研究者データベース

三寺 史夫(ミツデラ フミオ)
低温科学研究所 附属環オホーツク観測研究センター
教授

基本情報

所属

  • 低温科学研究所 附属環オホーツク観測研究センター

職名

  • 教授

学位

  • 理学博士(東北大学)

ホームページURL

J-Global ID

研究キーワード

  • 海洋循環モデル   海氷予報   油汚染   数値シミュレーション   予測モデル   物質循環   高密度水   流出油   水位   サハリン油田   海鳥   海洋レーダー   鉄仮説   高密度陸棚水   温暖化   中規模渦   海洋中層   風成循環と熱塩循環の結合   時計回り循環   多重平衡   千島列島   風成循環   北太平洋   潮汐混合   宗谷暖流   中層循環   熱塩循環   北太平洋中層水   東樺太海流   オホーツク海   

研究分野

  • 環境・農学 / 環境動態解析
  • 自然科学一般 / 大気水圏科学

職歴

  • 2003年03月 - 現在 北海道大学 低温科学研究所 教授

研究活動情報

論文

  • Shungo Fukumoto, Shin Sugiyama, Shuntaro Hata, Jun Saito, Takayuki Shiraiwa, Humio Mitsudera
    Journal of Glaciology 1 - 14 2022年07月04日 
    Abstract On the Kamchatka Peninsula in the Russian Far East, 405 glaciers with an estimated total mass of 49 Gt were reported in the 1970s. These have been retreating at an accelerated rate since the start of the 21st century. Because glacier studies in this region are scarce, ice loss and its influence on sea level rise and regional environments is poorly understood. In this study, we analyzed satellite data to quantify glacier mass change from 2000 to 2016 in six major glacier-covered regions on the peninsula. The mean rate of the glacier mass change over the study period was −0.46 ± 0.01 m w.e. a−1 (total mass change was −4.9 ± 0.1 Gt, −304.2 ± 9.1 Mt a−1), which is slightly lower than other regions in mid-latitude and subarctic zones. The mass loss accelerated from >−0.33 ± 0.02 m w.e. a−1 in the period 2000–2006/2010 to <−1.65 ± 0.12 m w.e. a−1 in 2006/2010–2015/16. The increase in mass loss is attributed to a rise in average decadal summer temperatures observed in the region (+0.68°C from 1987–99 to 2000–13). Moreover, a recent trend in Pacific decadal oscillation suggests future acceleration of mass loss due to a decline in winter precipitation.
  • Hatsumi Nishikawa, Humio Mitsudera, Takeshi Okunishi, Shin-ichi Ito, Taku Wagawa, Daisuke Hasegawa, Toru Miyama, Hitoshi Kaneko, Ren-Chieh Lien
    Progress in Oceanography 199 102691 - 102691 2021年12月
  • Jun Nishioka, Toru Hirawake, Daiki Nomura, Youhei Yamashita, Kazuya Ono, Aiko Murayama, Alexey Shcherbinin, Yuri N. Volkov, Humio Mitsudera, Naoto Ebuchi, Masaaki Wakatsuchi, Ichiro Yasuda
    PROGRESS IN OCEANOGRAPHY 198 2021年11月 [査読有り]
     
    Analysis of observed iron (Fe) and nutrient data from the East Kamchatka Current (EKC) in the western Bering Sea, including the Kamchatka Basin, northeastern shelf slope, and Gulf of Anadyr, in the summers of 2014 and 2018 was conducted. The results indicate the location of the edge of high-nutrient and low-chlorophyll water (Fe-limited subarctic Pacific water), which expands into the EKC area. The supply of Fe was related to the freshwater supply from the Kamchatka Peninsula and caused nutrient depletion along the coast of the Kamchatka Peninsula in the EKC. Consistent with the subarctic Pacific, water with extremely high nitrate (N) and phosphate (P) concentrations and extremely low dissolved oxygen concentrations was observed in a wide density range in the intermediate layer (26.6-27.6 sigma(theta)) in the Kamchatka Basin and formed an intermediate nutrient pool. However, the concentrations of both dissolved Fe (dFe) and silicic acid (Si) were highest in the deep layer below the 27.6 sigma(theta) isopycnal surface in the basin, indicating that the dFe and Si cycles were decoupled from the N and P cycle in the basin intermediate water. The increase in dFe concentration with depth in the deep layer was probably due to a reduction in particulate Fe that occurred in the benthic pore water on the slope or in the settling particulate microenvironment, as indicated by a negative N* index. Based on the observed data around the Kamchatka Strait and EKC in the summer of 2018, the chemical properties of the water in the intermediate layers in the EKC on the Pacific side of the Kamchatka Peninsula were mainly influenced by circulation of the Western Subarctic Gyre, which flows from the southeastern Aleutian Islands, with minor influence from Kamchatka Basin water flowing through the Kamchatka Strait on the coastal side of the EKC. Our results also indicate that nutrient-rich water intrusion occurred from the Aleutian Basin intermediate nutrient pool to the bottom layer of the Gulf of Anadyr. Additionally, nitrate removal by via the denitrification process and the input of sedimentary Fe occurred in the intruding bottom water. The findings from this study are important for understanding the biogeochemical interactions occurring among the subarctic Pacific, Bering Sea, and Arctic Ocean.
  • Takuya Nakanowatari, Tomohiro Nakamura, Humio Mitsudera, Jun Nishioka, Hiroshi Kuroda, Keisuke Uchimoto
    Progress in Oceanography 197 102615 - 102615 2021年09月 [査読有り][通常論文]
  • Muqing Shi, Takayuki Shiraiwa, Humio Mitsudera, Yaroslav Muravyev
    JOURNAL OF HYDROLOGY-REGIONAL STUDIES 36 2021年08月 [査読有り]
     
    Study Region: Kamchatka Peninsula, Russian Federation.Study Focus: The strength of an overturning process of North Pacific sea water originating in the Sea of Okhotsk is determined by the sea water salinity in the area. Recent observations show that the overturning process is important to the entire North Pacific and is statistically related to the precipitation over the Kamchatka Peninsula (KP). This study uses estimations based on specific discharge and the Soil and Water Assessment Tool (SWAT) to verify and further explore the connection between the terrestrial freshwater discharge from KP and the overturning process.New Hydrological Insights for the Region: Previously inaccessible observed discharge data from 11 major rivers on the KP were obtained and analyzed for the first time. Based on the data we estimate annual discharge from the entire KP using the specific discharge and regression analysis, and we then simulate monthly discharge from the western KP using the SWAT model. Our result shows a significant negative correlation (r(2) = 0.36, p < 0.01) between the interannual variation of sea water salinity in the overturning process area, and the variation of annual freshwater discharge from the western KP, therefore successfully verifies that freshwater discharge from the western KP is a critical factor that affects the strength of the overturning process.
  • Hung-Wei Shu, Humio Mitsudera, Kaihe Yamazaki, Tomohiro Nakamura, Takao Kawasaki, Takuya Nakanowatari, Hatsumi Nishikawa, Hideharu Sasaki
    SCIENTIFIC REPORTS 11 1 2021年06月 [査読有り]
     
    The interbasin exchange between the Sea of Okhotsk and the North Pacific governs the intermediate water ventilation and fertilization of the nutrient-rich subpolar Pacific, and thus has an enormous influence on the North Pacific. However, the mechanism of this exchange is puzzling; current studies have not explained how the western boundary current (WBC) of the subarctic North Pacific intrudes only partially into the Sea of Okhotsk. High-resolution models often exhibit unrealistically small exchanges, as the WBC overshoots passing by deep straits and does not induce exchange flows. Therefore, partial intrusion cannot be solely explained by large-scale, wind-driven circulation. Here, we demonstrate that tidal forcing is the missing mechanism that drives the exchange by steering the WBC pathway. Upstream of the deep straits, tidally-generated topographically trapped waves over a bank lead to cross-slope upwelling. This upwelling enhances bottom pressure, thereby steering the WBC pathway toward the deep straits. The upwelling is identified as the source of joint-effect-of-baroclinicity-and-relief (JEBAR) in the potential vorticity equation, which is caused by tidal oscillation instead of tidally-enhanced vertical mixing. The WBC then hits the island chain and induces exchange flows. This tidal control of WBC pathways is applicable on subpolar and polar regions globally.
  • Sayaka Yasunaka, Humio Mitsudera, Frank Whitney, Shin-ichiro Nakaoka
    JOURNAL OF OCEANOGRAPHY 77 1 3 - 16 2021年02月 [査読有り]
     
    A compilation of surface water nutrient (phosphate, nitrate, and silicate) and partial pressure of CO2(pCO(2)) observations from 1961 to 2016 reveals seasonal and interannual variability in the North Pacific. Nutrients and calculated dissolved inorganic carbon (DIC) reach maximum concentrations in March and minimum in August. Nutrient and DIC variability is in-phase (anti-phase) with changes in the mixed layer depth (sea surface temperature) north of 30 degrees N, and it is anti-phase (in-phase) with changes in Chl-a north of 40 degrees N (in 30 degrees N-40 degrees N). Seasonal drawdown of nutrients and DIC is larger toward the northwest and shows a local maximum in the boundary region between the subarctic and subtropics. Stoichiometric ratios of seasonal drawdown show that, compared to nitrate, silicate drawdown is large in the northwestern subarctic including the Bering and Okhotsk seas, and drawdown of carbon is larger toward the south. Net community production in mixed layer from March to July is estimated to be more than 6 gC/m(2)/mo in the boundary region between the subarctic and subtropics, the western subarctic, the Gulf of Alaska, and the Bering Sea. Nutrient and DIC concentrations vary with the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation which cause changes in horizontal advection and vertical mixing. The DIC trend is positive in all analysis area and large in the western subtropics (> 1.0 mu mol/l/yr). Averaged over the analysis area, it is increasing by 0.77 +/- 0.03 mu mol/l/yr (0.75 +/- 0.02 mu mol/kg/yr).
  • Ryu Saiki, Humio Mitsudera, Ayumi Fujisaki-Manome, Noriaki Kimura, Jinro Ukita, Takenobu Toyota, Tomohiro Nakamura
    PROGRESS IN OCEANOGRAPHY 190 2021年01月 [査読有り]
     
    In polar oceans, ice-band patterns are frequently observed around the ice edge in the winter, where sea ice production and melting continually occur. A better understanding of such fundamental processes in marginal ice zones (MIZs) may be key to accurate predictions of sea-ice evolution. Ice bands exhibit approximately 10-km-scale regular band spacings, and their long axes turn to the counter-clockwise (clockwise) with respect to the wind direction in the Northern (Southern) Hemisphere. We formulate a theory that is relevant for a continuously stratified ocean and compare the theoretical results with the numerical-model results and satellite observations. The numerical results quantitatively agree well with the theory. In particular, resonance condition, on which the phase speed of internal wave matches with the ice-band propagation speed, is always satisfied even if wind speed becomes slow. This is because there are an infinite number of baroclinic modes in continuously stratified ocean. We also show that an ice-band pattern emerges from a random initial ice concentration even though the wind is homogeneous. Plume like ice bands along ice edge, which are frequently observed by satellites, are well explained by the pattern formation from random noise. Various effects of the ice band formation were explored with respect to the relationship between the initial ice concentration and the wind direction, ice roughness, ice thickness, temporal variation of wind, and the Coriolis parameter.
  • Yusuke Kawaguchi, Jun Nishioka, Shigeto Nishino, Shinzou Fujio, Keunjong Lee, Amane Fujiwara, Daigo Yanagimoto, Humio Mitsudera, Ichiro Yasuda
    Journal of Geophysical Research: Oceans 125 9 2020年09月 [査読有り]
  • Takuya Nakanowatari, Humio Mitsudera
    Atmosphere, Earth, Ocean & Space 19 - 56 2020年06月16日 [査読有り][招待有り]
  • Shota Katsura, Hiromichi Ueno, Humio Mitsudera, Shinya Kouketsu
    JOURNAL OF PHYSICAL OCEANOGRAPHY 50 1 95 - 109 2020年01月 [査読有り][通常論文]
     
    The spatial distribution and seasonality of halocline structures in the subarctic North Pacific (SNP) were investigated using Argo profiling float data and various surface flux data collected in 2003-17. The permanent halocline (PH) showed zonal patterns in the spatial distributions of its depth and intensity and tended to be shallow and strong in the eastern SNP but deep and weak in the west. Mean distributions of PH depth and intensity corresponded to the winter mixed layer depth and sea surface salinity, respectively, indicating that it forms in association with the development of the winter mixed layer. In the Western Subarctic Gyre and Alaskan Gyre, where a relatively strong PH formed, PH intensity and depth showed clear seasonal variations, and deepening of the mixed layer compressed the underlying PH during the cooling period, resulting in intensification and development of the PH in late winter. In both regions, upwelling of high-salinity water also contributed to PH intensification. The summer seasonal halocline (SH) showed distinct zonal differences in frequency and intensity, which were opposite to the PH distribution. While an SH formed in the western and central SNP and coastal regions, it was seldom present in the eastern area. This zonal contrast of SH corresponded to freshening of the mixed layer during the warming period, primarily reflecting freshwater flux. Geostrophic and Ekman advection play important roles in spatial differences in SH intensity and depth. SH development contributed to PH intensification in the following winter, by decreasing salinity above the PH through entrainment.
  • Toru Miyama, Humio Mitsudera, Hajime Nishigaki, Ryo Furue
    Journal of Physical Oceanography 48 4 807 - 830 2018年04月01日 [査読有り][通常論文]
     
    The dynamics of a quasi-stationary jet along the Subarctic Front in the North Pacific Ocean (the Western Isoguchi Jet) were investigated using an idealized two-layer model. The experiments suggested that a seafloor topography, which is 500m high, produces a jet along its eastern flank. The formation mechanism of the jet can be explained via baroclinic Rossby wave characteristics. Baroclinic Rossby waves propagate along characteristic curves, which are significantly distorted by anticyclonic barotropic flow on the seafloor topography. A baroclinic surface jet is formed where a characteristic curve originating in the subtropical gyre and one originating in the subpolar gyre meet because the pycnocline depth varies discontinuously at this location. The barotropic flow on the seafloor topography is induced by eddies.
  • Tatsuro Karaki, Humio Mitsudera, Hiroshi Kuroda
    Journal of Oceanography 74 4 1 - 12 2018年02月12日 [査読有り][通常論文]
     
    The Soya Warm Current (SWC), which is the coastal current along the northeastern part of Hokkaido, Japan, has a notable baroclinic jet structure during summer. This study addresses the formation mechanism of the baroclinic jet by analyzing a realistic numerical model and conducting its sensitivity experiment. The key process is the interaction between the seasonal thermocline and the bottom Ekman layer on the slope off the northeastern coast of Hokkaido the bottom Ekman transport causes subduction of the warm seasonal thermocline water below the cold lower-layer water, so the bottom mixed layer develops with a remarkable cross-isobath density gradient. Consequently, the buoyancy transport vanishes as a result of the thermal wind balance in the mixed layer. The SWC area is divided into two regions during summer: upstream, the adjustment toward the buoyancy shutdown is in progress downstream, the buoyancy shutdown occurs. The buoyancy shutdown theory assesses the bottom-mixed-layer thickness to be 50 m, consistent with observations and our numerical results. The seasonal thermocline from June to September is strong enough to establish the dominance of the buoyancy shutdown process over the frictional spindown.
  • Mitsudera, H., Miyama, T., Nishigaki, H., Nakanowatari, T., Nishikawa, H., Nakamura, T., Wagawa, T., Furue, R., Fujii, Y., Ito, S.
    Nature Communications 9 2018年 [査読有り][通常論文]
  • Takuya Nakanowatari, Kay I. Ohshima, Vigan Mensah, Yoko Mitani, Kaoru Hattori, Mari Kobayashi, Fabien Roquet, Yasunori Sakurai, Humio Mitsudera, Masaaki Wakatsuchi
    POLAR SCIENCE 13 56 - 65 2017年09月 [査読有り][通常論文]
     
    The Sea of Okhotsk is a challenging environment for obtaining in situ data and satellite observation in winter due to sea ice cover. In this study, we evaluated the validity of hydrographic observations by marine mammals (e.g., seals and sea lions) equipped with oceanographic conductivity-temperaturedepth (CTD) sensors. During 4-yr operations from 2011 to 2014, we obtained total of 997 temperature-salinity profiles in and around the Soya Strait, Iony Island, and Urup Strait. The hydrographic data were mainly obtained from May to August and the maximum profile depth in shelf regions almost reaches to the seafloor, while valuable hydrographic data under sea ice cover were also obtained. In strong thermoclines, the seal-derived data sometimes showed positive biases in salinity with spikelike signal. For these salinity biases, we applied a new thermal mass inertia correction scheme, effectively reducing spurious salinity biases in the seasonal thermocline. In the Soya Strait and the adjacent region, the detailed structure of the Soya Warm Current including the cold-water belt was well identified. Dense water up to 27.0 sigma(theta), which can be a potential source of Okhotsk Sea Intermediate Water, has flowed from the Soya Strait into the Sea of Okhotsk in mid-winter (February). In summer, around the Iony Island and Urup Strait, remarkable cold and saline waters are localized in the surface layers. These regions are also characterized by weak stratification, suggesting the occurrence of tidally induced vertical mixing. Thus, CTD-tag observations have a great potential in monitoring data-sparse regions in the Sea of Okhotsk. (C) 2017 The Authors. Published by Elsevier B.V.
  • Takuya Nakanowatari, Tomohiro Nakamura, Keisuke Uchimoto, Jun Nishioka, Humio Mitsudera, Masaaki Wakatsuchi
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 122 5 4364 - 4391 2017年05月 [査読有り][通常論文]
     
