Yohei Onuki | Last modified date：2024.06.03 |

Assistant Professor /
Environmental Prediction

Center for Oceanic and Atmospheric Research

Research Institute for Applied Mechanics

Center for Oceanic and Atmospheric Research

Research Institute for Applied Mechanics

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##### https://kyushu-u.elsevierpure.com/en/persons/yohei-onuki

Reseacher Profiling Tool Kyushu University Pure

Academic Degree

Ph. D.

Country of degree conferring institution (Overseas)

No

Field of Specialization

Physical Oceanography

ORCID(Open Researcher and Contributor ID)

0000-0002-9484-2887

Total Priod of education and research career in the foreign country

02years02months

Research

**Research Interests**

- Mathematical modeling of the deep ocean environment: basic comprehension and prediction

keyword : Geophysical fluid dynamics, Ocean dynamics, Nonlinear mechanics, Internal wave, Turbulence

2017.04.

**Academic Activities**

**Papers**

1. | Yohei Onuki, Jules Guioth, Freddy Bouchet, Dynamical large deviations for an inhomogeneous wave kinetic theory: linear wave scattering by a random medium, Annales Henri Poincaré: A Journal of Theoretical and Mathematical Physics, doi.org/10.1007/s00023-023-01329-7, 25, 1215-1259, 45 pages, 2024.01, The wave kinetic equation predicts the averaged temporal evolution of a continuous spectral density of waves either randomly interacting or scattered by the fine structure of a medium. In a wide range of systems, the wave kinetic equation is derived from a fundamental equation of wave motion, which is symmetric through time reversal. By contrast, the corresponding wave kinetic equations is time irreversible: Its solutions monotonically increase an entropy-like quantity. A similar paradox appears whenever one makes a mesoscopic description of the evolution of a very large number of microscopic degrees of freedom, the paradigmatic example being the kinetic theory of dilute gas molecules leading to the Boltzmann equation. Since Boltzmann, it has been understood that a probabilistic understanding solves the apparent paradox. More recently, it has been understood that the kinetic description itself, at a mesoscopic level, should not break time reversal symmetry (Bouchet in J Stat Phys 181(2):515-550, 2020). The time reversal symmetry remains a fundamental property of the mesoscopic stochastic process: Without external forcing, the path probabilities obey a detailed balance relation with respect to an equilibrium quasipotential. The proper theoretical or mathematical tool to derive fully this mesoscopic time reversal stochastic process is large deviation theory: A large deviation principle uncovers a time reversible field theory, characterized by a large deviation Hamiltonian, for which the deterministic wave kinetic equation appears as the most probable evolution. Its irreversibility appears as a consequence of an incomplete description, rather than as a consequence of the kinetic limit itself, or some related chaotic hypothesis. This paper follows Bouchet (J Stat Phys 181(2):515-550, 2020) and a series of other works that derive the large deviation Hamiltonians of the main classical kinetic theories, for instance, Guioth et al. (J Stat Phys 189(20):20, 2022) for homogeneous wave kinetics. We propose here a derivation of the large deviation principle in an inhomogeneous situation, for the linear scattering of waves by a weak random potential. This problem involves microscopic scales corresponding to the typical wavelengths and periods of the waves and mesoscopic ones which are the scales of spatial inhomogeneities in the spectral density of both the random scatterers and the wave spectrum, and the time needed for the random scatterers to alter the wave spectrum. The main assumption of the kinetic regime is a large separation of these microscopic and mesoscopic scales. For the sake of simplicity, we consider a generic model of wave scattering by weak disorder: the Schrödinger equation with a random potential. We derive the path large deviation principle for the local spectral density and discuss its main properties. We show that the mesoscopic process obeys a time reversal symmetry at the level of large deviations.. |

2. | Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Breaking of internal waves parametrically excited by ageostrophic anticyclonic instability, Journal of Physical Oceanography, doi.org/10.1175/JPO-D-22-0152.1, 53, 6, 1591-1613, 2023.06, A gradient-wind balanced flow with an elliptic streamline parametrically excites internal inertia-gravity waves through ageostrophic anticyclonic instability (AAI). This study numerically investigates the breaking of internal waves and the following turbulence generation resulting from the AAI. In our simulation, we periodically distort the calculation domain following the streamlines of an elliptic vortex and integrate the equations of motion using a Fourier spectral method. This technique enables us to exclude the overall structure of the large-scale vortex from the computation and concentrate on resolving the small-scale waves and turbulence. From a series of experiments, we identify two different scenarios of wave breaking conditioned on the magnitude of the instability growth rate scaled by the buoyancy frequency, λ/N. First, when λ/N ≳ 0.008, the primary wave amplitude excited by AAI quickly goes far beyond the overturning threshold and directly breaks. The resulting state is thus strongly nonlinear turbulence. Second, if λ/N ≲ 0.008, weak wave-wave interactions begin to redistribute energy across frequency space before the primary wave reaches a breaking limit. Then, after a sufficiently long time, the system approaches a Garrett-Munk-like stationary spectrum, in which wave breaking occurs at finer vertical scales. Throughout the experimental conditions, the growth and decay time scales of the primary wave energy are well correlated. However, since the primary wave amplitude reaches a prescribed limit in one scenario but not in the other, the energy dissipation rates exhibit two types of scaling properties. This scaling classification has similarities and differences with D’Asaro and Lien’s (2000) wave-turbulence transition model.. |

