地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
117 巻 , 6 号
選択された号の論文の12件中1~12を表示しています
表紙
  • 2008 年 117 巻 6 号 p. Cover06_1-Cover06_2
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     “foss”は滝を表すアイスランド語.Langjökull(jökullは氷河を表す)を起源とするHvítá川の中上流部にあり,氷河に由来する白濁した川が水しぶきをあげて豪快に流れ下る.当地は,アイスランドの代表的な観光ツアーであるゴールデンサークルツアーにも組み込まれており,多くの人々が訪れる観光地である.河岸には,柱状節理や板状節理の発達した溶岩,枕状溶岩を含む角礫岩などがみられ,氷河の下で起こった火成活動の片鱗がうかがえる.目を上げれば,水源であるLangjökullをみることができる.アイスランドを訪れて印象的であったことの一つが水の美しさ.氷河性河川の豪快な流れ,溶岩原に湧き出す清澄な湧水,青白色の温泉水,どれも火山と氷河の国らしい自然の美しさを感じさせる.
    (写真・解説:金丸龍夫 撮影日2008年8月19日)
論説
  • 野村 律夫, 蜷川 清隆, 西戸 裕嗣
    2008 年 117 巻 6 号 p. 967-984
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     Brackish foraminifera were investigated from surface sediments and cores collected in Kumihama Bay, northern Kyoto Prefecture, to clarify the impact on brackish organisms of sea-level changes and recent global warming. Foraminifera increased in the 1950s and the early 1970s, corresponding to periods of higher sea levels around the Japanese Islands. Despite continuous sea-level rises from the 1980s, however, they began to decrease and became eutrophic fauna as represented by Trochammina hadai, preferring highly fertilized water. This change may be related to annual changes in sea level and lake water circulation, which become more intense during periods with wider annual ranges of sea-surface atmospheric pressure. Sea-level changes observed in Maizuru Bay during the 1950s and 1970s are intimately related to long-term changes in the Aleutian Low Pressure system called the Pacific Decadal Oscillation (PDO), but this has not been the case since the late 1980s. Thus, the mechanism of modern sea-level rises since the 1980s is probably not the same as that of the pre-1980s, and is caused by modern lake water conditions with less circulation and an abundance of T. hadai. In addition, the widening and deepening of the channel opening to the Sea of Japan between 1972 and 1975 are also found to contribute to the modification of brackish foraminiferal fauna in the lake.
短報
  • 李 虎相
    2008 年 117 巻 6 号 p. 985-996
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     本研究の目的は都市間の空間的相互作用の分析を通して国際航空ネットワークの構造を明らかにすることである。このために,本研究は, 社会ネットワーク分析の修正モデルを用いて都市間の空間的相互作用を分析した。1992年と2004年の国際航空ネットワークにおける各都市の国際的ネットワーク性と国際路線の連結度を測定し,各都市の最大連結度に当たる航空路線を用いて都市間の連結体系を解明する。
     都市間の最短距離による分析の結果,ロンドンやパリ,ニューヨーク,アムステルダム,フランクフルトが1992年と2004年におけるグローバルネットワークの中核を形成していることが明らかになった。この連結体系はロンドンを中心に形成されている。しかし,アジアの都市群は,東京,シンガポール,香港などを中心とする別の連結体系を形成していた。グローバルネットワークにおいて,アジアの都市間の連結体系が最も顕著な変化を示した。
解説
  • 後藤 忠徳, 三ケ田 均
    2008 年 117 巻 6 号 p. 997-1010
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     Electromagnetic (EM) methods have been developed for estimating the apparent resistivity or conductivity of material composing subsurface structure depending on survey objectives, while seismic methods are applied mainly to understand geological structure or stratigraphy. Magnetotelluric subsurface sounding (MT) methods are proven tools detect high-conductivity anomalies in the Earth's interior; controlled source EM (CSEM) methods detect high-conductivity anomalies; and, nuclear magnetic resonance (NMR) methods estimate “the mean free path” of hydrogen atoms in various molecules composing underground materials. Regarding methodologies, EM methods frequently applied in practice are: (1) natural source MT methods, (2) artificial source MT or EM methods, and (3) borehole EM methods. The fundamental principles of these methods can be summarized as measuring the induction effects of a survey target using a natural or artificial electromagnetic source. Finally, we investigate examples of recent EM approaches in connection with seismic methods and discuss the integration of survey data from both methods as a key to exploring underground structures not only in terms of stratigraphic interpretation but also for estimating material physical properties.