    Iron (Fe) is an essential nutrient for marine phytoplankton and it constitutes an important element in the marine carbon cycle in the ocean. This study examined the mechanisms controlling seasonal variation of dissolved Fe (dFe) in the western subarctic North Pacific (WSNP), using an ocean general circulation model coupled with a simple biogeochemical model incorporating a dFe cycle fed by two major sources (atmospheric dust and continental shelf sediment). The model reproduced the seasonal cycle of observed concentrations of dFe and macronutrients at the surface in the Oyashio region with maxima in winter (February-March) and minima in summer (July-September), although the simulated seasonal amplitudes are a half of the observed values. Analysis of the mixed-layer dFe budget indicated that both local vertical entrainment and lateral advection are primary contributors to the wintertime increase in dFe concentration. In early winter, strengthened northwesterly winds excite southward Ekman transport and Ekman upwelling over the western subarctic gyre, transporting dFe-rich water southward. In mid to late winter, the southward western boundary current of the subarctic gyre and the outflow from the Sea of Okhotsk also bring dFe-rich water to the Oyashio region. The contribution of atmospheric dust to the dFe budget is several times smaller than these ocean transport processes in winter. These results suggest that the westerly wind-induced Ekman transport and gyre circulation systematically influence the seasonal cycle of WSNP surface dFe concentration.
  • Ryu Saiki, Humio Mitsudera
    JOURNAL OF PHYSICAL OCEANOGRAPHY 46 2 583 - 600 2016年02月 [査読有り][通常論文]
     
    Ice bands are frequently observed over marginal ice zones in polar seas. A typical ice-band pattern has a regular spacing of about 10 km and extends over 100 km in the marginal ice zone. Further, the long axis of an ice band lies to the left (right) with respect to the wind direction in the Northern (Southern) Hemisphere. Here, the study shows that the resonance between ice-band pattern propagation and internal inertia-gravity waves below the sea ice well explains the ice-band pattern formation. Internal waves are generated by the difference between the stress on the open water and the stress on ice-covered water. This in turn reinforces the formation of an ice-band pattern with a regular band spacing. Specifically, the authors have found the following: 1) A band spacing on the order of 10 km is selected by the resonance condition in which the ice-band pattern propagation speed coincides with the phase speed of internal inertia-gravity waves. 2) The ice bands tend to develop favorably when the wind direction and the band propagation direction are nearly parallel. The velocity acceleration caused by the periodic differential stress associated with the ice bands, driven by the wind parallel to the band propagation direction, is important. The wind direction may turn to the left (right) slightly in the Northern (Southern) Hemisphere as a result of the Coriolis force acting on ice. Satellite images confirmed that the band spacing of the ice-band pattern in the polar seas is consistent with this theory.
  • 馬目 歩美, 三寺 史夫, Wang Jia, 若土 正暁
    低温科学 74 55 - 65 低温科学第74巻編集委員会 2016年 [査読無し][招待有り]
     
    アムール川流出水の振舞いは,鉄輸送におけるその重要性にもかかわらず未知の部分が多い.本研究ではオホーツク海北部における海氷-海洋結合シミュレーションを行い,「アムール川流出水が高密度陸棚水の経路に物質を輸送しうる」という仮説を検証した.シミュレーションの結果,陸棚域で100 km 以上沖へ流出水を輸送しうる二つの経路を確認した.これらの経路は河川水,高密度陸棚水,西向きの季節風が力学的にバランスすることにより維持される.回転流体系では通常河川水は岸沿いを流れるが,本研究はこうしたユニークな結合効果が流出水を効率的に沖側へと運び,高密度陸棚水の経路へ鉄を含む物質を堆積しうることを示した.The paths of the Amur River discharge on the continental shelf in the Sea of Okhotsk are still unknown despitetheir significance in transporting dissolved and particulate iron. In this study, we conduct a coupled ice-oceansimulation for the northern Sea of Okhotsk from June 1998 to September 2000 to answer the question: Does the AmurRiver discharge deposit materials to the pathway of the dense shelf water? In a series of numerical experiments, weidentified two routes (the western and eastern routes) that could transport the river water more than 100 km offshoreover the northwestern continental shelf. The two routes share the clockwise gyre in the Sakhalin Gulf and thenortheastward flow on the northwestern continental shelf. These features are connected through the westward jetalong the slope from the Sakhalin Gulf (the western route) and the northward transport over the shelf break canyon(the eastern route). The river water, the dense shelf water, and the easterly wind are in a fine geophysical balance forthose features, and all are required for the formation of the two routes. The model results show that these uniquejoint effects in the Sea of Okhotsk allow the Amur River discharge to be effectively transported over the northwesterncontinental shelf, unlike a general river discharge that flows along the coast, and deposit materials into the pathway ofthe dense shelf water.
  • Shinichiro Kida, Humio Mitsudera, Shigeru Aoki, Xinyu Guo, Shin Ichi Ito, Fumiaki Kobashi, Nobumasa Komori, Atsushi Kubokawa, Toru Miyama, Ryosuke Morie, Hisashi Nakamura, Hisashi Nakamura, Tomohiro Nakamura, Hideyuki Nakano, Hajime Nishigaki, Masami Nonaka, Hideharu Sasaki, Yoshi N. Sasaki, Toshio Suga, Shusaku Sugimoto, Bunmei Taguchi, Koutarou Takaya, Tomoki Tozuka, Hiroyuki Tsujino, Norihisa Usui
    Hot Spots in the Climate System: New Developments in the Extratropical Ocean-Atmosphere Interaction Research 1 - 30 2016年01月 [査読有り][通常論文]
     
    © The Oceanographic Society of Japan and Springer Japan 2016. This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.
  • Nakamura H, Isobe A, Minobe S, Mitsudera H, Nonaka M, Suga T
    Hot Spots in the Climate System: New Developments in the Extratropical Ocean-Atmosphere Interaction Research v - xi 2016年 [査読有り][通常論文]
  • Hisashi Nakamura, Atsuhiko Isobe, Shoshiro Minobe, Humio Mitsudera, Masami Nonaka, Toshio Suga
    JOURNAL OF OCEANOGRAPHY 71 5 463 - 467 2015年10月 [査読有り][通常論文]
  • Shinichiro Kida, Humio Mitsudera, Shigeru Aoki, Xinyu Guo, Shin-ichi Ito, Fumiaki Kobashi, Nobumasa Komori, Atsushi Kubokawa, Toru Miyama, Ryosuke Morie, Hisashi Nakamura, Tomohiro Nakamura, Hideyuki Nakano, Hajime Nishigaki, Masami Nonaka, Hideharu Sasaki, Yoshi N. Sasaki, Toshio Suga, Shusaku Sugimoto, Bunmei Taguchi, Koutarou Takaya, Tomoki Tozuka, Hiroyuki Tsujino, Norihisa Usui
    JOURNAL OF OCEANOGRAPHY 71 5 469 - 497 2015年10月 [査読有り][通常論文]
     
    This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air-sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air-sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air-sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.
  • Takuya Nakanowatari, Humio Mitsudera, Tatsuo Motoi, Ichiro Ishikawa, Kay I. Ohshima, Masaaki Wakatsuchi
    JOURNAL OF PHYSICAL OCEANOGRAPHY 45 4 988 - 1008 2015年04月 [査読有り][通常論文]
     
    Using oceanographic observations and an eddy-resolving ice-ocean coupled model simulation from 1955 to 2004, the effects of the wind-driven ocean circulation change that occurred in the late 1970s during multidecadal-scale freshening of the North Pacific Intermediate Water (NPIW) at salinity minimum density (similar to 26.8 sigma(theta)) were investigated. An analysis of the observations revealed that salinity decreased significantly at the density range of 26.6-26.8 sigma(theta) in the western subtropical gyre, including the mixed water region (MWR). The temporal variability of the salinity is dominated by the marked change in the late 1970s. With results similar to the observations, the model, selectively forced by the interannual variability of the wind-driven ocean circulation, simulated significant freshening of the intermediate layer over the subtropical gyre. The significant freshening is related to the increase in southward transport of the Oyashio associated with the intensification of the Aleutian low. Accompanying these changes, the intrusion of fresh and low potential vorticity water, originating in the Okhotsk Sea, to the MWR increased, and the freshening signal propagated farther southward in the western subtropical gyre during the subsequent 6 yr, crossing the Kuroshio Extension. These results indicate that the multidecadal-scale freshening of the NPIW is partly caused by intensification of the wind-driven cross-gyre transport of the subarctic water to the subtropical gyre.
  • Junji Matsuda, Humio Mitsudera, Tomohiro Nakamura, Yuichiro Sasajima, Hiroyasu Hasumi, Masaaki Wakatsuchi
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 120 3 1462 - 1489 2015年03月 [査読有り][通常論文]
     
    Dense Shelf Water (DSW) formation in the northwestern continental shelf of the Sea of Okhotsk is the beginning of the lower limb of the overturning circulation that ventilates the intermediate layer of the North Pacific Ocean. The upper limb consisting of surface currents in the Okhotsk Sea and the subarctic gyre has not been clarified. Using a high-resolution North Pacific Ocean model with a curvilinear grid as fine as 3 km x 3 km in the Sea of Okhotsk, we succeeded in representing the three-dimensional structure of the overturning circulation including the narrow boundary currents and flows through straits that constitute the upper limb, as well as the lower limb consisting of DSW formation and ventilation. In particular, pathways and time scales from the Bering Sea to the intermediate layer via the ventilation in the Sea of Okhotsk were examined in detail using tracer experiments. Further, we found that the overturning circulation that connects the surface and intermediate layer is sensitive to wind stress. In the case of strong winds, the coastal current under polynyas where DSW forms is intensified, and consequently diapycnal transport from the surface layer to the intermediate layer increases. Strong winds also induce a positive sea surface salinity anomaly in the subarctic region, causing a significant decrease in the density stratification and increase in the DSW salinity (i.e., density). These processes act together to produce intense overturning circulation and deep ventilation, which may subduct even to the bottom of the Sea of Okhotsk if the wind is strong.
  • Takuya Nakanowatari, Tomohiro Nakamura, Keisuke Uchimoto, Hiroki Uehara, Humio Mitsudera, Kay I. Ohshima, Hiroyasu Hasumi, Masaaki Wakatsuchi
    JOURNAL OF CLIMATE 28 2 714 - 736 2015年01月 [査読有り][通常論文]
     
    Causes of the multidecadal-scale warming of the intermediate water in the Okhotsk Sea and the western subarctic North Pacific during 1980-2008 are investigated using an ice-ocean coupled model with interannually varying atmospheric forcing. A hindcast experiment qualitatively reproduces the warming and decadal fluctuations of the intermediate water that are similar to those of observations: the warming is significant along the western part of the Okhotsk Sea and subarctic frontal region. The effects of the thermohaline- and wind-driven ocean circulation on the warming are evaluated from perturbation experiments on thermohaline (turbulent heat and freshwater fluxes) and wind causes, respectively. The thermohaline causes are shown to contribute positively to warming in the Okhotsk Sea Intermediate Water (OSIW). The heat budget analysis for the OSIW indicates that the warming is related to a decrease in cold and dense shelf water (DSW) flux, which is caused by a decrease in sea ice and surface water freshening. In contrast, the wind cause has a cooling effect in the OSIW through an increase in DSW. In the subarctic frontal region, the warming is mainly caused by the wind stress change. The heat budget analysis indicates that the warming is related to an increase in the northward advection of the subtropical warm water. These results imply that both thermohaline- and wind-driven ocean circulation changes are essential components of the warming in the intermediate water. The atmospheric conditions responsible for the warming are related to a weakened Aleutian low and Siberian high in early and late winter.
  • Ayumi Fujisaki, Humio Mitsudera, Jia Wang, Masaaki Wakatsuchi
    PROGRESS IN OCEANOGRAPHY 126 8 - 20 2014年08月 [査読有り][通常論文]
     
    The paths of the Amur River discharge on the continental shelf in the Sea of Okhotsk are still unknown despite their significance in transporting dissolved and particulate iron. In this study, we conduct a coupled ice-ocean simulation for the northern Sea of Okhotsk from June 1998 to September 2000 to answer the question: Does the Amur River discharge deposit materials to the pathway of the dense shelf water? In a series of numerical experiments, we identified two routes (the western and eastern routes) that could transport the river water more than 100 km offshore over the northwestern continental shelf. The two routes share the clockwise gyre in the Sakhalin Gulf and the northeastward flow on the northwestern continental shelf. These features are connected through the westward jet along the slope from the Sakhalin Gulf (the western route) and the northward transport over the shelf break canyon (the eastern route). The river water, the dense shelf water, and the easterly wind are in a fine geophysical balance for those features, and all are required for the formation of the two routes. The model results show that these unique joint effects in the Sea of Okhotsk allow the Amur River discharge to be effectively transported over the northwestern continental shelf, unlike a general river discharge that flows along the coast, and deposit materials into the pathway of the dense shelf water. (C) 2014 Elsevier Ltd. All rights reserved.
  • Jun Nishioka, Humio Mitsudera, Ichiro Yasuda, Hongbin Liu, Takeshi Nakatsuka, Yuri N. Volkov
    PROGRESS IN OCEANOGRAPHY 126 1 - 7 2014年08月 [査読有り][通常論文]
  • Tomohiro Nakamura, Yuuki Takeuchi, Keisuke Uchimoto, Humio Mitsudera, Masaaki Wakatsuchi
    PROGRESS IN OCEANOGRAPHY 126 135 - 145 2014年08月 [査読有り][通常論文]
     
    Tidally induced vertical mixing is important for thermohaline circulation. Previous estimations of tidal mixing have aimed to obtain time-averaged values, and ocean general circulation models (OGCMs) typically parameterize such mixing using a temporally constant strength. However, tidal mixing is known to vary temporally during tidal or spring-neap cycles. Here, we investigate the effects of temporal change in tidally induced vertical diffusivity (K-t) in the Kuril Straits using an OGCM. The results demonstrate that variations of vertical mixing on diurnal, 2-week, and 1/2-year timescales induce significant differences in the net effect of mixing and, therefore, in the thermohaline circulation originating in the Okhotsk Sea. For diurnal and 2-week variations, the strength of the tidal mixing effect depends on (1) the period and length of the duration over which K-t is larger than the temporal average and (2) the amplitude of the temporal variation of Kt, even if the time-averaged values are the same. This is explained by the relative importance of two states. In a quasi-equilibrium state, a larger Kt results in weaker stratification and vice versa, and thus the net tidal mixing effect is weaker when K-t, is variable than when it remains constant. Conversely, in an adjustment stage just after an increase in K-t, a larger Kt acts on stronger stratification and vice versa, resulting in a stronger mixing effect. For a 1/2-year variation, the strength of the tidal mixing effect also depends on the phase relationship with seasonal variation in stratification. These results imply the necessity of considering temporal change when estimating tidal mixing from observations, specifying it in OGCMs, and understanding its effects. (C) 2014 Elsevier Ltd. All rights reserved.
  • Hiroki Uehara, Andrey A. Kruts, Humio Mitsudera, Tomohiro Nakamura, Yuri N. Volkov, Masaaki Wakatsuchi
    PROGRESS IN OCEANOGRAPHY 126 80 - 97 2014年08月 [査読有り][通常論文]
     
    The dense shelf water (DSW) produced in the Sea of Okhotsk causes the deepest ventilation in the North Pacific and plays a key role in overturning to the depth of intermediate layers. Salinity determines the density of DSW in cold seas such as this. However, the variability of DSW salinity has yet to be quantified and its causes remain unknown because of the paucity of available observation data. Here, we describe the variability of DSW salinity in the period from 1950 to 2005 through the analysis of a newly compiled hydrographic dataset, expanded with Russian measurements in the Sea of Okhotsk and the Bering Sea. DSW salinity exhibits a decreasing trend of -0.0024 +/- 0.0015 y(-1) over 1950-2005, in addition to decadal-scale variability with a typical magnitude of similar to 0.1. We have found that DSW variability is controlled largely by surface salinity anomalies that propagate along pathways associated with ocean currents from the Bering Sea to the Sea of Okhotsk. These salt pathways can be traced farther upstream to the Alaskan Stream and western subarctic gyre. We suggest that this is a possible pathway which consists of a previously undefined upper branch of the North Pacific overturning cell. Along this pathway, the effects of the nodal tide that has an 18.6-year period may also propagate from the Kuril Straits and the Aleutian Passes. Our results indicate that long-term atmospheric variations and enhanced hydrological cycles over the North Pacific, which are associated with global climate changes, are conducted to the intermediate layer through surface salt pathways and subsequent DSW ventilation and lead to variability in material circulation and biogeochemical cycles. (C) 2014 Elsevier Ltd. All rights reserved.
  • Keisuke Uchimoto, Tomohiro Nakamura, Jun Nishioka, Humio Mitsudera, Kazuhiro Misumi, Daisuke Tsumune, Masaaki Wakatsuchi
    PROGRESS IN OCEANOGRAPHY 126 194 - 210 2014年08月 [査読有り][通常論文]
     
    An ocean general circulation model coupled with a simple biogeochemical model was developed to simulate iron circulation in and around the Sea of Okhotsk. The model has two external sources of iron: dust iron at the sea surface and sedimentary iron at the seabed shallower than 300 m. The model represented characteristic features reasonably well, such as high iron concentration in the dense shelf water (DSW) and its mixing, which extends southward in the intermediate layer from the northwestern shelf along Sakhalin Island and finally flows into the Pacific. Sensitivity experiments for the solubility of dust iron in seawater suggest that a solubility of 1% is appropriate in our simulation. Higher solubilities (5% and 10%) result in too low phosphate in the northwestern North Pacific in summer as well as too high iron concentrations at the sea surface, compared with observations. Besides, these experiments show that dust iron hardly contributes to the high iron concentration in the intermediate layer. To investigate locations from which the iron in the intermediate layer originates, the fate of sedimentary iron input from four regions in the Okhotsk Sea was examined. Results suggest that the western and central parts of the northern shelf are important. (C) 2014 Elsevier Ltd. All rights reserved.
  • Yagnesh, R. Y, T.Nakamura, H. Mitsudera, M. Kawashima
    HydrologicalResearchLetter 8 2 84 - 89 2014年07月 [査読有り][通常論文]
  • Hiroshi Kuroda, Daisuke Takahashi, Humio Mitsudera, Tomonori Azumaya, Takashi Setou
    FISHERIES SCIENCE 80 2 127 - 138 2014年03月 [査読有り][通常論文]
     