3. | Antoine Venaille, Yohei Onuki, Nicolas Perez, Armand Leclerc, From ray tracing to waves of topological origin in continuous media, SciPost Physics, 10.21468/SciPostPhys.14.4.062, 14, 4, 36 pages, 2023.04. |

4. | Yohei Onuki, Irreversible energy extraction from negative temperature two-dimensional turbulence, Physical Review E, 10.1103/PhysRevE.106.064131, 106, 064131, 17 pages, 2022.12. |

5. | Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Simulating turbulent mixing caused by local instability of internal gravity waves, Journal of Fluid Mechanics, 10.1017/jfm.2021.119, 915, A77, 14 pages, 2021.05. |

6. | Yohei Onuki, Quasi-local method of wave decomposition in a slowly varying medium, Journal of Fluid Mechanics, 10.1017/jfm.2019.825, 883, A56, 27 pages, 2020.01, [URL], The general asymptotic theory for wave propagation in a slowly varying medium, classically known as the Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) approximation, is revisited here with the aim of constructing a new data diagnostic technique useful in atmospheric and oceanic sciences. Using the Wigner transform, a kind of mapping that associates a linear operator with a function, we analytically decompose a flow field into mutually independent wave signals. This method takes account of the variations in the polarisation relations, an eigenvector that represents the kinematic characteristics of each wave component, so as to project the variables onto their eigenspace quasi-locally. The temporal evolution of a specific mode signal obeys a single wave equation characterised by the dispersion relation that also incorporates the effect from the local gradient in the medium. Combining this method with transport theory and applying them to numerical simulation data, we can detect the transfer of energy or other conserved quantities associated with the propagation of each wave signal in a wide variety of situations.. |

7. | Yohei Onuki, Yuki Tanaka, Instabilities of Finite-Amplitude Internal Wave Beams, Geophysical Research Letters, 10.1029/2019GL082570, 46, 13, 7527-7535, 2019.07, [URL], Beam-like internal waves are commonly generated by tides in the ocean, but their dissipation processes that cause vertical mixing remain poorly understood. Previous studies examined small-amplitude beams to find that parametric subharmonic instability (PSI) induces latitude-dependent wave dissipation. Using a novel approach based on Floquet theory, this study analyzes the stability of finite-amplitude beams over a wide range of parameters. If beam amplitude is small, PSI is indeed the principal mode under the condition f∕𝜎 ≤ 0.5, where f is the Coriolis parameter and 𝜎 is the beam frequency, and the growth rate is maximum when equality holds. However, as beam amplitude is increased, instability arises even when f∕𝜎 > 0.5, but the location of maximum instability shifts toward lower f∕𝜎; thus, the latitudinal dependence of instability is significantly altered. Furthermore, the resulting energy spectrum is strongly Doppler shifted to higher frequencies, which therefore distinguishes this configuration from the common cases of PSI.. |

8. | Yohei Onuki, Toshiyuki Hibiya, Parametric subharmonic instability in a narrow-band wave spectrum, Journal of Fluid Mechanics, 10.1017/jfm.2019.44, 865, 247-280, 2019.04, [URL]. |

9. | Yohei Onuki, Toshiyuki Hibiya, Decay Rates of Internal Tides Estimated by an Improved Wave-Wave Interaction Analysis, Journal of Physical Oceanography, 10.1175/JPO-D-17-0278.1, 48, 11, 2689-2701, 2018.11, [URL], 回転成層流体における波数空間内のエネルギーカスケード過程について、密度成層構造の空間変化や上下境界の影響を取り入れた形で、弱非線形乱流理論に基づく運動学的方程式の定式化を行った。これを用いて、海洋内部で潮汐によって励起された波動が非線形共鳴を経てエネルギーを失う割合を全球的に計算したところ、観測研究に整合する形で、中緯度海域に卓越したエネルギー損失率のピークを再現することに成功した。. |

10. | Yohei Onuki, Toshiyuki Hibiya, Excitation mechanism of near-inertial waves in baroclinic tidal flow caused by parametric subharmonic instability, Ocean Dynamics, 10.1007/s10236-014-0789-3, 65, 1, 107-113, 2015.01. |

**Presentations**

**Membership in Academic Society**

- Japan Geoscience Union

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