Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2022JC018556

Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2022JC018556

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.csr.2021.104644

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：SPRINGERNATURE  

Although previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. Here, we conducted a series of intensive in-situ turbulence observations using a state-of-the-art tow-yo microstructure profiler in the Kuroshio flowing over the seamounts of the Tokara Strait, south of Kyusyu Japan, in November 2017, June 2018, and November 2019, and employed a high-resolution numerical model to elucidate the turbulence generation mechanisms. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. The observed turbulence could supply nutrients to sunlit layers, promoting phytoplankton primary production and CO2 uptake.Persistent and intense mixing hotspots are generated where the Kuroshio flows over steep seamounts, with an increase in energy dissipation by two to three orders of magnitude, according to tow-yo microstructure measurements combined with numerical simulations.

DOI： 10.1038/s43247-021-00230-7

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Internal waves with high vertical wavenumber structures on the northern part of the eastern ridge in Luzon Strait were investigated using shipboard observations and a wave propagation model. Detailed repeat observations across the ridge revealed complex undulations of isopycnals near the ridge that were characterized by vertically alternating concave and convex structures. In addition, strong eastward and westward flow areas with horizontal and vertical scales of 3-5 km and 150-250 m, respectively, occurred alternately on the downstream side of the ridge. These high-wavenumber structures in flow and density fields were associated with internal waves generated by interactions between diurnal tidal flow and the meridional submarine ridge in the strait; an acoustic Doppler current profiler recorded no signals of the Kuroshio Current and semi-diurnal tides in the zonal flow on the western slope of the eastern ridge. Many temperature inversions around the ridge exhibited vertical scales of predominantly similar to 30 m. Simple calculations of internal wave motion reveal that each internal wave mode tends to stagnate near the summits of the ridge depending on the direction and strength of the background tidal flow. This suggests that the observed high-wavenumber structure in the flow profiles near the ridge is ascribed to the superposition of transient internal wave modes, although this is not the main cause of the temperature inversions. As a possible mechanism to explain the temperature inversions, we propose that higher-mode internal waves emanating from the unresolved topographic undulations are trapped by the observed high-wavenumber structures, resulting in turbulent mixing.Y

DOI： 10.1007/s10872-021-00615-4

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Vertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin-Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo-sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10−1 m2 s−1), resulting in strong nitrate fluxes of O(1–103 mmol m−2 day−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10−3 m s−1), as well as an upwelling nitrate flux of O(102 mmol m−2 day−1) in the entire subsurface layer.

DOI： 10.1029/2020GL092063

Scopus

Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020GL092063

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>In the stratified ocean, vertical eddy diffusivity in the subsurface pycnocline plays a major role in new production through transporting nutrients upwards from the darker/nutrient-rich layer. In order to evaluate the diffusivity that was less available in the subsurface layers where indirect estimations are difficult, we conducted direct microstructure measurements in the upper 300 m of the open Pacific during summer, from 40°S to 50°N along 170°W and from 137°E to 120°W across the subtropical North Pacific (21°30′N–23°N). The subsurface pycnocline in the mid- and low-latitude regions was occupied primarily by the Subtropical Underwaters characterized by high salinity, and the Subtropical Mode Waters appeared with an increasing depth and latitude. In the North Pacific, low-salinity subarctic water with shallow seasonal pycnocline was observed in the high-latitude region. The base level of the diffusivity was 0.14–0.47 × 10^–5 m² s^–1, while elevations up to 0.51–13 × 10^–5 m² s^–1 were observed at the equator where the Equatorial Undercurrents formed the strong shear in the subsurface layer, and in areas high internal tide energy such as the Hawaiian Ridge. The effect of wind on the diffusivity was less clear, probably because the wind energy is generally low during summer. The shear-to-strain ratio showed dome-shaped profiles with respect to latitude and an increasing trend from west to east in the subtropical North Pacific. The geographical distributions of the diffusivity presented in this study will contribute to better understanding biogeochemical cycles in the stratified upper ocean through improving estimations of the material transport.

DOI： 10.1007/s10872-020-00589-9

Scopus

Other Link： http://link.springer.com/article/10.1007/s10872-020-00589-9/fulltext.html

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>
Steep oceanic ridges and tidal currents in the Luzon Strait generate some of the world’s strongest turbulent mixing. To evaluate the impacts of the turbulence intensity on the marine ecosystem, we carried out measurements of microstructure turbulence and biogeochemical hydrography along 21°N in the Luzon Strait during the R/V <italic>Hakuho Maru</italic> cruise, KH-17-5-2, in November 2017. We found a turbulent kinetic energy dissipation rate exceeding <italic>O</italic>(10⁻⁷) W kg⁻¹ and vertical eddy diffusivity exceeding <italic>O</italic>(10⁻³) m² s⁻¹, two orders of magnitude larger than those in the open ocean, above a shallow sub-ridge on the eastern ridge of the Luzon Strait. In addition, a clear chlorophyll <italic>a</italic> bloom was identified in the surface layer above the sub-ridge from in situ measurements and satellite observations. High values of nitrate (4.7 mmol N m⁻² d⁻¹) and phosphate (0.33 mmol P m⁻² d⁻¹) fluxes estimated near the base of the surface chlorophyll <italic>a</italic> bloom strongly suggest that enhanced turbulent mixing promotes nutrient supply to the euphotic zone and generates new production within the surface layer, contributing to the formation of a quasi-permanent local chlorophyll <italic>a</italic> bloom north of Itbayat Island on the eastern ridge.