    We developed a realistic 1/50A degrees high-resolution ocean model capable of resolving submesoscale variability, and performed particle-tracking experiments based on this ocean model to identify elements that significantly affect the transport of the eggs and larvae of the Japanese Pacific walleye pollock Theragra chalcogramma into Funka Bay. The high-resolution model reproduced representative features of the oceanographic conditions of the main spawning area and season. A comparison of particle-tracking experiments performed under the passive transport condition based on high-resolution (1/50A degrees) and low-resolution (1/10A degrees) ocean models showed that high-resolution modeling is essential in order to realistically simulate the transport process. In this regard, however, the vertical motion of particles cannot be explained by the passive transport condition, as it leads to unrealistically deep sinking of particles in the simulation. Turning our attention to feasible non-passive transport conditions, we then incorporated the buoyancy motion of particles and conducted additional experiments that mainly differed in the particle density adopted. We clarified that buoyancy is an important factor in the retention of particles near the sea surface, and that the ratio of the particles that remain in Funka Bay to the number of particles released is sensitive to the vertical motions/positions of the particles, implying that it is necessary to model this vertical motion more accurately by incorporating more realistic biological processes or a statistical distribution into the particle-tracking model.
  • Jun Ono, Kay I. Ohshima, Keisuke Uchimoto, Naoto Ebuchi, Humio Mitsudera, Hajime Yamaguchi
    JOURNAL OF OCEANOGRAPHY 69 4 413 - 428 2013年08月 [査読有り][通常論文]
     
    To conduct the simulation of oil spills in the Sea of Okhotsk, we developed a three-dimensional, high-resolution ocean circulation model. The model particularly improved the reproducibility of velocity field during the strong stratification period. Particle-tracking experiments with the effects of evaporation and biodegradation were performed using the combined data of daily ocean currents from the present model and the hourly diurnal tidal currents from the tidal model. The results are shown by the relative concentration of the particles averaged over the 8 years of 1998-2005 based on the ensemble forecast idea. For the case of particles released from the Sakhalin II oil field, the particles deployed in September-January are carried southward by the East Sakhalin Current, finally arriving at the Hokkaido coast, after 60-90 days. The particles deployed in March-August are diffused offshore by the synoptic wind drift, and hardly transported to regions south of Sakhalin. For the case of particles released from the region off Prigorodnoye, the oil export terminal, after the diffusion by the synoptic wind drift, a part of them are carried offshore of Hokkaido by the Soya Warm Current. The particles released in November-April flow out to the Japan Sea through the Soya Strait, mainly by the synoptic wind drift and secondly by the diffusion due to strong tidal currents around the Soya Strait. By considering the effects of evaporation and biodegradation, the relative concentration of the particles is considerably decreased before arriving at the Hokkaido coast, particularly in the case of drift from the Sakhalin II oil field.
  • Hiroki Uehara, Andrey A. Kruts, Yuriy N. Volkov, Tomohiro Nakamura, Tsuneo Ono, Humio Mitsudera
    JOURNAL OF OCEANOGRAPHY 68 6 869 - 886 2012年12月 [査読有り][通常論文]
     
    This paper introduces a new hydrographic climatology of the Okhotsk Sea; this climatology was constructed from the Far Eastern Regional Hydrometeorological Research Institute (FERHRI) database. The FERHRI database has a volume of data three to five times larger than the data used in previous studies because unpublished Russian observation data have been included in the FERHRI database. After removing erroneous data from the database by pertinent quality control methods, the climatology for 1/4A degrees A xA 1/4A degrees grids is produced by applying objective analysis procedures. Features similar to those in previous studies are seen in the intermediate layers in the Okhotsk Sea, whereas our climatology provides values that fill in gaps in previous climatologies. It is obvious from the monthly climatologies that temperature and salinity distributions evolve in accordance with seasonal variations in the Eastern Sakhalin Current and inflow from the North Pacific. We also reconstructed climatologies for the winter mixed layer and dense shelf water from data obtained from the temperature minimum waters identified as the remnants of these two layers. Free access to the 1A degrees A xA 1A degrees versions of all climatologies constructed in this study is available through the website.
  • T. Nakamura, J. P. Matthews, T. Awaji, H. Mitsudera
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 117 2012年12月 [査読有り][通常論文]
     
    High spatial resolution thermal infrared (TIR) images derived by the LANDSAT Thematic Mapper (TM) sensors show the presence of numerous small-scale eddies near the Kuril Islands. As the diameters of these eddies range from around 2 to 30 km (i.e., submesoscale), they are much smaller than the eddies previously reported in this region (several tens to some hundreds of kilometers in diameter). Our simulations suggest that small-scale eddies similar to those observed in the satellite data are generated by diurnal barotropic tides. The eddy generation is well defined from Etorofu/Friza to the Onnekotan Straits, and it is caused by the effects of coastal boundaries and the stretching of water columns, which lead to eddy growth even after eddies have left the coast. We find that the counterclockwise eddies are generally larger in number and size and stronger in vorticity and surface height than the clockwise eddies in both the gradient and cyclostrophic wind balance regimes. The possible causes of such asymmetry are (1) the effect of planetary-vorticity tube stretching, which can be significant even when the final relative vorticity becomes much greater than planetary vorticity and (2) asymmetric advection by the rotating tidal flow, which advects counterclockwise eddies offshore but clockwise eddies onshore. These eddies induce strong stirring with a maximum apparent diffusivity of 10(8) cm(2) s(-1). The numbers, properties, and mixing effects of eddies vary greatly with the spring-neap cycle. Citation: Nakamura, T., J. P. Matthews, T. Awaji, and H. Mitsudera (2012), Submesoscale eddies near the Kuril Straits: Asymmetric generation of clockwise and counterclockwise eddies by barotropic tidal flow, J. Geophys. Res., 117, C12014, doi:10.1029/2011JC007754.
  • 中村 知裕, 古関 俊也, 三寺 史夫
    沿岸海洋研究 = Bulletin on coastal oceanography 50 1 71 - 77 日本海洋学会沿岸海洋研究部会 2012年08月31日 [査読無し][通常論文]
  • Shunya Koseki, Tomohiro Nakamura, Humio Mitsudera, Yuqing Wang
    JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 117 2012年03月 [査読有り][通常論文]
     
    In summer the Okhotsk Sea is often covered by low-level clouds, which occasionally co-occur with the Okhotsk high. We investigate the formation of low-level clouds and their effects on the Okhotsk high in July using reanalysis, satellite data, and a regional climate model. Statistical analysis suggests that the amount of low-level clouds over the Okhotsk Sea has a positive relationship with the strength of the Okhotsk high; however, the formation processes of the Okhotsk high and low-level clouds are not dependent on each other. A simulation focusing on July 2003, when the Okhotsk high was the strongest in the past decade, showed low-level cloud formation and resulting strong cooling over most of the Okhotsk Sea, which can be attributed to longwave radiation. Sensitivity experiments with reduced cloud amounts reveal that this radiative flux results in the cooling of the cloud top boundary layer (CBL), thereby reinforcing the Okhotsk high within the CBL. Trajectory analyses show that unsaturated air reaches saturation mainly because of the downward sensible heat flux. After cloud formation, radiative cooling causes an upward sensible heat flux below the clouds. Such cooling and heating roughly balance with the cooling due to evaporation of drizzle and cloud water and the heating due to condensation. Eventually, the CBL achieves a low-temperature steady state over the Okhotsk Sea. Although the latent heat flux is positive over the Okhotsk Sea irrespective of the presence or absence of low-level clouds, associated moisture flux is insignificant for achieving saturation. This positive latent heat flux is enhanced under cloudy conditions and compensates for the loss of water vapor due to condensation.
  • Shang-Ping Xie, Atsushi Kubokawa, Fumiaki Kobashi, Humio Mitsudera
    JOURNAL OF OCEANOGRAPHY 68 1 1 - 3 2012年02月 [査読有り][通常論文]
  • Hajime Nishigaki, Humio Mitsudera
    JOURNAL OF PHYSICAL OCEANOGRAPHY 42 2 306 - 320 2012年02月 [査読有り][通常論文]
     
    The dynamics of subtropical western boundary currents over slopes detaching from coasts with inshore pool regions, where the water of the subtropical gyre does not enter and the velocity is small, are investigated. This study is intended to understand the dynamics of the nearshore path of the Kuroshio, which has a distinct boundary between the boundary current and the coastal water. Numerical experiments under idealized conditions are made. The results show flow patterns with pool regions similar to the Kuroshio under simple conditions. A deep countercurrent is present on the lower bottom slope, which represents observed deep currents. This is part of a deep cyclonic recirculation north of the jet, which extends to the lower bottom slope despite steep topography. This extension can be explained by the geostrophic contours. In this region, the upper boundary current feels the bottom slope and the westward intensification is blocked. In the other region, where the bottom-layer velocity is very small, the upper boundary current is free from the bottom slope and westward intensification occurs at the coast. The sensitivity to the volume transport of the boundary current is investigated by case studies. The pool regions are broken in cases with large volume transports. It is indicated that these unsteady inshore regions are produced by instability caused by an outcrop of the upper isopycnal, which is led by a large baroclinic volume transport.
  • T. Nakamura, J. P. Matthews, T. Awaji, H. Mitsudera
    Journal of Geophysical Research: Oceans 117 12 2012年 [査読有り][通常論文]
     
    High spatial resolution thermal infrared (TIR) images derived by the LANDSAT Thematic Mapper (TM) sensors show the presence of numerous small-scale eddies near the Kuril Islands. As the diameters of these eddies range from around 2 to 30 km (i.e., submesoscale), they are much smaller than the eddies previously reported in this region (several tens to some hundreds of kilometers in diameter). Our simulations suggest that small-scale eddies similar to those observed in the satellite data are generated by diurnal barotropic tides. The eddy generation is well defined from Etorofu/Friza to the Onnekotan Straits, and it is caused by the effects of coastal boundaries and the stretching of water columns, which lead to eddy growth even after eddies have left the coast. We find that the counterclockwise eddies are generally larger in number and size and stronger in vorticity and surface height than the clockwise eddies in both the gradient and cyclostrophic wind balance regimes. The possible causes of such asymmetry are (1) the effect of planetary-vorticity tube stretching, which can be significant even when the final relative vorticity becomes much greater than planetary vorticity and (2) asymmetric advection by the rotating tidal flow, which advects counterclockwise eddies offshore but clockwise eddies onshore. These eddies induce strong stirring with a maximum apparent diffusivity of 108 cm2 s-1. The numbers, properties, and mixing effects of eddies vary greatly with the spring-neap cycle. © 2012. American Geophysical Union. All Rights Reserved.
  • Shunya Koseki, Tomohiro Nakamura, Humio Mitsudera, Yuqing Wang
    Journal of Geophysical Research Atmospheres 117 5 2012年 [査読有り][通常論文]
     
    In summer the Okhotsk Sea is often covered by low-level clouds, which occasionally co-occur with the Okhotsk high. We investigate the formation of low-level clouds and their effects on the Okhotsk high in July using reanalysis, satellite data, and a regional climate model. Statistical analysis suggests that the amount of low-level clouds over the Okhotsk Sea has a positive relationship with the strength of the Okhotsk high however, the formation processes of the Okhotsk high and low-level clouds are not dependent on each other. A simulation focusing on July 2003, when the Okhotsk high was the strongest in the past decade, showed low-level cloud formation and resulting strong cooling over most of the Okhotsk Sea, which can be attributed to longwave radiation. Sensitivity experiments with reduced cloud amounts reveal that this radiative flux results in the cooling of the cloud top boundary layer (CBL), thereby reinforcing the Okhotsk high within the CBL. Trajectory analyses show that unsaturated air reaches saturation mainly because of the downward sensible heat flux. After cloud formation, radiative cooling causes an upward sensible heat flux below the clouds. Such cooling and heating roughly balance with the cooling due to evaporation of drizzle and cloud water and the heating due to condensation. Eventually, the CBL achieves a low-temperature steady state over the Okhotsk Sea. Although the latent heat flux is positive over the Okhotsk Sea irrespective of the presence or absence of low-level clouds, associated moisture flux is insignificant for achieving saturation. This positive latent heat flux is enhanced under cloudy conditions and compensates for the loss of water vapor due to condensation. Copyright 2012 by the American Geophysical Union.
  • Humio Mitsudera, Keisuke Uchimoto, Tomohiro Nakamura
    JOURNAL OF PHYSICAL OCEANOGRAPHY 41 11 2120 - 2136 2011年11月 [査読有り][通常論文]
     
    The Soya "Warm Current" (SWC) flows through a shallow strait between the Japan Sea and the Sea of Okhotsk. The SWC has a jet structure downstream of the strait along the northern coast of Hokkaido with a maximum speed exceeding 1 m s(-1) at its axis in summer and fall. A surface cold belt with a subsurface doming structure forms offshore of the SWC axis. Mechanisms of the cold belt formation are discussed from a point of view of resonant interaction between a barotropic stratified flow and a shallow sill and subsequent baroclinic adjustment along the SWC. When a stratified current rides a slope upstream, the thermocline displaces upward greatly and outcrops owing to resonant generation of internal Kelvin waves if the upper layer is thinner than the lower layer. The control section, where a Froude number is unity, occurs "upstream" from the sill crest when the ambient inflow has a barotropic flow component. These upwelling features closely resemble those along the southwestern coast of Sakhalin Island. The SWC then flips from an upwelling-type to a downwelling-type structure; in doing so, it transits from the west coast of Sakhalin to the east coast of Hokkaido. It is this transition that leads to the offshore doming structure, which propagates downstream as a vorticity wave, manifesting the cold belt at the surface.
  • K. Misumi, D. Tsumune, Y. Yoshida, K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, F. O. Bryan, K. Lindsay, J. K. Moore, S. C. Doney
    JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES 116 2011年07月 [査読有り][通常論文]
     
    Mechanisms controlling the dissolved iron distribution in the North Pacific are investigated using the Biogeochemical Elemental Cycling (BEC) model with a resolution of approximately 1 in latitude and longitude and 60 vertical levels. The model is able to reproduce the general distribution of iron as revealed in available field data: surface concentrations are generally below 0.2 nM; concentrations increase with depth; and values in the lower pycnocline are especially high in the northwestern Pacific and off the coast of California. Sensitivity experiments changing scavenging regimes and external iron sources indicate that lateral transport of sedimentary iron from continental margins into the open ocean causes the high concentrations in these regions. This offshore penetration only appears under a scavenging regime where iron has a relatively long residence time at high concentrations, namely, the order of years. Sedimentary iron is intensively supplied around continental margins, resulting in locally high concentrations; the residence time with respect to scavenging determines the horizontal scale of elevated iron concentrations. Budget analysis for iron reveals the processes by which sedimentary iron is transported to the open ocean. Horizontal mixing transports sedimentary iron from the boundary into alongshore currents, which then carry high iron concentrations into the open ocean in regions where the alongshore currents separate from the coast, most prominently in the northwestern Pacific and off of California.
  • Yusuke Kawaguchi, Takeshi Tamura, Shigeto Nishino, Takashi Kikuchi, Motoyo Itoh, Humio Mitsudera
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2011年07月 [査読有り][通常論文]
     
    Winter water formation is examined in the Chukchi Sea for the winters of 1992-2006 using a primitive equation ocean model forced by NCEP wind and surface salinity flux derived from SSM/I thin ice thickness estimates. The model is also forced by an external inflow of 0.8 Sv through the Bering Strait. The model successfully reproduces the oceanic circulation on the Chukchi shelf, thus providing numerous insights into behaviors of salt-enriched water produced on the shelf. The experiments show that under northeasterly winds, northward throughflow across Barrow Canyon is reduced. This results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the Arctic Basin. The flow and salinity enhancement of the flow through Herald Canyon is less extreme but more steady than through Barrow Canyon. Together with moored salinity in the Bering Strait, the model results estimate the actual salinity to be 32.9 +/- 0.8 psu and 32.7 +/- 0.3 psu, respectively, for waters moving through the Barrow and Herald Canyons. Both estimates are less than 33.1 psu that is typically observed for the cold halostad layer in the Canada Basin, suggesting the importance of diapycnal mixing with saltier Atlantic origin water.
  • Ayumi Fujisaki, Humio Mitsudera, Hajime Yamaguchi
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2011年03月 [査読有り][通常論文]
     
    Formation process of the dense shelf water (DSW) in the Sea of Okhotsk was investigated with an ice-ocean coupled model. The hindcast through 1998-2000 modeled the anomalous ice productions being controlled by air temperature and mean ice speed over the coastal area. Ice production was larger by 30% in 1998-1999, while less developed ice cover in 1999-2000 allowed larger heat loss from the ocean. The compensating heat loss sustained the similar production of the DSW for > 26.75 sigma(theta) in 1998-1999. However, ice production thickened up the density constitution of the DSW, which was significantly denser in 1998-1999. An experiment without brine rejection suggested such modification of the density constitution plays a rather more important role in brine rejection for the DSW property than an increase of the volumetric production. The signal of brine rejection reached the 27 sigma(theta) layer farther south in the Kuril Basin. The model also showed that when winter outflow of the DSW from the continental shelf was neglected, as is the case in observational estimations, the annual production was underestimated by 20% compared with actual productions in 1998-2000. Ice production was increased as the air-ice drag coefficient C(Dai) increased and as the ice-water drag coefficient C(Diw) decreased because of the intensified polynya activity. In contrast, the density constitution of the DSW was lightened with the increased C(Dai), as a linear balance of dominantly intensified advection and slight increase of ice production. Consequently, the DSW property seemed insensitive to C(Dai) and C(Diw) compared with the anomalous air conditions year by year.
  • K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, M. Yamamoto-Kawai, K. Misumi, D. Tsumune
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2011年02月 [査読有り][通常論文]
     
    Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200-400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer.
  • Yusuke Kawaguchi, Takeshi Tamura, Shigeto Nishino, Takashi Kikuchi, Motoyo Itoh, Humio Mitsudera
    Journal of Geophysical Research: Oceans 116 7 2011年 [査読有り][通常論文]
     
