Researcher Profile and Settings

Master

Affiliation (Master)

• Faculty of Fisheries Sciences　Marine Bioresource and Environmental Science　Marine Environmental Science

Affiliation (Master)

• Faculty of Fisheries Sciences　Marine Bioresource and Environmental Science　Marine Environmental Science

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Profile and Settings

Degree

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Profile and Settings

• Name (Japanese)

  Isoda

• Name (Kana)

  Yutaka

• Name

  200901012011385385

Achievement

Research Interests

• 海洋物理   海洋力学   Physical Oceanography   Ocean Dynamics   

Research Areas

• Natural sciences / Atmospheric and hydrospheric science

Education

•        - 1988  Ehime University
•        - 1988  Ehime University  Graduate School, Division of Engineering

Awards

• 2004 日仏海洋学会学会賞
  
• 1999 水産海洋学会論文賞
  

Published Papers

• Vertical circulation induced by negative shear flow at the downside of Ooma-zaki in the Tsugaru Strait

  Asano Ryuta, Isoda Yutaka, Kobayashi Naoto, Nakamura Tomohiro, Itoh Kaoru

  Bulletin of Fisheries Sciences, Hokkaido University 北海道大学大学院水産科学研究院 70 (1) 63 - 76 2020/08/24
• Model experiments and observations of Oyashio barotropic flow intruding to Hidaka Trough via steep shelf slope area off Cape Erimo

  Miyake Masato, Isoda Yutaka, Imai Keiri, Oguma Kenji, Sawada Kouki

  Bulletin of Fisheries Sciences, Hokkaido University 北海道大学大学院水産科学研究院 70 (1) 1 - 11 2020/08/24 [Not refereed]
  
• Behavior of Water Mass Beneath the Tsushima Warm Current in the Japan Sea

  Xiaorong Fang, Yutaka Isoda, Isao Kudo, Takafumi Aramaki, Keiri Imai, Naoto Ebuchi

  Water 12 (8) 2184 - 2184 2020/08/03 

  To better understand the behavior of water mass beneath the Tsushima Warm Current (TWC), we use the vertical cross-sections of potential temperature, salinity and dissolved oxygen in the Japan Sea obtained by the T/V Oshoro Maru of the Hokkaido University during 8–29 June in 2011 to analyze its origins and variations. The results show that the potential temperature and salinity beneath the TWC varies little, but the dissolved oxygen varies largely with the geographical location. There are two deep water masses with different dissolved oxygen content below the TWC. One is on the coastal side with the low dissolved oxygen, and the other is on the offshore side with the high dissolved oxygen. It is inferred that the former one is relatively old water and the latter is the new water. By using the phosphate (PO4) and the apparent oxygen utilization (AOU) relationship, we calculate the PO40 (preformed PO4) as a water mass tracer. These results suggest that the water masses beneath the TWC with high and low dissolved oxygen originate from the same surface water mass in the central Japan Sea.
• Observation and model experiments on an internal-wave packet accompanied by streak bands over the sill topography of Tsugaru Strait

  Yamaguchi Takuya, Isoda Yutaka, Itoh Umihiko, Mukai Touru, Kobayashi Naoto

  Oceanography in Japan 日本海洋学会 29 (3) 71 - 90 0916-8362 2020 

  The Synthetic Aperture Radar (SAR) around the western entrance of the Tsugaru Strait has yielded well-defined images of an internal-wave packet accompanied by two or three surface-streaks (surface convergence within the same water mass), which is active during the stratified season, and whose wavelength is in the order of several hundred meters. Most of the waves in the packet were observed near the topographically shallow parts of the sill. Temporal changes were repeatedly observed in the internal waves confined to the sill using a high-frequency echosounder profiler within one-day in the summer 2017. The acoustic images suggest that a wave packet of extraordinary amplitude (> 150 m) has developed transiently around the downstream side of the sill at the ascending passage flow. This wave packet consists of two or three successive streak bands, with very disturbed sea surface conditions overlying wave troughs, i.e., strong downwelling areas. The dynamics of such waves developing over the sill is studied through a fully nonlinear nonhydrostatic numerical model. The vertical fluid stratification and temporal change of the barotropic passage flow were adjusted to approximate our observation conditions. The results suggest that the wave packet is effectively amplified near the downstream side of the sill, where the Froude number becomes a critical point, because upstream propagating waves on the sill slope stagnate and overlap efficiently. In this dynamical process, however, even if the wave grows to large amplitude, it does not form a well-organized solitary wave, but is rather scattered due to the strong dispersion of waves.

• Numerical experiments of surface clockwise circulation in Funka Bay during early summer

  Kobayashi Naoto, Isoda Yutaka, Horio Kazuki

  Oceanography in Japan 日本海洋学会 28 (4) 51 - 74 0916-8362 2019 

  A current system consisting of surface clockwise circulation is the most remarkable feature observed in Funka Bay during early summer. The present study investigates its formation process using a numerical model driven by the following three factors during the typically strati fied season, i.e., (1) freshening of coastal water because of river discharge, (2) density inflow of Tsugaru Gyre water, and (3) surface heat flux. It was found that the "topographic heat accu mulation effect" resulting from surface heating is essential for the genesis of the surface clock wise circulation. Because of the surface offshore flow generated by the thermal contrast be tween the shallow coastal and deep central regions, a weak anti-clockwise geostrophic flow is initially formed. Nevertheless, with continuous thermal forcing, after a few months, this offshore flow gradually reinforces the upslope transport of cold dense water. When the cooling resulting from the dense water upslope dominates in comparison to the downward heating resulting from vertical diffusivity around the coastal sea bottom, the coastal water is relatively colder than the offshore surface water. Therefore, shallowing of the interface toward the coast drives the geos trophic flow proceeding along the coast to the left-handed side. In response to this change, an isolated clockwise circulation begins to establish from the surface layer of the northern bay head, while an initially formed anti-clockwise flow migrates to the deeper region.

• Diurnal tidal currents attributed to free baroclinic coastal-trapped waves on the Pacific shelf off the southeastern coast of Hokkaido, Japan

  Hiroshi Kuroda, Akira Kusaka, Yutaka Isoda, Satoshi Honda, Sayaka Ito, Toshihiro Onitsuka

  Continental Shelf Research 158 45 - 56 1873-6955 2018/04/15 [Refereed][Not invited]
   

  To understand the properties of tides and tidal currents on the Pacific shelf off the southeastern coast of Hokkaido, Japan, we analyzed time series of 9 current meters that were moored on the shelf for 1 month to 2 years. Diurnal tidal currents such as the K1 and O1 constituents were more dominant than semi-diurnal ones by an order of magnitude. The diurnal tidal currents clearly propagated westward along the coast with a typical phase velocity of 2 m s–1 and wavelength of 200 km. Moreover, the shape and phase of the diurnal currents measured by a bottom-mounted ADCP were vertically homogeneous, except in the vicinity of the bottom boundary layer. These features were very consistent with theoretically estimated properties of free baroclinic coastal-trapped waves of the first mode. An annual (semi-annual) variation was apparent for the phase (amplitude) of the O1 tidal current, which was correlated with density stratification (intensity of an along-shelf current called the Coastal Oyashio). These possible causes are discussed in terms of the propagation and generation of coastal-trapped waves.
• Upward transport of iron at the west shelf edge–slope of the Okinawa Trough in the East China Sea

  Ryohei Sasayama, Nanako Hioki, Yuichiroh Morita, Yutaka Isoda, Keiri Imai, Atsushi Ooki, Kenshi Kuma

  Journal of Oceanography 74 (4) 1 - 13 1573-868X 2018/02/16 [Refereed][Not invited]
   

  We studied the behavior of chemical substances in the upper 300 m of the water column across the continental shelf–slope interface in the East China Sea off the Okinawa Trough. The behaviors of iron, inorganic nutrients, and humic-like fluorescent dissolved organic matter were strongly influenced by the extensive water exchange between the East China Sea and the Kuroshio Current across the shelf break and slope via upwelling and frontal processes. We attributed the high humic-like fluorescent intensity at the subsurface of the shelf break and slope regions to the lateral supply of humic-like fluorescent dissolved organic matter from the shelf sediments to the outer shelf region due to the intrusion of shelf water into Kuroshio subsurface water. We found that the behavior of iron at the continental shelf–slope was remarkably different from the conservative mixing of inorganic nutrients and humic-like fluorescent dissolved organic matter. In deep and bottom waters at the shelf–slope, high total iron concentrations, which were closely related to water transmittance, possibly resulted from the swept transport of iron-rich resuspended sediments over the shelf floor from the slope by the invading Kuroshio Intermediate Water close to the bottom.
• Observations and model experiments of the diurnal eddy of the Cold Water Belt along the offshore side of the Soya Warm Current

  Iida Hiroyuki, Isoda Yutaka, Kobayashi Naoto, Horio Kazuki

  Oceanography in Japan 日本海洋学会 27 (4) 155 - 174 0916-8362 2018 

  The detailed distributions of flow, temperature fields, and the temporal change of diurnal eddies accompanied with the Cold Water Belt (CWB) are observed by CTD and one-day repeated XBT/ADCP measurements across the Soya Warm Current (SWC) in the summer of 2016. Saline water originating from the Japan Sea Intermediate Water (JSIW) was found below the CWB. Tracer experiments using numerical models predict the offshore-side water of the SWC is supplied by onshore advection and upwelling of the deeper JSIW. The nearly homogeneous JSIW is taken into a counter-clockwise isolated eddy, which is periodically generated by the diurnal tidal current around the Soya Strait and is advected downstream along the offshore side of the SWC. Therefore, the majority of the water below the CWB, which is weakly stratified, is composed of this eddy street.

• Subarctic wintertime dissolved iron speciation driven by thermal constraints on Fe(II) oxidation, dissolved organic matter and stream reach

  Yuichiroh Morita, Kei Yamagata, Atsuki Oota, Atsushi Ooki, Yutaka Isoda, Kenshi Kuma

  GEOCHIMICA ET COSMOCHIMICA ACTA 215 33 - 50 0016-7037 2017/10 [Refereed][Not invited]
   

  We studied the seasonal variations in Fe(II), Fe(III), humic-like dissolved organic matter (DOM), nitrate and nitrite (NO3+ NO2), and silicate (Si(OH)(4)) in river waters of three subarctic rivers flowing into Hakodate Bay in southwestern Hok-kaido, Japan from May 2010 to February 2014. High Fe(II) concentrations were detected in winter at the sampling sites where the river bottom was comprised of sandy or silty sediment, primarily the lower and middle reaches of the rivers. Conversely, from early spring to late autumn Fe(II) levels were low or undetectable. We infer that soluble Fe(II) concentration in these subarctic river waters is driven by the balance between the influx of Fe(II) to the river and the Fe(II) oxidation rates that determines the dynamics in Fe(II) concentration in the river water. The Fe(II) may originate from reductive dissolution of Fe(III) in the river sediment or from Fe(II)-bearing groundwater. The latter seems to be the most likely source during winter time. The high Fe(II) concentrations during winter is predominantly attributed to the extremely slow oxidation rate of Fe(II) to Fe(III) at low water temperature rather than to an actual increase in the flux of reduced Fe(II). Nevertheless, we propose that the flux of reduced Fe(II) from river sediments and groundwater in lowland area of the catchment to overlying river waters might be the most important sources of iron in river waters. This provides an important insight into the role of river processes and the interaction between climate and river morphology in determining the inputs of iron to subarctic coastal marine waters. (C) 2017 Elsevier Ltd. All rights reserved.
• Migration area of the Tsushima Warm Current Branches within the Sea of Japan: Implications from transport of Ra-228

  M. Inoue, Y. Shirotani, Y. Furusawa, K. Fujimoto, H. Kofuji, K. Yoshida, S. Nagao, M. Yamamoto, Y. Hamajima, N. Honda, A. Morimoto, T. Takikawa, A. Shiomoto, Y. Isoda, M. Minakawa

  CONTINENTAL SHELF RESEARCH 143 167 - 174 0278-4343 2017/07 [Refereed][Not invited]
   

  We investigated lateral profiles of Ra-228 (half-life; 5.75 years) activity and Ra-228/Ra-226 (1600 years) activity ratio using 241 surface water samples collected in/around the Sea of Japan and the East China Sea (ECS) during June October of 2009-2014. In the ECS, the Ra-228/Ra-226 ratio in the surface waters exhibited markedly wide variation ( < 0.05-3.5) in June, predominantly reflecting the mixing between the Ra-228-rich continental shelf water and the Ra-228-depleted Kuroshio Current water. In July, the surface waters of the central Sea of Japan (135-138 degrees E) became separated into three currents: the Offshore Branch of the Tsushima Warm Current (OBTWC) (Ra-228/Ra-226 =0.7-1.2) at 39-41 degrees N, the Coastal Branch of the TWC (CBTWC) (similar to 0.7) on the southern side, and sub-Arctic Current (similar to 0.7) on the northern side. From the central to northeastern Sea of Japan, the Ra-228/Ra-226 ratio at the surface (0.8-1.0) was within a range between that of the CBTWC and OBTWC. The fraction of continental shelf water in the CBTWC, OBTWC, and in their combined current was estimated to be 11-16%, similar to 8%, and 10-11%, respectively.
• Generation Mechanism of the Anti-clockwise Nonlinear Eddy Street in the Region of the Soya Warm Current

  伊田 智喜, 有田 駿, 磯田 豊

  北海道大学水産科学研究彙報 北海道大学大学院水産科学研究院 66 (3) 99 - 114 1346-1842 2016/12 

  The generation mechanism of anti-clockwise nonlinear eddy street along the Soya Warm Current is examined using the inflow-forced numerical model incorporating the simple shelf-slope topography. According to the conservation of potential vorticity, a strong positive vorticity is produced off the inflow entrance by vortex stretching as the water column is pushed out to deeper offshore. It rapidly grows up an isolated anti-clockwise circular eddy, i.e., a nonlinear eddy, during advection time traveling round the eddy. Therefore, this advection time controls the oscillated period, and each isolated eddy is periodically generated and stably advected downstream by the mean shear-flow. Such flow system accompanied by the eddy street is interpreted as the self-excited oscillation.
• Numerical model experiments for seasonal variation of the North Pacific wind-driven circulation

  舘野 愛実, 藤原 将平, 磯田 豊, 朝日 啓二郎

  北海道大学水産科学研究彙報 北海道大学大学院水産科学研究院 66 (3) 87 - 97 1346-1842 2016/12 

  A two-layer model ocean with Izu-Ogasawara Ridge(IOR) and the Emperor Seamounts(ESs), driven by seasonally varying wind stress, is used to explain the observed seasonal variations of barotropic and baroclinic response in the North Pacific. Since isostasy is achieved for annual mean state, the annually averaged subarctic and subtropical surface gyres are separated without any bathymetric influence. For a short time scale such as seasonal variation, the barotropic flow is inhibited from ascending the bottom slope because of the restriction of the potential vorticity conservation, and generates the baroclinic Annual Rossby Waves(ARW) through the impinging process on the shelf slope. The most conspicuous feature in barotropic response is the southward invasion of the subarctic anti-clockwise circulation strengthened in winter into the subtropical region along the IOR and the western shelf slope. It is also found that the westward propagating baroclinic ARW from the southern ESs is appeared to be enhanced in spring by superimposing on the seasonal change of Ekman pumping. These model results are consistent with those estimatied from the observation.
• Soya Warm Current Effectively Weakened by Easterly Wind

  小野 祐嗣, 磯田 豊, 工藤 勲, 宮園 章, 有田 駿

  北海道大学水産科学研究彙報 北海道大学大学院水産科学研究院 65 (2) 47 - 59 1346-1842 2015/08 

  The subinertial variation of the Soya Warm Current(SWC) is investigated through the analysis of current records obtained from the moored meters settled at three sites along the east coast of Hokkaido, during a period from April to October 2011. We found that the rapid decreases of southeastward SWC were frequently observed due to the strong easterly wind blowing, but the inverse(westerly) wind with the similar amplitude never grew the strong SWC. To understand such asymmetrical response of the SWC to the wind forcing, the numerical model experiments including the nonlinear drag law of bottom friction were carried out. In the circumstances combined with the SWC generated by sea level difference across the Soya Strait and wind-induced current, the model result suggests that the asymmetrial action of bottom friction term causes the different dynamic response. When the momentum balance between easterly wind stress and sea level gradient across the atrait is just realized, the SWC effectively is weakened, and hence such dynamic responce becomes the frictional free. In the case of westerly wind forcing, the SWC is superimposed by the eastward wind-induced current so that the southeastward flow speed tends to increase. However, its increase rate is gradually suppressed, because the bottom friction term on the momentum balance dominants in proportional as the square of flow speed.
• Wind-driven current in coastal sea in the period of non-stratification : Channel and Box models with the bottom friction

  矢幅 寛, 磯田 豊

  北海道大学水産科学研究彙報 北海道大学大学院水産科学研究院 65 (2) 61 - 76 1346-1842 2015/08 

  We re-examine the maintenance mechanism for the vortices in steady state, taking into considertion of the dissipation term including bottom stress : τb under the forced conditions with a constant and shear wind stress along the channel axis. In case of channel-topography without the bay-head, the flow pattern according to "dynamic balance" of τs=-τb forms. Therefore, the vortices are never produced even by the vortices supply from uniform wind stress due to the bathymetry torque. We also compare two models based on a single-layer and a multi-layer respectively. In case of closed box-topography, the essential flow pattern of vortices can be explained by "vorticity balance" of curl(τs/H)～-curl(τb/H)(H is a depth), and the divergence term generates the lower mode shelf wave and distorts the position of vortices.
• Formation of the bottom water in the abyssal circulation in the Japan Sea

  Shoji K, Y. Isoda, K. Kuma, T. Aramaki

  Bulletin of Fisheries Sciences, Hokkaido University 北海道大学大学院水産科学研究院 65 (1) 17 - 29 1346-1842 2015/03 [Not refereed][Not invited]
   

  The abyssal waters in the Japan Sea vertically consist of the homogeneous Bottom Water (BW) and the weak-stratifiedDeep Water (DW). The formation of BW was inferred based on the spatial difference of their recent warming trends in potentialtemperature from 2000 to 2007 and the snapshot hydrographic observation in 2007 around a benthic front on the shallowersill region between the Yamato Basin (YB) and the Japan Basin (JB). It is found that there is no significant change in the thicknessof BW, but both BW and DW showed clear warming trends. Geothermal heating from the seabed with order of 102 mWm-2 is plausible candidate having potential to cause the vertical mixing of BW and its reasonable warming trend with order of 10-3°C yr-1. Considering upward advection of such heating from BW, the warming trends of DW at YB and JB are respectivelyexamined using the one-dimensional advection-diffusive equation. The results suggest the existence of horizontal abyssal densitycurrent between both basins, i.e., on the sill, which was consistent with the snapshot spatial distributions of salinity and dissolvedoxygen. If this local abyssal circulation may be continuously generated by the horizontal density gradient between bothbasins, the thickness of warming and vertical mixed BW will be always restricted by the sill depth.
• fN-Oscillation trapped in the benthic front in the Abyssal Japan Sea

  Yamauchi Y, Shoji K, Isoda Y, Arita S, Kawano K, Fujiwara S, Fang X, Asahi K, Ida S, Kuma K, Tateno M, Imai K, Owada M

  Oceanography in japan 日本海洋学会 24 (4) 147 - 169 2186-3105 2015 [Refereed][Not invited]
   

  本研究では,非静水圧の力学領域においてN(浮力振動数)と/(慣性振動数)がほぼ等しい成層状態で浮力振動と慣性振動の性質をあわせ持つ「fN振動」について,その特性と実在の可能性を検証した。まず,N→fの極限において内部波の分散関係から水平方向の群速度が極大となる場合にfN振動の水平波数kと鉛直波数mの比,k/m,が1/√<3>に等しいことと,N=fの理想的な成層条件下では群速度が零となるため,この振動は強制領域内に捕捉されることを示した。ついで,局所的なフロントを模した数値モデル実験によって,フロントに捕捉されたfN振動のエネルギーの一部が内部波として離脱することを示した。この内部波の海洋現場での発生源の候補として大和海盆底層水と日本海盆底層水の境界域に形成されている底層フロントに注目して,その近傍で観測を行い,fN振動の間接的な証拠と考えられるk/m～1/√<3>を満たす内部慣性重力波の擾乱の存在を水温と溶存酸素の変動から確認した。Because the vertical homogeneous Bottom Water (BW) with N～0 (N is the buoyancy frequency) is formed in the Japan Sea, the abyssal water always has a depth of N～f (f is the inertial frequency) above the BW. In the present study, we propose the theoretical existence of buoyancy/inertial oscillation under the weak stratification with N～f in the non-hydrostatic dynamics and term it "fN-Oscillation". Based on the dispersion relation of internal inertial waves in the limit of N→f, it is concluded that fN-Oscillation with lateral maximum group velocity fulfills the requirement of a finite spatial scale for a ratio of horizontal/vertical wavenumber k/m = 1/√<3>. In the idealized stratification of N=f, in particular, this oscillation is confined to the generation area because it contains zero group velocity. Testing of numerical model experiments, including a localized front under the stratification with N～f, suggests that some of the fN-Oscillation energy trapped in this front escapes as internal waves. We observed that the benthic front existed stably between the Yamato Basin BW (YBBW) and the Japan Basin BW (JBBW) and was the plausible origin of energetic upward-propagating internal waves. The benthic front has a notably sharp structure with a horizontal scale of several hundreds meters. In the vicinity of this front, the periodic variations of temperature and dissolved oxygen were confirmed according to the internal waves, with the wavenumber satisfying approximately k/m～1/√<3>. Such disturbances represent indirect evidence for the trapping fN-Oscillation.
• Inverse Estuary Circulation in Mutsu Bay-(1) Hydrographic Observations during the Summer-time-

  河野航平, 磯田豊, 工藤勲, 藤原将平, 有田駿, 小林直人, 吉田達, 扇田いずみ, 高津哲也

  海と空 海洋気象学会 90 (1) 11 - 16 0503-1567 2014/08/31 [Refereed][Not invited]
  
• Chemical evidence for the origin of the cold water belt along the northeastern coast of Hokkaido

  Kenshi Kuma, Ryohei Sasayama, Nanako Hioki, Yuichiroh Morita, Yutaka Isoda, Tohru Hirawake, Keiri Imai, Takafumi Aramaki, Tomohiro Nakamura, Jun Nishioka, Naoto Ebuchi

  Journal of Oceanography Springer 70 (4) 377 - 387 0916-8370 2014/01/01 [Not refereed][Not invited]
   

  In the southwestern Okhotsk Sea, the cold water belt (CWB) is frequently observed on satellite images offshore of the Soya Warm Current flowing along the northeastern coast of Hokkaido, Japan, during summertime. It has been speculated that the CWB is upwelling cold water that originates from either subsurface water of the Japan Sea off Sakhalin or bottom water of the Okhotsk Sea. Hydrographic and chemical observations (nutrients, humic-type fluorescence intensity, and iron) were conducted in the northern Japan Sea and southwestern Okhotsk Sea in early summer 2011 to clarify the origin of the CWB. Temperature-salinity relationships, vertical distributions of chemical components, profiles of chemical components against density, and the (NO3 + NO2)/PO4 relationship confirm that water in the CWB predominantly originates from Japan Sea subsurface water. © 2014 The Oceanographic Society of Japan and Springer Japan.
• Abyssal water mass exchange between the Japan and Yamato Basins in the Japan Sea

  Tomoharu Senjyu, Takafumi Aramaki, Shinichi S. Tanaka, Jin Zhang, Yutaka Isoda, Yuichiro Kumamoto, Sho Hibino, Toshiya Nakano

  JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 118 (10) 4878 - 4888 2169-9275 2013/10 

  Bottom water exchange between the Japan and Yamato Basins in the abyssal Japan Sea is investigated based on data from two research cruises in June 2010 and June 2011. Distributions of water properties and velocity profiles from CTD/LADCP observations revealed a two-layer water mass exchange in a rotating system. The cold Japan Basin Bottom Water (JBBW) with high dissolved oxygen (DO) concentrations flowed into the Yamato Basin along the northern boundary of the channel connecting the basins, while the warmer, lower-DO Yamato Basin Bottom Water (YBBW) extended over the JBBW along the southern periphery of the channel. A benthic front between the JBBW and YBBW was observed in both cruises, though the front shifted to the northeast in 2011 compared with its 2010 location. Further, the bottom flows in the channel were not unidirectional in 2011, though consistent southwestward flows were observed in 2010. The JBBW transport into the Yamato Basin was also larger in 2010 than in 2011. Analysis of historical hydrographic data showed that the benthic front exists mostly in the narrowest region of the channel, though it fluctuates temporarily to the northeast or southwest. The fluctuation of the benthic front appears to be associated with the JBBW transport because the observations in June 2010 and June 2011 were carried out during periods of southwestward extension of cold water and northeast protrusion of warm water, respectively. A relationship between benthic front displacements and mesoscale warm eddy migrations is suggested.
• The Kyucho passing through the bay mouth of Funka Bay-(3) Numerical model experiment II-

  小林直人, 磯田豊, 高津哲也, 木村修, 亀井佳彦

  海と空 88 (3) 57 - 63 0503-1567 2013/03/22 [Refereed][Not invited]
  
• Water column iron dynamics in the subarctic North Pacific Ocean and the Bering Sea

  UCHIDA Ren, KUMA Kenshi, KUMA Kenshi, OMATA Aya, ISHIKAWA Satoko, HIOKI Nanako, UENO Hiromichi, ISODA Yutaka, SAKAOKA Keiichiro, KAMEI Yoshihiko, TAKAGI Shohgo

  J Geophys Res Ocean Amer Geophysical Union 118 (3) 1257 - 1271 2169-9275 2013/03 [Refereed][Not invited]
   

  We measured water-column iron concentrations from west to east along 47 degrees N in the subarctic North Pacific, and in the Bering Sea. In the North Pacific dissolved Fe (D-Fe) showed surface depletion, mid-depth maxima at 1000-1500 m (west, 1.3-1.6 nM; east, 0.9-1.1 nM), and a gradual decrease with depth below 3500-4000 m depth (west, 1.1-1.4 nM; east, 0.6-0.7 nM). D-Fe and total soluble Fe (T-Fe) in deep water showed a decreasing trend eastward. The higher iron concentrations in western deep waters probably result from higher inputs of dissolved Fe through atmospheric deposition or lateral transport. In contrast, D-Fe throughout the Bering Sea showed a consistent depth regime characterized by a rapid increase with depth to mid-depths, a gradual increase with depth in intermediate water to a maximum of 1.6-1.7 nM at 1500-2250 m, and a gradual decrease with depth to 1.3-1.4 nM at 3700 m. Higher iron concentrations and deeper D-Fe maxima in the Bering Sea are likely due to higher biological productivity and greater and deeper D-Fe input from the decomposition of sinking particulate organic matter in deep water. We suggest that the higher concentrations and deeper input of D-Fe as well as PO4 and humic-type fluorescent dissolved organic matter in the Bering Sea probably results from the longer time for the accumulation of decomposition products resulting from iron supply from the organic-rich downslope sediment along the steep continental slopes and slow replacement of the deep water in the Bering Sea Basin.
• The Kyucho passing through the bay mouth of Funka Bay-(2) Numerical model experiment I-

  小林直人, 磯田豊, 高津哲也, 木村修, 亀井佳彦

  海と空 海洋気象学会 88 (2) 43 - 48 0503-1567 2012/11/30 [Refereed][Not invited]
  
• The Kyucho passing through the bay mouth of Funka Bay-(1) Data analysis-

  小林直人, 磯田豊, 小林雅行, 佐藤千鶴, 木村修, 山口秀一, 高津哲也, ROSA Ana Luisa

  海と空 海洋気象学会 88 (1) 9 - 20 0503-1567 2012/07 [Refereed][Not invited]
  
• Flow fields around the Emperor Seamounts detected from current data

  Taku Wagawa, Yutaka Yoshikawa, Yutaka Isoda, Eitarou Oka, Kazuyuki Uehara, Toshiya Nakano, Kenshi Kuma, Shogo Takagi

  JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 117 2169-9275 2012/06 [Refereed][Not invited]
   

  Bathymetric influences of the Emperor Seamounts (ESs) on the local and large-scale flow fields in the subarctic North Pacific were investigated by analyzing data of directly measured velocity obtained using Argo floats, near-surface buoys, and moored current meters. The local influences were evident in notable deflections of two relatively strong (similar to 0.05 m s(-1)) eastward flows over ESs. A Taylor cap induced by the eastward flow impinging against the Nintoku Seamount (around 42.0 degrees N, 170.5 degrees E) was clearly identified in the climatological flow field at 1000 m depth. This flow pattern was also identified in the near-surface flow field (15 m depth) and dynamic height field at 200 dbar relative to 2000 dbar, indicating the Taylor cap is stable or frequently generated and extends to the surface. An anticyclonic flow was found over the Suiko Seamount (around 45.0 degrees N, 170.5 degrees E). The influences on the large-scale flow field were found in data obtained from current meters moored over the southeastern flank of the Suiko Seamount and Argo float trajectories. These data indicate for the first time from velocity observations that there is a seasonally varying boundary current along the eastern side of ESs partially compensating for the Sverdrup transport to the east of ESs, and hence, the seasonally varying component of the subarctic gyre in the North Pacific is divided into two subgyres by ESs.
• Intrusion of the Oyashio water into the eastern mouth of Tsugaru Strait in early summer, 2003

  Kuroda Hiroshi, Isoda Yutaka, Takeoka Hidetaka, Kuma Kenshi, Honda Satoshi, Matsuura Hiromi, Saitoh Yukiyo, Takata Hyoe, Iwade Shoei, Wagawa Taku, Kobayashi Naoto

  CONTINENTAL SHELF RESEARCH 32 36 - 46 0278-4343 2012/01/01 [Refereed][Not invited]
  
• 簡略モデルを用いた噴火湾底泥ＡＶＳ変化の検討

  奥村 裕弥, 磯田 豊, 工藤 勲, 宮園 章

  沿岸海洋研究 49 91 - 21 2011 [Refereed][Not invited]
  
• Controls on iron distributions in the oceanic deep water colummn: Iron(III) hydroxide solubility and marine humic-like disolved organic matter

  KUMA Kenshi, ISODA Yutaka

  KAIYO MONTHLY 494 718 - 728 2011 [Not refereed][Not invited]
  
• Iron distributions in the water column of the Japan Basin and Yamato Basin (Japan Sea)

  Satoshi Fujita, Kenshi Kuma, Satoko Ishikawa, Shotaroh Nishimura, Yuta Nakayama, Satomi Ushizaka, Yutaka Isoda, Shigeyoshi Otosaka, Takafumi Aramaki

  Journal of Geophysical Research Wiley-Blackwell 115 (C12) C12001  0148-0227 2010 [Refereed][Not invited]
   

  In the Japan and Yamato basins (Japan Sea), dissolved Fe ([D-Fe], <0.22 μm fraction) was characterized by surface depletion, mid-depth maxima, then a slight decrease with depth in deep water and uniform concentration in bottom waters because of biological uptake in the surface water and release by microbial decomposition of sinking organic matter in mid-depth waters. Total Fe concentrations ([T-Fe]) in the surface water of the Japan Sea were 1-4 nM, a little higher than those in the surface waters of the nutrient-deficient subtropical western North Pacific and extremely higher than the nutrient-rich subarctic western North Pacific and the nutrient-deficient subtropical central North Pacific, resulting from high atmospheric Fe input to nutrient-depleted surface water of the Japan Sea. In the Japan Basin, the [T-Fe] in bottom water were lower than those in deep water, resulting from (1) the injection of new bottom water with the lower [T-Fe] into the Japan Basin bottom water, (2) the particulate Fe removal by particle scavenging during the bottom water circulation of the Japan Basin, or (3) the injection of deep water with the higher [T-Fe] into the Japan Basin deep water. On the other hand, the [T-Fe] in deep water of the Yamato Basin and the slope regions were variable with different [T-Fe] levels among stations and depths. We found a significant relationship between [T-Fe] and water transmittance in deep water, probably resulting from the iron supply into the deep water because of the lateral transport of resuspended sediment from the slope.
• Seasonal Variations of Shelf Circulation in Hidaka Bay, Hokkaido, Japan, with an Interpretation of the Migration Route of Juvenile Walleye Pollock

  Ana Luisa Rosa, Yutaka Isoda, Naoto Kobayashi

  JOURNAL OF OCEANOGRAPHY 65 (5) 615 - 626 0916-8370 2009/10 [Refereed][Not invited]
   

  The outer shelf of Funka Bay, located at the bay head of Hidaka Bay, is a recognised main winter spawning ground for walleye pollock (Theragra chalcogramma), whose newly hatched juveniles migrate eastward along the Hidaka shelf to the nursery ground located in the Doto area. To examine the seasonal change of the coastal current along this migration route, four current moorings were deployed along the shelf in Hidaka Bay from April, 2004 to June, 2006. Since these mooring sites were close to the coast, the circulation was estimated after removal of the wind-driven component. It was found that the winter coastal flow, forced by a north-westerly monsoon wind, is clockwise along the Hidaka shelf. However, this flow is weak due to the superimposition of the opposite Coastal Oyashio flow trapped on the shelf. In summer, a bifurcation of the coastal current occurs along the north-eastern Hidaka shelf with a south-eastward flow, associated with the Tsugaru Gyre, and a north-westward flow, consisting of a branch from this gyre. Our results provide a new understanding of the migration of juvenile walleye pollock: (1) very slow transport of juveniles along the Hidaka shelf from winter to spring, and (2) selection of earlier spawning survivors due to the bifurcated flow in early summer.
• Recent observations in the straits of the East/Japan Sea: A review of hydrography, currents and volume transports

  Hanna Na, Yutaka Isoda, Kuh Kim, Young Ho Kim, Sang Jin Lyu

  JOURNAL OF MARINE SYSTEMS 78 (2) 200 - 205 0924-7963 2009/09 [Refereed][Not invited]
   

  Recent observations of hydrography, currents and volume transports in the straits of the East/Japan Sea are reviewed. It is newly found that bottom cold water in the Korea/Tsushima Strait originating from the northern region of the East/Japan Sea appears not only in summer and autumn but also in winter. Intensive observations in the Korea/Tsushima Strait revealed two distinct cores of northeastward currents in the upper layer of the western and eastern channels. Mean volume transport through the Korea/Tsushima Strait is calculated as 2.5 +/- 0.5 Sv from four-year direct and indirect measurements. As continuous monitoring has started in the Tsugaru and Soya Straits, understanding of temporal variability of currents and volume transports through the straits is in progress. For the first time, simultaneous time series of volume transports are available in the Korea/Tsushima and Tsugaru Straits during the winter of 1999-2000. Ouflow through the Tsugaru Strait accounts for about 70% of inflow through the Korea/Tsushima Strait for this period. (C) 2009 Elsevier B.V. All rights reserved.
• Seasonal variation in Tsushima Warm Current paths over the shelf off the San'in coast, Japan

  Akihiko Morimoto, Yutaka Isoda, Tatsuji Tameishi, Shimpei Moriwaki

  CONTINENTAL SHELF RESEARCH 29 (11-12) 1437 - 1447 0278-4343 2009/06 [Refereed][Not invited]
   

  Using long-term sea surface temperature (SST) and acoustic Doppler current profiler (ADCP) data, we examined variations in the current axis of the Tsushima Warm Current (TWC) off the San'in coast of Japan, near the entrance to the Japan Sea. There were large horizontal temperature gradients along the shelf edge in the southwestern Japan Sea from October to May, suggesting that the second branch of the TWC appears not only in spring and autumn but also in winter. From the ADCP data analysis, we found that currents with speeds of approximately 20 cm s(-1) and greater appeared around the shelf edge off San'in coast in all seasons. The SST and ADCP data analyses suggested that the second branch of the TWC exists around the shelf edge off the San'in coast throughout the year. This finding differed from those of previous studies. A relatively strong current (speed greater than 15 cm s(-1)) appeared on the shore side in all seasons, except at line W in winter. This current might be the first branch of the TWC. The first branch seemed to occur around in 100 m isobaths, but shifted northward and southward because the bottom topography around lines W and M was relatively flat and the shelf was broad. The first branch was very obscure, and it was difficult to define the two branches of the TWC off the San'in coast from the seasonally averaged vectors. However, snapshots of current distribution derived from the ADCP data clearly showed these branches. Hence, both the first and second branches might occur throughout the year off the San'in coast. (C) 2009 Elsevier Ltd. All rights reserved.
• The Kyucho occurred off Kameda Peninsula, Hokkaido, at the end of May, 2006

  小林直人, 磯田豊, 小林雅行, 佐藤千鶴, 木村修, 山口秀一, 高津哲也, ROSA Ana Luisa

  北海道大学水産科学研究彙報 北海道大学大学院水産科学研究院 = Research Faculty of Fisheries Sciences, Hokkaido University 58 (3) 29 - 41 1346-1842 2009/02 [Not refereed][Not invited]
   

  A large damage to fishing set-nets caused by the Kyucho occurred off Kameda peninsula on 30 May, 2006. Such Kyucho seldom occurs in this area and hence it is unusual phenomenon. Two current-meters setting off Usujiri located at Kameda peninsula and off Shiraoi faced the Hidaka Bay captured this sudden and swift current. The meteorological and oceanographical conditions in 2005 without the Kyucho and those in 2006 with the Kyucho were compared. The difference in both years was the oceanic stratification. That is, the surface halocline was thinly formed by a large quantity of rainfall water accompanied by the atmospheric disturbance on 28 May in 2006, before the Kyucho occurrence. In the case of such discontinuity subsurface thinner layer than usual, we will expect that the particle velocity of wind-induced internal Kelvin wave increases, but its phase propagation speed rather decreases. In fact, the observed surface current velocity (0.3-1.2 m/s) is almost as same as the estimated propagation speed (0.3-0.8 m/s).
• Controls on iron distributions in the deep water column of the NorthPacific Ocean: Iron(III) hydroxide solubility and marine humic-type dissolved organic matter

  Saori Kitayama, Kenshi Kuma, Eri Manabe, Koji Sugie, Hyoe Takata, Yutaka Isoda, Kenji Toya, Sei-ichi Saitoh, Shohgo Takagi, Yoshihiko Kamei, Keiichiro Sakaoka

  Journal of Geophysical Research 114 (C08019) doi:10.1029/2008JC004754  2169-9275 2009 [Refereed][Not invited]
   

  Dissolved Fe in the western and central North Pacific Ocean was characterized by surface depletion, middepth maxima and, below that, a slight decrease with depth similar to the vertical distributions of nutrients, apparent oxygen utilization, Fe(III) hydroxide solubility, and humic-type fluorescence (H-flu) intensity. Dissolved Fe concentrations ([D-Fe], <0.22-mu m fraction) in the deep water column were one-half lower in the central region (0.3-0.6 nM) than the western region (0.5-1.2 nM) although the Fe(III) solubility ([Fe(III)sol], <0.025-mu m fraction) levels and distributions in deep waters were almost the same between both regions with middepth maxima (similar to 0.6 nM) at 500-1500-m depth range and then a gradual decrease to similar to 0.3 nM at 5000-m depth. Higher [D-Fe] than [Fe(III)sol] in the deep water column of the western region results from the higher production of dissolved Fe from the decomposition of sinking particulate organic matter in the western region than the central region because of the high atmospheric and/or lateral Fe inputs in the western region. Similarity between [D-Fe] level and [Fe(III)sol] value at each deep water depth in the central region may be attributed to [D-Fe] being nearly in the solubility equilibrium with Fe(III) hydroxide in seawater. Strong linear correlation between [D-Fe] and H-flu intensity in the central region and relatively similar linear relationships between [Fe(III)sol] and H-flu intensity in the western and central regions are the first confirmation that humic-type fluorescent dissolved organic matter may be responsible for [D-Fe] in the deep water column as natural organic ligands complexing with Fe(III).
• Processes Influencing Iron Distribution in the Coastal Waters of the Tsugaru Strait, Japan

  Saitoh Yukiyo, Kuma Kenshi, Isoda Yutaka, Kuroda Hiroshi, Matsuura Hiromi, Wagawa Taku, Takata Hyoe, Kobayashi Naoto, Nagao Seiya, Nakatsuka Takeshi

  JOURNAL OF OCEANOGRAPHY 64 (6) 815 - 830 0916-8370 2008/12 [Refereed][Not invited]
  
• Analysis of Inertial Oscillation Observed Near the Mouth of Funka Bay

  AKABANE Hiroko, ISODA Yutaka, MIYAKE Hideo, TAKAHASHI Daisuke, KIDO Kazuo, KURODA Hiroshi, AZUMAYA Tomonori, Hiroko Akabane, Yutaka Isoda, Hideo Miyake, Daisuke Takahashi, Kazuo Kido, Hiroshi Kuroda, Tomonori Azumaya, Hokkaido National, Fisheries Research Institute, Graduate School, of, Fisheries Science, Hokkaido University, Graduate School of, Fisheries Science, Hokkaido University, Hydrospheric Atmospheric Research Center Nagoya University, Geological Survey of Hokkaido, National Research, Institute of, Fisheries Science, Hokkaido National, Fisheries Research Institute

  Oceanography in Japan 日本海洋学会 17 (4) 265 - 279 2186-3105 2008/07/01 [Not refereed][Not invited]
   

  2001年の夏季,噴火湾湾口近くの水深85mの地点に海底設置型ADCPを設置し,計測された約4か月間の流速記録を解析した。流速のスペクトル解析において,慣性周期帯のエネルギーは,全層で半日潮流と同程度であった。狭い周期帯の変動成分を抽出可能なバンドパスフィルター(HAB法)を適用し,慣性振動流(慣性周波数;f=2π/18時間)を抽出した結果,観測期間中に流速振幅の顕著な増大(以下,イベント)が4回確認された。Pollard and Millard(1970)の単純な風強制モデルがこれらイベントのほとんどを再現できたことから,観測された慣性振動流は主に時間変化する風強制により発生したと推測される。慣性振動流の鉛直構造とその時間変化はイベントごとに異なり,大きく二つに分類できる。一つは,中層の振幅極大と鉛直上方への位相伝播で特徴付けられ内部波エネルギーの斜め下方への伝播を推測させるビーム状の構造であり,もう一つは内部波の水平伝播を連想させるモード状の構造である。主成分解析に基づく鉛直モード分解を行った結果,いずれの流速構造に対しても鉛直第2モード以上の高次モードが大きく寄与していることが示唆され,各イベントで最大振幅が観測される時刻は次数の大きな鉛直モードほど遅れる傾向があった。さらに,ビーム状の構造から検出された振幅極大の水深と特性曲線の傾きを用いて見積もった内部波エネルギーの水平伝播距離から,内部波の励起場所は噴火湾内にあることが推定された。A bottom-mounted ADCP was deployed near the mouth of Funka Bay at a depth of 85 m for about four months during the summer of 2001. Spectra analysis shows that the inertial energy dominates in all layers and the energy is in the same order of magnitude as the semi-diurnal tidal current energy. The inertial oscillations are band-passed using a HAB method capable of extracting narrow-band variations. It was found that the inertial currents were amplified four times (hereinafter, "events") during the entire period of the current observation. Moreover, it was shown that the simple wind-induced model of Pollard and Millard (1970) can well reproduce the observed events in the surface mixed layer. This suggests that the observed inertial currents were generated by the time variation of the wind stress above Funka Bay. It should be noted that the vertical current structure and its time variation of the individual events were different from each other, and these seem to be classified into two patterns related to the internal waves. One is referred to as the "beam structure", characterized by the maximum amplitude in the middle layer and the upward phase propagation of the near-inertial current, i.e., the energy of internal waves propagates downward. The other is referred to as the "mode structure", associated with the horizontal propagation of the internal vertical mode. The vertical mode decomposition based on a principal component analysis revealed that higher vertical modes (> 2nd mode) significantly contribute to all the events, regardless of the beam and mode structures. In addition, the higher vertical mode tends to be observed after the vertical mode with a lower order appears. The horizontal propagation distances of the internal wave from the generation point to the observation point were estimated from the depth of the maximum amplitude in the middle layer and the gradient of characteristic curve during the beam-structure period. The distance suggests that the generation of the internal wave related to the observed inertial current was limited to the waters within the Funka Bay.
• Diurnal Tidal Current on the Eastern Shelf of Hidaka Bay : Can juvenile walleye pollock, Theragra chalcogramma, move southeastward with the diurnal tidal current?

