研究者データベース

中村 知裕(ナカムラ トモヒロ)
低温科学研究所 附属環オホーツク観測研究センター
講師

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

  • 講師

学位

  • 博士(理学)(京都大学)

ホームページURL

J-Global ID

研究キーワード

  • 数値シミュレーション   潮汐   渦   高密度水   宗谷暖流   千島列島   物質循環   大気海洋相互作用   風成循環と熱塩循環の結合   オホーツク海   内部重力波   アリューシャン列島   散乱   鉛直混合   混合   海洋物理   通気   北太平洋中層水   熱塩循環   東樺太海流   中層循環   潮汐混合   北太平洋   鉄仮説   接地境界層   風成循環   

研究分野

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

職歴

  • 2005年 - 現在 北海道大学 低温科学研究所 環オホーツク観測研究センター 講師
  • 2004年 - 2005年 JAMSTEC地球環境フロンティア研究センター ポスドク研究員
  • 2001年 - 2003年 日本学術振興会特別研究員(PD) 日本学術振興会特別研究員

学歴

  • 1998年 - 2001年   京都大学   大学院理学研究科   地球惑星科学専攻 博士後期課程
  • 1996年 - 1998年   京都大学   大学院理学研究科   地球惑星科学専攻 修士課程
  • 1992年 - 1996年   京都大学   理学部

所属学協会

  • 米国気象学会   米国地球物理学連合   日本気象学会   日本海洋学会   

研究活動情報

論文

  • Low ocean-floor rises regulate subpolar sea surface temperature by forming baroclinic jets
    H. Mitsudera, T. Miyama, H. Nishigaki, T. Nakanowatari, H. Nishikawa, T. Nakamura, T. Wagawa, R. Furue, Y. Fujii, S. Ito
    Nature Communications 9 1190  2018年 [査読有り][通常論文]
  • 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.
  • 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.
  • 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.
  • 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.
  • Kenshi Kuma, Ryohei Sasayama, Nanako Hioki, Yuichiroh Morita, Yutaka Isoda, Tohru Hirawake, Keiri Imai, Takafumi Aramaki, Tomohiro Nakamura, Jun Nishioka, Naoto Ebuchi
    JOURNAL OF OCEANOGRAPHY 70 4 377 - 387 2014年08月 [査読有り][通常論文]
     
    In the southwestern Okhotsk Sea, the cold water belt (CWB) is frequently observed on satellite images offshore of the Soya Warm Current flowing along the northeastern coast of Hokkaido, Japan, during summertime. It has been speculated that the CWB is upwelling cold water that originates from either subsurface water of the Japan Sea off Sakhalin or bottom water of the Okhotsk Sea. Hydrographic and chemical observations (nutrients, humic-type fluorescence intensity, and iron) were conducted in the northern Japan Sea and southwestern Okhotsk Sea in early summer 2011 to clarify the origin of the CWB. Temperature-salinity relationships, vertical distributions of chemical components, profiles of chemical components against density, and the (NO3 + NO2)/PO4 relationship confirm that water in the CWB predominantly originates from Japan Sea subsurface water.
  • 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.
  • 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.
  • 中村知裕
    Hydrological Research Letters 8 2 84 - 89 2014年 [査読有り][通常論文]
  • S. Abe, T. Nakamura
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 118 1 316 - 331 2013年01月 [査読有り][通常論文]
     