    Winter water formation is examined in the Chukchi Sea for the winters of 1992-2006 using a primitive equation ocean model forced by NCEP wind and surface salinity flux derived from SSM/I thin ice thickness estimates. The model is also forced by an external inflow of 0.8 Sv through the Bering Strait. The model successfully reproduces the oceanic circulation on the Chukchi shelf, thus providing numerous insights into behaviors of salt-enriched water produced on the shelf. The experiments show that under northeasterly winds, northward throughflow across Barrow Canyon is reduced. This results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the Arctic Basin. The flow and salinity enhancement of the flow through Herald Canyon is less extreme but more steady than through Barrow Canyon. Together with moored salinity in the Bering Strait, the model results estimate the actual salinity to be 32.9 ± 0.8 psu and 32.7 ± 0.3 psu, respectively, for waters moving through the Barrow and Herald Canyons. Both estimates are less than 33.1 psu that is typically observed for the cold halostad layer in the Canada Basin, suggesting the importance of diapycnal mixing with saltier Atlantic origin water. Copyright 2011 by the American Geophysical Union.
  • Ayumi Fujisaki, Humio Mitsudera, Hajime Yamaguchi
    Journal of Geophysical Research: Oceans 116 3 2011年 [査読有り][通常論文]
     
    Formation process of the dense shelf water (DSW) in the Sea of Okhotsk was investigated with an ice-ocean coupled model. The hindcast through 1998-2000 modeled the anomalous ice productions being controlled by air temperature and mean ice speed over the coastal area. Ice production was larger by 30% in 1998-1999, while less developed ice cover in 1999-2000 allowed larger heat loss from the ocean. The compensating heat loss sustained the similar production of the DSW for > 26.75σθ in 1998-1999. However, ice production thickened up the density constitution of the DSW, which was significantly denser in 1998-1999. An experiment without brine rejection suggested such modification of the density constitution plays a rather more important role in brine rejection for the DSW property than an increase of the volumetric production. The signal of brine rejection reached the 27 σθ layer farther south in the Kuril Basin. The model also showed that when winter outflow of the DSW from the continental shelf was neglected, as is the case in observational estimations, the annual production was underestimated by 20% compared with actual productions in 1998-2000. Ice production was increased as the air-ice drag coefficient CDai increased and as the ice-water drag coefficient CDiw decreased because of the intensified polynya activity. In contrast, the density constitution of the DSW was lightened with the increased CDai, as a linear balance of dominantly intensified advection and slight increase of ice production. Consequently, the DSW property seemed insensitive to CDai and CDiw compared with the anomalous air conditions year by year. Copyright 2011 by the American Geophysical Union.
  • K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, M. Yamamoto-Kawai, K. Misumi, D. Tsumune
    Journal of Geophysical Research: Oceans 116 2 2011年 [査読有り][通常論文]
     
    Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200-400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer. Copyright 2011 by the American Geophysical Union.
  • Uchimoto, K, T. Nakamura, J. Nishioka, H. Mitsudera, M. Yamamoto-Kawai, K. Misumi, D. Tsumune
    Journal of Geophysical Research 116 C2 C02034  2011年 [査読有り][通常論文]
  • Misumi, D, D. Tsumune, Y. Yoshida, K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, F. O. Bryan, K. Lindsay, J. K. Moore, S. C. Doney
    Journal of Geophysical Research 116 G3 G03005  2011年 [査読有り][通常論文]
  • 宗谷暖流の物理:冷水帯形成のメカニズム
    三寺史夫, 内本圭亮, 中村知裕
    沿岸海洋研究 49 10 3 - 12 2011年 [査読有り][招待有り]
  • Uchimoto, K, T. Nakamura, H. Mitsudera
    Hydrological Res. Lett. 5 1 - 5 THE JAPAN SOCIETY OF HYDROLOGY AND WATER RESOURCES 2011年01月 [査読有り][通常論文]
     
    A lot of sedimentary particles are known to be mixed into dense shelf water (DSW) produced in the northern part of the Sea of Okhotsk as a result of brine rejection during sea ice formation. To investigate the behavior and distribution of materials in DSW, tracer experiments with an ice-ocean coupled model have been conducted. It was shown that a tracer injected in winter over the northwestern shelf flows on the shelf until early summer with the concentration higher near the bottom. Then, it goes down along the slope to the intermediate layer along the east coast of Sakhalin; the core of tracer is settled around a depth of 400 m. These features are consistent with observations. The experiment in which tidal mixing is absent along the Kuril Islands shows a shallower core of the tracer at a depth of about 200 m owing to the density decrease of DSW.
  • Sachiko Oguma, Tsuneo Ono, Yutaka W. Watanabe, Hiromi Kasai, Shuichi Watanabe, Daiki Nomura, Humio Mitsudera
    ESTUARINE COASTAL AND SHELF SCIENCE 91 1 24 - 32 2011年01月 [査読有り][通常論文]
     
    In this study, we examined the relationship between the low salinity water in the shelf region of the southern Okhotsk Sea which was seasonally sampled (0-200 m), and fluxes of low salinity water from Aniva Bay. To express the source of freshwater mixing in the surface layer, we applied normalized total alkalinity (NTA) and stable isotopes of seawater as chemical tracers. NTA-S diagrams indicate that NTA of low salinity water in the upper 30 m layer just off the Soya Warm Current is clearly higher than in the far offshore region in summer and autumn. Using NTA-S regression lines, we could deduce that the low salinity and high NTA water in the upper layer originates from Aniva Bay. For convenience, we defined this water as the Aniva Surface Water (ASW) with values S < 32, NTA > 2450 umol kg(-1). Formation and transport processes of ASW are discussed using historical data. The interaction between the maximum core of high NTA water on the bottom slope of eastern Aniva Bay and an anticyclonic eddy at the mouth of Aniva Bay are concluded to control ASW formation. Upwelling of the Cold Water Belt water at the tip of Cape Krillion is considered to cause ASW outflow from Aniva Bay. (C) 2010 Elsevier Ltd. All rights reserved.
  • Yusuke Kawaguchi, Sohey Nihashi, Humio Mitsudera, Kay I. Ohshima
    JOURNAL OF PHYSICAL OCEANOGRAPHY 40 11 2451 - 2465 2010年11月 [査読有り][通常論文]
     
    This paper investigates the formation mechanism of broad coastal polynyas beyond 100 km in offshore width. It is known that two regimes for wind-driven polynya opening exist: one is a convergent regime at the polynya edge in which inner frazil ice catches up with outer consolidated ice, whereas the other is a divergent regime in which the consolidated ice drifts offshore faster than the frazil ice at the edge. In this study, the authors focus on the latter, divergent polynya-edge regime. Because in the divergent regime the polynya possibly evolves without bound, they consider a thermal growth for inner frazil ice to find a finite solution of offshore width. Then, the authors investigate responses of the polynya opening for various wind angles phi from the offshore direction from the viewpoint of the polynya-edge regimes. At first, the authors estimate the deviation angle and wind factor for the frazil and consolidated ice based on each momentum balance, because sea ice motion driven by wind varies depending on the ice thickness due to relative effect of the Coriolis force. It was found that, when the surface wind deviates leftward about 10 degrees or greater from offshore, the divergent regime at the polynya edge generates a great polynya evolution. Otherwise, the convergent regime takes place yielding small offshore extent. These theoretical results were verified for the Okhotsk northwestern polynya using thin-ice-thickness data derived from Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) and the Japanese 25-year Reanalysis (JRA-25) surface wind. The data show that the polynya development distinctively changes at phi similar to 0 degrees, and the largest width is shown in the leftward wind angle of phi = 0 degrees-30 degrees. This clear relationship between the surface wind angle and the offshore extent of polynya nicely supports the theory.
  • Shigeru Aoki, Yoshikazu Sasai, Hideharu Sasaki, Humio Mitsudera, Guy D. Williams
    OCEAN DYNAMICS 60 3 743 - 757 2010年06月 [査読有り][通常論文]
     
    Flow structure in the Australian-Antarctic basin is investigated using an eddy-resolving general ocean circulation model and validated with iceberg and middepth float trajectories. A cyclonic circulation system between the Antarctic Circumpolar Current and Antarctic Slope Current consists of a large-scale gyre in the west (80-110A degrees A E) and a series of eddies in the east (120-150A degrees A E). The western gyre has an annual mean westward transport of 22 Sv in the southern limb. Extending west through the Princess Elizabeth Trough, 5 Sv of the gyre recirculates off Prydz Bay and joins the western boundary current off the Kerguelen Plateau. Iceberg trajectories from QuickScat and ERS-1/2 support this recirculation and the overall structure of the Antarctic Slope Current against isobath in the model. Argo float trajectories also reveal a consistent structure of the deep westward slope current. This study indicates the presence of a large cyclonic circulation in this basin, which is comparable to the Weddell and Ross gyres.
  • Ayumi Fujisaki, Hajime Yamaguchi, Humio Mitsudera
    OCEAN DYNAMICS 60 2 377 - 394 2010年04月 [査読有り][通常論文]
     
    Sensitivity study of the air-ice drag coefficient C (Dai) is presented with an ice-ocean coupled model for the Sea of Okhotsk. The C (Dai) x 10(3) value is varied from 2 to 5 based on the direct measurements in the region. The maximum volume transport of the East Sakhalin Current and the mean sea ice velocity were intensified as C (Dai) increased. The sensitivity experiment with the ice-water drag coefficient C (Diw) showed that the East Sakhalin Current volume transport is hardly affected by C (Diw) but significantly intensified by C (Dai). While the ice drift in the off-ice-edge direction was intensified by the increase in C (Dai) and the decrease in C (Diw), the ice edge location was nearly unchanged. This was due to melting caused by the relatively warm water inflow from the North Pacific. That is, sea ice extent in the region is strongly influenced by melting caused by a large ice-ocean heat transfer. In the active melting regions, the ice-ocean heat transfer of more than 100 W/m(2) occurred even in mid-winter. This is the same order as the cooling by air in winter, and a heat insulation capacity of sea ice is weakened in such regions.
  • T. Nakamura, Y. Isoda, H. Mitsudera, S. Takagi, M. Nagasawa
    GEOPHYSICAL RESEARCH LETTERS 37 2010年02月 [査読有り][通常論文]
     
    Diapyenal mixing caused through breaking of large-amplitude internal lee waves generated by sub-inertial diurnal tides, which are modulated with a 18.6-year period, is hypothesized to be fundamental to both the intermediate-layer ventilation and the bi-decadal oscillation around the North Pacific Ocean. The first observational evidence of such wave breaking is presented here. The breaking wave observed had similar to 200 m height and similar to 1 km width, and its associated diapycnal mixing was estimated to be similar to 1.5 m(2) s(-1), with a temporal average similar to 10(4) times larger than typical values in the open oceans. Our estimate suggests that a similar mixing process occurs globally, particularly around the Pacific and Antarctic Oceans. Citation: Nakamura, T., Y. Isoda, H. Mitsudera, S. Takagi, and M. Nagasawa (2010), Breaking of unsteady lee waves generated by diurnal tides, Geophys. Res. Lett., 37, L04602, doi: 10.1029/2009GL041456.
  • Small-scale eddies around the Kuril Straits generated by barotropic tidal flow
    Nakamura T, Y.Kanno, J.P.Matthews, H.Mitsudera, T.Awaji
    submitted to J. Geophys.Res. 2010年 [査読有り][通常論文]
  • Y. Kawaguchi, H. Mitsudera
    Journal of Geophysical Research: Oceans 114 10 2009年10月08日 [査読有り][通常論文]
     
    It is known that salinity anomaly under a polynya reaches and remains an equilibrium value S* after termination of its initial increase associated with brine rejection at the surface. In this paper, we investigate effect of along-shore (downwelling-favorable) wind on the equilibrium salinity anomaly by idealized numerical calculations and scale-based estimates. Numerical calculations showed that high saline water beneath polynya is advected downstream by wind-driven circulations over the shelf besides baroclinically developed eddies, which consequently induces a decrease in S* beneath polynya. The downwelling-favorable wind generates an offshore overturning flow through lower layers, referred to as Ekman Compensation Flow (ECF), which causes a great offshore salinity flux, as well as an along-shore current. We also constructed an equation for estimation of S* from the viewpoint of salinity budget over the polynya region, in which lateral salinity fluxes caused by ECF, along-shore wind-driven current and baroclinic eddies, are scaled. The solution S* was also verified by a series of numerical calculations. Furthermore, we investigated the effects of along-shore wind on dense water generation beneath the Okhotsk coastal polynyas. We conducted simplified numerical experiments assuming the Okhotsk situation, in which Fs and offshore width b of polynya are predicted by a thermodynamic polynya model with ECMWF meteorological variables. The simulated salinity shows a good agreement with the direct measurements. The theoretical estimates for S* was also applied to two Okhotsk polynyas, northwestern polynya (NWP) and northern polynya (NP). In conclusion, we found that the along-shore wind causes greater salinity decrease in NP than in NWP, whose variations substantially depend on the Aleutian Low activity. Copyright 2009 by the American Geophysical Union.
  • Y. Kawaguchi, H. Mitsudera
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 114 2009年10月 [査読有り][通常論文]
     
    It is known that salinity anomaly under a polynya reaches and remains an equilibrium value S* after termination of its initial increase associated with brine rejection at the surface. In this paper, we investigate effect of along-shore (downwelling-favorable) wind on the equilibrium salinity anomaly by idealized numerical calculations and scale-based estimates. Numerical calculations showed that high saline water beneath polynya is advected downstream by wind-driven circulations over the shelf besides baroclinically developed eddies, which consequently induces a decrease in S* beneath polynya. Thedownwelling-favorable wind generates an offshore overturning flow through lower layers, referred to as Ekman Compensation Flow (ECF), which causes a great offshore salinity flux, as well as an along-shore current. We also constructed an equation for estimation of S* from the viewpoint of salinity budget over the polynya region, in which lateral salinity fluxes caused by ECF, along-shore wind-driven current and baroclinic eddies, are scaled. The solution S* was also verified by a series of numerical calculations. Furthermore, we investigated the effects of along-shore wind on dense water generation beneath the Okhotsk coastal polynyas. We conducted simplified numerical experiments assuming the Okhotsk situation, in which F-s and offshore width b of polynya are predicted by a thermodynamic polynya model with ECMWF meteorological variables. The simulated salinity shows a good agreement with the direct measurements. The theoretical estimates for S* was also applied to two Okhotsk polynyas, northwestern polynya (NWP) and northern polynya (NP). In conclusion, we found that the along-shore wind causes greater salinity decrease in NP than in NWP, whose variations substantially depend on the Aleutian Low activity.
  • Junji Matsuda, Humio Mitsudera, Tomohiro Nakamura, Keisuke Uchimoto, Takuya Nakanowatari, Naoto Ebuchi
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 56 9 1401 - 1418 2009年09月 [査読有り][通常論文]
     
    Dense shelf water (DSW) produced from sea-ice formation on the northern shelves of the Sea of Okhotsk flows out to intermediate depths (200-500 m) along the Sakhalin coast and is the densest water mass formed in the North Pacific. This intermediate-layer intrusion forms the lower limb of a meridional overturning circulation in the Sea of Okhotsk that is enhanced by strong tidal mixing near the Kuril Islands and closed by a surface wind-driven circulation that returns salinized water to the DSW formation regions. This meridional overturning circulation is referred to as the intermediate-layer overturning. This paper presents results from a series of numerical experiments that investigated the dynamics of this intermediate-layer overturning circulation. The effects of wind, air temperature, Amur River discharge and tidal mixing along the Kuril Island were examined and were all found to influence the overturning. In particular, it was found that stronger wind forcing enhances the DSW intrusion because (1) intensified circulation increases northward salinity flux from the Kuril Islands where saline water upwells from the intermediate layer, and consequently raises background salinity in the northern shelves where DSW forms and (2) the DSW volume flux from the northern polynyas increases under increased winds. The observed interannual time-scale variability (< 10 years) of the intermediate-layer overturning is also discussed. (C) 2009 Elsevier Ltd. All rights reserved.
  • Wind- and buoyancy-driven overturning circulation in the Sea of Okhotsk
    Mitsudera, H, J. Matsuda, T. Nakamura, K. Uchimoto, T. Nakanowatari, N. Ebuchi
    Proceedings of 24th International Symposium on Okhotsk Sea and Sea Ice 121 - 124 2009年02月 [査読無し][通常論文]
  • Yusuke Kawaguchi, Humio Mitsudera
    TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY 60 4 789 - 802 2008年08月 [査読有り][通常論文]
     
    In polar regions low-pressure systems drive sea ice divergence, which can accelerate summer sea ice melt through energy absorption at resulting open water areas. This paper examines the mechanisms that cause the ice divergence and its seasonal change with a Lagrangian ice model. We focus on the effects of initial ice concentration, ice strength and ocean stratification. A series of idealized simulations (initially at 5 km resolution) are carried out with a Rankine combined vortex as external wind forcing. We have found a characteristic length scale r(1)* in free drift, based on the influence of the Coriolis term. The results show that ice concentration decreases most greatly within the range of r(1)*. In addition, ice divergence becomes small in the inner region for high concentrations (i.e. over 0.95), due to inward internal force blocking divergent deformations. The effects of ocean stratification on ice-drift divergence are also examined. Numerical results show that as the density stratification increases divergent Ekman flows beneath ice further promote the ice-drift divergence and lead to more reduction in the ice concentration through thinning of surface Ekman layer.
  • An integrated system study of the Kuroshio Extension
    Donohue, K. A, N. Bond, S. Chen, M.Cronin, P. Hacker, N.G. Hogg, S.R. Jayne, J.-H. Park, J. McClean, H. Mitsudera, B. Qiu, L. Rainville, Y. Tanimoto, K. Tracey, D.Randolph, M. Wimbush, S.-P. Xie
    EOS 89 17 161 - 162 2008年 [査読有り][通常論文]
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Takeshi Nakatsuka, Shigenobu Takeda, Takeshi Yoshimura, Koji Suzuki, Kenshi Kuma, Shigeto Nakabayashi, Daisuke Tsumune, Humio Mitsudera, W. Keith Johnson, Atsushi Tsuda
    Journal of Geophysical Research: Oceans 112 10 2007年10月08日 [査読有り][通常論文]
     
    Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991 Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea. Copyright 2007 by the American Geophysical Union.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Takeshi Nakatsuka, Shigenobu Takeda, Takeshi Yoshimura, Koji Suzuki, Kenshi Kuma, Shigeto Nakabayashi, Daisuke Tsumune, Humio Mitsudera, W. Keith Johnson, Atsushi Tsuda
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 112 C10 2007年10月 [査読有り][通常論文]
     