小特集:グローバル気候変動(Part II)
論説
  • 本井 達夫, 鬼頭 昭雄, 緑川 貴, 荒川 理, 笹井 義一, 陳 永利
    2008 年 117 巻 6 号 p. 1015-1028
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     Climate model experiments are carried out to understand the relationship between large-scale topography and climate variation. Mountain uplift experiments show that sea surface temperature, surface wind fields, precipitation and sea surface salinity are strongly influenced by mountain uplift. An enhanced Asian monsoon due to mountain uplift causes stronger seasonal coastal upwelling in the Indian Ocean and freshening in the Bay of Bengal, Yellow Sea and East China Sea. Mountain uplift experiments using a higher resolution atmospheric general circulation model reveal that the spatial pattern of precipitation becomes finer as resolution increases, and that there is a sharper contrast in the salinity distribution near coastal regions. Experiments in which the Panamanian Gateway is closed, opened and re-closed suggest that reorganization of the ocean current due to closure of the Panamanian Gateway induces a cooler and drier climate with a permanent halocline and sea ice in the subarctic Pacific.
総説
  • 柏木 洋彦, 小川 泰正, 鹿園 直建
    2008 年 117 巻 6 号 p. 1029-1050
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     The global carbon cycle controls the climate change in the Earth's environment on a geological timescale and is mainly associated with greenhouse effects produced by atmospheric carbon dioxide (CO2) and methane (CH4). This paper reviews the relationship between the global carbon cycle and presumed climate events during the Cenozoic.
     The global carbon cycle is primarily regulated by the balance between weathering and metamorphism-volcanism. Moreover, the organic carbon subcycle involving oxidative weathering and burial is of secondary importance. The balance of these geochemical processes results in variations of atmospheric CO2.
     The past climate on a geological time scale is reconstructed by several geochemical and paleontological methods or proxies. For example, sea-surface and deep-water temperature are deduced from oxygen isotope ratio and Mg/Ca ratio of foraminiferal tests. Terrestrial atmospheric temperature is estimated from leaf fossil and paleovegetation. Atmospheric CO2 level is calculated from carbon isotope ratios of phytoplankton and soil carbonate, stomatal density of leaf fossil, boron isotope ratio of foraminiferal test, Ce anomaly, and global carbon cycle modeling. It is important to consider their advantages and disadvantages in order to evaluate the paleoclimate adequately.
     Next, we discuss climate change based on these proxies. As a general trend, the Cenozoic climate change is characterized by a transition from ice-free to ice-covered conditions across the Eocene/Oligocene boundary. The Earth's surface environment was significantly warmed from the Paleocene to the Eocene by high levels of atmospheric CO2. Thereafter, it gradually cooled towards the present, which is possibly attributed to changes in ocean currents and other marine environments accompanying continental drift. This trend has been punctuated by several short-term climate events. The Paleocene-Eocene Thermal Maximum (PETM) was a remarkable warming event at the Paleocene/Eocene boundary, possibly attributed to the release of methane from hydrates into the atmosphere. Rapid cooling occurred at the Eocene/Oligocene boundary to form extensive continental ice sheets including the Antarctica, which seems to have been caused by atmospheric CO2 and change of oceanographic circulation and marine environment. After a moderate period from the late Oligocene to the early Miocene, there was a transient but significant warming in the middle Miocene. Since then, the Earth's environment has gradually cooled towards the present accompanied by the evolution of glaciations and marine environmental changes but a causal link between cooling and global carbon cycle has recently been pointed out. Although the carbon cycle including atmospheric CO2 and CH4 cannot explain all of the global climate changes in Cenozoic, it has undoubtedly played a dominant role on the Earth's climate.
論説
  • 山崎 孝治
    2008 年 117 巻 6 号 p. 1051-1062
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     The Arctic Oscillation (AO; Northern hemisphere annular mode), which is the most dominant mode of climate variability in the Northern extratropics, is reviewed. The AO is a seesaw pattern between the Arctic region and the mid-latitude regions. It is an atmospheric internal mode caused by interaction between mean flows and eddies. In winter, it extends to the stratosphere, and the tropospheric AO interacts with the stratospheric AO. Since the mid-20th century, the AO has shown an increasing trend in winter and summer. Climate models predict a future positive trend due to global warming. Recent sea ice loss in the Arctic Ocean is also discussed. The decline of the Arctic sea ice cover in late summer has accelerated recently and a record-low ice cover was observed in September 2007. The rate of decrease is much faster than climate model predictions, and it might pass a tipping point.