    The transition to turbulence after excitation of large-amplitude (similar to 200 m) unsteady lee waves in Amchitka Pass, Alaska, is investigated using a nonhydrostatic vertically two-dimensional model with realistic topography. The model resolves motions two orders smaller than a large-amplitude unsteady lee wave, which is excited in the lee of the ridge, and shows that transition processes near the ridge top and downstream of the first trough of the unsteady lee wave are different. Near the ridge top, three stages of transition are identified. In the first stage, convection begins on the upstream sides (forward wave breaking) and downstream sides (backward wave breaking) of the crests of the unsteady lee wave. In the next stage, Kelvin-Helmholtz (KH) waves develop in regions of enhanced shear between statically unstable regions and downslope flow on the bottom. In the last stage, Tollmien-Schlichting (TS) waves develop on the bottom, under the KH waves, and form vortices, which finally break down. To the best of the authors' knowledge, this is the first paper to report on the occurrence of backward wave breaking and the possibility of TS wave excitation in the ocean. Downstream of the first trough of the unsteady lee wave, flow is separated from the bottom by an adverse pressure gradient attributed to the unsteady lee wave. The separated flow forms vortices, which are shed quasi-periodically. Diapycnal mixing is enhanced by the development of KH and TS waves and flow separation, as well as by convection due to overturning isopycnals induced by the unsteady lee wave.
  • 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 D05208  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.
  • オホーツク海における大気海洋相互作用:夏季の下層雲-海面水温フィードバック
    中村知裕, 古関俊也, 三寺史
    沿岸海洋研究 1 71 - 77 2012年 [査読有り][招待有り]
  • 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 3 G03005  2011年09月01日 [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. Copyright 2011 by the American Geophysical Union.
  • 三寺 史夫, 内本 圭亮, 中村 知裕
    沿岸海洋研究 49 1 3 - 12 日本海洋学会沿岸海洋研究部会 2011年08月 [査読無し][通常論文]
     
    宗谷暖流の沖側には,夏季,冷水帯が形成される.これは,栄養塩が枯渇しがちな夏季の海洋表層に栄養塩豊富な海水を湧昇させるものであり,北海道オホーツク沿岸の生物生産に重要な役割を持つ.ここではそのような冷水帯の形成メカニズムについて考察した.まず,現実的なシミュレーションが冷水帯を再現していることを示し,冷水帯の特徴を抽出した.次に,サハリン沖での湧昇のメカニズムを,内部ケルビン波と海底地形の非線形共鳴の観点から理論的考察と理想化実験を行うことによって明らかにした.さらに,宗谷海峡で生じた湧昇は,順圧流に沿って傾圧渦度波として伝搬することにより,下流の北海道沿岸に密度境界面変位を励起すること,そして表層に傾圧ジェット,亜表層にはドーム状構造を生じることを明らかにした.冷水帯は,傾圧波によるそのような調節過程の,海表面への顕れである.
  • 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 2011年01月 [査読有り][通常論文]
  • 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]
  • Y. Sasajima, H. Hasumi, T. Nakamura
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 115 C11 C11007  2010年11月 [査読有り][通常論文]
     
    Sensitivity of Dense Shelf Water (DSW) formation to tidal mixing around the Kuril Straits, wind stress, and river runoff in the Okhotsk Sea is examined by an ice-ocean coupled model. Horizontal resolution of the model is set to 3-8 km in the northern Okhotsk Sea for well resolving the coastal polynyas which is believed to be the principal region of DSW formation. The model shows a good performance in terms of sea ice production and the consequent DSW formation. DSW is also found to be formed in the offshore region apart from the coastal polynya. DSW is defined independently for each experiment such that it identifies the water influenced by brine. By introducing such definition the sensitivity of the DSW formation is assessed separately for change of density and that of formation rate. The density of DSW exhibits high sensitivity to all the elements considered herein, while its formation rate is sensitive only to winds. Winds affect the DSW formation rate mainly by influencing that occurs in the offshore region.
  • Tomohiro Nakamura, Yasuhiro Kawasaki, Tokihiro Kono, Toshiyuki Awaji
    CONTINENTAL SHELF RESEARCH 30 6 598 - 607 2010年04月 [査読有り][通常論文]
     