    Iron is an essential nutrient and plays an important role in the control of phytoplankton growth ( Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.
  • Keisuke Uchimoto, Humio Mitsudera, Naoto Ebuchi, Yasumasa Miyazwa
    JOURNAL OF OCEANOGRAPHY 63 3 379 - 391 2007年06月 [査読有り][通常論文]
     
    An OGCM of the Sea of Okhotsk with a 1/12 degrees horizontal resolution and 45 vertical levels has been constructed, which successfully represents characteristics of its circulations. This article focuses particularly on an anticyclonic eddy forming off Hokkaido in the OGCM. It forms in late summer when the warm and saline Soya Warm Current Water separates from the northeastern coast of Hokkaido. This eddy is identified as the eddy studied by Wakatsuchi and Martin (1991) since its location, evolution and vertical structure are consistent with their description. Numerical experiments with varying transport of the Soya Warm Current (SWC) have been carried out, where the transport of the SWC is changed by applying the Island Rule around Hokkaido. It is shown that the eddy formation depends on the transport of the SWC. When the transport is large, an eddy forms off the Shiretoko Peninsula. When the transport is small, however, the saline Soya Warm Current Water does not reach that area, nor does a distinct eddy form. It is when the inflow transport of the Soya Warm Current is larger than the outflow from the Nemuro Strait and the Kunashiri Channel that the anticyclonic eddy forms.
  • Shigeru Aoki, Daisuke Fukai, Toru Hirawake, Shuki Ushio, Stephen R. Rintoul, Hiroshi Hasumoto, Takashi Ishimaru, Hideharu Sasaki, Takashi Kagimoto, Yoshikazu Sasai, Humio Mitsudera
    Journal of Geophysical Research: Oceans 112 5 2007年05月08日 [査読有り][通常論文]
     
    The spatial and temporal characteristics of cyclonic eddies in the Antarctic Divergence off Adélie Coast are described using repeated in situ observations between 2001 and 2003, satellite observations, and results of a high-resolution ocean general circulation model. Satellite ocean color images and altimetry clearly revealed a series of cyclonic eddies, with diameters of about 100-150 km that were separated by about 150 km in the zonal direction. The eddies are found at almost the same locations and can be traced for at least two months in the spring-summer period. Cold and fresh anomalies were found throughout the water column in the cores of the cyclonic eddies. The water properties in the core of the eddies are similar to water found over the continental slope, 150-200 km to the south. The geopotential anomaly of the cold features was smaller than the sea level anomaly detected with satellite altimetry, indicating that significant barotropic flow is associated with the eddies. The Ocean general circulation model for the Earth Simulator (OFES) reproduces a series of cyclonic eddies similar to those observed, including a significant barotropic component. In the model, the series of eddies appear to originate further west over the continental slope region with no consistent phase propagation. The combination of repeat in situ observations, remote sensing and high-resolution model results confirms the existence of persistent cyclonic eddies near 140°E and suggests the eddies play an important role in the exchange of water across the Antarctic Divergence. Copyright 2007 by the American Geophysical Union.
  • Shigeru Aoki, Mieko Hariyama, Humio Mitsudera, Hideharu Sasaki, Yoshikazu Sasai
    GEOPHYSICAL RESEARCH LETTERS 34 10 2007年05月 [査読有り][通常論文]
     
    We investigated the distribution of the deep mixed layer in the Southern Ocean, where Subantarctic Mode Water (SAMW) forms, by using Argo profiling floats and the ocean general circulation model for the Earth simulator (OFES). The mixed-layer depth (MLD) generally increased eastward from 50 degrees - 180 degrees E and from 180 degrees - 80 degrees W, with distinct local maxima near major bathymetric features. A sudden transition from deep to shallow mixed layer was found along the equatorward limit of the deep mixed layer. This diagnosed MLD front coincides with the line of u(g) . del sigma(s) = 0, indicating the distributions of the general deepening and local maxima of MLD were affected by near-surface geostrophic flow. The flow is steered by the major bottom topographies, and the local MLD maxima occurred where the flow direction was deflected southward. This implies a link between the bathymetric features and formation of thick SAMW.
  • Shigeru Aoki, Daisuke Fukai, Toru Hirawake, Shuki Ushio, Stephen R. Rintoul, Hiroshi Hasumoto, Takashi Ishimaru, Hideharu Sasaki, Takashi Kagimoto, Yoshikazu Sasai, Humio Mitsudera
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 112 C5 2007年05月 [査読有り][通常論文]
     
    The spatial and temporal characteristics of cyclonic eddies in the Antarctic Divergence off Adelie Coast are described using repeated in situ observations between 2001 and 2003, satellite observations, and results of a high-resolution ocean general circulation model. Satellite ocean color images and altimetry clearly revealed a series of cyclonic eddies, with diameters of about 100-150 km that were separated by about 150 km in the zonal direction. The eddies are found at almost the same locations and can be traced for at least two months in the spring-summer period. Cold and fresh anomalies were found throughout the water column in the cores of the cyclonic eddies. The water properties in the core of the eddies are similar to water found over the continental slope, 150-200 km to the south. The geopotential anomaly of the cold features was smaller than the sea level anomaly detected with satellite altimetry, indicating that significant barotropic flow is associated with the eddies. The Ocean general circulation model for the Earth Simulator ( OFES) reproduces a series of cyclonic eddies similar to those observed, including a significant barotropic component. In the model, the series of eddies appear to originate further west over the continental slope region with no consistent phase propagation. The combination of repeat in situ observations, remote sensing and high-resolution model results confirms the existence of persistent cyclonic eddies near 140 degrees E and suggests the eddies play an important role in the exchange of water across the Antarctic Divergence.
  • Yusuke Kawaguchi, Humio Mitsudera
    RECENT DEVELOPMENT OF OFFSHORE ENGINEERING IN COLD REGIONS, VOLS 1 AND 2, PROCEEDINGS 660 - + 2007年 [査読有り][通常論文]
     
    In polar regions low-pressure systems drive sea ice divergence, which can accelerate the sea ice melt through energy absorption at the resulting open water areas. This paper examines the mechanisms that cause the ice divergence and its sesonal change with a Lagrangian ice model. In particular, we focus on the dependency of ice divergence on the effects of change in initial ice concentration, ice strength and ocean stratification. A series of idealized simulations (initially 5 km resolution) are carried out with a geostrophic Rankine combined vortex as an external wind-forcing.
  • Humio Mitsudera, Bunmei Taguchi, Takuji Waseda, Yasushi Yoshikawa
    JOURNAL OF PHYSICAL OCEANOGRAPHY 36 11 2042 - 2059 2006年11月 [査読有り][通常論文]
     
    This paper discusses the role of the Izu Ridge in blocking the Kuroshio large meander from propagating eastward across the ridge. It is shown that a combination of the sloping bottom with baroclinicity in the Kuroshio flow is important for blocking of the large meander. It produces a cyclonic torque over the western slope of the ridge when the large meander impinges upon it. That is, the cyclonic torque is formed ahead of the large meander, which results in blocking and amplification of the meander upstream of the Izu Ridge. The baroclinicity of the Kuroshio over the ridge is caused by baroclinic topographic Rossby waves generated when the large meander encounters the ridge.
  • Fumiaki Kobashi, Humio Mitsudera, Shang-Ping Xie
    Journal of Geophysical Research: Oceans 111 9 2006年09月08日 [査読有り][通常論文]
     
    Subsurface fronts in the subtropical North Pacific were investigated by constructing a high-resolution temperature climatology. Three distinct subtropical fronts (STFs) are identified, which are the southern, northern and eastern STFs according to their relative geographical locations. The southern STF extends along 19°-21°N west of the dateline, while the northern and eastern STFs appear along 21°-24°N and 26°N in the western and central subtropical gyre, respectively. Our analysis showed that each of these STFs is associated with large negative meridional potential vorticity (PV) gradient in the thermocline below the front. The northern STF is located on the southern edge of the Kuroshio recirculation, where the negative PV gradient occurs within a narrow density range and is maintained by the low PV core of the subtropical mode water (STMW). On the other hand, the negative PV gradient at the eastern and southern STFs spans over a wide density range. The eastern STF forms near the center axis of the subtropical gyre, north of which the upper and lower portions of the central mode water (CMW) converge, forming a thick low PV pool that maintains the negative PV gradient on the front. Similarly, at the southern STF, the negative PV gradient is due to a low PV pool to the north, with the southernmost portion of the STMW stacked above the upper CMW advected from the central subtropical gyre. It is concluded that the mode waters are essential for the North Pacific STFs. Copyright 2006 by the American Geophysical Union.
  • Fumiaki Kobashi, Humio Mitsudera, Shang-Ping Xie
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 111 C9 2006年09月 [査読有り][通常論文]
     
    Subsurface fronts in the subtropical North Pacific were investigated by constructing a high-resolution temperature climatology. Three distinct subtropical fronts (STFs) are identified, which are the southern, northern and eastern STFs according to their relative geographical locations. The southern STF extends along 19 degrees-21 degrees N west of the dateline, while the northern and eastern STFs appear along 21 degrees-24 degrees N and 26 degrees N in the western and central subtropical gyre, respectively. Our analysis showed that each of these STFs is associated with large negative meridional potential vorticity (PV) gradient in the thermocline below the front. The northern STF is located on the southern edge of the Kuroshio recirculation, where the negative PV gradient occurs within a narrow density range and is maintained by the low PV core of the subtropical mode water (STMW). On the other hand, the negative PV gradient at the eastern and southern STFs spans over a wide density range. The eastern STF forms near the center axis of the subtropical gyre, north of which the upper and lower portions of the central mode water (CMW) converge, forming a thick low PV pool that maintains the negative PV gradient on the front. Similarly, at the southern STF, the negative PV gradient is due to a low PV pool to the north, with the southernmost portion of the STMW stacked above the upper CMW advected from the central subtropical gyre. It is concluded that the mode waters are essential for the North Pacific STFs.
  • B Qiu, P Hacker, SM Chen, KA Donohue, DR Watts, H Mitsudera, NG Hogg, Jayne, SR
    JOURNAL OF PHYSICAL OCEANOGRAPHY 36 3 457 - 473 2006年03月 [査読有り][通常論文]
     
    Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 in near the center of the gyre. The lower boundary of STMW, corresponding to sigma(theta) similar or equal to 25.5 kg m(-3), Was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated front a budget analysis of potential vorticity to be in the range of similar to 2-5 X 10(-4) m(2) s(-1). The latitudinal extent of the STMW formation was narrow, extending from 30 degrees N to the Kuroshio Extension jet near 35 degrees N. South of 30 degrees N, STMW did not form locally but was transported from the recirculation gyre by lateral induction.
  • B Taguchi, SP Xie, H Mitsudera, A Kubokawa
    JOURNAL OF CLIMATE 18 15 2979 - 2995 2005年08月 [査読有り][通常論文]
     
    The response of the Kuroshio Extension (KE) to large-scale Rossby waves remotely excited by wind stress changes associated with the 1970s climate regime shift is studied using a high-resolution regional ocean model. Two ensemble simulations are conducted: The control run uses monthly climatological forcing while, in the second ensemble, anomalous forcing is imposed at the model eastern boundary around 165 degrees E derived from a hindcast of decadal changes in subsurface temperature and salinity using a coarser-resolution model of the Pacific basin. Near the KE, ocean adjustment deviates strongly from the linear Rossby wave dynamics. Most notably, the eastward acceleration of the KE is much narrower in meridional extent than that associated with the incoming Rossby waves imposed on the eastern boundary. This KE acceleration is associated with an enhanced potential vorticity (PV) gradient across the front that is consistent with the inertial western boundary layer theory: the arrival of the Rossby waves at the western boundary causes the eastward current to accelerate, leading to enhanced advection of low (high) PV water of subtropical (subarctic) origin along the western boundary layer. The meridional dipole of PV anomalies results in a pair of anomalous recirculations with a narrow eastward jet in between. A three-layer quasigeostrophic model is used to demonstrate this inertial adjustment mechanism. Finally, transient eddy activity increases significantly and the eddy momentum transport acts to strengthen the mean flow response. The result that ocean physical response to broad-scale atmospheric forcing is large near the KE front has important implications for fisheries research.
  • A Ishida, H Mitsudera, Y Kashino, T Kadokura
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 110 C7 2005年07月 [査読有り][通常論文]
     
    We analyzed the results of a high-resolution global ocean circulation model with 1/4-degree grid spacing and 55 vertical levels to investigate the structure and the mechanism of subsurface countercurrents (SCCs) in the equatorial Pacific Ocean. The model SCCs have observably similar structures; that is, they are associated with the pycnostad between 26.0 and 26.8 sigma(theta), deep in the west and shallow and in the east, and the densities in the SCCs' cores decrease from west to east. The volume transport in the density layer shows that each of the north SCC and the primary south SCC accompanies a westward current at their equator side; the SCC and the westward current comprise the recirculation gyre there. High mesoscale eddy activity results in a mean flow acceleration corresponding to the SCCs, especially to the north SCC in the eastern Pacific Ocean. The structures are consistent with the low potential vorticity (PV) in the equatorial region; thus the recirculations are suggested to be related with the homogenization of the PV in the equatorial region by eddy activity. In contrast, the secondary south SCC flows southeastward along the thickness front of the layer between 26.0 and 26.8 sigma(theta) and ends near the coast around 11 degrees S, 80 degrees W. The coastal undercurrent and upwelling drain the water from it there. This suggests that the secondary south SCC is not a part of recirculation, but rather driven by force near the eastern boundary.
  • T Waseda, H Mitsudera, B Taguchi, K Kutsuwada
    JOURNAL OF OCEANOGRAPHY 61 3 539 - 548 2005年06月 [査読有り][通常論文]
     
    Motivated by an analysis of a satellite sea surface temperature image suggesting that a train of extra-tropical cyclones induces amplification of the Kuroshio meander, a regional Kuroshio/Oyashio general circulation model was used to investigate the impact of high-frequency wind on the Kuroshio path variations. Near Japan, the standard deviation of the wind stress curl can be 10 times larger than the monthly mean, so the synoptic variations of the wind stress curl cannot be neglected. With the bimodal Kuroshio case realized in the model, sensitivity tests were conducted using monthly and daily mean QuikSCAT-derived wind stress forcings. The comparison showed that the high-frequency local wind perturbed the Shikoku recirculation gyre (SRG) and caused a transition of the path from straight to meander. The strong anticyclonic eddy within the SRG triggered the meander in the latter case. The high-frequency wind perturbed the motion of the eddy that would have otherwise detached from the Kuroshio, migrated south and terminated the meandering state. The result reinforces the suggestion from previous studies that the anticyclonic eddy within the SRG plays an active role in controlling the Kuroshio path variations.
  • SP Kumar, A Ishida, K Yoneyama, MRR Kumar, Y Kashino, H Mitsudera
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 52 14-15 2031 - 2047 2005年 [査読有り][通常論文]
     
    The time evolution of the Indian Ocean warm pool, studied using a global high-resolution general circulation model, shows strong seasonality. The warm pool has the largest spatial extent during April-May, and least in December. The spatio-temporal variability of its surface area is tightly coupled to incoming solar radiation, except during the summer monsoon (June-September) when Ekman dynamics dominate. The vertical extension of the warm pool, on the other hand, appears to be controlled by the equatorial currents. The heat budget of the warm pool shows that the local heat storage is entirely accounted for by the surface heat flux and advection (horizontal and vertical) in all the three regions, viz. northern, equatorial and southern, considered in the study. Surface heat flux has a dominant role in determining the local heat storage and SST in the northern region, while advection plays an important role in equatorial region. In the southern region both surface heat flux as well as advection are equally important in regulating the local heat storage and SST. (c) 2005 Elsevier Ltd. All rights reserved.
  • H Mitsudera, B Taguchi, Y Yoshikawa, H Nakamura, T Waseda, TD Qu
    JOURNAL OF PHYSICAL OCEANOGRAPHY 34 5 1174 - 1196 2004年05月 [査読有り][通常論文]
     
    In this paper, results of a high-resolution regional model of the Kuroshio-Oyashio confluence, where the mixed water region (MWR) forms off the northeastern coast of Japan, are discussed. The model simulates major characteristics of the Kuroshio and the Oyashio system well, such as the separation of the Kuroshio Extension from the Japanese coast and southward intrusion of the Oyashio. Further, potential temperature and salinity structures in the intermediate layer sigma(theta) = 27.0 resemble those obtained from historical data. Upon the success of this simulation, the authors focus on the diagnosis of the Oyashio water pathways intruding into the subtropics. It is found that the pathways of the Oyashio water form in the vicinity of the Japanese coast, where warm core rings and the Oyashio intrusion are active. These pathways are shown to be primarily eddy driven. Of particular interest is the water that originates in the Sea of Okhotsk, characterized by low potential vorticity (PV). Impacts of the Okhotsk water are identified by conducting an experiment in which the exchange of waters between the Pacific Ocean and the Sea of Okhotsk is blocked. The impacts are striking. If the exchange were blocked, the pathways would not form in the MWR. Instead, a strong cyclonic recirculation, caused by separation of the Kuroshio from the Japanese coast, dominates the MWR and advects warm and salty Kuroshio water north-westward, letting it occupy the entire MWR. It is found that the low-PV flux from the subpolar region tends to reduce the cyclonic circulation in the MWR. As a result, a southward intrusion of the Oyashio is induced. Concurrently, this intrusion blocks the northward advection of the Kuroshio water, maintaining the Oyashio water pathways in the MWR.
  • YY Kim, TD Qu, T Jensen, T Miyama, H Mitsudera, HW Kang, A Ishida
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C3 2004年03月 [査読有り][通常論文]
     
