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 estimated in this study were 0.04–0.07 °C /year in the inner bay of Tokyo Bay. These rates were the same as those reported in other areas worldwide. The heat budget in the inner bay of Tokyo Bay also increased significantly from 2004 to 2019; this increase was related to the decrease in the loss of latent heat because of the increases of the atmospheric temperature and the decreases of wind speed. Because formation of the Tokyo Bay thermohaline front has been affected by the increase of water temperature and because the temperature increases have differed between months, the program of monthly monitoring should be continued to document future environmental changes in Tokyo Bay.

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 has not been examined in real oceans. We examine if this newly proposed instability mechanism exists in the subarctic North Pacific by analyzing our fine- and micro-scale turbulence measurement data. Vertical inversions were cautiously detected in seawater density profiles and used as a proxy for instability events for gradient Richardson number larger than the critical value of 1/4. We found that a portion of inversions were associated with active DC. Such DC-related inversions exhibited elevated levels of turbulent kinetic energy dissipation rate even for gradient Richardson number largely exceeding 1/4. Our estimate suggested that the thermohaline–shear instability contributes to roughly only 10% of the dissipation of turbulent kinetic energy in the diffusively convective layer in our observation site.

鉛直渦拡散係数の強さは海洋大循環の構造に寄与し,ひいては気候変動に影響を及ぼすことが知られている。そのため鉛直渦拡散係数の全球的な推定が求められているが,限られた航海時間で推定に用いる乱流(粘性散逸率ε)の直接観測を重点的に行うことは難しい。また,観測を行えたとしても乱流のエネルギー源となる風・潮汐・内部波の砕波・海面冷却など多岐にわたる現象を理解するとともに,膨大な量の乱流自身を特徴付けるパラメータについて理解しておく必要があるため,統一的に理解しづらいのが現状である。そこで本総説では,海洋乱流現象を解析する上で用いられる数mm から数10m に及ぶ長さスケール,無次元数の持つ意味について整理するとともに,特に船舶観測データを用いた海洋乱流エネルギー散逸率の推定方法について解説を行う。

Surface circulation in Sagami Bay located in the south of Honshu, Japan, was analyzed using high-frequency (HF) radar located at Izu Oshima Island and at Arasaki (the Miura Peninsula). Prominent current direction indicated counterclockwise circulation in the western part and could not be defined in the eastern part of Sagami Bay. EOF 1st mode score indicated that surface circulation is influenced by the inflow of the Kuroshio water. Stronger currents inflow via Oshima east channel generated counterclockwise circulation in the northern part of Sagami Bay. Weaker currents inflow via Oshima east channel led to clockwise circulation. Kuroshio warm water inflow from Oshima west channel led to counterclockwise circulation. These circulations were defined by sea level anomalies at Mera, Oshima, and Kozu islands. The EOF 2nd mode implied wind-driven current. Higher modes indicated dipole modes (North–South or East–West) and eddy-like mode. Consequently, three or more patterns for surface currents were found in Sagami Bay.

Blue shark (Prionace glauca) and shortfin mako shark (Isurus oxyrinchus) are recognized as pelagic sharks and highly migratory species, but these sharks appear in coastal areas. It is suggested that sharks migrate in the growth stage, but we do not know the details and how sharks use coastal areas. A fishing survey of these species in coastal areas like Sagami Bay is rare, and thus we carried out two types of longline operations to survey catch trends about the shark in Sagami Bay from August 2011 to July 2017: vertical and horizontal longline operations. Both species were caught throughout the year, with peaks in July and December for both blue sharks and shortfin mako sharks. Many of the male juvenile blue sharks caught had not reached sexual maturity, and pregnant blue sharks were also present among the females. The birthing period of blue sharks is reported to be from April to July, and the young are believed to be born and grow up in the open ocean of the North Pacific. However, the fact that we caught pregnant blue sharks close to giving birth suggests that they may give birth in coastal areas such as Sagami Bay. We were unable to catch young shark larvae because of the size selectivity of the longline fishing gear. Future studies, using net sampling, will be necessary to search for new-born blue sharks in this coastal area.

