Hisaki Yukiharu

写真a

Researcher Number(JSPS Kakenhi)

60305183
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Current Affiliation Organization 【 display / non-display 】

  • Concurrently   University of the Ryukyus   Faculty of Science   Physics and Earth Sciences   Professor  

  • Concurrently   University of the Ryukyus   Graduate School of Engineering and Science   Physics and Earth Sciences   Professor  

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Academic degree 【 display / non-display 】

  • Tohoku University -  Doctor of Science

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External Career 【 display / non-display 】

  • 1998.04
    　
    　

    - , University of the Ryukyus, Faculty of Science, Department of Physics and Earth Sciences, Meteorology Oceanography Sedimentology, Professor  

  • 1998.04
    　
    　

    University of the Ryukyus, Faculty of Science, Department of Physics and Earth Sciences, Meteorology Oceanography Sedimentology, Professor  

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Research Interests 【 display / non-display 】

  • 海洋物理学,海洋リモートセンシング

  • 海洋リモートセンシング

  • 海洋物理学

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Research Areas 【 display / non-display 】

  • Natural Science / Atmospheric and hydrospheric sciences

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Published Papers 【 display / non-display 】

  • The Western Pacific North Equatorial Countercurrent Responses to Two Forms of El Niño during the Period 1978 to 2017

    Yusuf Jati Wijaya, Ulung Jantama Wisha, Yukiharu Hisaki

    Climate ( Climate )  10 ( 10 )   2022.10 [ Peer Review Accepted ]

    Type of publication: Research paper (scientific journal)

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    This research aims to examine how the Western Pacific North equatorial countercurrent (NECC) flow reacts to two different forms of El Nino (EN) over a 40-year period. To establish the prevailing modes for each season, we implemented Empirical Orthogonal Function (EOF) analysis on the eastward current component of the Ocean Reanalysis System 5 (ORAS5) dataset. In comparison to the Central Pacific (CP) episode, the time series principal component of the first mode (PC1) demonstrated that the strongest NECC's magnitude often emerged during the development period (spring to fall) of the Eastern Pacific (EP) EN event. However, in episode CP 2002/2003, we witnessed an abnormal behavior in which the stronger NECC manifested. This was due to the emergence of a strong anomalous westerly wind, which differed from other CP events and forced the NECC's magnitude to be greater. When approaching the peak stage, on the other hand, the magnitude of the NECC during the CP episode was typically greater than that of the EP episode. The NECC's magnitude fell greatly in the second year of the EP episode, particularly during the spring season, since most EP episodes would transition into an La Nina (LN) event in the succeeding event. During the EP EN, it was found that the strength of the westerly wind had a bigger effect on the NECC than during the CP EN.

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  • Real-Time Properties of Hydraulic Jump off a Tidal Bore, Its Generation and Transport Mechanisms: A Case Study of the Kampar River Estuary, Indonesia

    Ulung Jantama Wisha, Yusuf Jati Wijaya, Yukiharu Hisaki

    Water (Switzerland) ( Water (Switzerland) )  14 ( 16 )   2022.08 [ Peer Review Accepted ]

    Type of publication: Research paper (scientific journal)

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    Since the hydraulic jump off a tidal bore in the Kampar Estuary has never been well-described, real-time measurements of hydraulic jump properties are crucial to understanding the tidal bore characteristics. This study aims to determine the real-time properties of a tidal bore generation, hydraulic jump, and transport mechanism in the Kampar River estuary. Tidal harmonic and range are analyzed using least-square-based tidal modeling. The tidal bore height and turbulent velocity records based on ADCP surveys in the estuary and upstream area are used to determine the hydraulic jump properties. Furthermore, an acoustic-based approach is also employed to quantify the suspended sediment concentration and flux during the passage of the bore. Kampar Estuary is predominated by semidiurnal co-tidal components (M2 and S2), where, based on the phase lag magnitude, it is categorized as an ebb-dominant estuary. This finding is proven by the more intense and prolonged ebb phases, especially during spring tidal conditions where the tidal range reaches 4 m. Of particular concern, the tidal bore height declines by 1.5 m every 20 km upstream with an erratic turbulent velocity. A sudden increase in transverse and vertical velocity during the passage of bore (ranging from -0.9 to 0.2 m/s) reflects the potency of sediment resuspension in the surrounding river edge marked by the significant increase in suspended sediment flux of about 3.7 times larger than at the end of the ebb tide. However, long-term measurement and regular bathymetry surveys are crucial to monitor the tidal bore behavior and morpho-dynamics in the Kampar River estuary.