    The variation of the North Equatorial Current (NEC) bifurcation is investigated using results from a high-resolution ocean general circulation model (OGCM). The bifurcation occurs at about 15.5degreesN for the annual average and is easily identifiable in the upper 500 m, but it varies with time and depth. In agreement with recent observations, during the summer season the NEC bifurcation moves equatorward with a weak poleward shift with depth, while a large poleward movement with a poleward shift with depth is found during the winter season. Vertical mode decomposition indicates that the seasonal variation of the NEC bifurcation is dominated by the first two baroclinic modes. On the interannual timescale, the meridional migration of the NEC bifurcation is strongly influenced by El Nino/Southern Oscillation (ENSO); its correlation with the Southern Oscillation Index exceeds 0.8 in magnitude at depths around the thermocline. The NEC bifurcation occurs at its northernmost position during El Nino years and at its southernmost position during La Nina years. This variation is mainly accounted for by westward propagation of upwelling (downwelling) Rossby waves generated by winds in the central equatorial Pacific and by an anomalous anticyclone (cyclone) located in the western North Pacific when a warm (cold) event matures. The interannual variability of the NEC transport is highly correlated with that of the Mindanao Current (MC) and the Kuroshio transports. It is also found that the interannual variability of the NEC bifurcation latitude is highly correlated with the variations of transports in the NEC and the Kuroshio, but is less correlated with transport variations in the MC.
  • T Endoh, H Mitsudera, SP Xie, B Qiu
    JOURNAL OF PHYSICAL OCEANOGRAPHY 34 2 360 - 371 2004年02月 [査読有り][通常論文]
     
    The Miami Isopycnic Coordinate Ocean Model configured with 18 horizontal resolution and 23 layers is used to examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic North Pacific. The model successfully reproduces the mesothermal structure consisting of a shallow temperature minimum and an underlying temperature maximum that are called the dichothermal and mesothermal waters, respectively. The mesothermal water is formed through cross-gyre exchange between the subtropical and subarctic gyres, whereas the dichothermal water originates from cold and low-salinity waters formed in the winter mixed layer. The horizontal distribution of the passive tracer injected into the subsurface layers south of Japan shows that warm and saline water of the Kuroshio in the density range of 26.8-27.0 sigma(theta) is the source of the mesothermal water. There are three pathways through which the Kuroshio waters enter the subarctic region. First, the Kuroshio waters that cross the gyre boundary in the western boundary region are carried to the Alaskan gyre by the northern part of the North Pacific Current. Second, the Kuroshio waters carried by the southern part of the North Pacific Current enter the Alaskan gyre through a cross-gyre window in the eastern basin. Third, the Kuroshio waters that diffuse along the isopycnal in the Kuroshio-Oyashio Extension enter the western subarctic gyre. The mesothermal water thus formed in the subarctic region is entrained into the winter mixed layer and returns to the subtropics as surface water by the southward Ekman drift, forming the subpolar cell.
  • Masumoto Yukio, Sasaki H, Kagimoto T, Komori N, Ishida A, Sasai Y, Miyama T, Motoi T, Mitsudera Humio, Takahashi K
    Journal of the Earth Simulator 1 35 - 56 2004年 [査読有り][通常論文]
  • T Yamane, H Mitsudera, T Shundoh
    TETRAHEDRON LETTERS 45 1 69 - 73 2004年01月 [査読有り][通常論文]
     
    An unprecedented C4-methyl regioselective halogenation of 4,5-dimethyl-2-aryl-1,3-thiazoles (1) has been accomplished. The reaction of compound 1 with N-chlorosuccinimide and N-bromosuccinimide under mild conditions provides an efficient and operationally simple method for obtaining 4-chloromethyl-5-methyl-2-aryl-1,3-thiazoles (2) and 4-bromomethyl-5-methyl-2-aryl-1,3-thiazoles (3), respectively, in good yields without the formation of 4-methyl-5-halomethyl regioisomers. (C) 2003 Elsevier Ltd. All rights reserved.
  • KV Lebedev, M Yaremchuk, H Mitsudera, Nakano, I, G Yuan
    JOURNAL OF OCEANOGRAPHY 59 6 751 - 763 2003年12月 [査読有り][通常論文]
     
    A finite-difference quasigeostrophic (QG) model of an open ocean region has been employed to produce a dynamically constrained synthesis of acoustic tomography and satellite altimetry data with in situ observations. The assimilation algorithm is based upon the 4D variational data interpolation scheme controlled by the model's initial and boundary conditions. The data sets analyzed include direct and differential travel times measured at the array of five acoustic transceivers deployed by JAMSTEC in the region of the Kuroshio Extension in 1997, Topex/Poseidon altimetry, CTD soundings, and ADCP velocity profiles. The region monitored is located within the area 27.5degrees-36.5degreesN, 143degrees-155degreesE. The results of assimilation show that mesoscale variability can be effectively reconstructed by five transceivers measuring direct and reciprocal travel times supported by relatively sparse in situ measurements. The misfits between model and data lie within the observational error bars for all the data types used in assimilation. We have compared the results of assimilation with the statistical inversion of travel time data and analyzed energy balances of the optimized model solution. Energy exchange between the depth-averaged and shear components of the observed currents reveals a weak decay of the barotropic mode at the rate of 0.2 +/- 0.7.10(-5) cm(2)/s(3) due to topographic interaction. Mean currents in the region are unstable with an estimate of the available potential energy flux from the mean current to the eddies of 4.7 +/- 2.3.10(-5) cm(2)/s(3). Kinetic energy transition has the same sign and is estimated as 2.8 +/- 2.5 .10(-5) cm(2)/s(3). Potential enstrophy is transferred to the mesoscale at a rate of 5.5 +/- 2.7 . 10(-18)s(-3). These figures provide observational evidence of the properties of free geostrophic turbulence which were predicted by theory and observed in numerical experiments.
  • T Waseda, H Mitsudera, B Taguchi, Y Yoshikawa
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 108 C7 2003年07月 [査読有り][通常論文]
     
    [1] The Kuroshio flows along the southern coast of Japan during its non-large-meandering state, then separates from the coast near the Kii Peninsula and attaches at the north of the Izu Ridge. The amplitude of the offshore displacement of the Kuroshio changes as a result of mesoscale perturbations. Satellite SSH and SST observations (TOPEX/Poseidon: T/P and NOAA AVHRR) have suggested that the short-term Kuroshio meander formation is triggered by anticyclonic eddies originating in the Kuroshio Extension. To simulate such an event, a numerical experiment using a high-resolution regional GCM was conducted by initializing the eddy with the observed T/P sea level anomaly. An anticyclonic eddy was injected to the south of the Kuroshio (140degreesE, 30degreesN) by means of sequential data assimilation of the T/P data of October 1992 for 30 days. The volume transport of the model Kuroshio was kept constant at 25 Sv, a condition which would not cause large meander of the Kuroshio. The simulation successfully reproduced the four phases of the interaction: (1) westward propagation of the eddy; (2) advection of the eddy by the Kuroshio; (3) meander formation; and (4) detachment of the eddy from the Kuroshio and their repetition. The analyses revealed that the inshore high potential vorticity (PV) water is generated at the sharp coastal topography (the Kii peninsula). The cyclonic eddies shed will eventually coalesce with the existing inshore cyclonic circulation and the meander grows. During the growth of the meander, the necessary barotropic kinetic energy is produced through the shallowing of the thermocline of the anticyclonic eddy as it elongates and splits. The growth of the meander ceases when the split anticyclonic eddies merge, the thermocline deepens, and the eddy detaches itself from the Kuroshio as a result of its own westward thrust. Simultaneously, the accumulated high PV inshore is released to the Kuroshio Extension region as cyclonic eddies. This study provides evidence for the active role of the anticyclonic eddy in causing the variability of the Kuroshio path and suggests a mechanism of rapid discharge/recharge of the available potential energy of the eddy and production/release of high PV inshore Kuroshio to cause the short-term Kuroshio meander.
  • T Waseda, L Jameson, H Mitsudera, M Yaremchuk
    JOURNAL OF OCEANOGRAPHY 59 2 187 - 200 2003年04月 [査読有り][通常論文]
     
    Wavelet Analysis provides a new orthogonal basis set which is localized in both physical space and Fourier transform space. Empirical Orthogonal Functions (EOFs), on the other hand, provide a global representation of data sets. Here we investigate the various ways in which one can combine these basis sets for optimal representation of data. EOFs represent the global large scale information and wavelet analysis are used to supplement this large scale information with local fine scale information. Here we begin to explore the application of these two basis sets for outputs from an Ocean General Circulation Model and we explore various applications, including data assimilation.
  • T Waseda, H Mitsudera
    JOURNAL OF OCEANOGRAPHY 58 5 627 - 638 2002年10月 [査読有り][通常論文]
     
    The shallow coastal water of the Enshu-Nada Sea (ENSW) recirculates regardless of whether the Kuroshio path is straight or has meanders. The recirculation is formed as a result of flow separation at the sharp coastline. The outputs of a recent numerical simulation of the Kuroshio current, including case of a short-term meander caused by an anticyclonic eddy, were analysed to track the motion of the ENSW. In contrast to the steady-flow cases, the unsteady cases showed that the ENSW discharges into the Kuroshio Extension region and intrudes further north into the Kuroshio-Oyashio confluence region due to chaotic advection. Two hyperbolic stagnation points of the velocity field characterise the transport paths; one south of the Izu peninsula and the other at the Kuroshio Extension. This mechanism exists even without the Ekman drift and may play an important role in the transportation of the fish eggs and larvae from the southern Japan spawning ground to the food abundant Kuroshio-Oyashio transition zone.
  • T Waseda, H Mitsudera, B Taguchi, Y Yoshikawa
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 107 C8 2002年08月 [査読有り][通常論文]
     
    [1] Presented are results from a numerical study of the interaction of a mesoscale eddy and the Kuroshio using a high-resolution regional general circulation model (GCM). The distinct evolution of the mesoscale eddies is not new in the literature, but most of the previous studies were conducted with simpler model configurations. To our knowledge the present study is the first to use a high-resolution GCM, proven to replicate a realistic ocean circulation. An anticyclonic eddy was injected to the south of Kuroshio (140degreesE, 30degreesN) by means of sequential data assimilation of TOPEX/Poseidon altimeter data of October 1992 for 30 days. The strength of the assimilation was varied to produce five eddies (Rossby number epsilon similar to 0.012-0.014) comparable in scale to those observed in this region. The two results (epsilon = 0.0131, 0.0141) resemble the observed onset of short-term meandering events in 1993 and 1998. The westward propagating anticyclonic eddy collides with the Kuroshio southeast of Kyushu, propagates downstream, and triggers the short-term Kuroshio meander (occurring between the Kii Peninsula and Izu ridge with duration of half a year). The sequence resembles the scenario hypothesized on the basis of altimeter observations and other in situ measurements during Tokyo Ogasawara Line Experiment. The description of these cases is important, but we focus here on the differentiation between the strong eddy cases (e = 0.0131, 0.0141) and the weak eddy cases (epsilon = 0.0118, 0.0124, 0.0125) classified on the basis of their subsequent evolutions: the strong geddy (meandering case) propagates west as it elongates zonally and barotropic tripolar vortices form, and the weak eddy (nonmeandering case) makes an abrupt southward migration while the eddy core splits as a result of advection by lower layer geostrophic motions. Such distinct evolutions are conjectured a result of a competition of the effects of nonlinearity, dispersion, and barotropicity of which their relative importance varied among the simulated eddies. Because the scale variation of the injected eddies are natural, we expect that even the weak-eddy case exists in nature; in fact, such a case was observed in 1994.
  • L Jameson, T Waseda, H Mitsudera
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 19 5 747 - 758 2002年05月 [査読有り][通常論文]
     
    The second generation of a new approach to data assimilation where wavelet analysis is used for error estimation is presented here. The first generation is known as EEWADAi. This modified and optimized method uses wavelet analysis to not only estimate numerical error but to also acquire an estimate of the variation at various scales of the model simulation. In the original EEWADAi, wavelet analysis on the finest scale was used to estimate numerical error. In the second-generation version, called SUgOiWADAi, wavelet analysis is used on a variety of scales to not only obtain an estimate of numerical error, finest-scale information, but to also obtain an estimate of model variation, information from coarser scales. This new algorithm is computationally very inexpensive and is very effective.
  • KA Donohue, E Firing, GD Rowe, A Ishida, H Mitsudera
    JOURNAL OF PHYSICAL OCEANOGRAPHY 32 4 1252 - 1264 2002年04月 [査読有り][通常論文]
     
    An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model's ability to simulate SCCs.
  • TD Qu, SP Xie, H Mitsudera, A Ishida
    JOURNAL OF PHYSICAL OCEANOGRAPHY 32 3 746 - 763 2002年03月 [査読有り][通常論文]
     
    The annual subduction rate in the North Pacific is estimated using five-day outputs from a high-resolution general circulation model (GCM). Two maxima (>200 m yr(-1)) are found in the western North Pacific: one is responsible for the formation of the subtropical mode water (STMW) and the other for the formation of the central mode water (CMW). A local maximum (>75 m yr(-1)) is also found in the formation region of the eastern subtropical mode water (ESMW). These results are compared with a calculation using the winter mixed layer depth and annual mean velocity fields to examine the effect of mesoscale eddies. Although the mesoscale eddies do not markedly affect the general subduction pattern, they enhance the annual subduction rate by up to 100 m yr(-1) in the formation region of the STMW/CMW, a 34% increase in a regional average (30degrees-44degreesN, 140degreesE-170degreesW). Further analysis shows that the effects of the mean seasonal cycle and smaller-scale (,30 days) eddies are generally small. The authors suggest that the two peaks in the subduction rate are related to a double-front structure on the intergyre boundary in the western North Pacific.
  • H Hashimoto, T Ikemoto, T Itoh, L Maruyama, T Hanaoka, M Wakimasu, H Mitsudera, K Tomimatsu
    ORGANIC PROCESS RESEARCH & DEVELOPMENT 6 1 70 - 73 2002年01月 [査読有り][通常論文]
     
    A new and efficient synthesis of 4-[N-methyl-N-(tetrahydropyran-4-yl)aminomethyl]aniline dihydrochloride, a key intermediate for the CCR5 antagonist TAK-779, is described. Reductive alkylation of methylamine with tetrahydro-4H-pyran-4-one followed by alkylation of N-methyl-N-(tetrahydropyran-4-yl)amine with 4-nitrobenzylbromide and reduction of N-(4-nitrobenzyl)-N-(tetrahydropyran-4-yl)amine results in a 78% isolated yield from the starting materials by a scalable method, using only commercially available reagents.
  • A Kamimura, F Kawahara, Y Omata, N Murakami, R Morita, H Otake, H Mitsudera, M Shirai, A Kakehi
    TETRAHEDRON LETTERS 42 48 8497 - 8500 2001年11月 [査読有り][通常論文]
     
    Unsymmetrically substituted fumaric esters underwent highly regioselective conjugate addition of thiols in the presence of a lithium cation in non-coordinative media. (C) 2001 Elsevier Science Ltd. All rights reserved.
  • T Qu, H Mitsudera, B Qiu
    JOURNAL OF PHYSICAL OCEANOGRAPHY 31 9 2575 - 2589 2001年09月 [査読有り][通常論文]
     
    Time-averaged structure of the Kuroshio/Oyashio system east of Japan was examined using historical hydrographic data. Unlike most of the earlier climatological analyses, the data were averaged along isopycnal rather than pressure surfaces in a 0.5 degrees x 0.5 degrees grid. As a result, most of the detailed phenomena associated with the narrow western boundary currents were revealed. Water from the Oyashio is seen to overshoot the zero zonally integrated wind-stress-curl line by more than 5 degrees, approaching as far south as 36 degrees -38 degreesN at the western boundary. Water from the Kuroshio Extension, by contrast, tends to feed into the Oyashio Front in the interior ocean. This exchange of waters leads to a zero of zonally integrated (western boundary-180 degrees) meridional transport at about 44 degreesN, reasonably coinciding with the zero of zonally integrated wind stress curl in the western North Pacific. A well-defined double-front structure is seen at depths of the thermocline, but it does not appear to have a strong signature in the surface dynamic topography. Though always accompanied by strong temperature and salinity gradients, water density changes little across the Oyashio Front near the surface. Both the Kuroshio Extension and Oyashio Front have a significant deep component, but below 1000 m the former seems to be dominated by eddy features associated with the Kuroshio Extension recirculation gyre.
  • H Mitsudera, T Waseda, Y Yoshikawa, B Taguchi
    GEOPHYSICAL RESEARCH LETTERS 28 10 2025 - 2028 2001年05月 [査読有り][通常論文]
     
    A meander-formation mechanism of the Kuroshio that involves anticyclonic eddies propagating from the Kuroshio Extension, is presented using the analysis of observations of the TOPEX/POSEIDON (T/P) altimeter data and in-situ data. Support of the mechanism comes also from a numerical simulation. In October 1997. a current meter mooring over the Izu Ridge at 500-m-depth captured large velocity fluctuations of about 50 cm s(-1) that were associated with a strong anticyclonic eddy. The observations from T/P show that the anticyclonic eddy propagated westward and collided with the Kuroshio off Kyushu. At collision, the anticyclonic eddy coupled with a cyclonic circulation accompanied by a small meander and formed a vortex pair. They were then advected downstream and finally formed a large-meander-like path off the south coast of Honshu. The meander lasted for a few months. subsequently, the anticyclonic eddy was detached, and the meander grew smaller in amplitude. Numerical experiments successfully simulate this interaction between the Kuroshio and anticyclonic eddies.
  • T Shinke, Y Yoshikawa, T Kamoshida, H Mitsudera
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 40 5B 3835 - 3841 2001年05月 [査読有り][通常論文]
     
    A data assimilation scheme for ocean acoustic tomography (OAT) data is developed and evaluated by an identical twin experiment to understand the dynamical processes and the evolution of flow pattern in the Kuroshio extension region. The scheme consists of three components, i.e., the two-layer quasi-geostrophic (QG) oceanic numerical model, the travel time data obtained from OAT, and the data assimilation technique which consists of the Kalman filter and observation equations, relating the model variables (stream function and interface displacement) to the OAT-derived reciprocal travel time. The identical twin experiment using two different model fields made from the same model demonstrates both the stability and the availability of the scheme developed in the present study.
  • M Feng, H Mitsudera, Y Yoshikawa
    JOURNAL OF PHYSICAL OCEANOGRAPHY 30 9 2257 - 2276 2000年09月 [査読有り][通常論文]
     
