Yamazaki Hidekatsu

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Yamazaki
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Hidekatsu
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  • Article
    Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front
    (American Geophysical Union, 2012-08-09) Nagai, Takeyoshi ; Tandon, Amit ; Yamazaki, Hidekatsu ; Doubell, Mark J. ; Gallager, Scott M.
    Direct observations of microstructure near the Kuroshio Front were conducted in August 2008 and October 2009. These show negative potential vorticity (PV) in the mixed layer south of the front, where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind-scaling. These elevated dissipation rates scale better with an empirical scaling, which considers local wind and Ekman buoyancy flux driven by downfront wind. Near-zero PV in the thermocline under the Kuroshio mainstream is observed at 200–300 m depth, with dissipation exceeding open ocean thermocline values by factors of 10–100. Overall, the large turbulent dissipation rates measured in the Kuroshio can be categorized into two groups, one characterized by low Richardson number along the Kuroshio Front thermocline, and the other characterized by high stratification away from the Kuroshio mainstream. The former is attributed to mixing by unbalanced frontal ageostrophic flows, and the latter is attributed to internal wave breaking. On average, both groups appear in regions of large horizontal density gradients. Observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues, extending upward from 500 to 100 m depth in a narrow (20 km) zone, suggesting down and upwelling driven by geostrophic straining, which is confirmed by Quasigeostrophic-Omega equation solutions. This implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the Kuroshio Front likely contribute to NPIW formation.
  • Article
    Dynamics and energetics of trapped diurnal internal Kelvin waves around a midlatitude lsland
    (American Meteorological Society, 2017-10-12) Masunaga, Eiji ; Fringer, Oliver B. ; Kitade, Yujiro ; Yamazaki, Hidekatsu ; Gallager, Scott M.
    The generation of trapped and radiating internal tides around Izu‐Oshima Island located off Sagami Bay, Japan, is investigated using the three-dimensional Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier–Stokes Simulator (SUNTANS) that is validated with observations of isotherm displacements in shallow water. The model is forced by barotropic tides, which generate strong baroclinic internal tides in the study region. Model results showed that when diurnal K1 barotropic tides dominate, resonance of a trapped internal Kelvin wave leads to large-amplitude internal tides in shallow waters on the coast. This resonance produces diurnal motions that are much stronger than the semidiurnal motions. The weaker, freely propagating, semidiurnal internal tides are generated on the western side of the island, where the M2 internal tide beam angle matches the topographic slope. The internal wave energy flux due to the diurnal internal tides is much higher than that of the semidiurnal tides in the study region. Although the diurnal internal tide energy is trapped, this study shows that steepening of the Kelvin waves produces high-frequency internal tides that radiate from the island, thus acting as a mechanism to extract energy from the diurnal motions.
  • Article
    Recommendations for plankton measurements on OceanSITES moorings with relevance to other observing sites
    (Frontiers Media, 2022-07-22) Boss, Emmanuel S. ; Waite, Anya M. ; Karstensen, Johannes ; Trull, Thomas W. ; Muller-Karger, Frank E. ; Sosik, Heidi M. ; Uitz, Julia ; Acinas, Silvia G. ; Fennel, Katja ; Berman-Frank, Ilana ; Thomalla, Sandy J. ; Yamazaki, Hidekatsu ; Batten, Sonia ; Gregori, Gerald ; Richardson, Anthony J. ; Wanninkhof, Rik
    Measuring plankton and associated variables as part of ocean time-series stations has the potential to revolutionize our understanding of ocean biology and ecology and their ties to ocean biogeochemistry. It will open temporal scales (e.g., resolving diel cycles) not typically sampled as a function of depth. In this review we motivate the addition of biological measurements to time-series sites by detailing science questions they could help address, reviewing existing technology that could be deployed, and providing examples of time-series sites already deploying some of those technologies. We consider here the opportunities that exist through global coordination within the OceanSITES network for long-term (climate) time series station in the open ocean. Especially with respect to data management, global solutions are needed as these are critical to maximize the utility of such data. We conclude by providing recommendations for an implementation plan.