Jiro Yoshida

Tokyo University of Marine Science and Technology · Graduate School of Marine Science and Technology

We investigated hydrographic conditions in Tokyo Bay recorded from 1992 to 2019 by the training vessel Seiyo Maru (retired in October 2021). We reviewed the basic features of hydrographic conditions discussed in the literature. The long-term trend of temperature differed from that reported in previous studies. The rates of temperature increase esti...

The strength of the vertical eddy diffusivity determines the structure of oceanʼs general circulation, which affects the climate change. Therefore, global estimation of vertical eddy diffusivity is required; however, focusing on the direct observation of turbulence (dissipation rates) for estimation within limited ship time is difficult. Even if ob...

A recent linear stability analysis and a numerical simulation suggest that diffusive convection (DC), a regime of the double-diffusive convection, plays a potential role in onset of thermohaline–shear instability, implying that DC could contribute to turbulence production in the oceans. However, an existence of such a thermohaline–shear instability...

In this note, we provide an overview of the theoretical, numerical, and observational studies focused on oceanic eddy diffusivity, with an emphasis on double-diffusive convection (DDC). DDC, when calculated using the turbulent kinetic energy (TKE) equation, produces a negative diffusion of density. A second-moment closure model shows that DDC is ef...

In this study, we revisited the rotating-tank experiments reported by FALLER（1960） and SENJYU（1988）and qualitatively discussed the abyssal circulation in the Japan Sea by focusing on the geometric similarity of their partial barrier experiments. A point source of water near the apex of the pie-shaped rotating-tank formed the so called STOMMEL-ARONS...

Direct current measurements by a shipboard and bottom-mounted acoustic Doppler current profiler and concurrent hydrographic observations with a CTD were conducted off southeastern Hokkaido, Japan, between January and May 2005 to reveal temporal variations in the current structure and volume transport of the Coastal Oyashio (CO). The CO, which has a...

The turbulent energy dissipation rates a in the western North Pacific Ocean were observed using a microstructure profiler at 49 casts, and the measured values were converted into diffusivities of heat K and salt K5. We obtained a new relationship between the Richardson number R and the buoyancy Reynolds number R5, which enables us to use K, instead...

The Kuroshio Current path has been classified into three passes: typical Large Meander path (tLM), nearshore Non-Large Meander path (nNLM) and offshore Non-Large Meander path (oNLM). We investigated these paths in more detail statistically by Discriminant Analysis using Maharanobis Distance and K-mean method (Nonhierarchical Cluster Analysis). We u...

We examined detailed hydrographic structures data along 137_E from 3_N to 34_N From 1990 to 2007 by using CTD. The general oceanic features found along this line agreed well with the results of previous studies using Nansen bottle data.We investigated the activity of double diffusive convection through the histogram plots of the density ratio（Rρ）an...

Hydrographic data obtained by high-resolution shipboard observations and Argo profiling floats have been analyzed to study the mesoscale structure and circulation of the North Pacific Subtropical Mode Water (STMW). The float data show that in the late winter of 2008, STMW having a temperature of approximately 18.8°, 17.7°, and 16.6°C formed west of...

The surface circulation of Sagami Bay is influenced by the path of the Kuroshio (Kawabe and Yoneno, 1987; Iwata and Matsuyama, 1989). While in the intermediate-deep layer, intrusion of the low salinity water (LSW) distributed at the depth of 320-500 m was observed (Senjyu et al., 1998), and the mean flows on the bottom of Sagami Trough showed an in...

We estimated the spatial distribution of the density ratio (Rρ) in the upper 1000 db of the North Pacific from the WOCE data set. The mode value of Rρ was equivalent to those reported in former studies (3-4), meaning that the double diffusive convection is moderate or weak; however, the "hot spots" of double diffusive convection were found off east...

Microstructure measurements were made in the Mixed Water Region of the Oyashio/Kuroshio/Tsugaru currents system where both turbulence and double diffusion are involved in mixing. While intense turbulence is observed near the front between the Oyashio and the Tsugaru Current, double diffusion occupies a noticeable fraction in both the Tsugaru Water...

In this study we test Talley's hypothesis that Oyashio winter mixed-layer water (26.5–26.6σ θ) increases its density to produce the North Pacific Intermediate Water (NPIW) salinity minimum (26.7– 26.8σθ) in the Mixed Water Region, assuming a combination of cabbeling and double diffusion. The possible density change of Oyashio winter mixed-layer wat...

The vertical mixing process induced by internal tides was investigated by repeated conductivity, temperature, and depth (CTD) measurements and bottom-mounted acoustic Doppler current profiler (ADCP) in Uchiura Bay from July 24 to 25, 2001. Internal tides were observed with a wave height of 40 m and a horizontal current of 0.3 ms−1. Density inversio...

The mixing processes in the Mixed Water Region (MWR) that lead to changes in the properties of North Pacific Intermediate Water (NPIW) have been studied using observational data sets obtained in May–June 1998. Neutral surfaces, the equation of water mass conversion rate on neutral surfaces and the equation of vertical velocity across neutral surfac...

