The environmental state of freshwater resources in Greece (rivers and lakes)   总被引：1，自引：0，他引：1  

N. T. Skoulikidis  I. Bertahas  T. Koussouris 《Environmental Geology》1998,36(1-2):1-17

 The dominant factor regulating Greek riverine dissolved inorganic solid concentrations is chemical weathering, which depends on catchment climate and petrography. The majority of Greek rivers are moderately polluted and only a few of them show significant human impact and an increase in pollutant concentrations over time. Due to droughts in recent years and anthropogenic influence, the mean annual riverine dissolved solid concentrations exhibit a general interannual increase and significant long-term variations in their intra-annual hydrochemical fluctuations. The shallow Greek lakes are eutrophic, while the deep ones are oligo-mesotrophic. The majority of the lakes are monomictic and show summer thermal stratification, while the shallow ones are characterized by anoxic hypolimnetic conditions. Catchment petrography is not a dominant factor in the composition of lake waters since biochemical processes prevail. The majority of Greek lakes are also moderately polluted and are characterized by phosphorous-limited photosynthesis. Received: 17 March 1997 · Accepted: 16 January 1998  相似文献   

Surface Boundary-Layer Characteristics Over a Tropical Inland Station: Seasonal Features     

Muvva Venkata Ramana  Praveena Krishnan  P. K. Kunhikrishnan 《Boundary-Layer Meteorology》2004,111(1):153-157

Some characteristic features of the atmospheric surfacelayer over an inland tropical station, Lucknow (27°N, 81°E)in India, are investigated for different seasons using micrometeorological data collected at the 10-m levelof an instrumented tower. Seasonal monsoonal variations in wind circulation (north-easterliesand south-westerlies) dominate the flow over this part of the region. This study utilizesthe wind, temperature and humidity data obtained for a period of one year from February 1996 to January 1997. The diurnal variation of mean parameterssuch as temperature, relative humidity, and wind speed are studied for different seasonsnamely pre-, south-west, post- and north-east monsoon periods. Turbulent statistics are computedusing the eddy correlation technique, and are studied under the framework of Monin–Obukhov similarity theory with results compared with otherexperimental studies reported in the literature. It is found that the turbulence statistics arenearly independent of season and the near-neutral values of the normalized standard deviations ofthe three wind components, temperature and specific humidityare found to be 1.00 ± 0.04 (vertical), 2.63 ± 0.36(longitudinal), 2.19 ± 0.06 (lateral), 6.56 ± 0.33and 6.45 ± 0.33 respectively.  相似文献   

江淮地区夏季雨量与北半球500 hPa环流遥相关的不稳定性   总被引：2，自引：0，他引：2       下载免费PDF全文

杨秋明 《应用气象学报》2004,15(5):612-622

用50年 (1951~2000年) 观测资料, 分成1951~1976年和1977~2000年两个时段, 研究了江淮地区夏季 (6~8月) 雨量与北半球500 hPa季平均环流时滞遥相关的年代际变化, 同时提出了遥相关不稳定指数, 讨论了遥相关不稳定性的空间分布和季节变化。结果表明, 这两个时段遥相关的空间分布存在显著差异, 且前一时期时滞遥相关强度明显强于后一时期, 特别是江淮地区夏季雨量与前期冬季西太平洋型 (WP) 的相关仅在1976年以前显著, 1977年以后迅速减弱消失, 而对于后一时期, 与春季欧亚遥相关型 (EU) 的相关明显加强。另外, 后一时期江淮地区夏季雨量与夏季EAP波列的相关明显强于前一时期。它反映了太平洋年代际振荡 (PDO) 对东亚季风降水和环流年际相关的影响, 这种遥相关的不稳定性与年代际和年际时间尺度振荡之间的相互作用有密切联系。  相似文献   

Investigation of temporal gravity variations in SG-records     

Bruno Meurers 《Journal of Geodynamics》2004,38(3-5):423

Not all of the gravity signals commonly observed in records of superconducting gravimeters have a physically significant meaning. Instrumental noise or signal components generated by specific processing techniques can have a strong effect. This paper analyzes selected case studies from gravity data acquired during the first period of GGP and tries to give an answer to the question of which signals are significant. It is focused on small amplitude variations in the time domain, which are compared with those of conventional spring-type gravimeters. Coherent atmospheric gravity signals due to atmospheric processes associated with strong vertical convection could be observed by different sensors. The second part deals with temporal variations of main tidal parameters, which can be seen clearly in the tidal analysis results from SG data. Synthetic case studies show that they are not strongly influenced by the analysis procedure and therefore mainly reflect ocean loading processes.  相似文献   