  KURODA Hiroshi, ISODA Yutaka, HONDA Satoshi, TAKEOKA Hidetaka, SHIMIZU Manabu

  La mer. 日仏海洋学会 46 (1) 37 - 47 0503-1540 2008/05/28 [Not refereed][Not invited]
  
• Iron in the Japan Sea and its implications for the physical processes in deep water

  Hyoe Takata, Kenshi Kuma, Yutaka Isoda, Shigeyoshi Otosaka, Tomoharu Senjyu, Masayuki Minagawa

  Geophysical Research Letters Wiley-Blackwell 35 (2) 2008/01 [Refereed][Not invited]
  
• 北海道西岸沖日本海における繁殖期のウトウの分布

  田中遊山, 本田聡, 磯田豊, 伊藤元裕, 綿貫豊

  日本鳥学会誌 57 148 - 153 2008 [Refereed][Not invited]
  
• Intrusion of the Tsushima warm water into the Tsugaru strait

  NAKAJIMA Yobun, ISODA Yutaka, MATSUURA Hiromi, WAGAWA Taku, AIKI Tomokazu, YABE Itsuka, TAKATA Hyoue, KUMA Kenshi, KOBAYASHI Naoto, YAHABA Hiroshi

  Umi to Sora 海洋気象学会 84 (1) 1 - 16 0503-1567 2008 [Refereed][Not invited]
  
• Seasonal variations of water system distribution and flow patterns in the southern sea area of Hokkaido, Japan

  Ana Luisa Rosa, Yutaka Isoda, Kazuyuki Uehara, Tomokazu Aiki

  JOURNAL OF OCEANOGRAPHY 63 (4) 573 - 588 0916-8370 2007/08 [Refereed][Not invited]
   

  Hydrographic data and composite current velocity data (ADCP and GEK) were used to examine the seasonal variations of upper-ocean flow in the southern sea area of Hokkaido, which includes the "off-Doto" and "Hidaka Bay" areas separated by Cape Erimo. During the heating season (April-September), the outflow of the Tsugaru Warm Current (TWC) from the Tsugaru Strait first extends north-eastward, and then one branch of TWC turns to the west along the shelf slope after it approaches the Hidaka Shelf. The main flow of TWC evolves continuously, extending eastward as far as the area off Cape Erimo. In the late cooling season (January-March), part of the Oyashio enters Hidaka Bay along the shallower part of the shelf slope through the area off Cape Erimo, replacing almost all of the TWC water, and hence the TWC devolves. It is suggested that the bottom-controlled barotropic flow of the Oyashio, which may be caused by the small density difference between the Oyashio and the TWC waters and the southward migration of main front of TWC, permits the Oyashio water to intrude along the Hidaka shelf slope.
• Seasonal Variations of Surface Flow around Toyama Bay

  AIKI Tomokazu, ISODA Yutaka, YABE Itsuka, KURODA Hiroshi, Tomokazu Aiki, Yutaka Isoda, Itsuka Yabe, Hiroshi Kuroda, Graduate School, of, Fisheries Science, Hokkaido University, Graduate School of, Fisheries Science, Hokkaido University, Graduate School of, Fisheries Science, Hokkaido University, National Research, Institute of, Fisheries Science

  Oceanography in Japan 日本海洋学会 16 (4) 291 - 304 2186-3105 2007/07/05 [Not refereed][Not invited]
   

  対馬暖流の沿岸分枝流の形成には陸棚地形が重要な役割を果たしているが,この分枝流の流路にある陸棚域は富山湾周辺で一端途切れている。過去の水温データ解析から,富山湾の東側における分枝流は沖合へ離岸した傾圧流と推測されているが,実際の流れのパターンとその季節変化については明らかにされていない。本研究では,長期間に蓄積されたGEK(Geomagnetic Electrokineto Graph)・ADCP(Acoustic Doppler Current Profiler)・沿岸水位のデータを用いて,富山湾周辺海域における表層流の季節変化を解析した。佐渡島(小木)と日本列島沿岸(輪島,富山,柏崎)における水位差の季節変化には,東向き表層流の強まる時期が7月と11月の2回あることが示された。この結果をもとに,一年間を5季節に区分し,GEK・ADCP両データを用いて表層流の季節空間分布図を作成した。まず,年間を通した流れとして,"能登半島の西側における39°N付近を境界とした東向き沖合流と,日本列島付近の東向き沿岸流とに二分岐した流れが,佐渡島周辺(能登半島の東側)で合流する",と特徴付けられる流れのパターンが認められた。すなわち,佐渡島の西側には一年を通して北東流が存在する。一方,富山湾周辺における流れパターンには明瞭な季節変化が示され,その流れ解析の空間解像度を高めることによって,二つの流れパターンの存在も明らかに示された。その第一の流れは富山湾内へ流入する流れ(湾の西側に沿った沿岸分枝流)であり,4月～9月に顕著にみられ,9月には弱まる。そして,他の流れは湾口を横断する流れ(能登半島北部から佐渡海峡に向かう傾圧流)で年間を通して存在し,湾内流入流が消滅後の秋季に最も明瞭になることも判明した。The shelf topography plays an important role in the formation of the coastal branch flow of the Tsushima Warm Current. This shelf breaks a part of the branch flow around Toyama Bay. Although its path east of Toyama Bay has been inferred as the offshore baroclinic flow from temperature distributions, its flow pattern and seasonal variability around Toyama Bay have not been clarified. By using long-term GEK, ADCP and sea level data, we describe the seasonal change of surface current around the Bay. Sea level differences between Sado Is. (Ogi) and coastal stations (Wajima, Toyama, Kashiwazaki) reveal that there are two periods in which the eastward surface current velocity is enhanced in July and November. Based on this feature, we classify the annual cycle into five seasons in order to examine temporal change in spatial distributions of surface flow using GEK and ADCP data. The annual common flow pattern shows that there are two main paths, the one consisting of the eastward offshore flow (the boundary being 39°N latitude) and the other. a coastal branch flow on the Japanese shelf joining near Sado Is (east of Noto Peninsula). This means that there is the northward flow west of Sado Is. throughout the year. On the other hand, the flow pattern around Toyama Bay has a prominent seasonal cycle, and our gridded current field with fine spatial resolution shows that there are actually two flow patterns. One is the inflow into Toyama Bay (the near-shore branch along the western bay coast) which is identified in April to September and weakens in September. The other is a flow across the bay mouth (the baroclinic flow from the north of Noto peninsula into Sado strait), which exists throughout the year and is clearly seen in autumn after the disappearance of Toyama Bay inflow.
• 津軽海峡通過流の海水変質過程

  松浦宏美, 磯田 豊, 黒田 寛, 久万健志, 斉藤雪美, 小林直人, 相木智一, 和川 拓, 矢部いつか, 干場康博

  海と空 83 2135  2007 [Refereed][Not invited]
  
• Effect of high iron concentrations on iron uptake and growth of a coastal diatom Chaetoceros sociale

  Shouei Iwade, Kenshi Kuma, Kenshi Kuma, Yutaka Isoda, Masahiko Yoshida, Isao Kudo, Isao Kudo, Jun Nishioka, Jun Nishioka, Koji Suzuki, Koji Suzuki

  Aquatic Microbial Ecology Inter-Research 43 (2) 177 - 191 0948-3055 2006/06/27 [Not refereed][Not invited]
   

  The growth and iron uptake of the coastal marine diatom Chaetoceros sociale were experimentally measured in batch experiments at 10°C to which an acidic Fe(III) stock solution was added. The direct input of Fe(III) into the culture media induced the highest iron uptake rate (∼3.4 to 4.2 × 10-16 mol Fe cell-1 d-1) by C. sociale during the first day of the incubation, resulting from the supply of bioavailable inorganic Fe(III) species at levels above its expected equilibrium value (∼0.1 nmol l-1) with solid amorphous Fe(III) hydroxide in seawater. The iron uptake rate during the first day of incubation in solid amorphous Fe(III) hydroxide medium aged for 1 d at 10°C was approximately 50 % lower than that in the direct Fe(III) input media. We used a modified approach in which further iron uptake by C. sociale from external iron in the direct Fe(III) input media was prevented by adding hydroxamate siderophore desferrioxamine B (DFB) during cultivation. After the addition of DFB, the highest growth rate (∼0.5 to 0.6 d-1) of C. sociale by intracellularly stored Fe in the direct Fe(III) input media was maintained for a few days since no iron uptake was observed after the DFB addition. The growth rate was independent of the amount of intracellularly stored Fe. However, the maximal cell yields appeared to be relatively dependent on the amount of intracellularly stored Fe, suggesting the presence of a critical concentration of intracellular Fe (minimum cellular Fe for growth) for phytoplankton growth (∼1 × 10-16 mol Fe cell-1 for C. sociale). In the present study, maximal and minimal Fe quotas were 3.4 to 4.2 × 10 -16 and ∼1 × 10-16 mol Fe cell-1 (the maximal/minimal Fe ratio of 3.4 to 4.2), respectively. The high iron uptake and storage capacity in C. sociale allows this species to accumulate excess iron at high concentrations of bioavailable inorganic Fe species and to support up to 1.8 to 2.1 cell divisions without any additional iron uptake. In addition, we attempted to model the effect of luxury uptake on growth, as biodilution of cellular Fe eventually decreases the Fe quota to a critical threshold. © Inter-Research 2006.
• Hydrographic Structural Change in the Transition Domain around the Emperor Seamounts

  和川拓, 磯田豊, 齋藤菜那, 東屋知範, 坂岡桂一郎

  海の研究 日本海洋学会 15 (3) 267 - 281 0916-8362 2006/05/05 [Not refereed][Not invited]
   

  北部北太平洋には亜寒帯前線と亜寒帯境界の間として定義される移行領域(Transitional Domain: TD)が存在し,TDを南北に横切るように170°Eに沿って天皇海山列が存在している。本研究では155°E,165°E,170°E,175.5°E,180°線における長期間の海洋観測データを解析し,TDにおける水塊構造が天皇海山列を境界に大きく変化していることを明らかにした。天皇海山列の東西海域ではTDを明確に特定できるのに対して,天皇海山列上においてTDはしばしば存在せず,南から北に向かって亜熱帯水から亜寒帯水に急激に変化する水塊前線を形成していた。さらに,TDにおける東向き輸送に沿って,水塊は西から東に向かって亜寒帯水から亜熱帯水へ急激に変化していた。TD付近に存在する強い東向きの傾圧流は,天皇海山列上の43°N付近(推古海山と仁徳海山の間)を通過した後,北側にシフトする。この傾圧流の強弱や分布位置は経年変動したが,北側にシフトする傾向は何れの年においても同じであった。傾圧流を挟んだ西側と東側のTDで起こる水塊の変化は,亜表層において特徴的な水塊である中央モード水が重い密度帯から軽い密度帯へ変化することに起因していた。すなわち,傾圧流の北側へのシフトと中央モード水の密度帯の変化は密着に関連していることが示唆された。Spatial distributions of baroclinic flow and water mass in the "Transition Domain" (TD), defined between the Subarctic Front and Subarctic Boundary in the northern North Pacific, were investigated through analysis of long-term hydrographic data taken in 155°E, 165°E, 170°E, 175.5°E, and 180° meridians. Along the 170°E meridian, the Emperor Seamounts is located across the TD in a strict north to south direction. Although the TD could be clearly detected on both sides of the Emperor Seamounts, it frequently disappeared and had a remarkable watermass front, i.e., abrupt and meridional, changing from subtropical to subarctic water masses, in the location of the Emperor Seamounts. Furthermore, along the eastward transport of the TD, an abrupt zonal change from subarctic to subtropical water took place. In each year, the estimated eastward baroclinic flow around the TD suggested a shift to the north after passage through the trench of the Emperor Seamounts around 43°N (between Suiko Seamount and Nintoku Seamount), although small year-to-year changes were shown in its intensity and horizontal distribution. This shift in flow might explain the above zonal water mass change in the TD. It was also shown that the density of the "Central Mode Water", formed at the sea surface mixed layer in winter, was gradually lighter from the west to the east.
• Effect of aging time on the availability of freshly precipitated ferric hydroxide to coastal marine diatoms

  Yoshida Masahiko, Kuma Kenshi, Iwade Shouei, Isoda Yutaka, Takata Hyoe, Yamada Masumi

  MARINE BIOLOGY SPRINGER 149 (2) 379 - 392 0025-3162 2006/05 

  Cell growth and iron uptake of the coastal marine diatoms Chaetoceros sociale and Thalassiosira weissflogii were studied in the presence of short-aged amorphous ferric hydroxide (am-Fe(III)) media. These were prepared by aging for 1 day, 3 days, and 3 weeks after adding a small amount of ferric iron acidic stock solution to autoclaved filtered seawater and were experimentally measured in culture experiments at 10 degrees C for C. sociale and 20 degrees C for T. weissflogii. The order of cell yields for both species was: 1-day aged am-Fe(III) > 3-day aged am-Fe(III) >> 3-week aged am-Fe(III) media. The iron uptake rates by C. sociale during 0-1 day in 1 day and 3-day aged am-Fe(III) media were about two-thirds and one-fourths, respectively, lower than that in the direct Fe(III) input medium containing C. sociale into which an acidic Fe(III) stock solution was added directly. The longer aging time of am-Fe(III) in media results in reducing the supply of bioavailable iron in the media by the slower dissolution rate of am-Fe(III) with the longer aging time. These results suggest that the chemical and structural changes of freshly precipitated amorphous ferric hydroxide with short aging time affect their ability, such as iron solubility and dissolution rate to supply bioavailable iron for the phytoplankton growth. The chemical and structural conversion of solid iron phases with time is one of the most important processes in changing the supply of available iron to marine phytoplankton in estuarine and coastal waters and in iron fertilization experiments.
• Interannual Variation of Water Temperature and Salinity in Deep Water West of the Tsugaru Strait

  森 康輔, 磯田 豊, 清水 学

  Reports of Research Institute for Applied Mechanics,Kyushu University 九州大学応用力学研究所 (130) 1 - 10 1345-5664 2006/03 

  Interannual variation of water-temperature and salinity structures in deep water was investigated by means of CTD data along N-Line (41°50'N, 138°00'E-140°00'E) and S-Line (40°36.5'N, 138°00'E-139°48'E) in spring and autumn from 1988 through to 1995. There were uneven isotherms below the depth of 500m, and the water less than 0.2°C exists relatively shallow region on shore, which seemed to be correlated to the bottom topography. The deep cold water was getting warm slightly throughout the observation period because of the heat conduction from surface layer, since the Japan Sea Proper Water might have not been formed for the last several decades. Comparing the interannual variation of the potential temperature at 500m-depth with the monsoon index, it was found that the water on shore becomes cold in moderate winter and warm in severe winter. It is suggested that the intensity of the convection in the Tsushima Warm Current alters the thickness of the surface layer. Salinity of the High-Salinity Intermediate Water (HSIW) was increasing throughout the observation period, especially in autumn, which indicates that high salinity water might have not been entrained to deep water due to the moderate winter.
• Seasonal Variations of the Flow Pattern and Water Masses off Kameda Peninsula, Hokkaido

  Kobayashi Naoto, Isoda Yutaka, Kuroda Hiroshi, Kimura Osamu, Yamaguchi Hidekazu, Onishi Mitsuyo, Rosa Ana Luisa

  Bulletin on coastal oceanography 日本海洋学会沿岸海洋研究部会 44 (1) 59 - 67 1342-2758 2006 [Refereed][Not invited]
   

  Seasonally varying nearshore currents, which have not been described in the oceanographic literature, are believed to exist off Kameda Peninsula, Hokkaido; a northwestward current called the "Shikabe-jio", and a southeastward current called the "Esan-jio". The purpose of this study is to describe these currents quantitatively and to examine the relationship between seasonal variations in currents and water mass distribution by analyzing CTD and ADCP data. We have confirmed the seasonal variation of flow patterns with opposite current direction, i.e., the baroclinic southwestward flow (Esan-jio) from spring to summer and the barotropic weak northwestward flow (Shikabe-jio) in winter. Tsugaru Warm Current Water appears off the Peninsula with transitional changes of current in autumn, but rapidly replaces Oyashio Water in winter.
• Comparing the vertical distribution of iron in the eastern and western North Pacific ocean

  TAKATA H.

  Geophysical Research Letters American Geophysical Union 33 (2) L02613 - L02613 0094-8276 2006 [Refereed][Not invited]
   

  Labile dissolved Fe (<0.22 μm) in the western (165°E) and eastern (165°W) North Pacific Ocean had nutrient- and apparent oxygen utilization (AOU)-like profiles characterized by surface depletion and deep enrichment (5–3000 m depth). Dissolved Fe concentrations in the deep-water column at the mid-latitudes were approximately one-half lower in the eastern region (0.5–0.8 nM) than in the western region (0.8–1.3 nM) although the dissolved Fe concentrations in the surface mixed layer in both regions were extremely depleted to 0.1–0.2 nM. Surprisingly, the labile particulate Fe concentrations (≤∼0.1 nM, total dissolvable Fe minus labile dissolved Fe) throughout the water column at low latitudes in the eastern region were extremely lower than those (∼0.5–1 nM) in the western region. It is suggested that the low Fe levels in the eastern mid-latitude oceanic region are primarily due to the lower atmospheric Fe supply in the eastern region.
• Benthic Front and the Yamato Basin Bottom Water in the Japan Sea

  SENJYU Tomoharu, ISODA Yutaka, ARAMAKI Takafumi, ARAMAKI Takafumi, OTOSAKA SHIGEYOSHI, FUJIO Shinzo, YANAGIMOTO Daigo, SUZUKI Takashi, KUMA Kenshi, MORI Kosuke

  J Oceanogr Oceanographic Society of Japan 61 (6) 1047 - 1058 0916-8370 2005/12 [Not refereed][Not invited]
  
• Comparative vertical distributions of iron in the Japan Sea, the Bering Sea, and the western North Pacific Ocean

  H Takata, K Kuma, S Iwade, Y Isoda, H Kuroda, T Senjyu

  JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 110 (C7) 2169-9275 2005/07 [Refereed][Not invited]
   

  日本海やベーリング海などの半閉鎖性海域と北太平洋での溶存鉄及び三価鉄溶解度の鉛直分布は，中深層での深度とともに徐々に増加することを特徴としている。中深層では，溶存鉄は主に天然有機配位子との錯体形成によって規定されていることが，各海域のこれらの強い相関性から推定される。更に三価鉄溶解度は，北太平洋の中層と深層での栄養塩との勾配が異なるが，三価鉄溶解度と海洋性腐植様傾向物質との関係は，海域での勾配の違いは確認できなかった。この結果は，腐植様蛍光物質の分布が，深海水中の三価鉄溶解度と溶存鉄濃度の規定要因であることを示唆している。
• The Lower Trophic Ecosystem in Funka Bay and Its Long-term Monitoring Index(Symposium: International Symposium on Long-term Variations in the Coastal Environments and Ecosystems)

  Kudo Isao, Miyazono Akira, Shimada Hiroshi, Isoda Yutaka

  Bulletin on coastal oceanography 日本海洋学会沿岸海洋研究部会 43 (1) 33 - 38 1342-2758 2005 [Refereed][Not invited]
   

  A monitoring program for scallop shellfish poison has been conducted since 1970's in Funka Bay, Hokkaido. High paralytic shellfish poison (PSP) values were detected in the 1980's, but the value decreased in the 1990's. This rise and fall of PSP coincided with the appearance of Alexandrium tamarense, a dinoflagellate species causative of PSP. Cyst density in sediment was high between 5 and 7cm, and between 12 and 14cm, suggesting a past multidecadal variation for this species in the past. As this species appears after the diatom spring bloom in Funka Bay, the occurrence of this toxic dinoflagellate bloom may be influenced by the nutrient utilization and regeneration process after the bloom. Another explanation for the variation of this species is as a reflection of an environmental change, such as in temperature.
• 「1994～2002年夏期におけるアラスカ湾の水温極小」

  橋本 洋平, 磯田 豊, 大西 広二, 坂岡 桂一郎

  海の研究 14 (4) 513 - 525 0916-8362 2005 [Refereed][Not invited]
  
• Examination of Harmonic Analysis Methods Using Semi-regular Sampling Data from an ADCP Installed on a Regular Ferry : Evaluation of Tidal and Residual Currents in the Eastern Mouth of the Tsugaru Strait

  KURODA Hiroshi, ISODA Yutaka, OHNISHI Mitsuyo, IWAHASHI Masayuki, SATOH Chizuru, NAKAYAMA Tomoharu, ITO Toshimichi, ISEDA Ken-ichi, NISHIZAWA Keisuke, SHIMA Shigeki, TOGAWA Orihiko, Hiroshi Kuroda, Yutaka Isoda, Mitsuyo Ohnishi, Masayuki Iwahashi, Chizuru Satoh, Tomoharu Nakayama, Toshimichi Ito, Ken-ichi Iseda, Keisuke Nishizawa, Shigeki Shima, Orihiko Togawa, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Present address)Center for Marine Environmental, Studies Ehime University, Mutsu Marine Laboratory Japan Marine Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Marine Research Laboratory Japan, Atomic Energy Research Institute, Mutsu Marine Laboratory Japan Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Marine Research Laboratory Japan, Atomic Energy Research Institute

  Oceanography in Japan 日本海洋学会 13 (6) 553 - 564 2186-3105 2004/11/05 [Not refereed][Not invited]
   

  津軽海峡東口周辺における実際の定期船ADCP流速データを用いて,黒田・磯田(2004)が提案した年周斯変動成分に関する調和解析上の注意点に基づき,どれだけ確からしい潮流と残差流(季節変動)が見積もれるかを検討した。日単位のデータサンプリングに依存した調和解析上の最大の問題点は,K_1とP_1分潮からSa分潮と同じ周期のエイリアシングが生じるため, K_1,P_1,Saの3分潮を同時に分離することが不可能なことである。ここにおいて, Sa,K_1の2分潮を選択し, O_1,M_2,Q_1,N_2の4分潮を加えて調和解析を行なった結果,構造的で卓越したK_1分潮流を見積もることができた。しかし,残差流に含まれる可能性があると推定されたP_1分潮の最大振幅は,残差流の年周期変動の振幅に対して無視できず,P_1分潮を調和解析によって分離する必要がある。そこで,津軽海峡東口周辺の検潮所におけるP_1とK_1分潮の潮位の振幅比と位相差の関係を潮流に適用した調和解析を再び行ない,過去に行われた短期の係留流速観測結果と比較して,両者の共通点と相違点を整理した。更に,このような方法で見積もられた潮流成分を除去することで,津軽海峡内における流れの季節変動を提示することが初めて可能となった。Some harmonic analysis methods using actual current data measured by an ADCP installed on a regular ferryboat were examined based on the suggestions made by Kuroda and Isoda (2004) regarding the harmonic analysis of annual variations. Hence the tidal and residual currents (seasonal variation) were estimated around the eastern mouth of the Tsugaru Strait. The biggest problem in the harmonic analysis is that the K_1, P_1, and Sa constituents cannot be separated simultaneously since a sun-synchronous infrequent semi-regular sampling causes K_1 and P_1 aliasing, the period of which is the same as that of the Sa constituent, i.e. 365.24 (d). First, harmonic analysis of the Sa, K_1, O_1, M_2, Q_1, and N_2 constituents could spatially estimate the systematic and dominant K_1 tidal current. However, the guessed maximum P_1 amplitude that may have been included in the residual current was not sufficiently less than the amplitude of the seasonal variation. Therefore, it was impossible to discuss the seasonal variations unless the P_1 constituent was estimated, based on assumptions and subsequently excluded. Another harmonic analysis of tidal currents was carried out using the amplitude ratio and phase difference of the P_1 tide relative to the K_1 tide. The harmonic constants were compared with those estimated from the mooring current measurements made in past years, and the points of commonality and difference between them were clarified. Thus, for the first time, the seasonal variations in the surface currents at the Tsugaru Strait were demonstrated by excluding the estimated tidal currents.
• Quick-Offer of Current Information using T/V ADCP-Program for Quality Control of ADCP Data-

  小林直人, 磯田豊, 高津哲也, 山口秀一, 木村修

  北海道大学水産科学研究彙報 北海道大学 55 (2) 97 - 103 1346-1842 2004/10 [Not refereed][Not invited]
  
• Comparison of HF Radar Observed and Computed Tidal Currents around the mouth of Funka Bay

  磯田 豊, 坂田 陽一郎, 磯野 哲郎, 清水 学, 他, 春日 一彦, 灘井 章嗣, 藤井 智史, 齊藤 誠一

  Bulletin of Fisheries Sciences, Hokkaido University 北海道大学 55 (1) 63 - 69 1346-1842 2004/08 [Not refereed][Not invited]
  
• Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron (III) hydroxide solubility

  Kenshi Kuma, Yutaka Isoda, Shigeto Nakabayashi

  Journal of Geophysical Research: Oceans 108 (9) 1 - 10 2169-9291 2003/09/15 

  The vertical profiles of nutrient, AOU (apparent oxygen utilization), and Fe(III) hydroxide solubility (Fe(III) solubility) in the western North Pacific commonly show that their concentrations increased with depth below the surface mixed layer and have strong gradients around the North Pacific Intermediate Water (NPIW) in a potential density (σθ) range of 26.7-27.0. They had the maximum values at a σθ range of 27.0-27.5, and then rapidly decreased with depth at higher σθ than 27.5. The similarity of their profiles versus σθ suggests that they are controlled by similar processes with an intrinsic timescale such as deep-ocean circulation. The vertical profiles of dissolved Fe were also characterized by mid-depth maxima and, subsequently, a slight decrease with depth, which is markedly similar to nutrient and Fe(III) solubility depth profiles. This implies that the major source of dissolved Fe in the deep ocean is release during the remineralization of biogenic organic matter. In the present study, we attempted to confirm that the dissolved Fe depth profiles are controlled by the sinking particulate organic matter (POM), the production of dissolved Fe from POM during carbon remineralization, the scavenging of dissolved Fe and the temporal fixed Fe(III) solubility depth profile. Using the production rate (α) of dissolved Fe (1/α = 5-10 years), the calculated depth profile of dissolved Fe is in remarkable agreement with the observed profile of dissolved Fe with mid-depth maxima. Therefore we concluded that dissolved Fe concentrations in deep ocean waters are controlled primary by the Fe(III) solubility.
• Variation of velocity and volume transport of the Tsugaru Warm Current in the winter of 1999-2000