    Vertical sections of temperature and current velocity across the sill in the Kruzenshtem Strait were observed four times in one day in September 1997. The observations captured nonlinear internal waves near the sea surface, which are likely to be tidally generated. Two of these waves had maximum amplitudes near the surface and each contained a trapped core, suggesting that a significant volumetric flux was induced. At mid depths, isotherms moved vertically more than 50 m, which is consistent with the generation of internal lee waves. From water-mass movements, the currents are estimated to flow from the North Pacific to the Sea of Okhotsk in the temperature-minimum layer at a mean speed of 0.2-0.3 m s(-1). The mean flow was weaker and the oscillatory component was stronger at mid depths, with the amplitude of the latter around 0.4 m s(-1). leading to the above lee wave generation. Active temporal variations in potential temperature, particularly those associated with small-scale patches, suggest the occurrence of vigorous water-mass transformation. Vertical and horizontal diffusivities are estimated to be O(10(-2) m(2) s(-1)) and O(10(2)-10(3) m(2) s(-1)), respectively. Because the tidal amplitude was close to the annual minimum at the time of observation, larger-amplitude internal waves and more vigorous water-mass transformation could take place at other times of the year. (C) 2009 Elsevier Ltd. All rights reserved.
  • T. Nakamura, Y. Isoda, H. Mitsudera, S. Takagi, M. Nagasawa
    GEOPHYSICAL RESEARCH LETTERS 37 4 L04602  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.
  • Tomohiro Nakamura, Takahiro Toyoda, Yoichi Ishikawa, Toshiyuki Awaji
    JOURNAL OF OCEANOGRAPHY 66 1 41 - 60 2010年02月 [査読有り][通常論文]
     
    A steady quasi-geostrophic 2.5-layer model, forced by both Ekman pumping and a mass source/sink situated at the western boundary has been constructed to investigate the effect of diapycnal transport due to convection in the Okhotsk Sea and tidal mixing at the Kuril Straits on the intermediate layer in the North Pacific. The model illustrates a combined effect of the wind-driven and mass-driven circulations. First, net mass input induces a "barotropic" mode inter-gyre flow along the western boundary through the dynamical influence of Kelvin waves. This flow creates characteristic curves (geostrophic contours) that facilitate inter-gyre communication through the western boundary layer from the location of the mass source to the subtropical gyre. Due to the effect of wind-driven circulation, the offshore part turns eastward into the interior, encircles the outer rim of the region (which would otherwise be the pool region in the absence of mass input), and then encounters the western boundary. Eventually, the water fed into the lower layer flows mostly along this path and later flows away to the equatorial region. Conversely, in the upper layer, water is fed from the equator to the subtropics, and to the subpolar interior region through the western boundary current. The water then circulates along the outer rim and is absorbed into the mass sink. The model is controlled mainly by three nondimensional parameters: (1) the ratio of net mass input rate to the maximum Sverdrup transport (Q/T-Sv(max)), which affects the inter-gyre communication by altering the paths of geostrophic contours, (2) the ratio of a mass input rate into the lower layer to that in total (Q(2)/Q), which controls the vertical structure of the inter-gyre flow, and (3) the measure of the wind forcing effect relative to the beta effect, which determines the horizontal extent of the area influenced by the mass input. The other parameter regimes with respect to Q/T-Sv(max) and Q(2)/Q are also presented.
  • 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.
  • Tomohiro Nakamura, Toshiyuki Awaji
    JOURNAL OF PHYSICAL OCEANOGRAPHY 39 7 1574 - 1594 2009年07月 [査読有り][通常論文]
     