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  • Variability of wave height in the Sea of Japan

    Hisaki Y.

    Continental Shelf Research ( Continental Shelf Research )  233   2022.01 [ Peer Review Accepted ]

    Type of publication: Research paper (scientific journal)

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    The Sea of Japan is a semi-enclosed sea located in the northwest Pacific area. There has been no study conducted to examine the relationship between the interannual variability of the wind field and the spatial variability of wave height in the Sea of Japan. We examined the relationship between the spatiotemporal variability of the wave height and the wind field in the Japan Sea using monthly mean ERA5 reanalysis data. Firstly, it is demonstrated that the ERA5 wave height closely matches that of moored buoys. Following that, we investigated the relationship between the wave height and the wind anomaly distribution using empirical orthogonal function (EOF) and self-organizing map (SOM) analysis. This is the first study to examine the spatial variation of wave height using the SOM. The EOF's first mode is a mode that oscillates in the central region of the Sea of Japan, while the second mode is a north-south dipole mode. In each season, the wave height anomaly EOF mode had a significant correlation with a variety of climatic indices. Although the correlation between EOF of wave height anomaly and EOF of wind speed anomaly was not high throughout the year, the correlation between them was high for each season. The SOM revealed a pattern consistent with the EOF analysis's first and second modes. Throughout the year, the relationship between wave height and wind speed anomaly patterns is not strong. The link, however, varies with the seasons. The frequency of patterns with increased wave height anomalies in the central Japan Sea has increased in recent years. This is consistent with the EOF first mode's long-term trend.

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  • Tidal bore generation and transport mechanism in the Rokan River Estuary, Indonesia: Hydro-oceanographic perspectives

    Ulung J. Wisha, Yusuf J. Wijaya, Yukiharu Hisaki

    REGIONAL STUDIES IN MARINE SCIENCE ( ELSEVIER )  52   102309 - 102309   2022 [ Peer Review Accepted ]

    Type of publication: Research paper (scientific journal)

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    Studies related to the tidal bore phenomenon in the Rokan Estuary are limited. Therefore, revealing the physical characteristics and the environmental impact of a tidal bore is crucial. This study aims to determine the tidal regime in the estuarine area, the propagation of the tidal bore, and the transport mechanism throughout the Rokan River. A tidal harmonic analysis and a hydrodynamic model based on Saint-Venant equations were performed. The amplitude of M2 and S2 (semidiurnal co tidal constituents) showed a sufficiently high magnitude upstream (around 0.5-1.4 m) with a shorter and more intense flood propagation (flood dominant). The tidal range peaked on average 5 m with a 6-6.5-hour displacement time during the full moon phase and lowered in amplitude and period during the first (third) quarter of the moon phase. A sudden increase in water level and velocity observed represented a hydraulic jump off a tidal bore. Of particular concern, the tidal bore height in the estuary was approximately 3 m and lowered by approximately 1 meter every 15 km upstream. The turbulent velocities showed a sharp increase in magnitude at the end of the ebb tide peaking at +0.5 m/s and significantly declined to approach -0.5 m/s during the passage of bore observed in the estuary (station R1), showing the reversal changes in the flow direction and magnitude. These mechanisms were not applicable upstream due to the irregular bottom morphology of the river channel affecting the transverse and vertical turbulent velocity. Some bed erosion-induced turbulence may affect the higher suspended sediment concentration (SSC), advected by the "whelps "beneath the first wave crest, transported upstream. (c) 2022 Elsevier B.V. All rights reserved.

  • A non-linear method to estimate the wave directional spectrum by HF radar

    Yukiharu Hisaki

    Ocean Remote Sensing Technologies: High frequency, marine and GNSS-based radar ( Institution of Engineering and Technology )    179 - 189   2021.12 [ Peer Review Accepted ]

    Type of publication: Research paper (other science council materials etc.)

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