  T Ito, O Togawa, M Ohnishi, Y Isoda, T Nakayama, S Shima, H Kuroda, M Iwahashi, C Sato

  GEOPHYSICAL RESEARCH LETTERS 30 (13) 1678  0094-8276 2003/07 [Refereed][Not invited]
   

  [1] The variation of velocity and volume transport of the Tsugaru Warm Current (TWC) is investigated based on the data of continuous and cross-sectional current monitoring in the Tsugaru Strait. During November 1999-March 2000, the structure of the velocity transection across the strait was almost stable in which the TWC occupied the central part of the strait, while the returning flows existed in the northern and southern part of the strait, and the velocity of these flows decreases with time. The volume transport of the TWC varies from 2.1 to 1.1 Sv, with the mean value of 1.5 Sv, following the variation of the TWC. The temporal variation of the volume transport shows a linear correlation with that of the sea level difference between the Japan Sea and the Pacific Ocean. The contribution of the sea level difference to the volume transport is estimated to be roughly 70%.
• 10-, 25-, and 60-day Periodic Current variations on theWestern shelf in Hidaka Bay

  KURODA Hirosi, ISODA Yutaka, ONISHI Mitsuyo, IWAHASHI Masayuki, SATOH Chizuru, NAKAYAMA Tomoharu, ITO Toshimichi, ISEDA Ken-ichi, NISHIZAWA Keisuke, SHIMA Sigeki, TOGAWA Orihiko, Hiroshi Kuroda, Yutaka Isoda, Mitsuyo Onishi, Masayuki Iwahashi, Chizuru Satoh, Tomoharu Nakayama, Toshimichi Ito, Ken-ichi Iseda, Keisuke Nishizawa, shigeki Shima, Orihiko Yogawa, Mutsu Marine Laboratory Japan, Marine, Science Foundation, resent address)Graduate School of, Fishcries Science, Hokkaido University, Mutsu Marine Laboratory Japan, Marine, Science Foundation, resent address)Graduate School of, Fishcries Science, Hokkaido University, Mutsu Marine Laboratory Japan, Marine, Science Foundation, resent address)Graduate School of, Fishcries Science, Hokkaido University, Mutsu Marine Laboratory Japan, Marine, Science Foundation, resent address)Graduate School of, Fishcries Science, Hokkaido University, Mutsu Marine Laboratory Japan, Marine, Science Foundation, resent address)Graduate School of, Fishcries Science, Hokkaido University, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Marine resarch Laboratory Japan, Atomic Energy Research Institute, Mutsu Marine Laboratory Japan Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Mutsu Marine Laboratory Japan, Marine, Science Foundation, Marine resarch Laboratory Japan, Atomic Energy Research Institute

  Oceanogr. Japan 日本海洋学会 12 (2) 195 - 214 2186-3105 2003/03/05 [Not refereed][Not invited]
   

  青森室蘭間を運航する定期旅客船に搭載されたADCPによる2000年4月～2001年4月までの流速データを用いて,日高湾西部陸棚上こおける数十日(10日～60日)周期の流速変動を調べた。主要10分潮の潮流成分をADCP流速データから除去した後,2種類のスペクトル解析法を適用した。その結果,西部陸棚上において10日,25日,60日の三つの周期帯の流速変動が卓越していることが分った。本研究では,不等間隔データに適用可能な新しいバンドバス法(HAB法:Harmonic Analysis Bawl-pass Method)を提案し,各周期変動の水平流速・鉛直流速の構造を抽出した。日高湾西部陸棚上のADCP観測点間において,三つの周期帯の流速変動はいずれも水平的にほぼ同位相であった。また,10日と25日周期変動の鉛直的な位相差はほとんど見られなかったが,60日周期変動には数日の顕著な位相差が見られ,下層流が上層流よりも常に先行していた。流れと海上風とのクロススベクトル解析を行った結果,10日周期帯の流速変動のみが風との高い相関を示し,北西風が北西同流よりも約1.5日先行していた。10日周期帯の流速変動が秋～冬季にかけて卓越していたことと,この見積もられた位相差が,大島(三宅(1990)による日高湾内での風強制順圧モデル計算の見積もりとよく一致していたことから,この周期帯の変動は北西の季節風によって励起された陸棚波の伝播に伴う流速変動と推測される。Variability in intraseasonal current fluctuations (periods between 10 and 60 days) on the western shelf in Hidaka Bay was examined by using an Acoustic Dopplcr Current Profiler (ADCP) installed on a regular ferry shuttling between Aomori and Muroran from April 2000 to April 2001. After ten primary tidal current constituents were subtracted from the ADCP data, two types of spectral analysis were carried out. and three periodic variations, around 10-, 25-, and 60-day periods, respectively, were identified on the shelf. A new band-pass method, the Harmonic Analysis Band-pass (HAB) method, which can be applied to irregularly sampled data, is proposed to extract the horizontal and vertical current structures of each periodic variation. Very little phase lag between ADCP points was found in any of these periodic variations on the western shelf. Slight vertical phase lag was observed in the 10- and 25-day periodic variations, and a significant vertical phase lag of several days (i.e., lower current always leads upper current) was observed for the 60-day periodic variation. Cross-spectral analysis between current and sea surface wind showed that among signals in the periodic variations, only those in the 10-day period wore coherent, and their phase lag indicates that the northwesterly wind leads the northwestward flow by about, 1.5 days. This estimated phase lag agrees well with that of the wind-forced barotropic model of Hidaka Bay by Ohshima and Miyake (1990). Also, the current variation in the 10-day period was dominant from autumn to winter. Therefore, we infer that this 10-day current variation is effectively induced by the northwesterly monsoon wind.
• Formation of Abyssal Waters and Polar Front in the Japan Sea Inferred from the Route of Heat Transport.

  Isoda Yutaka

  Journal of the Oceanographical Society of Japan The Oceanographic Society of Japan 12 (1) 69 - 84 0916-8362 2003 

  In considering the equilibrium heat balance in the Japan Sea, net heat loss through the sea surface is accomplished under the following two conditions; 1) a closed deep basin, and 2) lateral heat inflow transport to the southern upper layer, i.e., inflow of the Tsushima Warm Current (TWC). This is realized by heat transport through abyssal waters, because the downward heat supply from the TWC to the abyssal water must be balanced by heat loss from the sea surface somewhere in the northern Japan Sea. Therefore, the northern deeper waters continuously transfer heat to upper waters and both waters are easily mixed in the vertical direction. Such a heat condition would produce a significant thermal front as the northern boundary of TWC, i.e., polar front. In relatively cold winter, the surface water in the northern Japan Sea becomes dense enough to sink to the bottom due to the surface cooling. This homogeneous cold and dense water is called Bottom Water (BW). In ordinary years, after BW is newly formed, the heat transport takes place in a relatively shallow depth above the BW. It is likely that the downward diffusive heat supply from this shallow heat transport path gradually decreases the thickness of the BW layer.
• 沿岸親潮の海洋構造とその季節変化

  磯田 豊, 黒田 寛, 明正達也, 本田 聡

  沿岸海洋研究 41 5 - 12 2003 [Refereed][Not invited]
  
• Distribution of fine-scale shear in the deep waters of the North Pacific obtained using expendable current profilers

  Maki Nagasawa, Toshiyuki Hibiya, Yoshihiro Niwa, Michio Watanabe, Yutaka Isoda, Shogo Takagi, Yoshihiko Kamei

  Journal of Geophysical Research: Oceans 107 (12) 2002/12/15 

  Recent numerical experiments [Hibiya et al., 1996, 1998, 2002] predicted that the energy cascade across the internal wave spectrum down to small dissipation scales was under strong control by parametric subharmonic instabilities that transfer energy from the low vertical wave number, double-inertial frequency wave band to a high vertical wave number, near-inertial frequency wave band. To test whether or not the numerically predicted energy cascade process is actually dominant in the real deep ocean, we deployed a total of 106 expendable current profilers over a large area in the North Pacific to examine the spatial distribution of high vertical wave number (vertical wavelength ∼25 m) shear. At midlatitudes, significant enhancement of the 25 m vertical shear was found over prominent generation regions of semidiurnal internal tides such as the Hawaiian Ridge and the Izu-Ogasawara Ridge where the semidiurnal tidal frequency exceeds twice the local inertial frequency. At high latitudes, in contrast, no significant enhancement of the 25 m vertical shear was found to occur even over another prominent generation region of semidiurnal internal tides, the Aleutian Ridge, where the semidiurnal tidal frequency is less than twice the local inertial frequency. We find that the spatial distribution of the intensity of the 25 m vertical shear correlates very well with that of the low vertical wave number, double-inertial frequency internal wave energy numerically predicted by Nagasawa et al. [2000] and Niwa and Hibiya [2001a, 2001b]. This is the first in situ evidence for the dominant role of parametric subharmonic instability in transferring deep ocean internal wave energy down to small dissipation scales. This study provides a theoretical framework for future attempts to determine the large-scale structure of mixing over the world's oceans.
• Low-Frequency Variations of the Deep Flow Southwest of the Tsugaru Straits.

  Mori Kosuke, Isoda Yutaka, Murakami Takashi, Ohtani Kiyotaka

  Journal of the Oceanographical Society of Japan The Oceanographic Society of Japan 10 (1) 1 - 13 0916-8362 2001 

  Form April 1994 to April 1995, we carried out direct current measurements southwest of the Tsugaru Straits with moored instruments at depths of 300m and 2, 100 m to investigate current variability of deep water below the Tsushima Warm Current. It was found that the deep current is very energetic with some low-frequency variations and a steady barotropic northward flow having a yearly mean speed of about 5 cm s^-1. Velocity fluctuations were prominent in the upper and lower layers and occurred with three periods of 26∼30, 15∼18, and 10∼12 days. The variations with periods of 26∼30 and 10∼12 days are dominant throughout the year and correlated to local wind variations. The results of a barotropic model experiment forced by the wind stress suggest that the variation of 10∼12 days is caused by east-ward propagation of the topographic Rossby waves trapped on the west-east bottom slope. On the other hand, the variation of 15∼18 days appears to be baroclinic events associated with small meanders in July and August 1994.
• Temporal and spatial variability of phytoplankton pigment concentrations in the Japan sea derived from CZCS images

  Sang-Woo Kim, Sei-Ichi Saitoh, Joji Ishizaka, Yutaka Isoda, Motoaki Kishino

  Journal of Oceanography 56 (5) 527 - 538 0916-8370 2000 [Refereed][Not invited]
   

  Temporal and spatial variability of phytoplankton pigment concentrations in the Japan Sea are described, using monthly mean composite images of the Coastal Zone Color Scanner (CZCS). In order to describe the seasonal changes of pigment concentration from the results of the empirical orthogonal function (EOF) analysis, we selected four areas in the south Japan Sea. The pigment concentrations in these areas show remarkable seasonal variations. Two annual blooms appear in spring and fall. The spring bloom starts in the Japan Sea in February and March, when critical depth (CRD) becomes equal to mixed layer depth (MLD). The spring bloom in the southern areas (April) occurs one month in advance of that in the northern areas (May). This indicates that the pigment concentrations in the southern areas may increase rapidly in comparison with the northern areas since the water temperature increases faster in spring in the southern than in the northern areas. The fall bloom appears first in the southwest region, then in the southeast and northeast regions, finally appearing in the northwest region. Fall bloom appears in November and December when MLD becomes equal to CRD. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than those in summer. The low pigment concentrations dominate in summer.
• 日本海表面水温場の季節変化

  磯田 豊, 斉藤 誠一, 三原 正裕

  海と空 海洋気象学会 68 (3) p113 - 124 0503-1567 1992/12 [Not refereed][Not invited]
  
• Horizontal and Vertical Structures of a Warm Eddy above the Yamato Rise.

  Isoda Yutaka, Naganobu Mikio, Watanabe Hidetoshi, Nukata Kyoushi

  Journal of the Oceanographical Society of Japan The Oceanographic Society of Japan 1 (4) 141 - 151 0916-8362 1992 

  Spatial structure of a warm eddy which was located stably above the western side of the Yamato Rise was described in detail by examining hydrographic data, NOAA-infrared image, and current data in summer 1989. The doughnut-like flow structure surrounding a warm-core was found to play a significant role in the northward transport of warm and salin water in summer. However, it was inferred from the analysis of hydrographic data through the year that the above doughnut-like flow disappeared in winter and drastic structural variations of an eddy were brought by wintertime convection. During winter, not only the core-temperature but also the salinity decreased and the shape of an eddy became unclear, suggesting the exchange of a core water with the outer one.
• SST STRUCTURE OF THE POLAR FRONT IN THE JAPAN SEA

  Y ISODA, S SAITOH, M MIHARA

  OCEANOGRAPHY OF ASIAN MARGINAL SEAS 54 103 - 112 1991 [Refereed][Not invited]
  
• Variability of the Sea Surface Temperature Obtained by the Statistical Analysis of AVHRR Imagery -A Case Study of the South Japan Sea-

  Isoda, Y, Saitoh, S

  J. Oceanogr. Soc. Japan, 日本海洋学会 44 (2) 52 - 59 0029-8131 1988 [Refereed][Not invited]
  

MISC

• Transport and modification processes of dense Soya Current Water in the boundary region between shelf and basin areas off Abashiri

  千葉 彩, 堀尾 一樹, 磯田 豊, 小林 直人  海の研究  30-  (2)  15  -46  2021/03
• Inflow process of low potential vorticity water originating from the intermediate waters of the Japan Sea into the Tsugaru Strait

  伊田 智喜, 山下 慎司, 磯田 豊, 小林 直人  海の研究  25-  (4)  101  -122  2016/07
• Seasonal changes of sea surface height and sea surface geostrophic current anomalies in the Japan Sea

  朝日 啓二郎, 磯田 豊, 方 曉蓉  海の研究  25-  (3)  43  -61  2016/05
• Numerical experiment for a westward tail of the Tsugaru Gyre along the Hidaka shelf slope

  Naoto Kobayashi, Yutaka Isoda, Keijiro Asahi, Faculty of Fisheries Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University  海の研究 = Umi no Kenkyu (Oceanography in Japan)  24-  (5)  171  -187  2015/09/15  

  単純化した陸棚地形上への流入水の挙動に関わるf平面σ座標系数値モデル実験によって,初夏に津軽Gyreが日高湾陸棚斜面上に侵入したときに生じることが最近の海洋観測で確認された分岐流の形成・発達過程を調べた。成長を続ける津軽Gyreが侵入する北部陸棚斜面域には海底捕捉モードと表層捕捉モードの2種類の低周波擾乱が同時に存在し,斜面上の下層水柱は連続的に押し縮められ,負の相対渦度を生成する。その結果として,侵入域の西側ではほぼ順圧もしくは海底捕捉の時計回り渦流擾乱が形成され,分岐域から陸棚斜面に沿って西方へ伝播する。侵入域の東側では反時計回りの渦流擾乱が形成され,沖合の底層低温水を沿岸域へ定常的に供給する。この現象は津軽Gyreが北岸境界に沿った鏡像効果によって東方移動することを抑える働きをする。このようにして,分岐直後の渦流は日高湾陸棚斜面に沿って西方へ引き延ばされる津軽Gyreにまで発達することがわかった。A recent hydrographic survey indicated that a local bifurcation of the Tsugaru Gyre, which appeared over the Hidaka shelf slope in early summer, accompanied a clockwise, small eddy. To examine the bifurcation's formation and development processes, we performed a numerical experiment using an f-plane σ-coordinate model with idealized shelf topography. During the stage at which the Tsugaru Gyre grew and intruded into the northern shelf slope area, the lower-layer water column on the slope was continuously contracted to induce a negative relative vorticity. As a consequence, the nearly barotropic or bottom-trapped motion with clockwise rotation emanated west of the intrusion area and then propagated westward along the slope. This motion was associated with two types of low-frequency waves characterized by surface and bottom intensifications. On the contrary, an anti-clockwise eddy formed east of the intrusion area and stably fed on the offshore bottom cold water into the coastal area. These processes suppressed the eastward shift of the Tsugaru Gyre by the mirror effect of the lateral boundary. Thus, the above-mentioned small eddies were found to dominate the Tsugaru Gyre, shifting westward along the Hidaka shelf slope.
• Seasonal change of sea level anomalies in the western North Pacific

  Shohei Fujiwara, Yutaka Isoda, Manami Tateno, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Faculty of Fisheries Hokkaido University  海の研究 = Umi no Kenkyu (Oceanography in Japan)  23-  (6)  197  -216  2014/11/15  

  人工衛星高度計による海面高度偏差(Sea Surface Height Anomaly ; SSHA)とアルゴフロートブイ観測の密度場から計算された力学高度偏差(ΔD_;jは基準深度)を組み合わせて,西部北太平洋の風応力の季節変化に対する1年周期の順圧応答成分(ΔD_=SSHA-ΔD_<2000>)と傾圧応答成分(ΔD_=ΔD_<2000> - ΔD_;MLは表層混合層深度)を抽出した。順圧応答成分ΔD_の特記すべき空間分布は,晩冬季(2～3月)に亜寒帯循環領域の負偏差領域が,亜熱帯循環領域の伊豆小笠原海嶺付近の南方まで舌状に張り出していることである。伊豆小笠原周辺海域では,1～2ヶ月遅れた春季(4～5月)に傾圧応答成分ΔD_の正偏差領域が出現している。このように,順圧応答から数か月遅れた傾圧応答は,海嶺海底斜面上における順圧・傾圧モードのカップリングによる傾圧運動の励起(Impinging応答)を示唆している。また,シグナルとしては小さいものの,主として亜熱帯域にみられた他の傾圧応答ΔD_は,約40°N以南に理論上存在可能な傾圧第一モードの1年周期ロスビー波の海洋調節過程と推測された。Annual barotropic and baroclinic responses of sea level anomalies (ΔD_ and ΔD_) in the western North Pacific to seasonally varying wind stress curl are extracted by combining the sea surface height anomaly (SSHA) observed by satellite altimeter with the dynamic height anomaly (ΔD_;j is the reference depth) calculated by the density fields observed by Argo floats, i.e., ΔD_=SSHA - ΔD_<2000> and ΔD_ = ΔD_<2000> - ΔD_ (ML is the surface Mixed Layer depth). The most conspicuous feature in the seasonal change in ΔD_ is the tongue of negative values at the region of subarctic gyre in late winter (February to March) extending southward to the Izu-Ogasawara Ridge (IOR) in subtropical gyre. For annual ΔD_ variation, one of the much larger positive amplitude is also found in areas surrounding IOR in spring (April to May), suggesting the generation of the baroclinic activity through the coupling of the barotropic and baroclinic modes of motion on the steep bottom slope, i.e., the impinging response. The next prominent ΔD_ variability seems to be the adjustment of the ocean in terms of the baroclinic first-mode annual Rossby Waves, which can theoretically exist south of about 40°N
• Inverse Estuary Circulation in Mutsu Bay(2)Numerical Model Experiments

  河野 航平, 磯田 豊, 工藤 勲  海と空 = Sea and sky  90-  (1)  17  -25  2014/08
• The bifurcation of the Tsugaru Gyre over the Hidaka shelf slope

  小林 直人, 太田 紗生, 磯田 豊  海と空 = Sea and sky  90-  (1)  1  -10  2014/08
• 化学成分からみた宗谷暖流沖冷水帯水塊の起源

  久万健志, 笹山了平, 日置菜々子, 森田雄一朗, 磯田豊, 平譯享, 今井圭理, 荒巻能史, 中村知裕, 西岡純, 江淵直人  日本海洋学会大会講演要旨集  2013-  94  2013/08/30  [Not refereed][Not invited]
  
• 潮汐混合とオホーツク海・ベーリング海の物理・化学・生物過程―白鳳丸KH09‐4航海・おしょろ丸・クロモフ2006/2007シンセシス―アムチトカ海峡で観測された大振幅内部波の砕波とグローバル評価

  中村知裕, 磯田豊, 三寺史夫, 高木省吾, 長澤真樹  月刊海洋  43-  (12)  699  -704  2011/12/01  [Not refereed][Not invited]
  
• Wind-induced Shelf Waves off the West Coast of Hokkaido

  ISOGAI Yasuhiro, ISODA Yutaka, SHIMONO Manabu  海と空  86-  (3)  45  -60  2011/03/31
• 北海道西岸沖のホタテ貝種苗生産を支える産卵及び浮遊幼生の輸送過程

  磯貝 安洋, 磯田 豊, 下野 学, 小林 直人, 工藤 勲, 干場 康博  北海道大学水産科学研究彙報 = Bulletin of Fisheries Sciences, Hokkaido University  60-  (2)  23  -37  2010/12
• Seasonal Variation of Exchange Density Flow through the Bay-mouth of Mutsu Bay

  YAHABA Hiroshi, ISODA Yutaka, YOSHIDA Toru, KOSAKA Yoshinobu  海と空  85-  (4)  115  -129  2010/03/25
• Seasonal Variations of Surface Flow Patterns in the Nemuro Strait

  MORI Fumihiro, ISODA Yutaka, ABE Syoko, KOBAYASHI Naoto, YAHABA Hiroshi, ISOGAI Yasuhiro, Fumihiro Mori, Yutaka Isoda, Syoko Abe, Naoto Kobayashi, Hiroshi Yahaba, Yasuhiro Isogai, Graduate School of Environmental Science Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University, Graduate School of Environmental Science Hokkaido University, Faculty of Fisheries Sciences Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University  Oceanography in Japan  19-  (2)  89  -110  2010/03/15  

  根室海峡における表層流の季節変化を記述するために,長期の実測表層流(ADCPとGEK),風向風速,沿岸水位,沿岸水温,スナップショットの海洋観測データの解析を行なった。表層流が一年周期の正弦曲線で表現できるような季節変化は海峡南端付近にみられ,5-8月は北太平洋側から海峡内方向への流入,他の月は海峡側から北太平洋への流出を示した。しかしながら,この流入流出は海峡内における流れパターンの季節変化とは必ずしも連動していないようである。北西風が卓越する冬季の海峡内では南下する海峡通過流が示されたが,この南下流は風駆動による吹送流が海峡南端の流出流と一時的に繋がった流れと推測される。他の季節は春と秋に極大をもつ南西風が卓越する時期である。春季の海峡北部にある深い水深の海域(知床半島の南側)では,北側で北上流,南側で南下流となる二分岐流が形成され,この流れパターンは安定して秋季まで継続する。二分岐後の北上流は春と秋に南西風が強まる時期に同期して強まる傾向を示すことから,この二分岐流は吹送流として説明できる可能性が示唆される。一方,二分岐後の南下流は,夏季の海峡中間部の浅瀬付近から現れ,秋季には海峡南部で支配的となる西流によって次第に分断されていくようにみえる。Long-term direct current velocity (GEK and ADCP), wind, coastal sea level, coastal water temperature and snapshot hydrographic data were used to describe the typical seasonal variations of surface flow patterns in the Nemuro Strait. At the southern end of the Strait, we find a remarkable seasonal change with a clear one-year cycle in the inflow into the Strait (May to August) and the outflow to the North Pacific (other months). However, it seems that such in/outflow variation never synchronize with the seasonal flow patterns in the Strait. During the winter season when the northwesterly wind brows, we infer that the southward wind-induced passage-flow through the Strait may transiently connect to the outflow at the southern end of the Strait. During the other seasons when the southwesterly wind is intensified at two times in spring and autumn, stable bifurcation flow (i.e., the northern northward and southern southward branch flows) forms around the deeper area at the northern part of the Strait (i.e., south of the Shiretoko peninsula). Since the northward branch flow strengthens at the corresponding season of intensified southwesterly wind, one possible mechanism of bifurcation flow is the wind-driven current. On the other hand, the southward branch flow is gradually interrupted by the westward flow, which begins to appear around the shallower area at the central part of the Strait in summer and dominates at the southern Strait in autumn.
• Revision of Fujiwara et al. (1997)'s theoretical solution for surface anti-cyclonic gyre driven by estuarine circulation

