 Mar. 2017
    Mitsudera Humio; Fujii Yousuke; Nakanowatari Takuya; Nishikawa Hatsumi
    This study aimed to elucidate the overturning circulation originating in the Sea of Okhotsk that ventilates the intermediate layer of the North Pacific Ocean, where strength of the overturn is determined by salinity. However, our knowledge regarding salt transport pathways from low latitude to high latitude was still lacking. Here we present a salt pathway paying attention to the quasi-stationary jets in the transition region between the subtropical and subarctic gyres. The quasi-stationary jets flow northeastward regulated by bottom topography whose height is only 500 m although the ocean is deep. We have elucidated this process using theoretical models, numerical experiments and drifting buoy measurements. Numerical and chemical tracer analyses show that the saline Kuroshio waters are transported to the subarctic gyre through the quasi-stationary jets. The overturn is likely strengthened when the westerly wind is strengthened because of increase of the northward salt flux.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 26247076