Numerical experiments on double-diffusive intrusions are reviewed briefly. Though the number of studies is very limited at present, they have undoubtedly an advantage that a heat–salt system can be studied without undesired heat loss from the boundaries.Several possibilities for future numerical experiments are summarized.

Salt-finger convection is now widely recognized as an important mechanism for mixing heat and salt, both vertically and laterally in the ocean. This article reviews numerical simulations of salt-finger. Salt-fingers were first produced in the laboratory in the mid-1960s, when their structure and transport mechanisms were partially described. Howeve...

Seasonal variation of sea level was investigated at 68 tidal stations along the Japanese coast by using monthly mean data from 1981 to 1990. The main results mostly agree with TSUMURA's results (1963) by the data from 1951 to 1960, but the details are partly different from his results. The high sea level in winter is found along the Hokkaido coast...

This article reviews the historical microstructure measurements and our own recent development in a microstructure measurement profiler (TurboMAP). We show new data obtained from highly turbulent area in the wake of the Kuroshio behind Aoga-shima. We also mention how turbulent measurement data are applied to global circulation ocean models.

Intensive observations with high resolution XBT and CTD revealed the existence of intrusion processes of water mass in the Oyashio Frontal region off Sanriku coast. This frontal intrusion showed the characteristics produced by the double diffusive convection. By using simple advection and diffusion model, vertical and horizontal eddy diffusivities...

The behavior of double diffusive gravity current is investigated numerically. First, double diffusive lock exchange flow is produced with the initial density differences between each side of the barrier wall. After the lock-gate is withdrawn, the light hot salty water flows over the heavy cold fresh water with transporting salt as salt finger conve...

Studies of mixing were done at the northern flank of Georges Bank in the summer and autumn of 1988. Two time-series of the evolution and intensity of microstructure were examined over a tidal period in the context of tidal forcing and the evolution of the density and velocity field at the site. From the CTD, ADCP and microstructure observations (EP...

Double diffusive intrusion into a density gradient is investigated experimentally. We focus our attention on the dynamic structure of intrusion, although experimental conditions were essentially same as Turner [1978]. We released a sugar (salt) water body at its neutral buoyancy level into salt (sugar) stratified water. As an initial density anomal...

We investigated the behavior of double diffusive density current numerically. We released cold/fresh water on to the ambient warm/salty water, by parameterizing the effective heat and salt transport in double diffusive convection by an experimentally determined flux law due to Linden [1974]. It is found that the behavior of the current is well desc...

A comparison between Japan-equator XBT sections along 150E in late November 1989 and along 140E in early December 1991 is made. The warmest surface water above 29C diminished to the south of 2–4N and the surface mixed layer noticeably decreased in thickness in the equatorial region in December 1991; besides, the North Equatorial Countercurrent was...

The cross-sectional structure and the water type-distribution across warm-core ring WCR 86B to the east of Honshu, Japan, in September 1987 indicate that the ring consists of two parts: the central part which contains original homogeneous water, and the outer part which contains water added from the ring-ring interaction about 1 month earlier. In t...

The effects of Earth's rotation on the stability of a thermohaline front of finite width are studied by means of a linear theory. It is found that when the rotation is present, two different types of unstable modes are possible. When the front is narrow and a Rossby radius of deformation based on Ruddick and Turner's (1979) vertical scale is large...

The Kuroshio Front to the east of Honshu, Japan, commonly has a double structure. The front consists of three main parts: the temperature front at its northern edge, the density front at its southern edge and the inside zone between them. In a previous paper suggested that the structures of the density front in November, 1984, of the inside zone in...

An experiment on center lock-exchange flow is carried out when the density difference is very small. Two cases (one-component fluid and two-component fluid) are investigated for comparison. The experimental results indicate that there is a substantial difference in appearance between one-component gravity currents and two-component currents when th...

By using data with a relatively high spatial sampling rate, we show that the Kuroshio Front commonly has a double structure to the east of Honshu, Japan. Distributions of temperature, salinity, thermosteric anomaly and dissolved oxygen measured in July 1983 and November 1984 indicate that the Kuroshio Front consists of three main parts: the ‘temper...

The velocity fields and their variation in the two-dimensional forced plumes were measured by using hydrogen bubble technique. The experiments were done mainly for the single plume experiments with and without wall. The ensemble averaged horizontal profile of the observed vertical velocity can be regarded as Gaussian, except just near the vertical...

From the observation of frontal region between the Kuroshio and Oyashio Water, the active phase of horizontal and vertical mixing processes are captured. The microstructure activity is found to be very high inside of the Kuroshio Front.

The behavior of two plumes ejected into a thin water tank is investigated experimentally. As the time elapses, the plume axes deflect towards each other. Time evolution of this two-dimensional, dual forced plume is found to be similar to that of a two-dimensional, single plume ejected near a vertical wall. The symmetric plane of the two plumes in t...

The time evolution of the starting plume can be divided into four stages: The plume behaves like a usual starting plume without wall in Stage I; the axis deflects towards the wall very slowly in Stage II and rapidly in Stage III; and the plume achieves quasi-steady state in Stage IV. The authors construct physical models to explain plume characteri...