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 effective within a narrow range. Other parameterizations can use in the actual sea, but improvements are still needed. Mixing coefficients referring to mixing efficiency are key factors when distinguishing DDC from conventional turbulence. Here, we show that measurements involving the gradient Richardson number, the buoyancy Reynolds number, and density ratio play a crucial role in determining eddy diffusivity in the presence of DDC. Therefore, deployment of a microstructure profiler together with either an acoustic Doppler current profiler (ADCP), lowered ADCP, or electromagnetic current meter is essential when measuring eddy diffusivity in the ocean's interior.

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 baroclinic jet structure with southwestward speeds exceeding 90 cm s−1 and a width of 7–8 km, was associated with a surface-to-bottom density front and was formed on the offshore side of the shelf break. The volume transport of CO (T CO) was estimated by integrating the fluxes of lower-density water that was trapped against the coast along the density front represented by the 26.2 σ θ isopycnal line. This transport decreased monotonously from 0.79 Sv (1 Sv = 106 m3 s−1) in January to 0.21 Sv in March and subsequently to 0.12 Sv in May, possibly due to the decay of the East Sakhalin Current Water in the Okhotsk Sea. Accompanied by a decrease in T CO, the location of the jet structure associated with the density front moved toward the coast while the maximum speed of the jet decreased and the tilt of the front became more horizontal. Consequently, more saline offshore Oyashio water flowed into the deep part of the shelf area, and the current structure altered from relatively barotropic in winter to baroclinic in spring. This study is the first to estimate the observed volume transport of the CO from direct current measurements.

日本南岸の黒潮流路は代表的な3流路（典型的大蛇行流路: tLM，非大蛇行接岸流路: nNLM，非大蛇行離岸流路: oNLM）に区分されてきた。本研究では日本南岸の潮位データ，並びに，黒潮流軸データセットを用い，1970年1月から2009年12月までの黒潮流路を統計的手法である，マハラノビス距離を用いた判別分析とK-mean法を用いた非階層型クラスター解析により分類した。黒潮流路の指標として南限緯度（東経136度-142度間）と北限緯度（東経136度-140度間）を算出し，大蛇行西偏流路 (LMW)，非大蛇行北偏流路 (NLMN)，非大蛇行南偏流路（NLMS）に加えて，南限緯度が北緯32度以南を示す大蛇行東偏流路（LME）の4つに分類できた。LMWはtLMに対応し，NLMNはnNLMに対応するものであるが，LMEは北緯32度以南まで達しているという面ではLMWと類似し，八丈島の南を通過しているかという面ではNLMSと類似していた。しかしながら，八丈島の潮位，並びに串本以西でも低潮位の傾向を示したことから，LMEはoNLMに隠されていた4つ目の流路と判断された。本研究によって，LMEの流路が区分されたことにより，NLMNからLMWとなり，大蛇行が形成され，LMWが北東に偏することによりLMEを形成し，さらに北上しNLMS，そしてNLMNとなり大蛇行が消滅するに至ることが示された。