    The frequency change in internal gravity waves upon scattering from a rough topography is investigated analytically. For this, sets of appropriate and tractable governing equations for various parameter regimes are derived using the method of multiple scales under the assumption that the amplitude of the bottom topography is small. A solution is shown for a simple case in which an incident internal wave is approximately linear and monochromatic. The solution has the following features: the intrinsic frequencies of the scattered waves are given as the sum and difference of the incident-wave frequency and the Doppler shift ( or lee-wave frequency). This Doppler shift causes the change in the frequency. Hence, the assumption of frequency conservation is not valid if the Doppler shift is significant, that is, when the horizontal scale of the bottom roughness ( or the length scale in the plane of the slope) is on the order of or much less than that of the incident-wave flow excursion. This condition can be satisfied in a realistic parameter range. The occurrence of such a frequency change has the following implications: first, it affects the estimate of the boundary mixing induced by the scattering because the energy redistribution in the vertical wavenumber space on scattering differs from that estimated using the assumption of frequency conservation. This effect happens because for a given horizontal wavenumber, the change in the frequency alters the vertical wavenumber of the scattered waves through the dispersion relation. Furthermore, if the incident waves are not monochromatic, even the leading-order scattered waves cannot be obtained by the superposition of the solutions for all the Fourier components of the incident waves because of the difference in the Doppler shift. Second, the effects of the background flow associated with the incident and primary reflected waves are significant when the frequency change occurs such that the background flow can create a critical level and/or advect scattered waves. The former causes mixing and background-flow acceleration, and the latter is favorable for the amplification of the scattered waves through superposition. Third, the resulting energy redistribution in frequency space could modify the spectrum shape of the oceanic internal waves, which is considered to affect both interior and boundary mixing.
  • 中付 知裕, Tomohiro Nakamura, 北海道大学低温科学研究所環オホーツク観測研究センター, 北海道大学低温科学研究所, Pan-Okhotsk Research Center Institute of Low, Temperature Science, Hokkaido University
    海の研究 = Umi no Kenkyu (Oceanography in Japan) 17 2 133 - 146 日本海洋学会 2008年03月05日 [査読無し][通常論文]
     
    千島列島域の潮汐・潮流過程がオホーツク海-北太平洋間の海水輸送および列島域の鉛直混合に重要な役割を果たしうること,列島域の混合がオホーツク海・北太平洋中層の通気とそれに伴う水塊形成・循環駆動の要因の一つであることを示し,北太平洋中層水形成機構における新たなパラダイムを提出した筆者らの一連の研究について紹介する。Tidal processes around the Kuril Islands play an important role in both water transport between the Sea of Okhotsk and the North Pacific Ocean and diapycnal mixing around the Kuril Islands. The latter has been suggested to be one cause of the ventilation of the intermediate layer of the Sea of Okhotsk and the North Pacific, leading to a new paradigm for the formation mechanism of the North Pacific Intermediate Water. This paper reviews the studies of the author and the coauthors, who have investigated the tidal processes in the Kuril Islands and proposed the above paradigm.
  • 潮汐過程がオホーツク海・北太平洋間の海水交換に果たす役割
    中村知裕
    海の研究 17 2 133 - 146 2008年 [査読有り][招待有り]
  • 環オホーツク圏領域気候モデル
    中村知裕, 三寺史夫
    天気 55 7 555 - 560 2008年 [査読有り][招待有り]
  • T Nakamura, T Toyoda, Y Ishikawa, T Awaji
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 111 C4 C04003 - C04003 2006年04月 [査読有り][通常論文]
     
    [1] The effects of tidal mixing at the Kuril Straits on the North Pacific intermediate layer are investigated using an ocean general circulation model. A comparison of numerical experiments with and without a tidal mixing effect suggests that tidal mixing at the Kuril Straits enhances the ventilation of the North Pacific intermediate layer. The enhanced ventilation results in both freshening and cooling down to similar to 27.6 sigma(theta). In particular, the simulated North Pacific Intermediate Water (NPIW) becomes fresher and denser ( by 0.3 psu and 0.1 sigma(theta) at the maximum) and hence more realistic. The enhanced ventilation is caused both through the supply of the ventilated water from the Kuril Straits, which subsequently spreads along the subarctic and subtropical gyres, and through a modification of the circulation there. The ventilation of the supplied water in turn originates from a combination of tidally enhanced convection in the Okhotsk Sea and downward diffusion at the Kuril Straits as discussed in a previous paper. The former affects the upper part of the NPIW, while the latter is dominant in the denser layers. The circulation is modified through a dynamical adjustment to the mass input into an intermediate layer that is produced by the convergence of diapycnal transport due to the tidally enhanced convection and diffusion. The dynamical adjustment is conducted mainly through Kelvin waves which have the ability to induce intergyre flow along the western boundary and also by eastward moving long Rossby waves. The latter can be present under the influence of the wind-driven gyres for the second and higher baroclinic modes and act to spread information into the interior directly from the western boundary. As a direct consequence of this adjustment, transport of the ventilated water from the Kuril Straits to the subtropical gyre is enhanced by the intergyre flow along the western boundary, which appears as the southward intrusion of the Oyashio Current. The transported water leaves the coast to encircle the interior, and returns to the western boundary, eventually flowing into the equatorial region. Such equatorward transport associated with mass convergence in the intermediate layer is compensated by transport toward the Kuril Straits in the shallower and deeper layers, thereby enhancing both shallow and deep meridional overturning cells by 2 - 3 Sv. The above dynamical adjustment is thus central to our understanding of the ventilation of the intermediate layer and provides a basis for the analytical model developed in an accompanying paper.
  • T Nakamura, T Toyoda, Y Ishikawa, T Awaji
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 53 3 425 - 448 2006年03月 [査読有り][通常論文]
     
