Naoki Furuichi

Bispectral Analysis of Energy Transfer within the Two-Dimensional Oceanic Internal Wave Field

Abstract Bispectral analysis of the numerically reproduced spectral responses of the two-dimensional oceanic internal wave field to the incidence of the low-mode semidiurnal internal tide is performed. At latitudes just equatorward of 30°, the low-mode semidiurnal internal tide dominantly interacts with two high-vertical-wavenumber diurnal (near inertial) internal waves, forming resonant triads of parametric subharmonic instability (PSI) type. As the high-vertical-wavenumber near-inertial energy level is raised by this interaction, the energy cascade to small horizontal and vertical scales is enhanced. Bispectral analysis thus indicates that energy in the low-mode semidiurnal internal tide is not directly transferred to small scales but via the development of high-vertical-wavenumber near-inertial current shear. In contrast, no noticeable energy cascade to high vertical wavenumbers is recognized in the bispectra poleward of ∼30° as well as equatorward of ∼25°. A new finding is that, although PSI is possib...

Assessment of turbulence closure models for resonant inertial response in the oceanic mixed layer using a large eddy simulation model

Large eddy simulation (LES) of the resonant inertial response of the upper ocean to strong wind forcing is carried out; the results are used to evaluate the performance of each of the two second-order turbulence closure models presented by Mellor and Yamada (Rev Geophys Space Phys 20:851–875, 1982) (MY) and by Nakanishi and Niino (J Meteorol Soc Jpn 87:895–912, 2009) (NN). The major difference between MY and NN is in the formulation of the stability functions and the turbulent length scale, both strongly linked with turbulent fluxes; in particular, the turbulent length scale in NN, unlike that in MY, is allowed to decrease with increasing density stratification. We find that MY underestimates and NN overestimates the development of mixed layer features, for example, the strong entrainment at the base of the oceanic mixed layer and the accompanying decrease of sea surface temperature. Considering that the stability functions in NN perform better than those in MY in reproducing the vertical structure of turbulent heat flux, we slightly modify NN to find that the discrepancy between LES and NN can be reduced by more strongly restricting the turbulent length scale with increasing density stratification.

Assessment of the upper‐ocean mixed layer parameterizations using a large eddy simulation model

A large eddy simulation (LES) of the upper ocean mixed layer processes in the winter northwestern Pacific is carried out and compared with concurrent microstructure measurements. We find that dissipation rates of turbulent kinetic energy (e) and temperature variance from LES agree well with field observations in the areas where the Ozmidov length scale, calculated from the modeled e and buoyancy frequency, exceeds the grid size of LES. It is also found that, even though the Ozmidov length scale is less than the grid size of LES, model estimates of the vertical turbulent heat flux near the base of the mixed layer are very similar to observed values, suggesting that this LES model reproduces the intensity of entrainment well. This enables us to use the results from LES to assess the turbulence closure models of Mellor and Yamada [1982, MY] and Nakanishi and Niino [2009, NN] for several forcing scenarios consisting of strong winds, surface heating, and surface cooling. It is found that NN performs better than MY in reproducing the results from LES for each forcing scenario, particularly when the turbulent length scale is adjusted to be more restricted by density stratification.

Periodic behaviors of deep-ocean flow and turbulent mixing in hydrothermal field on the Okinawa Trough, Japan

In this study, the current fluctuations were measured at the hydrothermal fields in the Okinawa Trough (Hatoma Knoll, Irabu Knoll, and Iheya North field) to characterize the fluid environment. The purpose of this study was to reveal the fluctuations of the deep-sea currents using an acoustic Doppler current profiler (ADCP), and to correlate them with the development of a bottom mixed layer as predicted by a numerical model. An ADCP (Teledyne RD Instruments Workhorse Sentinel, 300 kHz) was installed on the seabed at the Hatoma Knoll (depth about 1500 m), Irabu Knoll (depth about 2000 m), and Iheya North field (depth about 1000 m). For each observation point, the deep-sea current data were successfully recovered in the layer at depths between the seafloor and 40-60 m above the seafloor. Notably, the measurement results obtained at the Hatoma Knoll and Iheya North field showed a remarkably interesting fluctuation. During a time period of approximately 3 h, the range in the depths at which the currents were successfully recorded increased in a pulse form, up to around the upper layer at 120 m, allowing measurements in almost all of the layers. The time period of this pulse corresponded to the shift from an ebb tide to high tide. With the exception of these pulses, the currents could only be recorded from the seafloor to about a 40 m layer. This result indicated that a suitable environment for ADCP measurements was formed only within this range. To investigate the cause, a numerical model experiment was performed for the bottom turbulence and subsequent occurrence of vertical mixing of the suspended particulate matter in the Okinawa Trough using a large-eddy simulation (LES) model. The simulated results suggested that the semidiurnal tide periodically enhances the bottom turbulence at intervals of approximately 6 h, with the result that the bottom mixed layer thickness increases to a maximum of approximately 40-60 m. It was concluded that a suitable environment for ADCP measurements near the deep seafloor was formed mainly by the bottom mixed layer corresponding to the tide.

Application of the Improved Vertical Mixing Scheme to the Modeling of the Japan Sea Under Traveling Typhoons

The surface mixed layer in response to two different mixed layer schemes in the Japan Sea under traveling typhoon is investigated using a high-resolution ocean circulation model. The simulated sea surface temperature in the original Mellor-Yamada (1982) mixed layer scheme model is obviously warmer than observation data. It agrees with the viewpoint of previous studies implying that the intensity of the turbulent mixing is underestimated in the Mellor-Yamada scheme. On the other hand, the improved Mellor-Yamada turbulence closure scheme, which proposed by Nakanishi and Niino (2009), provides improved sea surface temperature. Furthermore, the correlation between the model vertical temperature profile and the Argo Profiling Float data is relatively higher. This study concludes that the improved mixed layer scheme contributes to a certain extent to overcome the shortcomings of the original Mellor-Yamada scheme, namely, insufficient growth of the mixed layer and underestimates of the turbulent kinetic energy, showing a better performance compared with the original Mellor-Yamada scheme in modeling the ocean mixed layer.