DOI： 10.1038/s41598-020-74938-5

Scopus

PubMed

Other Link： http://www.nature.com/articles/s41598-020-74938-5

Publishing type：Research paper (scientific journal)   Publisher：Copernicus GmbH  

Abstract. The Kuroshio Current has been thought to be biologically
unproductive because of its oligotrophic conditions and low plankton
standing stocks. Even though vulnerable life stages of major foraging fishes
risk being entrapped by frontal eddies and meanders and encountering low
food availability, they have life cycle strategies that include growing and
recruiting around the Kuroshio Current. Here we report that phytoplankton
growth and consumption by microzooplankton are stimulated by turbulent
nitrate flux amplified by the Kuroshio Current. Oceanographic observations
demonstrate that the Kuroshio Current topographically enhances significant
turbulent mixing and nitrate influx to the euphotic zone. Graduated nutrient
enrichment experiments show that growth rates of phytoplankton and
microheterotroph communities were stimulated within the range of the
turbulent nitrate flux. Results of dilution experiments imply significant
microzooplankton grazing on phytoplankton. We propose that these rapid and
systematic trophodynamics enhance biological productivity in the Kuroshio.

DOI： 10.5194/bg-17-2441-2020

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2019, The Oceanographic Society of Japan and Springer Nature Singapore Pte Ltd. We found a good linear relation between the mean volume backscattering strength measured using a scientific echo sounder and the distribution density of moon jellyfish recorded simultaneously by an underwater video camera in small bays (Hokezu Bay and Mikame Bay) in Japan. This relation held in all of the surveys carried out in recent years (2013–2016), although the regression coefficient varied slightly from year to year due to the existence of ambient organisms. Using this relation and high-resolution echo sounder scan surveys across the bay, we obtained the spatial structures of moon jellyfish aggregations and categorized them into either patchy, layered, or wavy aggregations. We also examined possible physical mechanisms responsible for the moon jellyfish aggregations. In particular, passive particle tracking simulations of several idealized internal wave fields suggest that the formation of wavy aggregations is closely related to the presence of internal waves. However, internal waves alone cannot induce patchy and layered aggregations, indicating that particular biological processes (e.g., swimming behavior) are also necessary for such aggregations to form.

DOI： 10.1007/s10872-019-00507-8

Scopus

Language：Japanese   Publisher：Coastal Oceanography Research Committee, the Oceanographic Society of Japan  

Under the Grants-in-Aid for Scientific Research in Innovative Areas（MEXT）project “Ocean Mixing Study（OMIX）”, we
established a system to measure the turbulent nitrate flux by attaching a small nitrate sensor on a turbulence profiler. In this
paper, we explain the instrument setup, the data processing method considering measurement errors, and then demonstrate the
capability of the developed observational system.

DOI： 10.32142/engankaiyo.2019.8.008

CiNii Books

Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-17H01860/

Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Also, tidal currents over the rough topography may produce internal tides, and associated turbulence. The previous observations show energetic high vertical wavenumber near-inertial wave shear in the Kuroshio thermocline, which implies strong turbulent mixing. However, direct turbulence measurements in this region are very scarce. Using high lateral resolution (1-2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of &gt;O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. The turbulent kinetic energy dissipation rates within these turbulent layers are &gt;O(10-7 W kg(-1)), and estimated vertical eddy diffusivity shows &gt;O(10(-4) m(2) s(-1)) on average. These results suggest that the high vertical wavenumber near-inertial waves propagating in the upstream Kuroshio could have large impacts on the watermass modifications, momentum mixing, nutrient supply, and associated biogeochemical responses in its downstream.

DOI： 10.1038/s41598-017-15167-1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Turbulent mixing and background current were observed using a microstructure profiler and acoustic Doppler current profilers in the Tokara Strait, where many seamounts and small islands exist within the route of the Kuroshio in the East China Sea. Vertical structure and water properties of the Kuroshio were greatly modified downstream from shallow seamounts. In the lee of a seamount crest at 200 m depth, the modification made the flow tend to shear instability, and the vertical eddy diffusivity is enhanced by nearly 100 times that of the upstream site, to K-p similar to O(10(-3))-O(10(-2)) m(2) s(-1). A one-dimensional diffusion model using the observed eddy diffusivity reproduced the observed downstream evolution of the temperature-salinity profile. However, the estimated diffusion time-scale is at least 10 times longer than the observed advection time-scale. This suggests that the eddy diffusivity reaches to O(10(-1)) m(2) s(-1) in the vicinity of the seamount. At a site away from the abrupt topography, eddy diffusivity was also elevated to O(10(-3)) m(2) s(-1), and was associated with shear instability presumably induced by the Kuroshio shear and near-inertial internal-wave shear. Our study suggests that a better prediction of current, water-mass properties, and nutrients within the Kuroshio requires accurate understanding and parameterization of flow-topography interaction such as internal hydraulics, the associated internal-wave processes, and turbulent mixing processes.