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 degrees, 17.7 degrees, and 16.6 degrees C formed west of 140 degrees E, at 140 degrees-150 degrees E, and east of 150 E, respectively, in the recirculation gyre south of the Kuroshio Extension. After spring, the newly formed STMW gradually shift southward, decreasing in thickness. Simultaneously, the STMWs of 16.6 degrees and 17.7 degrees C are gradually stirred and then mixed in terms of properties. In late fall, they seem to be integrated to form a single group of STMWs having a temperature centered at 17.2 degrees C. Such STMW circulation in 2008 is much more turbulent than that in 2006, which was investigated in a previous study. The difference between the two years is attributed to the more variable state of the Kuroshio Extension in 2008, associated with stronger eddy activities in the STMW formation region, which enhance the eddy transport of STMW.High-resolution shipboard observations were carried out southeast of Japan at 141 degrees-147 degrees E in the early fall of 2008. To the south of the Kuroshio Extension, STMW exists as a sequence of patches with a horizontal scale of 100-200 km, whose thick portions correspond well to the mesoscale deepening of the permanent pycnocline. The western (eastern) hydrographic sections are occupied mostly by the 17.7 degrees C (16.6 degrees C) STMW, within which the 16.6 degrees C (17.7 degrees C) STMW exists locally, mostly at locations where both the permanent pycnocline depth and the STMW thickness are maximum. This structure implies that the STMW patches are transported away from their respective formation sites, corresponding to a shift in the mesoscale anticyclonic circulations south of the Kuroshio Extension. Furthermore, 20%-30% of the observed STMW pycnostads have two or three potential vorticity minima, mostly near temperatures of 16.6 degrees and 17.7 degrees C. The authors presume that such a structure formed as a result of the interleaving of the 16.6 degrees and 17.7 degrees C STMWs after they are stirred by mesoscale circulations, following which they are vertically mixed to form the 17.2 degrees C STMW observed in late fall. These results indicate the importance of horizontal processes in destroying the vertically uniform structure of STMW after spring, particularly when the Kuroshio Extension is in a variable state.

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 and the Mixed Water between the Oyashio and the Kuroshio. After determining a criterion to distinguish double diffusion from turbulence, vertical diffusivities and buoyancy fluxes are estimated using microstructure data. When turbulence is weak, double diffusion is observed around temperature and salinity anomalies, partly due to interleaving, and dominates the buoyancy flux. Vertical diffusivities due to double diffusion are parameterized as a function of the 10-m-scale density ratio. The 10-m-scale diffusivity estimates are consistent with the microstructure data when an appropriate criterion to reproduce a probability density function for the Turner angle is applied. A weighted-average diffusivity model is proposed to account properly for turbulence and double diffusion simultaneously.

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 water is discussed using an instantaneous ratio of the change of temperature and salinity along any particular intrusion (R l ). We estimate the range of R l DD required to convert Oyashio winter mixed-layer water to the NPIW salinity minimum due to double diffusion, and then assume double-diffusive intrusions as this conversion mechanism. A double-diffusive intrusion model is used to estimate R l DD in a situation where salt fingering dominates vertical mixing, as well as to determine whether Oyashio winter mixed-layer water can become the NPIW salinity minimum. Possible density changes are estimated from the model R l DD by assuming the amount of density change due to cabbeling. From these results, we conclude that Oyashio winter mixed-layer water contributes to a freshening of the lighter layer of the NPIW salinity minimum (around 26.70σθ) in the MWR.

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 surfaces have been used to distinguish dominant processes by assuming the horizontal scale to be the streamer scale (under 100 km). The possibility of double diffusive convection is also discussed in relation to the density ratio. These results may be summarized as follows: (1) the difference between the potential density surface and the neutral surface may rise to −0.04 kg/m3 around the source water of NPIW; (2) horizontal diffusion causes strong modifications of the source water of NPIW; (3) the density range within which strong modification of the source water of NPIW occurs becomes dense from the northern part of MWR near the Oyashio Front to the southern part near the Kuroshio Front, and to the eastern part. Our modeling of these processes shows that cabbeling has effects on the density increment of the source water of NPIW in the northern and southern part of MWR. Double diffusive convection has effects on the density increment of the source water of NPIW, mainly in the northern part of MWR. The possible density increment due to cabbeling in these areas is estimated to be 0.01≈0.03 kg/m3. The possible density increment due to double diffusive convection is 0.01≈0.03 kg/m3. The total density increment due to cabbeling and double diffusive convection amounts to 0.06 kg/m3.

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. (C) 2003 Elsevier Science Ltd. All rights reserved.

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. However, rapid diffusion of heat in the laboratory frustrates exact or conclusive understanding of many aspects of salt-fingers. Numerical simulation of salt-finger began in the 1980s. The rapid development of computational technology has brought great improvements in our understanding of salt-fingers. However, fully 3-D simulations that resolve all the scales present in salt-fingering, particularly in an oceanic environment where internal waves and turbulence are confounding influences, pose many more challenges for the future.