    Four years of mooring array measurements in Tokara Strait, south of Kyushu, Japan, from 1992 to 1996 are used to analyze the structure and temporal variability of the Kuroshio Current. The mean Kuroshio current in Tokara Strait shows a nearly permanent subsurface double-core structure, possibly due to topographic blockage effects. The dominant variations of the Kuroshio in Tokara Strait are separated into long-term variability (typically 100-day period) and short-term variability (10 days to 1 month) according to spectrum and wavelet analysis. The long-term variability has a large horizontal scale across the strait, with a striking twofold banded structure in spatial correlations. This is due to the north-south Kuroshio axis shift that advects the double-core structure of the mean current. The axis shift can be indexed with the northeastward current velocity at the northernmost station: a composite analysis using this index shows well-defined northward and southward axis shift structures of the Kuroshio current. From the composite of the TOPEX/Poseidon sea level anomaly in terms of this index, the Kuroshio axis shift and the current structure change are associated with a dipole-shape sea level anomaly east of Tokara Strait. On the other hand, the short-term variability of high kinetic energy only has a small horizontal scale within the northern part of the current, which is related to frontal variability. There exists a deep southwestward undercurrent below 600 m in the northern part of Tokara Strait. flowing along the isobaths. The undercurrent becomes stronger during the northward shift of the Kuroshio axis, while it almost disappears during the southward shift.
  • T Qu, H Mitsudera, T Yamagata
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 105 C3 6415 - 6424 2000年03月 [査読有り][通常論文]
     
    Water mass distribution was studied by analyzing historical hydrographic data in the South China Sea. Despite considerable modification of characteristics as a result of mixing, waters of both salinity maximum and minimum of the North Pacific origin were traced on the density surfaces around 25.0 and 26.73 sigma(theta), respectively. In the salinity maximum layer, property distribution suggests an intrusion into the South China Sea all year-round through the Luzon Strait. The seasonal variation of the intrusion contains a pronounced semiannual signal, with greater strength in winter and summer than in spring and fall. From spring to fall, the intrusion water from the Pacific is narrowly confined in the continental slope south of China; only in winter, when the northeast monsoon becomes fully developed, can it spread in the southern South China Sea. In the salinity minimum layer, water enters the South China Sea only in spring, when the intrusion in the salinity maximum layer is weakest. A combined use of the "island rule" with climatological data suggests a mean Luzon Strait transport of the order 4 Sv (1 Sv = 10(6) m(3) s(-1)).
  • TD Qu, H Mitsudera, T Yamagata
    JOURNAL OF PHYSICAL OCEANOGRAPHY 29 7 1488 - 1505 1999年07月 [査読有り][通常論文]
     
    This study provides a climatology of the circulation and water mass distribution by using historical data combined with observations from dozens of recent cruises near the Philippine coast. The most striking results are related to the poleward contraction of the subtropical gyre on denser surfaces, with the bifurcation of the North Equatorial Current moving from about 15 degrees N in the upper thermocline to about 20 degrees N at intermediate depths. Though time variability and the possible errors in the data are rather large, the Halmahera eddy (IIE) is clearly seen in the climatic mean fields, lying at about 3 degrees N, 130 degrees E near the surface and reaching the Mindanao coast on density surfaces around 27.2 sigma(theta). It seems that the previously observed Mindanno Undercurrent is merely a component of the recirculation associated with the HE, North Pacific Tropical Water (NPTW) and Intermediate Water (NPIW) enter the western ocean with their extreme properties centered at 15 degrees and 20 degrees N, respectively, and continue southward as far as the southern tip of Mindanao along the western boundary. The influence of South Pacific sources becomes increasingly important with depth, antarctic Intermediate Water (AAIW) is traced to about 12 degrees N off Mindanao: but, there is little indication of a northward Row of AAIW farther north. Salinity extremes are also used as an indicator of NPTW and NPIW and the primary result is that mixing of potential temperature and salinity are not jointly compensated, thus leading to an increase of density in NPTW and a decrease of density in NPIW in the flowpath from the North Pacific subtropical gpre to the Tropics along the Philippine coast.
  • A Kamimura, Y Gunjigake, H Mitsudera, S Yokoyama
    TETRAHEDRON LETTERS 39 40 7323 - 7324 1998年10月 [査読有り][通常論文]
     
    alpha-(N,N'-Bisalkoxycarbonyl)hydrazino-alpha,beta-unsaturated ketones are readily prepared via the aza-Baylis-Hillman reaction of alkyl vinyl ketones and azodicarboxylate esters. (C) 1998 Elsevier Science Ltd. All rights reserved.
  • A Kamimura, H Mitsudera, S Asano, A Kakehi, M Noguchi
    CHEMICAL COMMUNICATIONS 10 1095 - 1096 1998年05月 [査読有り][通常論文]
     
    A combination of a tert-butyl alpha,beta-unsaturated ester, an aldehyde and lithium thiophenolate in CH2Cl2 undergoes a one-pot Michael/aldol tandem reaction to give a condensation adduct of the three components, an alpha-phenylthiomethyl-beta-hydroxy ester, in good yield with high syn-selectivity.
  • TD Qu, H Mitsudera, T Yamagata
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 103 C4 7537 - 7548 1998年04月 [査読有り][通常論文]
     
    In this study we describe the velocity structure and transport of the North Equatorial Current (NEC), the Kuroshio, and the Mindanao Current (MC) using repeated hydrographic sections near the Philippine coast. A most striking feature of the current system in the region is the undercurrent structure below the surface flow. Both the Luzon Undercurrent and the Mindanao Undercurrent appear to be permanent phenomena. The present data set also provides an estimate of the mean circulation diagram (relative to 1500 dbar) that involves a NEC transport of 41 Sverdrups (Sv), a Kuroshio transport of 14 Sv, and a MC transport of 27 Sv, inducing a mass balance better than 1 Sv within the region enclosed by stations. The circulation diagram is insensitive to vertical displacements of the reference level within the depth range between 1500 and 2500 dbar. Transport fluctuations are, in general, consistent with earlier observations; that is, the NEC and the Kuroshio vary in the same phase with a seasonal signal superimposed with interannual variations, and the transport of the MC is dominated by a quasi-biennial oscillation. Dynamic height distributions are also examined to explore the dynamics of the current system.
  • PG Baines, SJ Majumdar, H Mitsudera
    JOURNAL OF FLUID MECHANICS 312 107 - 124 1996年04月 [査読有り][通常論文]
     
    We describe a mechanistic picture of the essential dynamical processes in the growing Tollmien-Schlichting wave in a Blasius boundary layer and similar flows. This picture depends on the interaction between two component parts of a disturbance (denoted 'partial modes'), each of which is a complete linear solution in some idealization of the system. The first component is an inviscid mode propagating on the vorticity gradient of the velocity profile with the free-slip boundary condition, and the second, damped free viscous modes in infinite uniform shear with the no-slip condition. There are two families of these viscous modes, delineated by whether the phase lines of the vorticity at the wall are oriented with or against the shear, and they are manifested as resonances in a forced system. The interaction occurs because an initial 'inviscid' disturbance forces a viscous response via the no-slip condition at the wall. This viscous response is large near the resonance associated with the most weakly damped viscous mode, and in the unstable parameter range it has suitable phase at the outer part of the boundary layer to increase the amplitude of the inviscid partial mode by advection.
  • Y KURODA, H MITSUDERA
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 100 C6 10801 - 10816 1995年06月 [査読有り][通常論文]
     
    Detailed structures of internal tides in the East China Sea were observed during autumn from 1986 to 1989, using an underwater sliding vehicle that can measure temperature, conductivity, and depth continuously, sliding up and down along a faired cable. A variety of structures were observed and are classified with internal Froude numbers of the lowest two modes, which depend on the barotropic tidal current velocities and the phase speeds of internal waves (i.e., stratification). In the central East China Sea, the second-mode waves are most strongly generated at the shelf break and then propagate shoreward onto the East China Sea continental shelf. Since the barotropic tidal current is faster than the phase speed of the second-mode waves on the shelf, the generation mechanism is consistent with conventional models. On the other hand, in the southern East China Sea the lowest-mode response is dominant, although the tidal current velocity is slower than the lowest-mode phase speed. A nonlinear model is proposed for this response.
  • PG BAINES, H MITSUDERA
    JOURNAL OF FLUID MECHANICS 276 327 - 342 1994年10月 [査読有り][通常論文]
     
    In homogeneous and density-stratified inviscid shear flows, the mechanism for instability that is most commonly invoked and discussed is Kelvin-Helmholtz instability, as it occurs for a simple velocity discontinuity. There is a second mechanism, the wave-interaction mechanism, which is much more general, and is the subject of this paper. This mechanism depends on two free waves that propagate in opposite directions in a stratified shear flow, and which may become stationary relative to each other because of the shear. If this occurs, and their relative phase is suitably chosen, the velocity field of each wave increases the displacement of the other, and so the disturbance grows. We show that this mechanism is responsible for instability in a general class of symmetric but otherwise arbitrary velocity and density profiles, provided that the Richardson number R(i) < 1/4 in a central region of arbitrarily small thickness. A critical layer exists in this central region for the growing disturbance, but its role in the instability process is incidental. When R(i) > 1/4 everywhere, the flow is stable because the free waves described above are absorbed by the critical layer, and hence are heavily damped. The necessary criteria of Rayleigh and Fjortoft for instability in homogeneous fluid are seen to provide a suitable geometry for two interacting waves. Some specific examples are given, including a succinct explanation of Holmboe waves.
  • H MITSUDERA, PG BAINES
    ELEVENTH AUSTRALASIAN FLUID MECHANICS CONFERENCE, VOLS 1 AND 2 1017 - 1020 1992年 [査読有り][通常論文]
  • PG BAINES, H MITSUDERA
    ELEVENTH AUSTRALASIAN FLUID MECHANICS CONFERENCE, VOLS 1 AND 2 75 - 78 1992年 [査読有り][通常論文]
  • H MITSUDERA, R GRIMSHAW
    JOURNAL OF PHYSICAL OCEANOGRAPHY 21 6 737 - 765 1991年06月 [査読有り][通常論文]
     
    The resonant interaction of a longshore baroclinic current with a topographic feature is investigated, using a quasi-geostrophic two-layer model, where the lower layer is assumed to be deep but is not stagnant. In this model the current may be baroclinically unstable. When a long-wave phase speed is close to zero (in a fixed reference frame), which is found to be realized when the current has almost zero velocity at the coast, there is an enhanced generation of mesoscale variability due to a combination of resonant topographic forcing and baroclinic instability. A forced evolution equation of the KdV-type, which includes an additional coupling term with the lower-layer equation, describes the behavior of the upper layer. On the other hand, the lower-layer motion is governed by a linear vorticity equatio, which in turn is coupled to the upper-layer equation. A stability analysis shows that a solitary wave is unstable when a parameter GAMMA (the phase speed in the absence of any coupling between the two layers) takes values in a certain range determined by considering a linear stability problem. A variety of numerical solutions are presented, covering stable and unstable cases, characterized by the property of the baroclinic current and the forcing mechanism, which is due either to a coastline perturbation or to bottom topography. It is found that upstream and downstream nonlinear waves are generated due to resonant forcing and may be further amplified by baroclinic instability if the wave parameter GAMMA meets the instability criterion. These destabilized nonlinear waves show very complicated interactive behavior.
  • H MITSUDERA, R GRIMSHAW
    PURE AND APPLIED GEOPHYSICS 133 4 635 - 664 1990年 [査読有り][通常論文]
  • H MITSUDERA, K HANAWA
    JOURNAL OF FLUID MECHANICS 198 453 - 469 1989年01月 [査読有り][通常論文]
  • H MITSUDERA, K HANAWA
    CONTINENTAL SHELF RESEARCH 8 2 113 - 129 1988年02月 [査読有り][通常論文]
  • H MITSUDERA, K HANAWA
    CONTINENTAL SHELF RESEARCH 7 7 699 - 714 1987年07月 [査読有り][通常論文]

書籍

その他活動・業績

共同研究・競争的資金等の研究課題

  • 表層と中層をつなぐ北太平洋オーバーターン:大陸からの淡水供給を介した陸海結合系
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2021年04月 -2024年03月 
    代表者 : 三寺 史夫, 白岩 孝行, 植田 宏昭, 中村 知裕
  • 黒潮・親潮等海洋前線帯の大気海洋結合系における役割とその経年変動の予測可能性
    日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2019年06月 -2024年03月 
    代表者 : 野中 正見, 三寺 史夫, 東塚 知己, 笹井 義一, 佐々木 英治, 碓氷 典久, 小守 信正, 田口 文明
     
    黒潮続流域の海面水温偏差の形成に重要な混合層厚偏差の形成メカニズムの調査を進めるとともに、北太平洋における十年規模の気候変動において重要な役割を果たす水温偏差の再出現過程の実態の解析を推進した。また、北太平洋亜寒帯循環表層の塩分躍層について、永年的な躍層に加え、夏季に発達する季節的な躍層を見出した。 水平解像度1/30度の高解像度北太平洋海洋モデルOFES2の1990年から2016年までの過去再現実験の出力を用い,北太平洋の亜熱帯反流域で冬季に活発なサブメソスケールの微細現象循環場の経年から十年規模変動を調べた。一方で、OFES2を用いた水平解像度1/10度の北太平洋海洋モデルに対して低次生態系の動的環境適応(FlexPFT)モデルと従来の低次生態系モデルをそれぞれ結合させ、それぞれシミュレーションを実施した。その結果、動的環境適応モデルは従来のモデルに比べて、特に亜熱帯域におけるクロロフィル濃度極大層をよく再現できることが示された。 2017年に発生した黒潮大蛇行のアンサンブル予測実験を実施し、現実同様、大蛇行の長期化を予測した。黒潮の低流量状態が持続していることが長期化の要因と考えられる。また、黒潮続流の下流域においては海洋中規模渦の活動度の経年・十年規模変動が数年前から予測出来る可能性があることを示唆した。 黒潮のような海盆西岸に形成される強い海流と同様に急峻な水平温度勾配を伴う内海と陸の境界に着目して大気大循環モデルを用いた数値実験を実施し、海岸域に表層風の収束・発散が生じること、その影響の及ぶ高度が夏季と冬季で異なること、などを見出した。また、大気大循環モデルAFESによる高解像度(約50km)アンサンブル過去再現実験を実施し、この実験結果と大気再解析データを組み合わせ、北太平洋域の大気変動の強制応答成分を評価した。
  • 北太平洋における塩分躍層の時空間変動とその生物生産への影響
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2018年04月 -2021年03月 
    代表者 : 上野 洋路, 三寺 史夫
     
    海洋の塩分躍層は、塩分が深さとともに急激に増加する層のことであり、北太平洋亜寒帯域では、成層構造や混合層深度の決定に重要な役割を果たしている。 2019年度は北太平洋亜寒帯域に注目し、2003~2017年のアルゴフロートデータ、海面フラックスデータ等を用いて、塩分躍層の分布と季節変動に関する研究を実施した。その結果、永年塩分躍層は、北太平洋亜寒帯域東部において浅く強く、西部において深く弱いことが示された。永年塩分躍層の平均深度と強さは、それぞれ冬季混合層深度と海面塩分に対応しており、冬季混合層の発達に伴って永年塩分躍層が形成されていることが示された。比較的強い永年塩分躍層が形成される西部亜寒帯循環とアラスカ循環においては、永年塩分躍層の強さと深度は顕著な季節変動を示し、海面冷却期における混合層の発達がその下の永年塩分躍層を圧縮、晩冬にむけて永年塩分躍層が強化発達されることが示された。西部亜寒帯循環とアラスカ循環では、高塩分水の湧昇も永年塩分躍層の強化に関係していた。また、永年塩分躍層とは異なり、夏季の季節塩分躍層はその頻度と強さにおいて顕著な東西差を示した。季節塩分躍層は西部・中央・沿岸域では形成されたが、東部海域ではほとんど存在していなかった。この季節塩分躍層の東西差は海面加熱期の混合層の低塩化、特に淡水フラックスに対応していた。また、地衡流とエクマン流は季節塩分躍層の強さと深さの空間分布に重要な役割を果たしていた。季節塩分躍層の発達は、エントレインメントを通じて永年塩分躍層上部の塩分を低下させることにより、冬季永年塩分躍層の強化に影響を与えていることが示された。
  • 数値シミュレーションによる北太平洋栄養物質循環の三次元構造と長期変動の解明
    日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2018年04月 -2020年03月 
    代表者 : 三寺 史夫
     
    本研究の目的は、オホーツク海・ベーリング海・親潮など北太平洋西部亜寒帯海域の「豊かな海の恵み」を生み出す仕組みを明らかにすることである。北西北太平洋における高い生物生産性を解明するためには、海洋の中深層に蓄えられている栄養塩や鉄分がどこで表層に湧き上がり、どの経路を通って西部亜寒帯海域に至るのか、という北太平洋全体の三次元的な物質循環の理解が不可欠である。その解明を目指し、栄養物質循環の数値モデリングを実施する。また、縁辺海を含めた新たな栄養塩データにより長期変動とモデル再現性の評価を行う。 数値実験に関しては、オホーツク海・ベーリング海を含む西部北太平洋亜寒帯循環の中解像度のモデル(水平解像度0.5°)を用い、1979年~2011年の熱・淡水フラックス、風応力を用いて経年変動実験を行った。モデルは観測されたリン酸塩の変動をよく再現し、大気経年変動の寄与が最も大きいことを示した。これに加え、海洋の中規模渦を解像する高解像度北太平洋物質循環モデルを開発している(水平解像度3~10㎞)。これまで中層の鉄濃度が観測よりもかなり低いという問題があった。H30年度は溶存鉄の濃度を決めるリガンド濃度を改良し、中層鉄循環の再現性向上を進めた。 栄養塩データ解析については、潮汐の18.6年周期変動に注目して解析を行った。従来の北太平洋表層栄養塩データセットを用いた予備解析結果、千島列島やアリューシャン列島に近いところで、潮汐混合が盛んな時期に、表層栄養塩濃度が上昇するシグナルが得られた。さらに詳しく調べるために、新たに縁辺海を含めた新たな北太平洋長期栄養塩データセットを作成した。なお、予備解析の結果は、国際シンポジウムや国内研究集会で発表された。
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2017年04月 -2020年03月 
    代表者 : 三寺 史夫, 白岩 孝行, 立花 義裕, 的場 澄人, 杉山 慎, 美山 透, 中村 知裕, 西岡 純
     