DOI： 10.1002/2017JC013049

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The climatology and linear trend of seasonal water temperature and heat budget in a semienclosed sea connected to the Kuroshio region (Seto Inland Sea: SIS) are investigated by constructing and analyzing a gridded data set of water temperature, salinity, and air-sea heat fluxes. In the SIS, winter-time water temperature and ocean heat content (OHC) showed a significant increasing trend while those in summer-time exhibited no significant change. By analyzing the heat budget between the tendency of ocean heat content OHC/t and air-sea net heat flux Q(net), the driving factor of the winter-time warming trend in the SIS was identified as autumn Q(net), while the unchanged summer-time OHC is mainly due to decreased spring-time oceanic heat transport to the SIS from the Kuroshio region. In addition, we showed that the decreased spring-time heat transport could have been induced by upwelling due to wind curl and wind speed in the shelf-slope region, heat transport by the Kuroshio south of Shikoku Island, and freshwater input to the SIS from rivers. The importance of both oceanic and atmospheric forcing mechanisms for explaining variation in the water temperature in coastal areas is demonstrated.

DOI： 10.1002/2016JC011748

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

CiNii Books

Language：Japanese   Publishing type：Research paper (scientific journal)  

CiNii Books

Other Link： http://id.nii.ac.jp/1141/00047208/

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The terms of the steady-state turbulent kinetic energy (TKE) budget in the oceanic convective boundary layer (CBL) are estimated by use of microstructure data obtained over the continental shelf of the East China Sea. The dissipation term is calculated from the micro-scale vertical shear of horizontal velocity measured directly using a freely-falling microstructure profiler, whereas the buoyancy flux and shear production terms are estimated indirectly by integrating vertically the one-dimensional conservation equation of density and by applying similarity theory, respectively. The transport term, calculated as the residual of the other three terms, vertically redistributes the TKE from the upper half of the CBL to the lower half, consistent with the TKE budgets in the atmospheric CBL and in shear-free and slightly-sheared CBLs simulated by large eddy-simulation models. The relatively large contribution of the transport term to the TKE budget shows that a local equilibrium form of the TKE equation is not appropriate for the TKE budget in the oceanic CBL.

DOI： 10.1007/s10872-013-0215-3

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Observations of turbulence, stratification, and mean current were made using a microstructure profiler and an acoustic Doppler current profiler (ADCP) during four cruises at a central location in the Ariake Sea, under weakly and strongly stratified conditions. Continuous measurements of the dissipation rate of turbulent kinetic energy (TKE), epsilon, were made. These revealed that frictional bed turbulence with quarterdiurnal variation in the bottom boundary layer (BBL) was one of the most energetic sources of vertical mixing in the sea. Thickness of the BBL was strongly confined by the stable stratification. We investigate a relationship between the BBL height h and the Ozmidov scale. We present a systematic argument that describes the vertical structure and characteristic scales of velocity and turbulence inside the frictional BBL, where the stratification persisted. Considerable deviation of observed vertical shear from the law of the wall indicated a modification of turbulent scales by the stratification. Shear stress calculated from the velocity data using vertical integration of the equation of motion was found to decrease approximately linearly with height. The TKE production rate P, estimated using the shear stress, was highly correlated with the dissipation rate. The buoyancy contribution to TKE balance in the BBL was quantified in terms of the flux Richardson number R-f as R-f = 0.12.

DOI： 10.1007/s10872-012-0103-2

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

An acoustic Doppler current profiler was deployed in summer and autumn in 2007-2009 at two stations on the East China Sea shelf. Clear velocity spirals that basically correspond to theoretical Ekman spirals were identified for both mean and tidal currents. From these spirals and the corresponding Ekman equation, we estimated the time-averaged eddy viscosity (mu) profiles. The estimated mu was largest (2-3 x 10(-3) m(2) s(-1)) around 5 m from the bottom and decreased almost exponentially with height. A qualitatively similar profile of the eddy diffusivity was also inferred from the acoustic Doppler current profiler data and microstructure profiler data. The flux Richardson number was estimated as 0.11 +/- 0.10 similar to 0.46 +/- 0.17, indicating relatively large buoyancy contribution to the turbulent kinetic energy budget. Citation: Yoshikawa, Y., T. Endoh, T. Matsuno, T. Wagawa, E. Tsutsumi, H. Yoshimura, and Y. Morii (2010), Turbulent bottom Ekman boundary layer measured over a continental shelf, Geophys. Res. Lett., 37, L15605, doi: 10.1029/2010GL044156.

DOI： 10.1029/2010GL044156

Web of Science

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Event date： 2022.7

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Event date： 2022.5 - 2022.6

Language：Japanese   Presentation type：Poster presentation