  磯田 豊  Bulletin of fisheries sciences, Hokkaido University  60-  (1)  1  -4  2010/03  

  Under the baroclinic process influenced by the Earth's rotation in enclosed coastal seas, Fujiwara et al. (1997) suggested that anti-cyclonic gyre (negative vorticity) was generated in the upper mixed layer by the horizontal divergence associated with upward entrainment, which was part of the estuarine circulation. They theoretically show that such vorticity (ζ) is close to a spatial uniform value of ζ=-f/2 (f>0 is the Coriolis parameter) at the steady state. However, this solution is basically given by infinitely small thickness of water column according to the potential vorticity conservation. We propose the revised solution in the finite upper mixed layer (H), i.e., ζ=-fx/[x+(u0/w0)H] is the increasing function of offshore distance from a river mouth (x) and depends on the ratio of river inflow velocity (u0) to upward entrainment velocity (w0).
• Seasonal variation of surface horizontal circulation in Mutsu Bay

  矢幅 寛, 磯田 豊, 吉田 達, 小坂 善信  Bulletin of fisheries sciences, Hokkaido University  59-  (3)  59  -65  2009/12  

  The field current observations were carried out to examine the seasonal variation of surface horizontal circulation in Mutsu Bay throughout the year. The observed flows in winter/summer heartily support the previous model studies for wind-induced current. That is, we could confirm that anti-clockwise/clockwise eddies at western/eastern parts of the bay, respectively, were generated by the horizontal shear easterly-wind in summer, and two vortices at the eastern part of bay were generated by the spatial uniform westerly-wind in winter.
• Mode Waters Observed around Musashi Rise West of Hokkaido

  HIGAKI Naoyuki, ISODA Yutaka, HONDA Satoshi, Naoyuki Higaki, Yutaka Isoda, Satoshi Honda, Graduate School of Fisheries Sciences Hokkaido University, National Research Institute of Fisheries Science Fisheries Research Agency  Oceanography in Japan  18-  (6)  335  -350  2009/11/05  

  北海道西岸沖において,冬季から春季の対馬暖流は鉛直混合により徐々に不明瞭となり,夏季から秋季の対馬暖流は石狩湾沖で局所的な時計回りの蛇行流を伴って北上している。このような遷移的な流れパターン変化を理解することを目的に,2005年から2008年の5～6月にXCTDやXBTによる調査を実施した。我々は季節躍層下において6℃以下の冷水が水温一様の厚い層として存在し、地理的には武蔵堆周辺海域に制限されて分布していることをみつけた。それゆえ、本研究ではこの水塊を「武蔵堆モード水」と呼ぶことにした。調査を実施した4年間の中で,2006年と2008年に観測された武蔵堆モード水は特に厚い層を形成しており、これは強い冬季海面冷却の結果であることが示された。対馬暖流の流れパターンが大きく変化する5～6月において,武蔵堆モード水の水平的な拡がりは北上する対馬暖流を二分岐させているようにみえる。沿岸側の分枝流は石狩湾沖で小さな暖水渦流を形成した後,北海道西岸沖の陸棚縁に沿って北上し,沖合側の分枝流は武蔵堆モード水の沖側端を時計回りに迂回する北上流となる。The Tsushima Warm Current (TWC) along the west of Hokkaido gradually dissipates from winter through spring because of vertical mixing of the surface layers. In summer and autumn, it extends northward accompanied by a local clockwise meandering off Ishikari Bay. In order to understand such seasonal changes of TWC flow pattern, XCTD and XBT surveys were carried out in May or June from 2005 to 2008. Cold (<6℃) water mass was found below the seasonal thermocline around Musashi Rise, which we call "Musashi Rise Mode Water (MRMW)" in this study. Because of stronger winter cooling during 2006 and 2008, the thickness of MRMW was larger in those years than in 2005 and 2007. In May and June, when the path of TWC changes, the MRMW seems to play a role in causing the bifurcation of the northward flow of TWC. The onshore branch of TWC forms a small warm eddy off Ishikari Bay, and then flows along the shelf edge west of Hokkaido. The offshore branch flows clockwise along the offshore edge of MRMW.
• The Subsurface Counter-flow along the Offshore Side of the Soya Warm Current in Spring

  ABE Shoko, ISODA Yutaka, YAHABA Hiroshi, Shoko Abe, Yutaka Isoda, Hiroshi Yahaba, Graduate School of Environmental Science Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University  Oceanography in Japan  18-  (4)  265  -286  2009/07/05  

  宗谷暖流(Soya Warm Current;SWC)域周辺における表層流の季節変動を調べるために,実測の表層流データ(ADCP・GEK),沿岸水位データ,海洋観測資料の解析を行なった。この解析により我々は,春季の一時期において,南東流であるSWC(沿岸捕捉流)の沖合側に北西向きの亜表層反流が存在することをみつけた。この季節は東樺太海流(East Sakhalin Current;ESC)水からSWC水への水塊交換が生じる遷移的な時期に当たる。ESCとSWCの体積輸送量の冬季から春季の時間変化を強制した数値モデル計算では,次のような結果が得られた。冬季において,低温低塩水で特徴付けられるESC水は海底ほど流れが強い海底捕捉流として北海道沿岸まで達し,北海道沖の浅い陸棚上に次第に蓄積される。春季に入ると,卓越していた北西季節風が弱まるため,風強制により励起されていたESCは急速に弱まる。その結果,わずかに高温高塩化したESC変質水は陸棚上に残留し,その流れ構造は表層ほど流れが強い密度流へと変化する。この水塊と流れの構造は資料解析結果と一致した。従って,SWC沖合の北西向き亜表層反流は陸棚上に形成される傾圧循環流の一部であることが推測される。Composite surface current velocity data (ADCP and GEK), coastal sea level data and hydrographic data were used to examine the seasonal variations of upper-ocean flow around the Soya Warm Current (SWC) region. We've discovered that the subsurface counter-flow (northwestward current) along the offshore side of SWC (coastal trapped southeastward current) transiently occurs in spring, i.e., the timing of dramatic exchange from East Sakhalin Current Water (ESCW) to SWC Water (SWCW). In winter, cold and fresh ESC water reached near Hokkaido coast and had the bottom-intensified/controlled flow structure. Then, it gradually accumulated on the shallower side of the shelf area off Hokkaido. In spring, the prevailing northwesterly wind was weakened and the flow energy of wind-driven ESC rapidly decreased. As a result, the modified ESCW (slightly warmer and saltier water) remained on the shelf area and its flow structure changed to the surface intensified density current, i.e., the northwestward counter-flow off the SWC. Our model simulation, which is forced by the temporal change in ESC and SWC volume transports from late-winter to early-summer, suggests that such counter-flow is formed as a part of the clockwise baroclinic circulation on the shelf area.
• Sea level difference inducing the passage-flow through the Tsugaru Strait

  IINO Rieko, ISODA Yutaka, YAHABA Hiroshi  海と空  85-  (1)  1  -19  2009/06/30
• Seasonal Variations of Water System Distributions and Flow Patterns off the West Coast of Hokkaido

  HIGAKI Naoyuki, ISODA Yutaka, ISOGAI Yasuhiro, YAHABA Hiroshi, Naoyuki Higaki, Yutaka Isoda, Yasuhiro Isogai, Hiroshi Yahaba, Graduate School of Fisheries Sciences Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University, Graduate School of Fisheries Sciences Hokkaido University  Oceanography in Japan  17-  (4)  223  -240  2008/07/01  

  北海道西岸沖表層流の流れパターンの季節変化を調べるために,海洋観測資料と直接測流(ADCP・GEK)データを解析した。海面加熱期である夏季の対馬暖流は,水深約50m付近に高塩分水を伴う北上流である。この北上流は津軽海峡の西側では沿岸に接岸して強化され,さらに北側の石狩湾沖では時計回りの蛇行流の形成によって離岸する。しかし,この高塩分水は海面冷却期である冬季に入るとすぐに消滅する。この原因は強い海面冷却によって表層の低塩分水と鉛直混合したためと推測される。それゆえ,冬季から春季の対馬暖流は津軽海峡の西側では幅広い北上流として認められるものの,石狩湾よりも北側の海域では次第に弱まり,不明瞭な北上流となる。Hydrographic data and direct current velocity data (ADCP and GEK) were used to examine the seasonal variations of upper-ocean flow off the west coast of Hokkaido, Japan. During the summer season when the sea surface temperature increases, the Tsushima Warm Current (TWO) flows northward carrying more saline water in the intermediate layer (about 50 m depth). The current flows swiftly along the west coast of the Tsugaru Strait and diverts offshore at Ishikari Bay. This is caused by the presence of the local clockwise meandering current of the Bay. This saline water, however, abruptly disappears in the winter season when the surface temperature cools. This could be explained by the strong vertical mixing between the saline water in the surface and less saline water in the subsurface layer. Therefore, during the winter season, the northern flow of the TWC still exists near the Tsugaru Strait, but dissipates as it nears the Ishigaki Bay due to the vertical mixing of the layers.
• Generation of Shelf Waves by Periodical Wind Forcing

  Isoda Yutaka, Satoh Chizuru  Bulletin on coastal oceanography  46-  (1)  59  -68  2008  

  We investigated the generation of shelf waves by periodical wind forcing using the method of linear response system with a unit impulse response function. The dispersion relation of the shelf wave has a maximum frequency where the alongshore group velocity vanishes. One of the questions we seek to answer is how response occurs when the periodical vortices are supplied to the shelf area at a higher frequency domain where shelf wave is not permitted. Shelf waves, whose dispersion depends on the forcing spatial scale, can be generated by vortices input at any frequency. At the higher frequency described above, however, propagation of the shelf wave disappears due to superimposition of the previously generated waves.
• Seasonal Variation in the Nearshore Branch of the Tsushima Warm Current as Inferred from Coastal Sea Level Data

  ISHIKAWA Kohei, ISODA Yutaka, AIKI Tomokazu, Kohei Ishikawa, Yutaka Isoda, Tomokazu Aiki, Graduate School of Environmental Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University  Oceanography in Japan  16-  (3)  223  -236  2007/05/05  

  対馬暖流沿岸分枝流の季節変化を明らかにするために,日本列島沿岸の26地点,沖合に点在する島の7地点,計33地点の検潮所における6年間(1998～2003年)の月平均水位資料を解析した。本解析では沖合島とその対岸の水位差,列島に沿った水位勾配の各経月変化をもとに,沿岸分枝流の波路に沿った表層流の季節変化を推測した。その結果,沿岸分枝流の北上流強化は夏季(4月～8月)と晩秋から冬季(11月～12月)にかけての年2回程度あることが推測され,これらの強化には地理的な違いがあることが明らかになった。6年平均でみた場合,夏季の強化は流入口(対馬海峡)から佐渡島付近までの南部海域で,晩秋から冬季にかけての強化は流入口付近と流出口(津軽海峡)を含む佐渡島以北の海域で主に卓越していた。ただし,両海域における冬季から晩秋にかけての強化はそれぞれ独立した現象であることが推測された。なお,個々の北上流強化の強さや出現海域は経年的に大きく変化していた。Seasonal variations of the Nearshore Branch (NB) of the Tsushima Warm Current (TWC) were investigated using the monthly mean coastal sea level records of 26 tide-stations along the Japanese coast and 7 tide-stations of off-shore islands during recent 6 years (1998～2003). To infer the seasonal variability of surface flow along the path of the NB, we examined the sea level difference between the offshore islands and their neighboring Japanese coast, and the coastal sea level gradient on the downstream of the NB. It is inferred that the seasonal variation of the NB is characterized by the biannual intensifications of northward flow, i.e., both during summer (April～August) and during winter (November～December). In the mean annual state during 6 years, the summer-time intensification is dominant around the southern region from the inflow-mouth (Tsushima Strait) to around the Sado-Island, while the winter-time intensification is dominant mainly around the inflow-mouth and north of the Sado-Island also including the outflow-mouth (Tsugaru Strait). However, it is suggested that these winter-time intensifications in both regions are independent phenomena of each other. However, strength and predominant area of individual intensifications varied interannually.
• Seasonal Variations of Surface Flow around Toyama Bay

  相木智一, 磯田豊, 矢部いつか, 黒田寛  海の研究  16-  (4)  2007
• Wind-driven Current in the Coastal Sea in the Period of Non-stratification : Channel Model without the Bottom Friction

  Isoda Yutaka, Satoh Chizuru  Bulletin on coastal oceanography  44-  (2)  211  -220  2007  

  The dynamics of wind-driven current in the coastal sea during the period of non-stratification are reviewed using the ideal channel model without bottom friction. We compared two models based on a single-layer and a multi-layer, respectively, and the temporal responses were examined under the forced conditions with a constant wind stress along the channel axis.
• Current variations with several-day period caused by wind in the Toyama Bay

  相木 智一, 磯田 豊, 小善 圭一  Bulletin of fisheries sciences, Hokkaido University  57-  (1)  9  -18  2006/11
• Hydrographic Structural Change in the Transition Domain around the Emperor Seamounts

  WAGAWA Taku, ISODA Yutaka, SAITOH Nana, AZUMAYA Tomonori, SAKAOKA Keiichiro, Taku Wagawa, Yutaka Isoda, Nana Saitoh, Tomonori Azumaya, Keiichiro Sakaoka, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Factory of Fisheries Hokkaido University, Factory of Fisheries Hokkaido University  Oceanography in Japan  15-  (3)  267  -281  2006/05/05  

  北部北太平洋には亜寒帯前線と亜寒帯境界の間として定義される移行領域(Transitional Domain: TD)が存在し,TDを南北に横切るように170°Eに沿って天皇海山列が存在している。本研究では155°E,165°E,170°E,175.5°E,180°線における長期間の海洋観測データを解析し,TDにおける水塊構造が天皇海山列を境界に大きく変化していることを明らかにした。天皇海山列の東西海域ではTDを明確に特定できるのに対して,天皇海山列上においてTDはしばしば存在せず,南から北に向かって亜熱帯水から亜寒帯水に急激に変化する水塊前線を形成していた。さらに,TDにおける東向き輸送に沿って,水塊は西から東に向かって亜寒帯水から亜熱帯水へ急激に変化していた。TD付近に存在する強い東向きの傾圧流は,天皇海山列上の43°N付近(推古海山と仁徳海山の間)を通過した後,北側にシフトする。この傾圧流の強弱や分布位置は経年変動したが,北側にシフトする傾向は何れの年においても同じであった。傾圧流を挟んだ西側と東側のTDで起こる水塊の変化は,亜表層において特徴的な水塊である中央モード水が重い密度帯から軽い密度帯へ変化することに起因していた。すなわち,傾圧流の北側へのシフトと中央モード水の密度帯の変化は密着に関連していることが示唆された。Spatial distributions of baroclinic flow and water mass in the "Transition Domain" (TD), defined between the Subarctic Front and Subarctic Boundary in the northern North Pacific, were investigated through analysis of long-term hydrographic data taken in 155°E, 165°E, 170°E, 175.5°E, and 180° meridians. Along the 170°E meridian, the Emperor Seamounts is located across the TD in a strict north to south direction. Although the TD could be clearly detected on both sides of the Emperor Seamounts, it frequently disappeared and had a remarkable watermass front, i.e., abrupt and meridional, changing from subtropical to subarctic water masses, in the location of the Emperor Seamounts. Furthermore, along the eastward transport of the TD, an abrupt zonal change from subarctic to subtropical water took place. In each year, the estimated eastward baroclinic flow around the TD suggested a shift to the north after passage through the trench of the Emperor Seamounts around 43°N (between Suiko Seamount and Nintoku Seamount), although small year-to-year changes were shown in its intensity and horizontal distribution. This shift in flow might explain the above zonal water mass change in the TD. It was also shown that the density of the "Central Mode Water", formed at the sea surface mixed layer in winter, was gradually lighter from the west to the east.
• Internal Kelvin Waves in the Near-inertial Period in Funka Bay

  Satoh Chizuru, Isoda Yutaka, Iwahashi Masayuki, Shimizu Manabu  Bulletin on coastal oceanography  44-  (1)  49  -58  2006  

  In this paper, the near-inertial motion of Funka Bay is discussed. Water temperature data were analyzed and compared with output from a reduced-gravity numerical model. Temperature measurements were taken at 9 stations along the bay coast at intervals of 15-20km from March-December 1999. Data analysis shows that the seasonal thermocline frequently displays large vertical displacement over 15-18 hours; the inertial period in this bay is 17.76 hours, and the spatial distribution suggests positive and negative displacement of the interface at alternate stations. The model was expressed by idealized coastal geometry, and the behavior of near-inertial motion was examined from an impulsive start. Two types of reduced-gravity model were applied-one modeled a circular basin and another included a bay mouth, represented by a channel joined to a circular basin. The model without a bay mouth showed only that the interface associated with first-mode baroclinic Poincare waves rose in one half of the bay; descending in the other half, and this motion rotated clockwise. On the other hand, the model with a bay mouth reproduced reasonably well the observed alternative displacement of the thermocline along the coast. Such near-inertial motion resulted from the combination with first-mode baroclinic Poincare waves (rapid clockwise rotation) and internal Kelvin waves (slow counter-clockwise rotation). We also inferred that this Kelvin wave was generated by the imbalance of boundary conditions around the bay mouth for the Poincare waves, and hence it had near-inertial periodicity similar to that of the Poincare waves and a very short wavelength of 20-30km (～2πλ_i; λ_i. internal radius deformation).
• Coastal Current on the Eastern Shelf of Hidaka Bay

  KURODA Hiroshi, ISODA Yutaka, TAKEOKA Hidetaka, HONDA Satoshi  日本海洋学会  62-  (5)  731  -744  2006  [Not refereed][Not invited]
  
• Seasonal variations of water properties and the baroclinic flow pattern in Toyama Bay under the influence of the Tsushima Warm Current

  Satoshi Nakada, Yutaka Isoda, Isamu Uchiyama  Journal of Oceanography  61-  (5)  943  -952  2005/10/01  [Not refereed][Not invited]
   

  The seasonal variations of water properties and the baroclinic flow pattern in the upper layer of Toyama Bay, where the shelf breaks in the passway of the eastward coastal branch of the Tsushima Warm Current, have been examined using temperature and salinity data from 26 local stations collected in the 32 years from 1963 through 1994. The results show that the flow pattern around the bay, as inferred from the distributions of the geopotential anomaly at 300 dbar and saline core water, changes remarkably from summer to autumn. There are two obvious inflows into Toyama Bay in a year. One is the surface inflow of less saline water from east of the Noto Peninsula as the coastal-trapped density-driven flow of the coastal branch during the transition from May to July. In September, this inflow is abruptly weakened by a transient northwestward reversal flow in the intermediate layer around 100 m depth. This reversal flow is accompanied by the temporary shallowness of the pycnoclines inside the bay. At that time, another inflow with more saline water of the year occurs in the intermediate layer. From November until January, this reversal flow disappears and a southeastward passing through-flow gradually intensifies across the bay mouth, accompanied by deepening of the pycnoclines inside the bay. According to our interannual analysis over the 32-year study period, this reversal flow has been a stable seasonal phenomenon, except for only 4 years, in which a local warm region or warm eddy developed just north of the Noto Peninsula. Copyright © The Oceanographic Society of Japan/TERRAPUB/ Springer 2005.
• 日本海の環境と生態系 (地球規模海洋生態系変動研究(GLOBEC)--海洋生態系の総合診断と将来予測)

  磯田 豊  海洋  37-  (8)  545  -552  2005/08
• Subtidal current variations of the eastern shelf of Hidaka Bay

  Kuroda Hiroshi, Kubo Masato, Isoda Yutaka, Takeoka Hidetaka, Honda Satoshi  Bulletin of Fisheries Sciences, Hokkaido University  56-  (2)  43  -53  2005/07/12
• Flow Variations off Cape of Esan, northeast of Tsugaru Strait

  Tannno Tomomi, Kuroda Hiroshi, Isoda Yutaka, Aiki Tomokazu  Bulletin of Fisheries Sciences, Hokkaido University  56-  (2)  33  -41  2005/07/12
• 隠岐海峡通過流の数値モデル

  矢部 いつか, 磯田 豊  海と空  81-  (1)  1  -12  2005/07
• Seasonal Variations of Flow Structure around the Oki Strait

  YABE Itsuka, ISODA Yutaka  海と空  80-  (4)  163  -174  2005/03/31
• Mesothermal Water in the Gulf of Alaska

  Isoda Yutaka, Kubose Kentarou, Takagi Shogo, Hibiya Toshiyuki  Bulletin of Fisheries Sciences, Hokkaido University  56-  (1)  7  -18  2005/03
• Application Limits of Linearized Vorticity Equation for Rotating Fluids

  Isoda Yutaka, Yabe Itsuka, Kuroda Hiroshi  Bulletin on coastal oceanography  43-  (1)  61  -67  2005  

  When a hydrostatic approximation can be made, a water column for a homogeneous rotating fluid conserves its potential vorticity; given by Q=(f+ζ)/h, where f is planetary vorticity, ζ is the relative vorticity and h is the height of a water column. The form of dQ/dt=0 (d/dt means the time derivative of a quantity) is the conservation equation of potential vorticity. However, a linearized form of a vorticity equation for a water column moving across a shelf slope, in the case of negligible surface motion, is not regarded as representing the conservation of potential vorticity. After introducing the physical meaning for this linearized vorticity equation, we point out some problems in discussing the steady-state solution of a uniform flow across a shelf slope using linear theory.
• Winter Transport and Tidal Current in the Tsugaru Strait

  Onishi Mitsuyo, Isoda Yutaka, Kuraoda Hiroshi, Iwahashi Masayuki, Satoh Chizuru, Nakayama Tomoharu, Ito Toshimichi, Iseda Kenichi, Nishizawa Keisuke, Shima Shigeki, Togawa Orihiko  Bulletin of Fisheries Sciences, Hokkaido University  55-  (2)  105  -119  2004/10
• Interannual Variations of the Yearly Mean Sea Level around the Japanese Islands

  Isoda Yutaka, Kitamura Fumiko, Murakami Takashi  Bulletin of Fisheries Sciences, Hokkaido University  55-  (2)  85  -95  2004/10
• Near-Inertial Oscillation Observed off Henashi-zaki of Aomori Prefecture

  Sakata Youichirou, Isoda Yutaka  Bulletin of Fisheries Sciences, Hokkaido University  55-  (2)  121  -137  2004/10
• Transition Domain on 155°E Meridian in 2002 and 2003

  Wagawa Taku, Isoda Yutaka, Takagi Shogo, Sakaoka Keiichirou  Bulletin of Fisheries Sciences, Hokkaido University  55-  (1)  23  -33  2004/08
• Simple Estimation of Net Heat Transport through the Sea Surface in Funka Bay

  磯田 豊, 末武 秀己, 東屋 知範  Bulletin of Fisheries Sciences, Hokkaido University  55-  (1)  43  -52  2004/08
• Internannual Variations of the Tsugaru Gyre