  • 川村 隆一
    2008 年 117 巻 6 号 p. 1063-1076
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     In the tropics, the El Niño-Southern Oscillation (ENSO) occupies a major part of the interannual air-sea interactive system. The ENSO plays a vital role in triggering the occurrence of extraordinary anomalous climates and weather not only in the tropics but also in the extratropical regions. In the South Asian monsoon region, the ENSO can influence the interannual variability of the monsoon system through at least two different impacts. During the decay phase of ENSO (from winter to summer) its delayed impact operates through large-scale air-sea interaction in the tropical Indian Ocean and land-surface hydrological processes over the Asian Continent, eventually bringing about change in summer monsoon activity in June and July. During the ENSO growth phase (from summer to winter), in contrast, its direct impact is considered to account for monsoon interannual variability especially in August and September. East Asian monsoon variability is also significantly affected by ENSO-related tropical forcing. Especially in early winter, ENSO-related anomalous convection can give rise to a change in the East Asian winter monsoon system through stationary Rossby wave propagation along the South Asian waveguide, but the remote response depends on the geographical configuration of the anomalous tropical convection. In summer, the ENSO's delayed impact is associated with excitation of an extratropical teleconnection, which causes anomalous weather in northeastern Asia.
     Midlatitude air-sea interactions and their potential impact on large-scale atmospheric circulations are also discussed. The coexistence of the East Asian winter monsoon flow and western boundary current makes air-sea heat exchanges in the Kuroshio extension very active. Due to enhanced baroclinicity and surface heat fluxes from the ocean, a number of extratropical cyclones tend to develop explosively in the vicinity of Japan. The activity of these extratropical cyclones contributes to the downstream development of upper-level teleconnections.
  • 菅野 洋光
    2008 年 117 巻 6 号 p. 1077-1093
    発行日: 2008/12/25
    公開日: 2010/04/26
    ジャーナル フリー
     Mean summer temperatures in northern Japan appear to exhibit cyclical variations after the regime shift—global-scale climatic jump of oceanography and meteorology fields—around the late 1970s. Alternation between cool and hot summers has resulted in cool and hot weather damage to rice crops, respectively. The temperature fluctuations are caused by the Rossby wave propagation (Pacific-Japan pattern) from the tropical western Pacific (Nitta, 1987). This teleconnection also affects other regions, hence many agricultural districts in Asia may experience simultaneous meteorological effects. This paper focuses on the influence of the teleconnection pattern on three areas: northern Japan, northeastern China, and Java, Indonesia. These areas are important in terms of food supplies (mainly paddy rice production) for their countries.
     In northern Japan, variations of summer temperatures correlate strongly with the 500 hPa height field and SST east-west contrast around the western tropical Pacific. A positive correlation between the SST contrast and northern Japan summer temperatures has occurred since the 1980s, and it is clear that after the regime shift northern Japan summer temperatures have been affected by Rossby wave propagation from the tropical Pacific Ocean.
     The relations between summer temperatures in northern Japan and in Heilongjiang, northeastern China, and the global meteorology field were analyzed. Summer temperatures in northern Japan and Heilongjiang were found not to exhibit simultaneous variations. Northern Japan summer temperatures have a negative correlation with summer 500 hPa heights over a wide tropical area centered on the Indochina Peninsula and around eastern Siberia, which are the results of Rossby wave propagation and formation of Okhotsk high pressure in summer. In the case of Heilongjiang, there is not a strong correlation with the tropical area and there is no correlation around eastern Siberia. Hence, the teleconnection pattern does not strongly affect summer temperatures in northeastern China. Summer temperatures in northern Japan and spring surface pressure show a negative correlation around the Arabian Sea and the western tropical Pacific. This may signal the pre-Indian monsoon and pre-Asian monsoon pressure field. On the other hand, summer temperatures in Heilongjiang and the spring pressure field show a strong positive correlation over the Tibetan Plateau. This result raises interesting issues about their relationship, such as whether a spring high pressure indicates a dry surface over the plateau and whether an increase of sensible heat from the land affects any feedbacks in the next season. In summary, the factors that cause summer temperature variations over northern Japan and Heilongjiang, respectively, are different.
     Precipitation in JJA in Java and global surface pressure show a positive correlation around northern Japan. This implies that high precipitation over Java is related to a high-pressure anomaly over northern Japan. This result may be due to the simultaneous influence of the Rossby wave on both areas.
     In conclusion, the factors affecting summer temperature variations in northern Japan and Heilongjiang in China differ, and summer climates in northern Japan and Java, Indonesia are closely connected through Rossby wave propagation. This study improves our understanding of simultaneous variations of climate via teleconnection patterns for these agriculturally important locations, which is of importance to understand variations of food supplies under possible future climatic variability.
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