    The role played by tide-induced diapycnal mixing at the Kuril Straits in the formation of the Okhotsk Sea water is examined using tin ocean general circulation model. The result shows that tidal mixing causes a freshening of intermediate water down to similar to 27.6(sigma 0). This implies all enhancement of ventilation by tidal mixing, since a freshening flux can collie only from above in the Okhotsk Sea. In fact, when compared with the case without tidal mixing, the production rate of Dense Shelf Water (DSW) increases from 0.01 to 0.13 Sv (10(6) m(3) s(-1)) with maximum DSW density increased from 26.6 to 26.8(sigma 0). This enhanced production of DSW allows Lis to successfully reproduce a layer of local potential vorticity minimum in the Okhotsk Sea, which characterizes the Okhotsk Sea Mode Water. The analysis reveals that tidal mixing at the Kuril Straits has two roles in enhancing ventilation. First, it preconditions the water properties of the DSW because tidal mixing at the Kuril Straits induces an upward salt flux from the saltier lower layer. The Subsequent transport of the saline water from the Straits to regions of sea-ice generation increases the density of subducting water, resulting in more realistic densities and production rates of DSW. Second, tidal mixing directly modifies water properties throughout the water column. The ventilation due to this mechanical mixing further enhances the intermediate layer ventilation, and is dominant ill the Kuril Straits and in the layer denser than the DSW. Although the effect of double diffusion on ventilation in the Okhotsk Sea is also examined, it is much less effective than the tidal mixing. Since the Okhotsk Sea is a key location for the ventilation of the North Pacific intermediate layer, the important implication here is that tidal mixing at the Kuril Straits enhances the meridional overturning in the entire North Pacific Ocean. (c) 2006 Elsevier Ltd. All rights reserved.
  • T Toyoda, T Awaji, Y Ishikawa, T Nakamura
    GEOPHYSICAL RESEARCH LETTERS 31 17 L17206  2004年09月 [査読有り][通常論文]
     
    The preconditioning of winter mixed layer in the formation of the eastern subtropical mode water (ESTMW) in the North Pacific Ocean is investigated using an eddy-permitting ocean general circulation model. The result shows that convergence in the northward Ekman transport of saltier water and weak summertime heat fluxes due to the presence of stratus cloud in the ESTMW formation region are both central to the initiation of significant mixed layer deepening that subsequently evolves into the ESTMW pycnostad in the main thermocline. These two factors originate from air-sea interactions inherent in the region of Northeast Pacific Basin. The approach described here has significant potential for determining the water-mass distribution and may lead to an explanation of the Sverdrup's Eastern Gyral in the subtropical North Pacific.
  • T Nakamura, T Awaji
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C9 C09S07  2004年07月 [査読有り][通常論文]
     