    親潮域は、世界でも稀な豊かな海である。その要因は「アムール川流域の湿原から流出し、オホーツク海大陸棚から海洋中層循環を通して供給される鉄分」を介した「陸海結合システム」にある。本課題では、大気変動を海洋に伝えるカムチャツカ半島の降水・雪氷・河川に着目し、海洋子午面循環を駆動する塩分変動の要因、またそれに基づく「陸海結合システム」に対する制御メカニズムの解明を目指した。その結果、(1)カムチャツカ半島からの河川流出量は、アムール川の約80%という大きなものであること、(2)中層循環を駆動する塩分に対して、アムール川よりはむしろカムチャツカ河川流出水からの影響が大きいこと、が明らかとなった。
  • 日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2016年04月 -2019年03月 
    代表者 : 豊田 威信, 三寺 史夫, 木村 詞明
     
    季節海氷域における数値海氷モデルの精度向上に資することを目的として、北極海やオホーツク海の観測データを基に変形氷を検出するアルゴリズムの開発および従来のモデルの海氷力学過程の検証を行った。前者については両海域の観測結果から氷厚値よりも氷厚と密接に関わる変形氷を検出する方が現実的と判断し、PALSAR画像を用いたアルゴリズムを開発して変形氷分布の特徴をある程度明らかにした。一方、後者については従来モデルで慣習的に用いられてきた海氷の塑性体としての振る舞いに着目し、降伏曲線の物理的な意味を見出すとともに、AMSR-E画像等から作成した海氷速度データを基にその妥当性を定量的に吟味した。
  • 日本学術振興会:科学研究費助成事業 若手研究(B)
    研究期間 : 2016年04月 -2019年03月 
    代表者 : 中野渡 拓也, 三寺 史夫, 中村 知裕, 内本 圭亮, 西岡 純, 渡邉 英嗣
     
    海氷・海洋結合物質循環モデルを用いて、オホーツク海・北太平洋亜寒帯域における鉄や栄養塩や基礎生産量の季節から経年変動の要因を調査した。親潮海域や北太平洋亜寒帯域における表層の溶存鉄や栄養塩濃度の変動には、その場の海面冷却による鉛直的な輸送プロセスだけでなく、その上流域であるオホーツク海からの水平的な輸送が同程度に影響していることが明らかになった。このような水平的な物質輸送プロセスは、数十年スケールで北太平洋亜寒帯域の広範囲の栄養塩や基礎生産の変化にも実質的に影響を及ぼしていることがわかった。一方で、海氷の生成・融解に伴う鉄輸送の影響は、オホーツク海南部で1-2%と有意な影響は見られなかった。
  • 北太平洋栄養物質循環の数値モデリング
    日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2016年04月 -2018年03月 
    代表者 : 三寺 史夫
     
    本研究では、オホーツク海をはじめとする北太平洋西部亜寒帯海域の「豊かな海の恵み」を生み出す仕組みの解明を目的として、栄養物質循環の数値モデリングを実施した。具体的には、(1)強い混合域で大きな鉛直拡散係数を与えるという従来型パラメタリゼーションを用いたモデルによる数値実験(以下、西部亜寒帯物質循環モデル)、(2)潮汐モデルを海洋海氷結合モデルに直接結合し、鉛直混合を起潮力から計算して物質循環を駆動するという、これまでにない北高解像度太平洋物質循環シミュレーションの開発を進めた。 西部亜寒帯物質循環モデルでは、経年変動実験を行った。風応力、熱、淡水フラックスとしてERA-interimを用いモデルを駆動した。また、千島列島における潮汐混合は、18.6年の周期で20%強度を変動させた。期間は1979年‐2011年である。モデルは、道東沖・冬季のリン濃度の年年変動(周期が7年以下)を良好に再現した。千島列島から親潮域にかけて、潮汐混合に伴ってリン濃度の南北勾配が大きい。このため、地衡流(親潮の強さ)の変動がリン酸塩移流の変動となり、最終的に親潮域のリン濃度変動を引き起こしていることがわかった。この地衡流変動の要因は、風の年々変動により励起される北部北太平洋沿岸の沿岸捕捉波であることが判明した。 また、高解像度北太平洋物質循環シミュレーションの開発を進め、亜寒帯循環の大部分はHigh Nutrient Low Chlorophyll(HNLC;高栄養塩にも関わらず生物生産が比較的低い)海域であることを再現した。一方で、親潮から北太平洋中央部にかけて帯状に高生産海域が現れることも再現した。このモデルを用いて潮汐の有り無し実験を行ったところ、潮汐が無い場合、オホーツク海の生物生産が大きく減少することがわかった。
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2014年04月 -2017年03月 
    代表者 : 三寺 史夫, 中村 知裕, 西岡 純, 西垣 肇, 美山 透, 伊藤 進一, 和川 拓, 藤井 陽介, 中野渡 拓也, 西川 はつみ
     
    オホーツク海で沈み込み北太平洋の中層を通気する北太平洋の子午面循環の解明を目指した。子午面循環の強度を決めるのは高緯度域の塩分であるが、北太平洋では低緯度からの高塩水供給経路は未解明であった。本研究では、北太平洋亜熱帯循環と亜寒帯循環の境界域である移行領域を流れる準定常ジェットに注目し、低緯度から亜寒帯への塩輸送過程を明らかにした。理論、数値実験、漂流ブイ観測から、準定常ジェットの流路は、たかだか500m程度の海底地形によって制御されていることが分かった。黒潮起源の高塩水は、この準定常ジェットを通って亜寒帯循環に侵入する。西風が強くなると亜寒帯への塩輸送が大きくなり、子午面循環は強化される。
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2014年04月 -2017年03月 
    代表者 : 西岡 純, 三寺 史夫, 安田 一郎
     
    東カムチャツカ上流域および千島海峡部の栄養物質(鉄や栄養塩など)の観測を実施し、水塊の混合過程を踏まえて、西部北太平洋亜寒帯域の水塊の化学的特徴を決める要因を調べた。また西部北太平洋亜寒帯域の水塊の特徴とアラスカ湾で過去に得られたデータを比較した。その結果、西部亜寒帯域には、北方縁辺海の影響を受けて「鉄:硝酸塩比」の高い水塊が中層に存在しており、その水が表層に影響することで、植物プランクトン増殖や生物的二酸化炭素吸収量が高くなっていることが明らかになった。これらの結果は、数値モデルでも表現されており、西部亜寒帯域が鉄制限海域でありながら植物プランクトン増殖が大きい理由が明らかとなった。
  • 日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2010年04月 -2016年03月 
    代表者 : 中村 尚, 磯辺 篤彦, 立花 義裕, 早坂 忠裕, 川村 隆一, 見延 庄士郎, 三寺 史夫, 野中 正見, 川合 義美, 吉田 聡
     
    1) 2015年10月日本海洋学会刊行の国際学術誌Journal of Oceanography 第71巻第5号に当新学術領域の成果発信のために特別セクション"Hot Spots" in the climate system ─ New development in the extratropical air-sea interaction researchを設け,計9編の査読付論文を掲載した.領域関係者計6名が特別セクションの編集委員を務めた.さらに,それら9編の論文を集めた書籍をSpringer社から2016年3月に刊行した. 2) 2016年夏刊行を目指し,日本気象学会のモノグラフ「気象研究ノート」の特集号の執筆・編集を進めた.9つの計画研究班と4つのワーキンググループ,さらには若手チームがそれぞれの章を執筆した.加えて,領域全体で実施した集中観測やそれに基づく大気同化実験からの公開データ,さらには気象研究所と東京大学が共同制作中の全球大気再解析JRA-55の追加プロダクトなどの,当領域からの公開データに関する章も設けた.現時点で原稿はほぼ出揃い,刊行へ向けた最終編集作業に入っている.
  • 日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2012年04月 -2015年03月 
    代表者 : 豊田 威信, 三寺 史夫, マイク メイラン, アリソン コホウト
     
    氷縁域において波―海氷相互作用が氷盤分布の形成および波活動に及ぼす影響を明らかにすることを目的として2012年晩冬季に東南極域の海氷域で波活動と氷盤分布の同時観測を実施した。ただし天候の事情で氷盤分布は波の影響が少ない海氷内部領域で観測を行い、同海域氷縁域の過去の結果と比較することにより波の影響を推定した。その結果、内部領域においても既に氷縁域で見られる氷盤分布の原型が見られ波―海氷相互作用は新たな氷盤分布を作るというよりはこれらを変調する働きがあること、ストームに伴う大波は内部領域まで侵入して氷盤形成に影響を及ぼし得ることなどが示された。
  • 日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2010年04月 -2015年03月 
    代表者 : 三寺 史夫, 中村 知裕, 浮田 甚郎, 木村 詞明, 小木 雅世, 金子 正美, 田口 文明, 小守 信正, 大島 慶一郎, 川島 正行, 高谷 康太郎
     
    冬季の季節海氷域の形成、および夏季のオホーツク海高気圧とそれに伴う下層雲の発生という、オホーツク海・北極圏における特徴的な季節性に関わる大気海洋(海氷)相互作用について探求した。夏季のオホーツク海では下層雲-海面水温フィードバックにより、海面水温が低く維持されることが明らかとなった。また、冬季のオホーツク海に特徴的なのは、その海氷変動が北極海・東シベリア海における先行する夏の海氷変動と連動している、ということである。気圧偏差の年平均で定義した北極振動(Annual AO)という新しい視点から、北極域・オホーツク海域における、海域・季節を越えてリンクするフィードバックシステムの解明を進めた。
  • 日本学術振興会:科学研究費助成事業 基盤研究(S)
    研究期間 : 2010年04月 -2015年03月 
    代表者 : 若土 正曉, 三寺 史夫, 西岡 純, 中村 知裕, 江淵 直人, 渡邉 豊, 鈴木 光次, 黒田 寛, 大島 慶一郎, 津旨 大輔, 的場 澄人, 江淵 直人, 小埜 恒夫
     
    オホーツク海から親潮域の豊かな水産資源を支える“奇跡ともいえる仕組み”を明らかにするため、オホーツク海と北太平洋をつなぐ熱塩/物質循環システムの実態解明を進めた。熱塩循環の強さを決めるオホーツク海北部の高密度水(DSW)の塩濃度は、亜寒帯循環を跨ぐ塩濃度偏差の長距離伝播、海氷生産量変動、降水量変動との「せめぎ合い」により決まる。アムール川河口・オホーツク北部陸棚上に堆積する光合成に必須の栄養物質・鉄は、このDSWに取り込まれ、北太平洋の広範囲に供給される。DSWと潮汐混合を介し強く結合した熱塩循環と物質循環が生物生産を北太平洋規模で規定するという、絶妙なシステムが本研究により明らかとなった。
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2009年 -2011年 
    代表者 : 藤吉 康志, 三寺 史夫, 川島 正行, 山口 一
     
    ドップラーレーダを用いて、様々な気象条件下でも確実に流氷域を検出できる方法を開発した。また、面相関法と実測した海氷のドップラー速度データを組み合わせた海氷の流動ベクトルの作成アルゴリズム(マッチング法)を完成させた。さらに、ドップラーレーダデータを毎時大気解析GPVに同化することによって作成された風の場を使って我々の流氷モデルを走らせ、流氷域の短時間予測を実施し、ドップラーレーダデータと比較した。
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2007年 -2009年 
    代表者 : 三寺 史夫, 大島 慶一郎, 中村 知裕, 小野 数也, 小埜 恒夫
     
    オホーツク海から北太平洋にかけての中層温暖化の実態を明らかにするとともに、熱塩(中層)循環の力学過程とその変動メカニズムの解明、および、その基礎となる高密度陸棚水(DSW)生成過程の解明を目的とした。北西陸棚域で生成されるDSWはここ50年間で約0.1PSU減少し軽くなっており、これが26.8-27.0σ_θでは温暖化シグナルとして現れている。数値実験の結果、DSWの塩分は、気温の上昇(海氷生成量の減少)、降水量の増加、風応力の変動の影響を受けて変動することが示された。一方、低緯度の亜熱帯循環中層では逆に低温化が顕著である。これは、70年代半ば以降、亜寒帯循環が強化され、親潮を通した亜寒帯から亜熱帯への低温水流入量が増加したためである。
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2006年 -2008年 
    代表者 : 江淵 直人, 中塚 武, 西岡 純, 三寺 史夫, 大島 慶一郎, 中村 知裕
     
    オホーツク海および北西北太平洋親潮域の高い海洋生物生産性を支えている物質循環のメカニズムを,海洋中層(400~800m)の循環と鉄の輸送過程に注目して,現場観測と数値モデルによる研究を行った.その結果,オホーツク海北西大陸棚起源の鉄分が,海氷の生成とともに作られるオホーツク海中層水によって移送され,千島海峡で広い深度層に分配された後,西部北太平洋に送り出されている様子が定量的に明らかとなった.
  • オホーツク海における、海水・海氷・油の流動予測システムの開発
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2005年 -2007年 
    代表者 : 大島 慶一郎, 若土 正曉, 江淵 直人, 三寺 史夫, 深町 康
     
    海水・海氷・油の流動予測の基盤となる3次元海洋モデルを開発・高精度化した。最終的には、分解能1/12度で、日本海と太平洋の海水交換も含み、日々の風応力と月平均の海面熱フラックスによって駆動されるモデルを開発した。係留系及び海洋レーダーより取得した測流データとモデルとの比較・検討により、東樺太海流・宗谷暖流に関しては非常に再現性のよいモデルを開発することができた。このモデルに粒子追跡法を取り入れて、サハリン油田起源の海水の漂流拡散を調べた。水平拡散の効果は、Markov-chain modelを仮定したランダムウォークを用いて取り入れた。表面下15mでは粒子の漂流はほとんど海流(東樺太海流)で決まり、粒子は東樺太海流に乗って樺太東を南下しあまり拡散せずに北海道沖に達する。表層(0m)では、風による漂流効果も受けるので、サハリン沖の粒子の漂流は卓越風の風向に大きく依存する。沖向き成分の風が強い年は、表面の粒子は東樺太海流の主流からはずれ、北海道沖まで到達する粒子の割合は大きく減じる。アムール川起源の汚染物質の流動予測も同様に行い、東樺太海流による輸送効果の重要性が示された。2006年2・3月に起こった知床沿岸への油まみれ海鳥の漂着問題に、後方粒子追跡実験を適用し、死骸は11-12月のサハリン沿岸から流れてきた可能性が高いことを示した。潮流による拡散効果を正しく評価するために、主要4分潮のオホーツク海の3次元海洋潮流モデルを、観測との比較・検討に基づいて作成した。この潮汐モデルと上記の風成駆動モデルを組み合わせて粒子追跡実験を行うことで、より正確な流動予測モデルとなる。オホーツク海の海氷予測に関しては、その最大面積を予測するモデルを提出した。秋の北西部の気温とカムチャッカ沖の海面水温を用いることで、3ヶ月前の時点で、高い確度で最大海氷面積を予測できることを示した。
  • オホーツク海南部における時計回り循環のメカニズムとその北太平洋へのインパクト
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2005年 -2006年 
    代表者 : 三寺 史夫, 大島 慶一郎, 中村 知裕, 小野 数也, 西垣 肇, 内本 圭亮
     
    オホーツク海南部の千島海盆は北太平洋中層水の源であり、物質循環においてもキーエリアである。この海域では時計回り循環や渦が卓越しており、それに対応するように中層水にあたる密度では分厚い混合層(低渦位層)が分布している。また、宗谷暖流、親潮、東樺太海流が流れこみ、非常に複雑な海域である。そのような千島海盆の時計回り循環が生ずるメカニズムを、単純化されたモデルと、現実的なシミュレーションを用いて研究した。 単純化されたモデルでは、千島列島を海嶺と考え、その直上での潮汐混合が引き起こす千島海盆内の循環を調べた。まず、局所的混合が小さければ線形解が成立することを確かめた。混合を大きくすると、渦位のインプットは中規模渦による混合領域から消散とバランスし、密度境界面の深さが決まること、海底地形がある場合には、地形に沿った下層の流れ場が上層の波動の特性曲線を変えるため、境界面深度に対して大きな影響を与えること、が明らかとなった。特に、特性曲線が閉じる場合にはロスビー波が海嶺上で捕捉され、注入された渦位をなかなか解消されず、境界画が大きく深まる。地形に沿った下層の流れ場が上層の波動の特性曲線を変えるため、境界面深度に対して大きな影響を与えること、が明らかとなった。特に、特性曲線が閉じる場合にはロスビー波が海嶺上で捕捉され、注入された渦位をなかなか解消されず、境界画が大きく深まる。地形に沿った時計回り循環は潮汐残差流で生じるので、本研究で述べた境界面深化のメカニズムは現実にも起こっている可能性が高い。 現実的な高解像度モデルでは、風成循環の影響を調べた。東樺太海流の一部がサハリン島南東端で東方へ離岸し続流を形成しており、続流の南方に千島海盆全体にわたるような時計回り循環が生じていた。また、宗谷暖流が知床半島から流出するときには暖水渦が形成していた。宗谷暖流は大きく季節変動するため、それに伴って暖水渦も秋に生じた。これらは観測結果と良い対応を示す。 このように、千島海盆はオホーツク海内の限られた海域ではあるが、内部潮汐による熱塩循環と風成循環が相俟って時計回り循環が作られている複雑な海域であることが明らかとなった。

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    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 風成循環、熱塩循環、ポテンシャル渦度、ロスビー波 potential vortisity (PV), Rossby wave, wind-driven circulation, thermohaline circulation
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    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 雪氷・寒冷圏、雪、氷、極地 cryosphere, snow, ice, polar regions

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  • 2014年4月1日 - 2016年3月31日 低温科学研究所環オホーツク観測研究センター長
  • 2016年4月1日 - 2018年3月31日 低温科学研究所環オホーツク観測研究センター長
  • 2018年4月1日 - 2020年3月31日 低温科学研究所環オホーツク観測研究センター長
  • 2020年4月1日 - 2022年3月31日 低温科学研究所環オホーツク観測研究センター長


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