  磯田 豊, 鈴木 鋼治  Bulletin of Fisheries Sciences, Hokkaido University  55-  (1)  71  -74  2004/08
• Evolution of cannibalism: referring to costs of cannibalism (vol 226, pg 291, 2004)

  K Nishimura, Y Isoda  JOURNAL OF THEORETICAL BIOLOGY  228-  (2)  291  -291  2004/05  [Not refereed][Not invited]
  
• Evolution of cannibalism: referring to costs of cannibalism

  K Nishimura, Y Isoda  JOURNAL OF THEORETICAL BIOLOGY  226-  (3)  293  -302  2004/02  [Not refereed][Not invited]
   

  A rational explanation for cannibalism is that it would be favored under conditions of crowding of conspecific individuals and/or low availability of alternative prey with the fear of starvation, so as to maximize individual fitness. Cannibalism has, however, not evolved and is not maintained by a simple individual optimization, while it has evolved and is maintained as a game among population members. We analysed the attainable state of an evolutionary cannibalism game within a framework that reflects the minimum essence of cause-effect in the cannibalism phenomenon. Cannibalism is predator-prey interaction among conspecifics. Immediate direct payoffs (survival in the interaction among conspecifics) and indirect payoffs (growth results in potential productivity and survival against the threat of starvation) would be included. No morphological specialization and no size priority of cannibalism individuals are assumed as conservative situations in which we analyse the possibility of cannibalism. Cannibalism would be possible under the conservative condition, if initially the wild population's cannibalism rate is not sufficiently lower than a threshold value. Crowding and/or low availability of alternative prey with the fear of starvation facilitates cannibalism evolution. Energy gain from conspecific prey would be attenuated by costs of counterattacks by conspecific victims and by challenge cost of its own. Discounting net intake energy required in the arms race for cannibalism challenge result in a relative disadvantage of having a high cannibalism rate and makes an evolutionary equilibrium of low cannibalism rate, even when potential profitability of conspecific prey is high. (C) 2003 Elsevier Ltd. All rights reserved.
• 隠岐海峡通過流の数値モデル

  矢部いつか, 磯田豊  日本海洋学会大会講演要旨集  2004-  2004
• 隠岐島周辺海域における流れ場の季節変化

  矢部いつか, 磯田豊  日本海洋学会大会講演要旨集  2004-  2004
• Variant evolutionary trees under phenotypic variance

  K Nishimura, Y Isoda  JOURNAL OF THEORETICAL BIOLOGY  226-  (1)  79  -87  2004/01  [Not refereed][Not invited]
   

  Evolutionary branching, which is a coevolutionary phenomenon of the development of two or more distinctive traits from a single trait in a population, is the issue of recent studies on adaptive dynamics. In previous studies, it was revealed that trait variance is a minimum requirement for evolutionary branching, and that it does not play an important role in the formation of an evolutionary pattern of branching. Here we demonstrate that the trait evolution exhibits various evolutionary branching paths starting from an identical initial trait to different evolutional terminus traits as determined by only changing the assumption of trait variance. The key feature of this phenomenon is the topological configuration of equilibria and the initial point in the manifold of dimorphism from which dimorphic branches develop. This suggests that the existing monomorphic or polymorphic set in a population is not an unique inevitable consequence of an identical initial phenotype. (C) 2003 Published by Elsevier Ltd.
• 北部北太平洋の移行領域における水塊特性

  海と空  79:65-75-  2004  [Not refereed][Not invited]
  
• Application of Harmonic Analysis to Annual Variations in Current using Semi-regular Sampling Data from an ADCP installed on a Regular Ferry

  Kuroda Hiroshi, Isoda Yutaka  Journal of the Oceanographical Society of Japan  13(2),151-161-  (2)  151  -161  2004  [Not refereed][Not invited]
   

  The relation between a sun-synchronous infrequent semi-regular sampling with an ADCP (acoustic Doppler current profiler) installed on a regular ferry and a harmonic analysis for the K₁ and P₁ constituents was clarified. Double aliasing of the same cycle with different phases was caused by the semi-regular sub-sampling of the K₁ or the P₁ constituent, where the aliased period of either constituent is 365.24 days; this is the same as that of another Sa constituent. Although it is impossible to separate the plurality of waves having the same period for a regularly sampled time series, it is possible to separate two arbitrary constituents out of these three (i.e., K₁, P₁, and Sa) for a semi-regularly sampled time series. As contamination by the non-separated constituent in analyzing the two constituents cannot be avoided, it is very important to decide what two constituents should be estimated. The selection of the Sa and K₁ constituents, by excluding the P₁ constituent, makes the estimation of the K₁ constituent relatively accurate by imposing the influence of seasonal current variations on the Sa constituent. Moreover, it is possible to determine the validity of eliminating the P₁ constituent from the harmonic analysis by comparing the amplitude of the P₁ constituent extrapolated, using the ratio of equilibrium tidal amplitude between the K₁ and the P₁ constituent with the amplitude of annual variation of residual data wherein only the K₁ constituent was deleted.
• 噴火湾沿岸域の水温変動

  沿岸海洋研究  42,73-83-  2004  [Not refereed][Not invited]
  
• Response of Fish Population to a Time-varying Survival Rate of Offspring or Fishing Rate of Parent

  Isoda Yutaka  Bulletin of Fisheries Sciences, Hokkaido University  54-  (3)  43  -52  2003/12
• Formation of Abyssal Waters and Polar Front in the Japan Sea Inferred from the Route of Heat Transport

  ISODA Yutaka, Yutaka Isoda, Graduate School of Fisheries Sciences Hokkaido University  海の研究 = Umi no Kenkyu (Oceanography in Japan)  12-  (1)  69  -84  2003/01/05  

  日本海の深層域が閉じているという地理的条件と対馬暖流による南側上層への熱供給という条件のために,定常状態の熱バランスを考えると,日本海の表面は必ず海面冷却となる。これは深層域が閉じているために,対馬暖流域から深層へ輸送された熱が北側表層へ輸送されなければならないという熱輸送経路で説明される。北部海域における下層から上層への熱輸送経路は下層加熱の状態にあり,鉛直的に水塊が混合され易く,形成される極前線(対馬暖流の北限)は非常に明瞭な水温前線となることが期待される。そしてある年の冬,比較的大きな海面冷却によって,底層にまで至る冷たく重い深層水が形成される。この水塊は均一な水温で特徴付けられ,底層水(Bottom Water)と呼ばれる。このような深い深層水形成後の熱輸送経路は,対馬暖流域から深層水上部を経由して極前線の北側海域の表層へと向かう浅い経路をとる。それゆえ,底層水は次回の深い深層水形成までの間,この浅い熱輸送経路から加熱され続け,その厚さは次第に減少することが推測される。In considering the equilibrium heat balance in the Japan Sea, net heat loss through the sea surface is accomplished under the following two conditions; 1) a closed deep basin, and 2) lateral heat inflow transport to the southern upper layer, i.e., inflow of the Tsushima Warm Current (TWC). This is realized by heat transport through abyssal waters, because the downward heat supply from the TWC to the abyssal water must be balanced by heat loss from the sea surface somewhere in the northern Japan Sea. Therefore, the northern deeper waters continuously transfer heat to upper waters and both waters are easily mixed in the vertical direction. Such a heat condition would produce a significant thermal front as the northern boundary of TWC, i.e., polar front. In relatively cold winter, the surface water in the northern Japan Sea becomes dense enough to sink to the bottom due to the surface cooling. This homogeneous cold and dense water is called Bottom Water (BW). In ordinary years, after BW is newly formed, the heat transport takes place in a relatively shallow depth above the BW. It is likely that the downward diffusive heat supply from this shallow heat transport path gradually decreases the thickness of the BW layer.
• Clockwise Circulation in the Upper Layer of Funka Bay in Summer

  Satoh Chizuru, Isoda Yutaka, Shimizu Manabu  Bulletin on coastal oceanography  40-  (2)  181  -188  2003  

  It is known that a stable clockwise circulation is formed in the upper layer of Funka Bay, after the freshwater discharge reaches the seasonal maximum from April to May. In this study, a variable-density, 1.5-layer model is used to investigate the possible formation mechanism of this clockwise circulation. The freshening of coastal waters is mostly due to rivers, i.e. many point sources in Funka Bay, so that in a first approximation the freshwater source uniformly distributes along the coastline and supply freshwater into a preexisting surface mixed layer. River water bends to the left, and a coastally trapped plume advances along the left-hand coast looking downstream. Some of the river water circulates about the plume nose to return the direction of right-hand coast along the offshore density front looking upstream. When the upward entrainment is included in the model, this clockwise flow around the plume nose is gradually developed in the wide area of the bay. This model result suggests that the following horizontal process, proposed by McCreary et al. (1997), plays the important role on the initial formatting of the clockwise circulation in Funka Bay. Shoreward decrease in density generates the geostrophic flow proceeding along the right-handed coast, and then shallowing of the interface to the coast drives the flow in the opposite direction. This shallowing of the interface is caused by the internal Kelvin-wave propagation.
• A late winter hydrography in Hidaka Bay,south of Hokkaido, Japan

  Manabu Shimizu, Yutaka Isoda, Kazumi Baba  海の研究 = Umi no Kenkyu (Oceanography in Japan)  11-  (2)  366  -366  2002/03/05  

  北海道南部に位置する日高湾において,沿岸親潮水(COW)と津軽暖流水(TWW)の空間分布と流動場,および両水塊の変質を調べるために,1996年と1997年の晩冬に海洋観測を行った。沿岸親潮水はオホーツク海の融氷水を起源とする低温で低塩な水であり,冬季から春季にかけて日高湾へと流入し,津軽暖流水は一年を通して津軽海峡から北太平洋へ高塩分水を供給していると言われている。本観測期間において,低温な表面混合層(σ_θ<26.2,0～100m)が,主に日高湾の陸棚斜面上に見られ,沿岸親潮が反時計廻りの流路を形成していることが確認された。一方,津軽暖流水は相対的に高塩分で低渦位であり,沖合いのσ_θ=26.6～26.8上で局所的な水温と塩分の極大を伴い,いくつかのパッチ状構造を呈していた。ターナーアングルの見積りから,津軽暖流水と沿岸親潮水の間で見られる水温と塩分の鉛直微細構造は強い鉛直混合(二重拡散混合)を示唆していることが分かった。また,1997年晩冬の津軽海峡出口では,津軽暖流水と変質された沿岸親潮水の間に明瞭な熱塩フロントが形成されており,津軽暖流が海底地形に捕捉されて三陸沿岸を南下していることが推測された。それゆえ,この時期に日高湾内で観測されたパッチ状の津軽暖流水は,津軽暖流から孤立した水塊であり,diapycnal mixingによって急激に変質されていることが分かった。
• Response of the Coastal Branch Flow to Alongshore Variation in Shelf Topography off Toyama Bay

  NAKADA Satoshi, ISODA Yutaka, KUSAHARA Kazuya, Satoshi Nakada, Yutaka Isoda, Kazuya Kusahara, KCS Co. Ltd, Graduate School, of, Fisheries Sciences, Hokkaido University, Graduate School of Environmental Earth Sciences Hokkaido University  海の研究 = Umi no Kenkyu (Oceanography in Japan)  11-  (2)  243  -258  2002/03/05  [Not refereed][Not invited]
   

  対馬暖流の沿岸分枝流の形成には陸棚地形が重要であると言われているが,この分枝流の流路にある陸棚域は富山湾沖で一端途切れている。はじめに,17年間(1978～1994年)の水温資料を用いて,対馬暖流全域における水平熱輸送量の季節変化を調べた。その結果,春季において正の熱輸送量の極大値(すなわち,局所的に大きな水温上昇域)が富山湾沖に出現し,夏季～秋季にかけて日本列島沿岸に沿って北上していることが。そこで,富山湾沖に出現したこの局所的な水温上昇域の形成・移動の物理メカニズムを簡単な2層モデルを用いて調べた。モデル計算は静止状態から始め,陸棚上の順圧的な沿岸分枝流を上流側で強制し続けた。この沿岸分枝流が富山湾沖を模した陸棚の途切れる海域に流入したとき,順圧的な流れ場は内部境界面を沿岸側へ下方に傾けた傾圧的な流れ場へと遷移した。数十日後には,この傾圧流は時計回りの渦を形成し,内部境界面の顕著な水平勾配から判断される沖合いフロント構造を保ったまま,陸棚斜面に沿ってゆっくりと北上する様子を再現した。このモデルで再現された時計回りの渦の形状と大きさは,上述の水温上昇域と同程度のスケールをもっている。富山湾沖の陸棚斜面上における渦度バランスは,富山湾に近い岸側の上層において負の相対渦度(すなわち,時計回りの渦)を供給する非線形項の卓越を示している。さらに,この時計回りの渦の北上には佐渡島付近の陸棚斜面上を北上する下層流の存在が必要であることが示唆された。It has been known that the shelf topography plays an important role in the formation of the coastal branch flow of the Tsushima Warm Current (TWC). However, this shelf in the pathway of the branch flow breaks at Toyama Bay. At first, using water temperature data for 17 years from 1978 to 1994, we drew seasonal maps of horizontal heat transport in the TWC region. These maps show that a local maximum of positive heat-content, i.e. a warming region, appears offshore of Toyama Bay in spring and moves northward along the Japanese coast from summer to winter. A simple two-layer model is used to investigate the physical processes governing the formation and movement of this warming region off Toyama Bay. The motion starts from rest and is forced by a constant barotropic coastal branch flow on the shelf on the up-stream side. As the coastal branch flow enters the shelf break area, it experiences a regime where the flow is transformed into a baroclinic flow with the deepening of the interface toward the coast. After several tens of days of integration, this flow forms a clockwise eddy, which gradually moves north along the shelf slope with an offshore boundary front detected by a singnificant horizontal gradient of the interface. The shape and dimensions of this eddy are comparable with those of the above mentioned warming region. In the vorticity balance on the continental slope in Toyama Bay, the nonlinear term that supplies negative relative vorticity (the clockwise eddy) dominates. Northward movement of this clockwise eddy requires northward flow in the lower layer over the wide shelf around Sado Ridge.
• Response of the Coastal Branch Flow to Alongshore Variation in Shelf Topography off Toyama Bay.

  中田聡史, 磯田豊, 草原和弥  海の研究  11-  (2)  243  -258  2002/03/05  [Not refereed][Not invited]
  
• 対馬暖流の輸送量と流路変動 (総特集 黒潮の生物輸送と生産機能) -- (1章 黒潮系の流動構造とその変動)

  磯田 豊, 伊藤 綾子, 中田 聡史  海洋 号外  (31)  22  -33  2002
• Modeling of the spring bloom in Funka Bay, Japan

  T Azumaya, Y Isoda, S Noriki  CONTINENTAL SHELF RESEARCH  21-  (5)  473  -494  2001/03  [Not refereed][Not invited]
   

  Effects of vertical stability and concentration of nutrient on the diatom bloom in Funka Bay, south Hokkaido, in spring were investigated using one-dimensional and three-dimensional ecosystem models. In the model, six compartments: two for phytoplankton (diatom and dinoflagellate), two for nutrients (silicate and nitrate), one for zooplankton and one for detritus were considered. Vertical stability depended on the net heat flux through the sea surface. Calculated results are compared with the observational results for 1981 (Tsunogai and Watanabe, Journal of the Oceanographical Society of Japan 39 (1983) 231-239). This comparison shows that the rapid spring diatom bloom corresponds to the timing when the net heat flux through the sea surface changes from cooling to heating. This result suggests that the stability of the water column due to warming play an important role in the onset of the diatom bloom in Funka Bay. In addition, the limiting factor of the diatom bloom during the spring bloom is not the silicate, but nitrate. The model can reproduce the observed data by changing the nutrient uptake rate. This suggests a dramatic increase in the assimilation rate of silicate after the consumption of nitrate. (C) 2001 Elsevier Science Ltd. All rights reserved.
• Temporal and spatial variability of phytoplamkton pigment concentrations in the Japan Sea derived from CZCS images

  Sang-Woo Kim, Sei-ichi Saito, Joji Ishizaka, Yutaka Isoda, Motoaki Kishino  海の研究 = Umi no Kenkyu (Oceanography in Japan)  10-  (1)  82  -82  2001/01/05
• A late winter hydrography in Hidaka Bay, south of Hokkaido, Japan

  Manabu Shimizu, Yutaka Isoda, Kazumi Baba  Journal of Oceanography  57-  (4)  385  -395  2001  [Not refereed][Not invited]
   

  Hydrographic observations in Hidaka Bay, south of Hokkaido, Japan were carried out in late winter 1996 and 1997 to examine the spatial distributions and circulation features of two different water masses, i.e., Coastal Oyashio Water (COW) and Tsugaru Warm Water (TWW), and their modifications. It is known that COW is mostly composed of cold and low-salinity water of the melted drift ice coming from the Okhotsk Sea and flows into Hidaka Bay from winter to spring and TWW with high-salinity continuously supplies from the Tsugaru Strait to the North Pacific. Cold surface mixed layers (< 26.2σθ, 0-100 m depth) were found mainly over the shelf slope, confirming that anti-clockwise flow of COW was formed. TWW was relatively high in salinity and low in potential vorticity, and had some patch-like water masses with a temperature and salinity maximum in the limited area in the further offshore at the deeper density levels of 26.6-26.8σθ. The fine structure of vertical temperature and salinity profiles appeared between TWW and COW is an indication of enhanced vertical mixing (double-diffusive mixing), as inferred from the estimated Turner angles. At a mouth of the Tsugaru Strait in late winter 1997, a significant thermohaline front between TWW and the modified COW was formed and a main path of TWW spreaded south along the Sanriku coast, probably as the bottom controlled flow. Hence, the patch-like TWW observed in late winter is isolated from the Tsugaru Warm Current and then rapidly modified due to a diapycnal mixing.
• Ocean Structural Change around the Diatom Bloom in Funka Bay(Symposium: Role of Spring Diatom Bloom in the Material Cycling of Funka Bay)

  Isoda Yutaka, Takeyama Youhei, Azumaya Tomonori  Bulletin on coastal oceanography  38-  (1)  3  -14  2000  

  Reviewing the recent research results of heat, fresh water, salt budgets and sea water exchange of Funka Bay, the changes of physical environment around the spring diatom bloom are summarized. Next, it is investigated that what kind of ecosystem model is able to express the time difference that the increase of meso-zooplankton (meso-Z) population is delayed for about two months from the diatom bloom. This time difference could be reproduced by the following two model conditions. (1) The condition that diatoms are not good food for meso-Z suppresses the increase of meso-Z at the time of diatom bloom. (2) The condition that meso-Z can graze micro-zooplankton from the microbial loop supports the increase of meso-Z in late time.
• Verification of Age and Spawning Hypothesis of Pacific Saury Based on an Offspring-Parent Model

  Bull Jpn. Soc. Fish. Oceanogr.  64-  (2)  77  -84  2000  [Not refereed][Not invited]
  
• Response of Fish Population to a Time-Varying Carrying Capacity

  Bull Jpn. Soc. Fish. Oceanogr.  64-  (2)  71  -76  2000  [Not refereed][Not invited]
  
• Cooling-induced current in the upper ocean of the Japan Sea

  Yutaka Isoda  Journal of Oceanography  55-  (5)  585  -596  1999/10  [Not refereed][Not invited]
   

  The most plausible scenarios for seasonal to interannual variabilities and their possible causes are investigated for the Tsushima Current system passing through the Japan Sea. The study is based on the north and south two-box model across the polar front in an idealized upper ocean of the Japan Sea. The boxes are connected by lateral diffusive heat transport and cooled by atmospheric forcing at the annual mean state. The south box, i.e. the Tsushima Current region, only interacts with the outside warmer box in the East China Sea and has an eastward thermal-driven current originating in the outside box. The magnitude of this current depends on the strength of the thermal gradient between the north and south boxes the inflow of warm waters can therefore be maintained by net heat loss through the sea-surface. I call such a thermal-driven inflow process a 'Cooling-Induced Current' system in the present study. Under periodical heat forcing, the perturbation response of the model to water temperature fields and inflow transport were examined. It is shown that the lateral diffusion time across the polar front (over a period of 10 years) is crucial to the interannual modeled response. An analysis of the seasonal heat budget suggests that the heat transported into the Japan Sea from the East China Sea in summer is stored mainly within the Tsushima Current region and contributes to heat loss by the sea-surface cooling in winter.
• Winter SST variations with 6-8 years period in the Japan Sea

  ISODA Yutaka  海と空  74-  (4)  156  -164  1999/03/01
• Flow Structure of the Coastal Oyashio on the Shelf Area of Hidaka Bay

  Shimizu Manabu, Isoda Yutaka  Bulletin on coastal oceanography  36-  (2)  163  -169  1999  

  The water of the Coastal Oyashio characterized by the cold and low salinity water flows into Hidaka Bay of the southern Hokkaido from February to March. To examine its horizontal and vertical flow structures, CTD and ADCP observations were carried out during 5-13 March 1996. The main part of the Coastal Oyashio is distributed on the shelf slope and its current structure is almost vertically uniform. The volume transport of the Coastal Oyashio is estimated about 0.4-0.5 sv by using the diagnostic model.
• Numerical Simulation of the Formation of a Northward Deep Flow off Cape Henashi, Japan

  MORI Kosuke, ISODA Yutaka  Bulletin of the Faculty of Fisheries, Hokkaido University  49-  (3)  107  -116  1998/12
• Numerical simulations of the transport process of walleye pollock eggs into Funka Bay

  SHIMIZU Manabu, ISODA Yutaka  水産海洋研究  62-  (4)  411  -411  1998/11/01
• Salt Budget and Water Exchange in Funka Bay

  ISODA Yutaka, HASEGAWA Nobuhiko, SHIMIZU Manabu  海と空  74-  (2)  27  -37  1998/09/21
• Numerical Simulations of the Transport Process of Walleye Pollock Eggs into Funka Bay.(COMPARISON BETWEEN OYASHIO REGION AND BERING SEA ECOSYSTEMS. I : Physical, Chemical Environment, Primary Production, Zooplankton and Their Coupling Model Studies. 10)

  Shimizu Manabu, Isoda Yutaka  Memoirs of the Faculty of Fisheries, Hokkaido University  45-  (1)  56  -59  1998/09  

  Ⅰ. Physical, Chemical Environment, Primary Production, Zooplankton and Their Coupling Model Studies
• Seasonal Variability of Phytoplankton Pigment Concentrations at the Sea Surface of the Japan Sea Based on the Combined Empirical Orthogonal Function Analysis

  KIM Sang-Woo, ISODA Yutaka  Bulletin of the Japanese Society of Fisheries Oceanography  62-  (3)  206  -217  1998/08/01
• A Numerical Model of Tides and Tidal Currents around Funka Bay

  SAKATA Youichirou, ISODA Yutaka  Bulletin of the Faculty of Fisheries, Hokkaido University  49-  (2)  51  -57  1998/08
• 東北・北海道沖における暖水渦分布と親潮第一貫入の経年変化 (総特集 黒潮親潮移行域の物理・生物・化学過程) -- (2章 北上暖水・暖水塊の物理および生物・化学過程)

  岩橋 雅行, 磯田 豊  号外海洋  (13)  56  -63  1998/08
• 東部対馬暖流域における暖水渦分布の経年変化 (総特集 日本海の大気海洋相互作用と表層海況・気象変動)

  磯田 豊  海洋  30-  (8)  498  -504  1998/08
• Internnual variations of the surface mixed layer in the Tsushima Current Region

  KIM Sang-Woo, ISODA Yutaka  Umi to Sora  74-  (1)  11  -22  1998/06/01
• Variations with a tidal and several-day periods in the Tsugaru Strait