    [1] Baroclinic processes generated by the K-1 tidal flow around the Kuril Straits and their role in the linkage between the Okhotsk Sea and the North Pacific are investigated using a three-dimensional nonhydrostatic model. Large-amplitude unsteady lee waves are generated around sill tops all along the Kuril Island Chain and induce significant density inversions from the sea surface down to the similar to 27.6 sigma(theta) density layer. Along with this wave breaking, a large flow of energy takes place from the swift tidal flow regions to greater depths as a result of the clockwise propagation of both the topographically trapped waves around the islands and the remaining unsteady lee waves. Such wave generation and energy exchange eventually cause intense diapycnal mixing over the shallow topographic features and near the bottom, leading to a maximum diapycnal diffusivity of over 10(3) cm(2) s(-1). Even in the deep Bussol' Strait, which is thought to be the main exit of the Okhotsk Sea water, considerable mixing ( corresponding to diapycnal diffusivity values of between 10 similar to 50 cm(2) s(-1)) occurs. Vigorous mixing in the shallow regions produces low potential vorticity (PV) water in the upper layer and high PV water below. Accordingly, PV fronts are formed along the island chain and sustain instabilities that in turn release baroclinic eddies with low core PV values. These features are indicated in recent satellite imagery and in situ observations, suggesting that the tide is the controlling factor in the formation of fronts observed in the Kuril Straits. Tidally induced mean transport across the straits is estimated to reach similar to 26 Sv (1 Sv = 10(6) m(3) s(-1)), which is much larger than in the barotropic case. However, most of the exchanged water tends to circulate around the islands due to the effect of bottom topography, whereas eddies pinched off from the tidal fronts carry the exchanged water offshore. The amount of eddy-driven offshore transport away from the islands reaches 14 Sv, with roughly half being directed to the North Pacific. This significant offshore transport spreads the effect of the diapycnal mixing taking place in the straits over wider areas of the North Pacific and the Okhotsk Sea. For example, the PV values generated in the eddies differ by 30 - 40% from the ambient water on average, whereas sea surface salinity within the eddies is 0.1 psu higher than the surroundings, corresponding to the density increase of 0.08 sigma(theta). Thus the eddy transport provides significant PV and salinity fluxes to both the North Pacific and the Okhotsk Sea. Hence we infer that a combination of tidally driven processes taking place at the passages leads to the formation of eddies that play a central role in water transport and transformation and are an essential aspect of the physical interplay between the Okhotsk Sea and the North Pacific.
  • T Nakamura, T Toyoda, Y Ishikawa, T Awaji
    JOURNAL OF OCEANOGRAPHY 60 2 411 - 423 2004年04月 [査読有り][通常論文]
     
    A numerical study using a 3-D nonhydrostatic model has been applied to baroclinic processes generated by the K, tidal flow in and around the Kuril Straits. The result shows that large-amplitude unsteady lee waves are generated and cause intense diapycnal mixing all along the Kuril Island Chain to levels of a maximum diapycnal diffusivity exceeding 10(3) cm(2)s(-1). Significant water transformation by the vigorous mixing in shallow regions produces the distinct density and potential vorticity (PV) fronts along the Island Chain. The pinched-off eddies that arise and move away from the fronts have the ability to transport a large amount of mixed water (similar to14 Sv) to the offshore regions, roughly half being directed to the North Pacific. These features are consistent with recent satellite imagery and in-situ observations, suggesting that diapycnal mixing within the vicinity of the Kuril Islands has a greater impact than was previously supposed on the Okhotsk Sea and the North Pacific. To examine this influence of tidal processes at the Kurils on circulations in the neighboring two basins, another numerical experiment was conducted using an ocean general circulation model with inclusion of tidal mixing along the islands, which gives a better representation of the Okhotsk Sea Mode Water than in the case without the tidal mixing. This is mainly attributed to the added effect of a significant upward salt flux into the surface layer due to tidal mixing in the Kuril Straits, which is subsequently transported to the interior region of the Okhotsk Sea. With a saline flux into the surface layer, cooling in winter in the northern part of the Okhotsk Sea can produce heavier water and thus enhance subduction, which is capable of reproducing a realistic Okhotsk Sea Mode Water. The associated low PV flux from the Kuril Straits to the open North Pacific excites the 2nd baroclinic-mode Kelvin and Rossby waves in addition to the 1st mode. Interestingly, the meridional overturning in the North Pacific is strengthened as a result of the dynamical adjustment caused by these waves, leading to a more realistic reproduction of the North Pacific Intermediate Water (NPIW) than in the case without tidal mixing. Accordingly, the joint effect of tidally-induced transport and transformation dominating in the Kuril Straits and subsequent eddy-transport is considered to play an important role in the ventilation of both the Okhotsk Sea and the North Pacific Ocean.
  • Toshiyuki Awaji, Shuhei Masuda, Yoichi Ishikawa, Nozomi Sugiura
    Journal of Oceanography 59 6 931 - 943 2003年12月 [査読有り][通常論文]
     
    A four-dimensional variational data assimilation system has been applied to an experiment to describe the dynamic state of the North Pacific Ocean. A synthesis of available observational records and a sophisticated ocean general circulation model produces a dynamically consistent dataset, which, in contrast to the nudging approach, provides realistic features of the seasonally-varying ocean circulation with no artificial sources/sinks for temperature and salinity fields. This new dataset enables us to estimate heat and water mass transports in addition to the qualification of water mass formation and movement processes. A sensitivity experiment on our assimilation system reveals that the origin of the North Pacific Intermediate Water can be traced back to the Sea of Okhotsk and the Bering Sea in the subarctic region and to the subtropical Kuroshio region further south. These results demonstrate that our data assimilation system is a very powerful tool for the identification and characterization of ocean variabilities and for our understanding of the dynamic state of ocean circulation.
  • 中村 知裕, 淡路 敏之, 豊田 隆寛, 石川 洋一
    沿岸海洋研究 41 1 13 - 22 日本海洋学会沿岸海洋研究部会 2003年08月 [査読無し][通常論文]
     
    高分解能京都大学海洋大循環モデルによる北太平洋シミュレーション実験結果を用いて,沿岸親潮の形成過程について検討した.シミュレートされた沿岸親潮は,春に低塩で秋に高塩かつ流速が最大となる等,観測から指摘されている特徴を良く再現していた.モデルの沿岸親潮は親潮の沿岸域での流れというよりも,むしろ春は海氷融解による低塩・低温水,秋は夏季の宗谷暖流水を起源とする高塩・高温水により駆動される季節変動に富んだ局所的な流れであった.この結果は,オホーツク海からの密度・渦位フラックスのみでも,沿岸親潮は駆動されることを示している.興味深いことに,春季には宗谷暖流水の密度が高すぎて西部千島海峡を横切れないため,北海道南東沿岸に現れる高塩水の起源は夏季の宗谷暖流水のみであった.このことは,オホーツク海中層水の形成や宗谷暖流と沿岸親潮のつながりを理解する上で重要である.また,従来の大循環モデルでは考慮されていない潮流過程が,千島列島における海水混合・交換を通して沿岸親潮に影響を与えうることが示唆された.
  • 海洋大循環モデルに見られた沿岸親潮
    中村知裕, 淡路敏之, 豊田隆寛, 石川洋一
    沿岸海洋研究 41 1 13 - 22 2003年 [査読有り][招待有り]
  • T Nakamura, T Awaji
    JOURNAL OF PHYSICAL OCEANOGRAPHY 31 8 2511 - 2524 2001年 [査読有り][通常論文]
     
    On the basis of ray tracing of individual waves generated at various phases of the tidal flow, an amplification mechanism is presented for a new class of topographically generated internal waves identified by Nakamura et al., which develop across a broad latitude range and can exist even above the critical latitude where the tidal frequency equals the inertial frequency. The results show that unsteady lee waves are always amplified when the maximum frequency is much smaller than the buoyancy frequency because their phase speeds (amplitudes) are equal (proportional) to the tidal flow speed at their time of generation. Fast mixed tidal-lee waves are also effectively amplified, when the rotation effect is significant. Accordingly, amplification of unsteady lee and fast mixed tidal-lee waves can occur even if the requirements of previous theories (e.g., the critical slope and critical Froude number conditions) are not satisfied. Since the result here covers the generation and amplification processes of topographic internal waves across a broader parameter range than earlier theories, it should contribute to a better understanding of boundary mixing processes, especially in high-latitude regions.
  • T Nakamura, T Awaji, T Hatayama, K Akitomo, T Takizawa
    JOURNAL OF PHYSICAL OCEANOGRAPHY 30 7 1622 - 1644 2000年07月 [査読有り][通常論文]
     