  ISODA Yutaka  Bulletin of the Japanese Society of Fisheries Oceanography  62-  (2)  147  -151  1998/05/01
• Response of Fish Population to the Time-Varying Fishing Effort

  Isoda Yutake, Azumaya Tomonori  Bulletin of the Faculty of Fisheries, Hokkaido University  49-  (1)  15  -29  1998/03
• Response of Fish Population to the Time-Varying Reproductive Rate

  Isoda Yutake, Azumaya Tomonori  Bulletin of the Faculty of Fisheries, Hokkaido University  49-  (1)  1  -14  1998/03
• Mooered Instrument Observation in the Tsushima Current Southwest of the Tsugaru Strait

  Yutaka Isoda, Tomoharu Nakayama, Takashi Murakami, Sang-Woo Kim, Kiyotaka Ohtani  Umi no Kenkyu  7-  (4)  237  -244  1998  [Not refereed][Not invited]
   

  Warm water of the Tsushima Current converges southwest of the Tsugaru Strait and flows relatively stable northward with most narrow cross section in the Japan Sea. Therefore, many Japanese oceanographers have evaluated the volume transport in this region to monitor the interannual or seasonal variation of the Tsushima Current system. However, the description on the water movements by the past studies have been only inferred from the dynamic caluclation on the basis of hydrographic data. From April 1992 to March 1993, we carried out the direct current measurement southwest of the Tsugaru Strait by using of a moored instrument at the depth of 130m (Apr.∼Oct.1992) and 180m (Oct.1992∼Mar.1993) to investigate the dynamical variability of the Tsushima Current. The large baroclinic events appear to be dominated mainly by passing through the seaward meander of the coastal current in May 1992 and by merging a warm eddy with the spatial scale of several hundred kilometers within the coastal curent in January 1993. Moreover, about one month-periodical fluctuations observed in the temparature and the on-offshore current corresponded with a Tsushima Current frontal eddy event that could be identified in the NOAA/AVHRR images. From a comparison between the current velocities observed by the current meter and the geostrophic ones, it is suggested that the Tsushima Current includes the evidence as the barotoropic northward flow with about 10 cm s^-1 tobe unrelated with the seasonal changes of stratification. This implies that the actual Tsushima Current transports are likely to be larger than those estimated by the past.
• Interannual Variations of Oceanic Conditions Related to the Walleye Pollock Population around the Pacific Sea Area, South of Hokkaido

  ISODA Y.  Bulletin of the Sapanese Society of Fisheries Oceanognaphy  62-  (1)  1  -11  1998  [Not refereed][Not invited]
  
• Thermal front structure formed at the mouth of Funka Bay in winter

  SHIMIZU Manabu, ISODA Yutaka  海と空  73-  (3)  103  -111  1997/12/25
• Fresh water budget of Funka Bay

  HASEGAWA Nobuhiko, ISHODA Yutaka  海と空  73-  (3)  113  -121  1997/12/25
• Coastal Sea-level Variations with a Several-day Period on the Tsugaru Strait

  Baba Kazumi, Isoda Yutaka  Bulletin of the Faculty of Fisheries, Hokkaido University  48-  (3)  29  -40  1997/12
• Generation of Linear Rossby Waves

  Isoda Yutaka  Bulletin of the Faculty of Fisheries, Hokkaido University  48-  (3)  41  -51  1997/12
• Spatial distributions and mixing energies of tidal currents over the eastern Bering Sea shelf

  AZUMAYA Tomonori, ISODA Yutaka  La mer.  35-  (2)  35  -47  1997/05/28
• The Transport Process of Walleye Pollock Eggs into Funka Bay in Winter

  SHIMIZU Manabu, ISODA Yutaka  Bulletin of the Japanese Society of Fisheries Oceanography  61-  (2)  134  -143  1997/05/20
• Relationship between SST and catch rates of migrant and stagnant species in the southern North Pacific off Hokkaido

  MATSUISHI Takashi, ISODA Yutaka  Bulletin of the Japanese Society of Fisheries Oceanography  61-  (1)  71  -75  1997/02/28
• Dispersion of Rossby Waves

  Isoda Yutaka  Bulletin on coastal oceanography  34-  (2)  173  -181  1997  

  Dispersion relation of Rossby waves is reviewed. The temporal change of potential vorticity is investigated within a small time △t according to a concept of differential equation to answer C_g(group velocity)>0 at the short wavelength when C(phase velocity)<0.
• 過去40年間における日本海表層の海況変動

  磯田豊  水産海洋研究  61-  303  -307  1997
• Heat Budget of Funka Bay

  72-  (3)  4,13-21  1997  [Not refereed][Not invited]
  
• Interaction of a warm eddy with the coastal current at the eastern boundary area in the Tsushima Current region

  Y Isoda  CONTINENTAL SHELF RESEARCH  16-  (9)  1149  -1163  1996/08  [Not refereed][Not invited]
   

  The Tsushima Current region in the Japan Sea is one of the eddy-rich areas. According to Isoda (1994) (Journal of Oceanography, 50, 1-15), when an eastward moving warm eddy reached its eastern boundary area, two or three mesoscale eddies frequently evolved and gradually disappeared into the coastal current. Such process associated with the eddy-current interaction are examined numerically using a reduced gravity model on the f-plane. Once a warm (anticyclonic) eddy influences the coastal current, the following two kinds of meanders evolve. In the nonlinear regime, an original eddy moves onshore and becomes part of the seaward meander. Furthermore, this eddy excites another small seaward meander or a new eddy at its northern part due to the vorticity control of the distorted coastal current. After that, a newly formed northern eddy travels downstream with an advective speed of the coastal current, while the potential vorticity of an original eddy becomes to lose within a larger potential vorticity anomaly onshore side of the coastal current. (C) 1996 Elsevier Science Ltd.
• Interannual Variations of Water Temperature,Salinity and Oxity

  Umi to Sora  71-  (2)  47  -57  1995  [Not refereed][Not invited]
  
• Tides and Tidal Currents off the San'in Coast

  Isoda Yutaka, Kawakami Masahiro  Bulletin on Coastal Oceanography  32-  (2)  177  -185  1995  [Not refereed][Not invited]
   

  The characteristics of the tides and tidal currents off the San'in coast are described, and their generation with relation to the shelf topography is investigated using a two-dimensional tidal model. The diurnal currents are generated by the direct vortex stretching of water columns across the shelf/slope region at the Korean side exit of the Tsushima/Korea Strait, and propagate as the double-Kelvin waves trapped along the shelf edge off the San'in coast. In the result, the amplitude of the diurnal currents is more remarkable on the slope than on the shelf. For the semidiurnal tides, the influence of the shelf topography is not large, and the co-oscillating tide attributed to forcing through the Tsushima/Korea Strait is dominant on the shelf as also shown by Odamaki (1989a)'s one-dimensional tidal model.
• Warm eddy movements in the eastern Japan Sea

  Yutaka Isoda  Journal of Oceanography  50-  (1)  1  -15  1994/02  [Not refereed][Not invited]
   

  Warm eddy movements and their areal extent in the eastern Japan Sea were described by presenting space-time diagrams for the warm eddy locations and magnitudes. The analyzed data were compiled from Japan Maritime Safety Agency thermal maps at 200 m depth from 1985 to 1992. Two to four warm eddies always existed in the eastern Japan Sea and exhibited both internnual and annual signals. We found that warm eddies were generated in spring around Oki Spur at least three times during the analyzed period of eight years, moved eastward, and interacted with neighboring warm eddies, which were involved in coalescences or separations. The warm eddy distributions off Noto Peninsula have clear seasonal preference. Warm eddies moved eastward from Noto Peninsula in winter-spring to North Japan in the next winter, with mean translation speeds of 0.5-2 cm s-1. Warm eddies reaching North Japan typically decayed during a few month after splitting into two or three mesoscale warm eddies. © 1994 Journal of the Oceanographic Society of Japan.
• Interannual SST variations to the north and south of the polar front in the Japan Sea

  Lamer  32-  285  -293  1994  [Not refereed][Not invited]
  
• Generation mechanism of higher mode nondispersive shelf waves by wind forcing

  Yutaka Isoda  Journal of Oceanography  49-  (5)  535  -549  1993/09  [Not refereed][Not invited]
   

  The higher mode predominance in the current velocity fields associated with wind-induced shelf waves in the nondispersive regime is studied with a special attention to the effect of the geographical boundary, e.g. wide strait or wide bank areas. The effect of such large topographic change is represented by wind forcing with a finite dimension near the geographical boundary. The time development processes of the wind-induced shelf waves is examined in the context of an initial-value problem, where a spatially finite wind stress is applied at t=0. Various modes of shelf waves excited at the boundary start propagating simultaneously and develop monotonically within the forcing region. After the passage of such wave, the energy of wind is used to maintain the attained equilibrium condition, i.e. the steady shelf circulation. The current evolution of the lower mode is restricted to the earlier stage because of the large propagation speed. In contrast, the higher mode waves can travel slowly within the forcing region so that the kinetic energy is supplied from wind stress for a long time before the equilibrium condition is established. Consequently, the observation at the fixed point near the geographical boundary would show that the higher mode waves gradually dominate as time goes on, i.e. for the long-term forcing. © 1993 Journal of the Oceanographic Society of Japan.
• The northward intruding eddy along the East coast of Korea

  Yutaka Isoda, Sei-ichi Saitoh  Journal of Oceanography  49-  (4)  443  -458  1993/08  [Not refereed][Not invited]
   

  The current structures and their seasonal variations in the East Korean Warm Current (EKWC) region, which plays a significant role in the northward transport of warm and saline waters, were described by combining the sea surface temperature (SST) data of consecutive satellite inferred (IR) images and hydrographic data. The SST patterns in winter-spring clearly showed that the small meander of thermal front originating from the Tsushima/Korea Strait formed close to the Korean coast and grew an isolated warm eddy with horizontal dimension of order 100 km. Such warm eddy began to intrude slowly northward from spring to summer. At that time, interactions with neighboring synoptic warm eddy [Ks] around the Ulleung Basin were found to have strongly influence the movement of the intruding eddy and its structural change. In autumn, after the northward movement stopped at the north of eddy [Ks], the relative stable northward current along the Korean coast were formed. The evidence from observational results does not support a persistent branching of the EKWC from the Tsushima/Korea Strait, but a seasonal episodic supply of warm and saline waters due to the northward intruding eddy process described above. © 1993 Journal of the Oceanographic Society of Japan.
• 大和海嶺上の暖水渦

  磯田 豊, 永延 幹男, 渡辺 秀俊  愛媛大学工学部紀要  12-  (3)  p355  -365  1992/02
• 日本海の暖水渦の挙動について

  磯田豊  海と空  67-  231  -243  1992
• Topographic Effects of the Tsushima Islands on the Tsushima Warm Current

  Isoda Yutaka  沿岸海洋研究ノート  27-  (1)  76  -84  1989  

  Data of temperature and salinity around the Tsushima Straits were analysed to investigate the topographic effects of the Tsushima Islands on the Tsushima Warm Current from the difference of density structure between west and east channels. In winter, the Tsushima Warm Current which is little affected by the Tsushima Islands inflows into the Japan Sea. In summer, the current in the outlet of west channel has the vertical shear flow due to the 2 layered density structure and that in the outlet of east channel has the horizontal shear flow which exhibits the coastal trapped density current. Besides, at the upper layer on the outlet of west channel a warm eddy is formed during a few months period with 60km to 80km in diameter and 50m to 70m in depth.

Association Memberships

• 水産海洋学会   日本海洋学会   

Research Projects

• How asymmetric currents in a deep sea channel affect on the land-deep sea materials transport and habitat for benthos?

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2022/04 -2025/03 

  Author : 千手 智晴, 張 勁, 堀川 恵司, 磯田 豊, 筒井 英人
• Occurrence mechanism of strong-year classes in flathead flounder Hippoglossoides dubius by the assessment of transport process and prey abundance for larvae and juveniles

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2013/04 -2016/03 

  Author : Takatsu Tetsuya, HOSHI NAOKI, NAKAYA MITSUHIRO, ISODA YUTAKA, INAGAKI YUTA, KANO YOTA, NAKATANI TOSHIKUNI, SUZUKI KOTA, TAMURA JUN'ICHI

   

  To clarify the occurrence mechanism of strong-year classes in flathead flounder Hippoglossoides dubius, transport process of pelagic larvae and juveniles and settled area of juveniles were investigated in Funka Bay and its vicinity in 2013-2015 FY. In May 2013, late pelagic stage larvae were collected with MOHT- and FMT-nets around the area off Muroran City located at the north-east end of the bay, but no larva was collected in 2014-2015. It might be important for the recruitment success in the 2013 year-class that the clockwise vortex off Muroran captured the larvae was retained by the predominantly northwesterly winds. In this vortex, suitable water temperature (4.1-5.6 degrees Celsius) and prey abundance were observed for larval growth. In contrast in August-December through 2013-2015, no settled juvenile was collected on the sea floor with an otter trawl net in and out of the bay with <98-m and 95-354-m bottom depths, respectively.
• Mechanism of oligotrphication elucidated in an intensive bivalve cultured bay

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2012/04 -2015/03 

  Author : KUDO Isao, ISODA Yutaka

   

  Oligotrophication, decreasing trend of nutrients was examined in Mutsu bay, a semi-enclosed bay where intensive bivalve culture was carried out. Average nutrient concentration has decreased by one-third since 1970s when scallop culture was introduced to the bay. The causes for this oligotrophication were landings of scallop and the increase in deposition of scallop feces to the sediment surface, resulting in the removal of N and P from the bay.
• Establishment of metacommunity approach toward conservation of coastal biological community in varying environment

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2009 -2012 

  Author : NAKAOKA Masahiro, TANAKA Norio, HORI Masakazu, YOTSUKURA Norishige, MIYASHITA Kazushi, ISODA Yutaka, NODA Takashi, NADAOKA Kazuo, YAMAMOTO Tomoko, HAMAGUCHI Masami

   

  To provide scientific information for the conservation and management of coastal ecosystems and resources, the present study carried out an integrated approach to investigate structure and dynamics of coastal marine community and their changes in relation to global climate change such as temperature rise. We specially targeted on seagrass bed, rocky intertidal and kelp forest communities as model systems. We conducted broad-scale, long-term monitoring of these communities based on periodical census and the use of remote sensing and GIS analyses, statistical analyses on meta-community structure, genetic analyses based on molecular data, and field manipulative experiments. The results demonstrated that structures of coastal communities are affected by multiple factors that operate at different spatial scales, and that their dynamics can be unstable with ongoing climate change through modification of species interaction processes between producers and consumers.
• Quantification of boundary mixing using expendable microstructure profilers for the deep ocean and its incorporation into the global thermohaline circulation model

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2009 -2011 

  Author : HIBIYA Toshiyuki, ISODA Yutaka, KITADE Yujiro, HASUMI Hiroyasu

   

  The global mapping of diapycnal diffusivities in the deep ocean is essential for accurate modeling of global thermohaline circulation. Nevertheless the global distribution of diapycnal diffusivities below a depth of ～2000 m remains unknown. In order to clarify this, we have deployed a free-fall multi-scale profiler VMP-5500 thatcan measure microscale turbulence as well as the background fine-scale shear and strain from the sea surface down to a depth of ～6000 m. Using a combination of the fine-scale and microscale data thus obtained simultaneously together with the aid of the concurrent numerical simulation of internal waves which are generated by tidal interaction with abyssal rough bottom and propagate through the background shear and strain fields, we have succeeded in developing an empirical formula for diapycnal diffusivities near abyssal rough sea beds expressed in terms of fine-scale parameters.
• Verification of the turbulence and light-intensity hypotheses on larval survival of flathead flounder

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2008 -2011 

  Author : TAKATSU Tetsuya, ISODA Yutaka, KOBAYASHI Naoto

   

  Faster-growing larvae of flathead founder Hippoglossoides dubius in the initial feeding stage showed higher survival rate than slower-growing ones within cohort in Funka Bay, Hokkaido. The larvae of 2007 year-class that experienced warm-water-temperature condition showed the fastest growth in 2007-2011 year-classes. Contrary to expectations, the 2008 year-class that showed slowest-growing in larval stage became a strong year class in mature stage. Growth rate in the initial feeding stage was weakly correlated with wind speed as a turbulence indicator however did not with water temperature, global solar radiation, or density of copepod nauplii as principal prey for flathead founder larvae. Match-mismatch and growth-mortality hypotheses in initial feeding stage have some degree of validity but are insufficient to explain the variability seen in recruitment. The survival in later stage may determine the recruitment success.
• Iron behavior and sources regulating primary production in marginal seas (Bering Sea, Okhotsk Sea, Japan Sea)

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2006 -2009 

  Author : KUMA Kenshi, SAITOH Seiichi, ISODA Yutaka, NAGAO Seiya, NAKATSUKA Takeshi, SUZUKI Koji, WATANABE yutaka, NISHIOKA Jun

   

  Surface mixed layer of the basin region in the Bering Sea is under a typical iron-deficient and nutrient-rich condition with low primary production because of low atmospheric iron input. However, phytoplankton bloom (Breen Belt) is observed at the shelf break region of the Southeastern Bering Sea for a long period in summer. It is suggested that the Green Belt is maintained by the supply of nutrient-rich basin water and iron-rich shelf deep water to the surface water of the shelf break. The tidal mixing in the straits of the Kuril Islands is an important role on supplying nutrient and iron to the surface water of the northwestern North Pacific Ocean and maintaining the high primary production in the Oyashio region. On the contrary, the surface mixed layer in the Japan Sea is characterized by low nutrient and high iron concentrations, resulting from high atmospheric iron input to nutrient-depleted surface water of the Japan Sea close to the Asian continent.
• Identification of mixing hotspots on the abyssal rough bottom using a microstructure profiler and their incorporation into global thermohaline circulation models

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2007 -2008 

  Author : HIBIYA Toshiyuki, ISODA Yutaka, KITADE Yujiro, HASUMI Hiroyasu

   

  By deploying a microstructure profiler over the Izu-Ogasawara Ridge and the Emperor Seamounts, we have succeeded, for the first time in Japan, in identifying mixing hotspots on the abyssal rough bottom. We have also found that these mixing hotspots can be best quantified in terms of Gregg-Heyney-Polzin (GHP) parameterization. Finally, abyssal mixing hotpots in the world's oceans quantified using GHP parameterization have been embedded into the ocean general circulation model to obtain the most accurate pattern of global thermohaline circulation.
• Historical transition and prediction of Northern Pacificecosystem associated with human impact and climate change

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2004 -2008 

  Author : KISHI Michio, SAKURAI Yasunori, SAITOH Sei-ichi, IIDA Kohji, KAERIYAMA Masahide, ISODA Yutaka, WATANUKI Yutaka, YAMAGUCHI Atsushi, IKEDA Tsutomu, AOKI Ichiro, KIMURA Shingo, KOMATSU Teruhisa, MATSUDA Hiroyuki, SUGIMOTO Takashige

   

  日本周辺の黒潮・親潮・対馬暖流の各生態系を対象として, (1) 気候変化に伴う海洋環境と生産力の歴史的変化が生態系構造と機能, 特に低次栄養段階生物の生産, 種多様性, 卓越種交替にどのような影響を与えてきたか(ボトムアップコントロール), (2) 高次生物と漁業活動が各生態系の構造と機能にどのように影響してきたか(トップダウンコントロール), (3) 物理-低次生産-高次生産-資源利用に関する多様なモデルを作成して20世紀以降の海洋生態系の歴史的変遷を評価し, (4) 将来の気候変化と人間活動に応答する生態系変化を予測するための生態系モデルを構築して, 日本周辺の海洋生態系の多様性保全と生物資源の最適な資源利用方策を策定することを目標とする。
• 沿岸及び外洋性植物プランクトン鉄摂取増殖並びに生体内鉄利用機構

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 2005 -2006 

  Author : 久万 健志, 磯田 豊

   

  ある特殊な場合を除き、植物プランクトンが摂取出来る鉄の形態は溶存無機鉄イオン種と言われており、海洋植物プランクトンによる鉄摂取速度は、海水中に存在する溶存無機態鉄(Fe(III)')濃度に依存すると考えられる。また昨年度の研究において、植物プランクトンは生体内に鉄をプールする能力を有することが明らかとなった。本年度の研究では、種類の異なる珪藻について過剰にプールした鉄による増殖能力を、生体内に蓄えられた鉄量の違いによる到達最大セル数を測定することにより求めた。また以前求めたセル当たりの臨界鉄量(植物プランクトンが増殖するのに最低限必要な鉄量)とそれぞれ種類の異なる珪藻の大きさを顕微鏡で測定し、それぞれの種のセル表面積及び体積当たりの鉄摂取速度を算出した。一般的に、植物プランクトンによる微量金属摂取速度は、そのセル表面に存在する金属取込みサイトの数によって決まると考えられる。また同じ沿岸性珪藻であれば、セル表面積当りの取込みサイト数は同じと考えると、セル表面積当たりの鉄摂取速度はどの種でも同じであると推察される。しかしながら、実験結果から算出されたセル表面積当たりの鉄摂取速度は、種によって異なっていることがわかった。このことは、種によってセル表面積当りの鉄取込みサイト数が異なるか、または鉄と鉄取込みサイトとの錯体形成能力が異なることを意味している。 また最近の我々の植物プランクトン培養実験では、栄養塩及び鉄が十分量存在するにも関わらず生長が制限され、マンガン欠乏であることが示された。植物プランクトンが高い比生長速度を維持するためには、栄養塩に加え鉄とマンガンが重要であることが明らかになった。
• Quantification of the intensity of mixing hotspots to clarify the dynamics of the global meridional overturning circulation

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2005 -2006 

  Author : HIBIYA Toshiyuki, ISODA Yutaka, KITADE Yujiro, HASUMI Hiroyasu

   

  The un-tethered free-fall microstructure profiler “TurboMAP-D" was used to collect a total of 53 velocity microstructure profiles in the interior of the Pacific Ocean. We found the diapycnal diffusivity at 20°-30°N was roughly in proportional to the logarithm of the locally available energy density of the semidiurnal internal tide and reached more than 10^<-4> m^2s^<-1> near the Izu-Ogasawara Ridge. In contrast, diapycnal diffusivity was found to remain of the order of〜10^<-5> m^2s^<-1> poleward of 30°N and equatorward of 20°N, irrespective of the amount of locally available semidiurnal internal tide energy. We have confirmed that these observed features are consistent with the global map of diapycnal diffusivity in the main thermocline predicted by Hibiya et al. (2006). Using an idealized ocean general circulation model, we next examined the effect of“mixing hotspots", predicted by Hibiya et al. (2006), on the meridional overturning circulation of the Pacific Ocean. We have found that the estimated upwelling in the mixing hotspots is not sufficient to balance deep-water production. In addition, the northward transport of the deep water crossing the equator was found to be much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation for these results is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated from observations. We raise doubts about the validity of previous ocean general circulation models where diapycnal diffusivity is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys.
• Global mapping of diapycnal mixing rates in the deep ocean

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2002 -2003 

  Author : HIBIYA Toshiyuki, HASUMI Hiroyasu, ISODA Yutaka, YOSHIDA Jiro

   