    The tidal exchange between the Okhotsk Sea and the North Pacific Ocean is studied numerically with particular emphasis on the predominant K-1, barotropic component. The calculated harmonic constants of the K-1, tide in and around the Okhotsk Sea agree well with those obtained from extensive tide gauge observations. The features of the simulated tidal fields are similar to those reported in the literature. Since the K-1, tide is subinertial in the Okhotsk Sea, topographically trapped waves are effectively generated, contributing to strong tidal currents with a maximum amplitude of over 1.5 m s(-1) in the Kuril Straits. The structures of tide-induced mean flows in most passages of the straits are characterized by "bidirectional currents" (in which the mean flow exhibits a reversal in direction across the passages). This feature is clearly indicated in NOAA infrared imagery. The mean transport shows significant net exchange of water via several straits in the Kuril islands. A transport of about 5.0 Sv (1 Sv = 10(6) m(3) s(-1)) toward the North Pacific is produced by the K-1, tide, primarily through the Bussol. Kruzenshterna, and Chetverty Straits. Analysis reveals that the bidirectional mean currents at shallow passages are produced through the well-known process of tidal rectification over variable bottom topography, whereas in deep passages such as Bussol Strait, propagating trapped waves along the islands are essential for generating the bidirectional mean currents. Particle tracking clearly demonstrates these features. The tidal current is therefore thought to play a major role in water exchange processes between the Okhotsk Sea and the North Pacific.
  • T Nakamura, T Awaji, T Hatayama, K Akitomo, T Takizawa, T Kono, Y Kawasaki, M Fukasawa
    JOURNAL OF PHYSICAL OCEANOGRAPHY 30 7 1601 - 1621 2000年07月 [査読有り][通常論文]
     
    Numerical experiments with a two-dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the Kuril Straits and their effect on vertical mixing. The results show that sill-scale internal waves at the K-1, tidal frequency are confined to the sill slopes because the K-1, tide is subinertial in the Kuril Straits. In contrast to previous theories, the authors show that intense short internal waves generated at the sill breaks by the subinertial K-1, tidal current can propagate upstream as the tidal current slackens. Theoretical considerations identify these short waves as unsteady lee waves, which tend to be trapped at the generation region and:row into large-amplitude waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking lending to a maximum vertical diffusivity of similar to 10(3) cm(2) s(-1). This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M-2, tidal current does not cause such strong vertical mixing, because most of generated internal waves propagate away as first-mode internal tides and because the barotropic flow amplitude is small. The authors therefore suggest the possibility that generation of lee waves through interactions between the K-1, current and the bottom topography of the Kuril Straits contributes to the observed modification of the Okhotsk Sea water required in the formation of the NPIW.

書籍

  • 低温環境の科学事典
    中村知裕 (担当:分担執筆範囲:オホーツク海の下層雲・霧と大気海洋相互作用, 第10章, 第2節)
    朝倉書店 2016年
  • 低温科学便覧
    中村知裕 (担当:分担執筆範囲:環オホーツク海域における海洋循環, 第8章, 第2節)
    丸善出版株式会社 2015年
  • オホーツクの生態系とその保全
    三寺史夫, 内本圭亮, 中村知裕, 西岡純, 三角和弘, 津旨大輔 (担当:分担執筆範囲:オホーツク海および親潮域における物質循環のモデリング)
    北海道大学出版会 2013年
  • 田畑 伸一郎, 江淵 直人, 大島 慶一郎, 西岡 純, 三寺 史夫, 中村 知裕, 庄子 仁, 南 尚嗣, 八久保 晶弘, 白岩 孝行, 本村 眞澄, 西内 修一, 封 安全, 田畑 朋子 
    北海道大学出版会 2012年 (ISBN: 9784832967700)

その他活動・業績

受賞

  • 2007年 日本海洋学会 岡田賞
     
    受賞者: 中村知裕

教育活動情報

主要な担当授業

  • 気候モデリング特論
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 数値モデリング、気候、大気、海洋


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