  Global mapping of diapycnal mixing rates in the thermocline is essential to improve the ability of global thermohaline circulation models in predicting future climate changes. For this purpose, we have first estimated diapycnal mixing rates over a large area in the interior of the North and South Pacific, the North Atlantic and the South Indian Ocean by incorporating the fine-scale vertical shear of horizontal velocity measured by expendable current profilers (XCP) into Gregg's formula (Gregg, 1989). The remarkable finding from the XCP survey is that the estimated diapycnal mixing rate in the thermocline is strongly dependent on the latitude. Then, these estimates have been related to the main energy source for diapycnal mixing, the semidiumal internal tide energy numerically calculated at each location, and an empirical formula has been found to predict the global distribution of diapycnal mixing rates in the deep ocean. Incorporating the numerically predicted semidiurnal internal tide energy at each longitude and latitude into the resulting empirical formula, we have found that strong diapycnal mixing (mixing hotspot) is limited to prominent topographic features at latitudes 20^0 -30^0 where the available semidiurnal internal tide energy can be efficiently transferred to dissipation scales by resonant interactions amongst internal waves called parametric subharmonic instability. Embedding the obtained distribution of diapycnal mixing rates into global thermohaline circulation models should yield significant improvement in their ability to predict future climate change.
• Stock Fluctuations throughout Reproduction and Recruitment Process of the Japanese Common Squid, Todarodes pacificus, related to Climate Changes

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2000 -2002 

  Author : SAKURAI Yasunori, BOWER John R., ISODA Yutaka, SAITOH Sei-ichi, YAMAMOTO Jun

   

  In short-lived squids, recruitment success most likely depends on the physical and biological environments at the spawning and nursery grounds. No egg masses have been observed in the natural habitat. During 2000-2002, MOCNESS and an ROV were used to examine the distribution of egg masses and paralarvae of Todarodes pacificus in the Sea of Japan. Gelatinous egg masses were observed at two stations in the Tsushima Current along the continental shelf off the Oki Islands in November, 2000. They occurred within the pycnocline (75 and 95 m depth, respectively), where temperatures were about 18℃, which was suitable for embryonic development. Upon hatching, paralarvae may rise to the surface layer, where they would be carried into the convergent frontal zone in the Tsushima Current. Annual catches of T. pacificus, have markedly increased since the late 1980s, and recent catches have equaled those of the 1960s. The winter spawning area of T. pacificus in the East China Sea shrank when adult stocks decreased during a cool regime that occurred before 1988, and that its fall and winter spawning areas extended and overlapped in the Sea of Japan and East China Sea when adult stocks increased during a warm regime after 1989. Successful hatching from egg masses was to be higher when the mixed layer depth (MLD)is shallower than the bottom depth. We examined the relationship between annual catch of the winter-spawning stock, and wind speed and air temperature in the East China Sea in February during 1980-2000 to determine how a series of calm and warm winters might promote a stock increase related to changes in the MLD. Annual catches markedly increased during a decade of weak wind speed and warm air temperature from the mid-1980s to mid-1990s, suggesting that the strength of winter wind stress affects the winter-spawning stock fluctuations in T. pacificus.
• Monitoring on Marine Living Resources and their environment

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1999 -2002 

  Author : TERAZAKI Makoto, SAKURAI Yasunori, KIMURA Shingo, SUGIMOTO Takashige, ISODA Yutaka

   

  In short-lived squieds, recruitement success most likely depends on the physical and biological environments at the spawning and nursery grounds. Annual catches of Japanese common squid, Todarodes pacificus, in Japanese and Korean waters have markedly increased since the late 1980s, and recent catches have equaled those of 1960s. A previous study suggested that the winter spawning area of T. pacificus in the East China Sea shrank when adult stocks decreased during a cool regime that occurred before 1988, and that its fall winter spawning areas extended and overlapped in the Japan and East Sea when adult stocks increased during a warm regime that occurred after 1989. Inter-calibration of four different plankton nets was carried out in order to compare the sampling efficiency of each net, quantitatively. Horizontal distribution, vertical distribution and seasonal variation of zooplankton, copepods and chaetognaths in the various waters in the Japan Sea, were reported. The Feeding activity of Sagitta elegans was high during night time and they consumed similar food organisms throughout the year. High zoopalnkton biomass was recognized in May after the spring bloom of phytopalnkton at Station T off Tsugaru and there was no influence of the Tsushima warm current in this station from January to July. Transportation mechanism of fish eggs and larval fish was analyzed by using CTD casts, ADCP, remote sensing and drifting buoys observations in the East China Sea.
• Role of diatom bloom irf marine ecosystem

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2000 -2001 

  Author : BAN Shuhei, ISODA Yutaka, KUDO Isao, MARUO Masahiro

   

  We examied zoo-and phytoplankton biomass, bacterial biomass, primary production, bacterial production, meso- and microzooplankton grazing during the period from pre- to post-spring-diatom bloom in Funka Bay, Hokkaido, Japan, to clarify the fate of spring diatom bloom. Diatom bloom started in February at 3 - ℃, lasted by mid-March. During the blooming period, mesozooplankton biomass was alyways low, and consequently its grazing was also quite low level. Microzooplankton increased during the blooming period, but did not graze diatoms Whereas, bacterial production increased during the bloomin period, but its biomass was constant. Using these results, we estimated the carbon budget through food web in the bay. During the blooming period, carbon produced by diatoms was carcely transported to mesozooplankton through grazing food chain due to low mesozooplankton biomass, while microbial loop dominated through bacterial production usingdissolved organic carbon (DOC) from diatoms. On the contrary, after the bloom, two distinct loops existed, i. e. one is dinofragellates to mesozooplankton though regenerated inorganic nutrients by mesozooplankton, while another one is bacteria to protists to mesozooplankton through DOC excreted by mesozooplankton. These results suggest that spring diatom bloom supports indirectly mesozooplankton production through DOC excreted by diatoms, while after the bloom nutrients and DOC regenerated by mesozooplankton supported dinofragellate production and subsequently mesozooplankton themselves.
• spatial distribution of diapycnal mixing rates in the deep waters of the North Pacific

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2000 -2001 

  Author : HIBIYA Toshiyuki, FURUE Ryo, ISODA Yutaka, YOSHIDA Jiro

   

  The mechanical energy which drives the interior mixing in the oceans is believed to be originally supplied at large scales and then transferred across the deep ocean internal wave spectrum down to small dissipation scales. In recent numerical experiments (Hibiya et al., 1996, 1998, 2002), this energy cascade process was shown to be dominated by parametric subharmonic instabilities which transfer energy from the low vertical wavenumber, double-inertial frequency wave band to a high vertical wavenumber, near-inertial frequency wave band. The resulting enhanced, high vertical wavenumber, nearinertial current shear was shown to play a key role in controlling turbulent mixing processes. In this study, to test whether or not the numerically predicted energy cascade process is actually dominant in the real deep we deployed a total of 106 expendable current profilers over a large area in the North Pacific to examine the spatial distribution of high vertical wavenumber (vertical wavelength 〜25 m ) shear. At mid latitudes, significant enhancement of the 25-m vertical shear was found over prominent generation regions of semidiurnal internal tides such as the Hawaiian Ridge and the Izu-Ogasawara Ridge where the semidiurnal tidal frequency exceeds twice the local inertial frequency. At high latitudes, in contrast, no significant enhancement of the 25-m vertical shear was found to occur even over another prominent generation region of semidiurnal internaUtides, the Aleutian Ridge, where the semidiurnal tidal frequency is less than twice the local inertial frequency. We find that the observed spatial distribution of the intensity of the 25-m vertical shear correlates very well with that of the low vertical wavenumber, double-inertial frequency internal wave energy numerically predicted by Nagasawa et al. [2000] and Niwa and Hibiya [2001]. This is the first in-situ evidence for the dominant role of parametric subharmonic instability in transferring deep ocean internal wave energy down to small dissipation scales. This study provides a theoretical framework for future attempts to determine the large-scale structure of mixing over the world's oceans.
• Study of the distribution characteristics of salmonids in the Gulf of Alaska.

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1999 -2000 

  Author : TAKAGI Shogo, KAERIYAMA Masahide, ISODA Yutaka

   

  The feeding ecology of pacific salmon (Oncorhyncus) in the Golf of Alaska was investigated during early summers of 1994-2001. The dominant prey of salmon were squids (mainly Berryteuthis anonychus) in the Subarctic Current, and zooplankton (e.g., euphausiids, copepods, and pteropods) in the Alaskan Gyre during 1994-1996 and 1998. Food competition occurred between sockeye and pink salmon. Pink salmon appeared to feed on more diverse prey at a lower tropic level than sockeye salmon. The feeding and growth of sockeye and pink salmon were affected by the El Nino event during spring and summer of 1997. Analysis of δ 13C and δ 15N isotope values and stomach contents of Pacific salmon indicate their summer feeding ecology and tropic dynamics have changed in the offshore waters of the Golf of Alaska since the 1990s. Scatter plots in the δ 13C - δ15N plane of Pacific salmon in 1999-2000 showed that the difference in isotope values was generally consistent with stomach content analyzes, i.e., Chinook salmon, steelhead trout, coho salmon, sockeye salmon, pink salmon, and shum salmon in order of tropic level. Our results suggest that sockeye, pink, and chum salmon show a high plasticity in their feeding strategy such as switching diets from micronekton or jellyfish to zooplankton according to changes in the ecosystem.
• Reproductive Strategy of the Squid, Todarodes pacificus, Produce Large Gelatinous Egg Masses

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1997 -1999 

  Author : SAKURAI Yasunori, ISODA Yutaka, SAITOH Sei-ichi

   

  Reproductive strategy related to stock fluctuation on the ommastrephid squid Todarodes pacificus are examined by field survey and captive experiments. Immature squid were collected and maintained in a raceway tank where they matured, mated, and spawned. Gelatinous egg masses were spherical and nearly buoyant. The largest egg mass measured 80 cm in diameter and contained approximately 200,000 eggs. The egg-mass surface layer effectively prevented crustaceans, protozoans, and bacteria from infesting the masses. Paralarvae hatched after 4-6 days at 18-19℃ and actively swam at once, with many individuals swimming at the surface. Egg masses disintegrated soon after hatching. Survival of eggs and paralarvae were examined at temperatures ranged from 3.5-29℃ to determine the optimum temperature range for development and survival. Highest embryonic survival rates occurred between 15 and 23℃. It is sugggested that T. pacificus spawns in waters warmer than 15℃, and egg masses maintain their structure for about 4-9.5 days before disintegrating at temperatures between 15 and 23℃. The inferred spawning sites of T. pacificus around Japan are assumed that egg masses and hatchlings occur at temperatures between 15 and 23℃, and above the continental shelf and slope, because captive females regularly sit on the tank bottom just before spawning. Annual catches of Todarodes pacificus in Japan have gradually increased since the late 1980s. Paralarval abundances have also been higher since the late 1980s than during the late 1970s and mid-1980s. We propose a possible scenario for the recent stock increase based on changing environmental conditions. First, we examined trends in the annual variations of stock and larval catches, and infer potential spawninig areas, assuming that egg masses and hatchlings occur at temperatures between 15 and 23℃, and over the continental shelf. We then infer changes in the spawning areas during 1984-95 based on GIS data. We conclude that since the late 1980s the fall and winter spawning areas have overlapped in the Tsushima Strait and near the Goto Islands, and that winter spawning sites have expanded over the continental shelf and slope in the East China Sea.
• Study on the transport mechanism of chemical substances to the deep sea above the 5m of continental shelf slope at the Japan Trench

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1996 -1997 

  Author : NORIKI Shinichiro, NARITA Hisashi, ISODA Yutaka

   

  Five time-series sediment traps were moored at Hidaka Basin (41ﾟ28'N,142ﾟ29'E,Water Depth 1.4km ; June/96-June/97) and northern Japan Trench (40ﾟ29'N,144ﾟ31'E,Water Depth 7.3km ; June/96-July/97). Settling particles collected with sediment traps were analyzed for aluminum, opal, calcium carbonate, manganeses and rare earth elements, especially La and Yb. Aluminum flux increased with depth at the station of Japan Trench. The particle collected at 1.2km depth in summer season was rich in Mn at the station of Hidaka Basin. The high Mn/Al particles were found at the deeper depth of the station of Japan Trench. The [Yb/La] of the particle at 1km depth of Japan Trench was similar to that of Asian continental dust, while the [Yb/La] of the particles at 6.7km depth was similar to that of Japan Island crust. These results indicate that lithogenic aluminosilicate are vertically and horizontally transported from coastal region to the deep Japan Trench above the continental shelf slope.
• Joint Study on the Bioclimatology in the Bering Sea

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1995 -1997 

  Author : OHTANI Kiyotaka, ONISHI Hiroji, SHIGA Nanobu, NAKATANI Toshikuni, SAITOH Sei-ichi, SAKURAI Yasunori

   

  During Bering Sea surveys by the T/S Oshoro-Maru of Hokkaido University in the summers of 1995,1996 and 1997, we conducted a joint study of the bioclimatology in the Bering Ses with sientists from the University of Alaska, Fairbanks. In this study, we examined the relationship between the physical environment, production structure and recruitment of keystone species, including walleye pollock Theragra chalcogramma, related to recent climatic changes in th Bering Sea ecosystem. The results will be published in teh Memories of the Faculty of Fisheries.Hokkaido University, in the summer of 1998. A summary of our joint study is as follows : 1. We studied coastal upwelling in the St.Lawrence Island Polynya in the summers of 1994 and 1995 by satellite and ship observations. 2, We clarified interannual variation and vertical distribution of appendicularians in the St.Lawrence Island Shelf in summer. 3. We examined biophysical processes relevant to recruitment dynamics of walleye pollock in the eastern Bering Sea during the summers of 1995,1996 and 1997. We will present a conceptual model of factors influencing pollock recruitment in this area. 4. We clarified the effect of temperature and salinity on development and survival of eggs and larvae of Arctic cod, Boreogadus saida as an indicator species related to climatic change in the Berin Sea. 5. We studied the bottom fish composition and food habits in the southern water off the St.Lawrence Island in the Bering Sea.
• 日本海内部の傾圧流量と海峡間の水位差に伴う通過流量の不一致に起因した流動場

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 1995 -1995 

  Author : 磯田 豊

   

  対馬海峡から流入し津軽・宗谷海峡から流出する対馬暖流の通過流量uηは両海峡間の水位差△η=ηs-ηn、すなわち太平洋側の外部条件に依存している。一方、日本海内部の対馬暖流は冷たい日本海固有水の上を密度流として東流し、その傾圧流量uρは南北方向の密度差△ρ=ρn-ρs、すなわち日本海の内部条件に依存している。私は△ηと△ρに依存したそれぞれの流量は、必ずしも一致する必要はないと考えた。実際に、対馬・津軽両海峡内における最近のADCP観測は通過流量の季節変化が小さいことを示しているが、日本海内部の傾圧流量は夏季の暖水流入と冬季の海面冷却による△ρの変化から、明らかに大きな季節変化をしている。 上述の両流量の不一致に起因した物理現象は、両流れ場の接点である対馬海峡付近において、夏季の成層の発達と同時に傾圧流が強まり、底層に日本海から冷水が侵入する現象として現れている。この冷水侵入の力学を理解する物理モデルとしてy方向の流れが消える、すなわちy方向の地衡流が存在しない(f=0)韓国沿岸に沿ったx-z断面Aを考えた。△ηによるuηは外部強制流Uoとして与え、△ρによる密度流はモデル内で表現される。その結果、4月観測と同程度の冷水侵入状態を再現するためには、鉛直粘性をko=200cm^2s^<-1>のオーダまで上げる必要があること、このケースで再現された日本海内部の約10℃以下の水は、日本海の底層水(0℃)と流入暖水(11℃)が海峡内で混合した結果、形成されていることがわかった。次に、このケースから10月を想定し流入水温をTo=21℃まで上げると、再び重力循環が強まり底部冷水の侵入が強まることもわかった。ただし、このモデルでは水平または鉛直拡散が大きいため、その様子は水温場には顕著に現れない。
• Mid and Deep Circulation in the Japan Sea

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1994 -1995 

  Author : YOON Jong-hwan, DANCHENKOV M, MYUNG Cheol-soo, VOLKOV Y.n, JEON Dong chull, KIM Kuh, SENJYU Tomoharu, ISOBE Atsuhiko, YAMAGUCHI Satoru, KOTETAYAMA Wataru, TAKEMATSU Masaki, AN Hui-soo, KIM Kyung-ryul, CHOI Byung-ho, KIM Cheol-ho, HONG Chol-hoon

   

  The Japan Sea is a mini ocean and a very good test basin for the numerical ocean prediction. However, so far, the intermediate and deep circulation haven't been well understood. We carried out 4 cruises to survey the circulation below the surface layr during two fiscal years (1994,1995) as international cooperative field researches between Japan, Korea nad Russia. The CTD and chemistry measurements during the summer cruise in the main part of the Japan Sea (1994,7.5-7.25) showed the increase of the temperature in the deep layr with the rate of 0.02 C per 10 years and the oxgen decrease, suggesting no formation of the deep water during at least last ten years. Strong mesoscale activities with the time scale of month and the velocity exceeding 20 cm/s were detected in the deep Japan Basin by analyzing the recovered long-term (1993.8-1994.8) current mooring data. Many informations about the distribution of the salinity minimum layr with the oxgen maximum were obtained in this cruise. The second winter cruise (1996.2.15-2.25) carried out off Vladivostok in '96 winter detected the deep convection reaching 600m depth, while no sign of deep convection could be found in the first winter cruise (1995.3.1-3.15), because the winter in 1995 was warm. The ADCP and CTD measurements carried out during the summer cruise off the wakasa bay (1995.5.26-6.8) observed the subsurface counter current below the nearshore branch of the Tsushima current over the continental shelf break with the speed exceeding 10 cm/s. The presence and the structure of this undercurrent correspond well to the results of several numerical models. Many new aspects about the intermediate and deep clrculation found in this research will contribute much to the future studies for the Japan Sea circulation.
• 冬季の海面冷却によって駆動される対馬暖流

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 1994 -1994 

  Author : 磯田 豊

   

  日本海において、冬季の海面冷却によって損失する熱量は対馬暖流の流入に伴う熱輸送によって補われている。申請者はこの熱収支過程を組み込んだ南北2ボックスモデルを作成し、日本海の水温場及び対馬暖流の流入量の経年変動と海面冷却変動との関係を調べた。まず、過去20年間の海面水温(SST)資料解析を行い、日本海には10年スケールと2〜4年スケールのSST変動が卓越していることをみつけた。10年スケールのSST変動の特徴は、対馬暖流域である南海海域よりも北部海域で卓越し、対馬海峡における流入量の変動にも顕著なシグナルとして現れる。一方、2〜4年スケールのSST変動の特徴は、南部と北部のSST変動の振幅、位相差がいずれも小さく、流入量の変動には顕著に現れない。このように変動特性が大きく異なる10年及び2〜4年スケールの変動は、海面冷却をのみを強制力とした同一のボックスモデルで再現することができた。モデル結果は、両時間スケールの応答の違いが海面冷却の強制周期と南北海域間の水平拡散時間との関係で説明できることを示している。
• Water & Material Circulation in the Japan and East China Sea

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1993 -1994 

  Author : TAKEMATSU Masaki, VOLKOV Y.n., KIM Kuh, GAMO Toshitaka, ISODA Yutaka, DANCHENKOV M.a., GONCHAREKO I.a., LI Rong-feng, JI Zhoug-zhen, ZATSEPIN A.g., MILLOT Claude, SU Ji-lan, OSTROVSKI A.g, MATSUNO Takeshi, YANAGI Tetsuo, YAMAGATA Toshio, NOZAKI Yoshiyuki, OOTANI Kiyotaka, KOTERAYAMA Wataru, IMAWAKI Shiro, MASTUDA Akira, YUNG John-fung, BYOG S-k., NA J.-y., KIM K.r., CHOI Byong-ho, CREPON Michel

   

  The Project "Water and Material Circulation in the Japan and East Chaina Seas" is an international cooperation research program supported by the Ministry of Education, Science and Culture, Japan. It is generally Kwown as CREAMS project and recognized as one of the important activities of IOC/WESTPAC 'Ocean Dynamics and Climate II'. The goal of the program is to clarify the circulation of the marginal seas, to study associated basic processes and to contrust reliable numerical models for physical, chemical and biological studies. A feature of the project is its intensive effort to collect new date sets. In fact, basin-wide joint observations of the Japan Sea were carried out in summer (1993,1994) and in winter (1995) on board Russian research vessels. The observaional work included precise CTD to the bottom, water sampling for chemical study, release of surface drifters, deployment of moorings with current meters and sediment traps and physical and chemical measurments by means of a specially designed towing vehicle. To back up the observations, satellite infra-red images and altimeter date were colleced during the past two project years. Those new date sets were discussed in relation to results of numerical models, date analyzes, laboratory experiments and theory at international (open) symposium in Fukuoka (1994) and Seoul (1995).
• Study on the Kyucho in the Bungo Channel

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 1988 -1989 

  Author : TAKEOKA Hidetaka, AKIYAMA Hideki

   

  Investigations were made on the Kyucho in the Bungo Channel. Its structure, periodicity of its generation, mechanism of its generation and its influences on the marine environment were studied by the field observations, such as long term monitoring of water temperature and CDT surveys, and analyses of the historical data. The following results were obtained. (1) The Kyucho in the Bungo Channel is a phenomenon of the intrusion of a warm, lighter water mass from the Pacific Ocean, and is fundamentally a gravity current. The warm water mass intrudes into eastern half of the channel looking the coast to its right side. (2) The Kyucho appears around neap tide. (3) The Kyucho appears mainly around summer. (4) Generation of the Kyucho is determined by the variation of the intensity of vertical mixing. But quantitative examination of the vertical mixing suggested that some other mechanism must work in addition to this effect. (5) The intensities of the Kyucho is influenced by the oceanic disturbances. The Kyucho is intensified when the supply of warm water mass from the ocean to the southern part of the Bungo Channel falls on neap tide. (6) The Kyucho gives many influences on the marine cultures along the eastern coast of the Bungo Channel such as disease or decease of the cultured fish due to the sudden change of water temperature. On the other hand, the Kyucho contributes to the renewal of the bay water maintaining water quality, and supports the large scale cultures. Study on the generation mechanism including the effect of earth's rotation is necessary in future.
• 深層大循環と深海水の年令

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 1987 -1987 

  Author : 武岡 英隆, 磯田 豊, 柳 哲雄

   

  非保存性物質によって決定される海水の年令分布は, 海洋の深層循環等の時空間スケールの大きな流動のトレーサーとして有効である. この年令分布と, 流動・拡散の関係を, 1次元と2次元の基礎的な解析モデルおよび数値モデルによって調べた. 1次元モデルでは, 境界のない場合には, 年令の空間的変化に拡散の影響を表れず, 年令の勾配は直ちに流速に換算できること, 境界がある場合には, 境界付近で年令の勾配が流速から期待されるものとは異ってくること, この影響範囲はペクレ数(拡散時間と移流時間の比)が小さいなど大きいことがわかった. 2次元モデルでは, 閉じた正方形領域で循環流を考え, 1つの隅に物質の出入口がある場合と, 1辺全体が出入口になっている場合の2通りを考えた. 前者の場合, ペクレ数が大きくなると年令分布から循環の向きは判定できるが, 流速が直ちにわかるのは特定のペクレ数の場合に限ること, 後者の場合は, 出入口と反対側の辺における流速は, ペクレ数が10程度以上になると年令分布から直ちにわかることなどがわかった. これらの結果の一部をフィリピン海盆の深層水の循環の問題に適用した. その結果, これまでに得られている知見や理論により推定される輸送場から判断すれば, 同海盆における年令分布から循環の向きや流速のオーダーは求め得る筈であることがわかった.
• 日本海,対馬暖流の力学構造
• Dynamical Structures of the Tsushima Current in the Japan Sea