北极海冰减少及其与相关气象场的联系   总被引：2，自引：7，他引：2  

方之芳  郭裕福  乔琪  方锦辉 《高原气象》2002,21(6):565-575

利用 195 3— 1998年北极海冰资料及相应的海平面气压场和我国东北 4 2°N以北 2 2个台站气温资料 ,应用统计分析方法 ,研究海冰和气象场的年际和年代际变化以及它们的联系。得到如下结论 :(1)高纬各纬度带和主要海域的海冰范围都呈现明显的衰减现象 ,6 0°N以北纬带趋势项的方差贡献超过总方差一半 ,远远大于周期项的方差贡献 ,此海域更明显显示近年海冰减少的现象。 (2 )巴伦支海和格陵兰海 ,海冰的年代际变化具有明显的 10年以上的周期变化特点 ,也存在明显的减少趋势 ;而拉布拉多海和白令海海冰主要是 10年以上的周期变化。 (3)自 90年代开始 ,海冰均发生陡然减少的现象 ,对全球气候变暖现象 ,海冰的变化是十分敏感的。 (4) 4 0°N以北的各纬度带的海平面气压的总体趋势是下降的 ,北冰洋涛动指数明显显示海平面气压场的减少趋势和 90年代前后的显著性差异。 (5 )与海平面气压的下降相对应 ,我国东北的温度是明显上升的。 (6 )北冰洋涛动能制约巴伦支海、格陵兰海和拉布拉多海域的海冰范围 ,也与我国东北温度有十分密切的联系。当AO指数偏大时 ,即冬季冰岛附近海平面气压偏低时 ,巴伦支海和格陵兰海海冰范围缩小 ,而拉布拉多海海冰范围扩大 ;我国东北冬半年的温度出现明显上升。  相似文献   

赤道中印度洋夏季变温对中国夏季降水影响的数值模拟   总被引：5，自引：0，他引：5       下载免费PDF全文

张卫青  钱永甫 《大气科学》2002,26(1):91-101

利用大气环流模式模拟了赤道中印度洋海区夏季变温与海温本身异常对中国夏季降水的影响.结果表明:当该海区夏季正(负)海温距平值呈递增(递减)变化时,激发出的降水场的正(负)异常幅度比正(负)海温距平强迫下相应的降水场的正(负)异常幅度大;但当正(负)海温距平呈递减(递增)趋势变化时,激发出的降水异常场与正(负)海温距平强迫下相应的降水异常场的分布形势类似(不同),异常幅度也有所差异.夏季中印度洋不同类型的海温异常对大气加热的不同,导致东亚季风和印度季风的不同变异,暖海温异常下引起东亚季风的明显偏强,而冷海温异常下引起印度季风的明显偏强.  相似文献   

The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth?     

Matthew T. Hurtgen  Michael A. ArthurNeil S. Suits  Alan J. Kaufman 《Earth and Planetary Science Letters》2002,203(1):413-429

The present study employs a method for analysis of the sulfur isotopic composition of trace sulfate extracted from carbonates collected in Namibia in order to document secular variations in the sulfur isotopic composition of Neoproterozoic oceanic sulfate and to assess variations in the sulfur cycle that may have accompanied profound climatic events that have been described as the snowball Earth hypothesis. The carbonates in the Otavi Group of Northwest Namibia contain 3-295 ppm sulfate. Positive excursions, to a high of 40‰ (CDT), occur above the lower (Chuos Formation) and upper (Ghaub Formation) glacial intervals in the Rasthof and Maieberg cap carbonates, respectively. Positive excursions at the top of the Rasthof Formation (reaching 51‰) and within the overlying Gruis Formation (34‰) do not appear to correspond to glaciation. The δ³⁴S_sulfate values within the Ombaatjie Formation exhibit shifts over relatively short stratigraphic intervals (tens of meters), varying between ∼15 and 25‰. Cap carbonates from Australia exhibit positive δ³⁴S_pyrite trends with amplitudes similar to those of Namibian δ³⁴S_sulfate, although, more data are necessary to firmly establish these δ³⁴S trends as global in nature. δ³⁴S_sulfate excursions found in Namibian cap carbonates are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an isolated, anoxic global ocean, although, there are other mechanisms that may have facilitated these large shifts in δ³⁴S_sulfate. Regardless, the low sulfate concentrations in Otavi carbonates, the high amplitude variability of the δ³⁴S_sulfate curve, and the apparently full reduction of sulfate (as implied from δ³⁴S_pyrite data), even in strata low in C_org, suggest that Neoproterozoic oceanic sulfate concentrations were much lower than modern values. Additionally, the buildup of ferrous iron and banded-iron formations during the Sturtian glacial event would indicate that Fe supply exceeded sulfide availability during the glacials and/or that all sulfide was fixed and buried. This could be construed as further evidence in support of low oceanic sulfate (and sulfide) at this time.  相似文献   

Variations in zonal mean and planetary wave properties of the stratosphere and links with the troposphere     

R. S. Quiroz 《Pure and Applied Geophysics》1980,118(1):416-427

Zonal mean data and amplitudes and phases of planetary zonal waves were derived from daily hemispheric maps for tropospheric and stratospheric levels, for the four winters 1975–76 to 1978–79. Important year-to-year fluctuation in zonal means and wave activity are described, most notable of which are the changes from 1975–76 to 1976–77. Comparison of the relative strengths of the stratospheric and tropospheric jet streams shows a strong negative correlation (–0.8) between monthly mean zonal stratospheric winds (at 10 mb, 65°N) and zonal tropospheric winds (at 200 mb, 32.5°N, in the jet core) and a positive correlation (+0.7) between the stratospheric 10 mb winds and the tropospheric 200 mb winds at 65°N. Parameters correlated were the departures from the climatological mean zonal winds. The structure of correlation between wave amplitudes in the same wave number (1, 2) at different altitudes and between wave numbers 1 and 2 is investigated. We find a high correlation (+0.93) between wave 1 in the stratosphere (10 mb height) and wave 2 (height) in the troposphere at 65°N; but only a weak correlation (+0.2) between wave 1 amplitudes in the stratosphere and troposphere. These results suggest the possible importance of wave-wave interactions in processes linking the stratosphere and troposphere. The wave correlations presented here are based on comparisons of monthly means of daily amplitudes; the correlation structure in individual wave developments may differ, in view of the likelihood of altitudinal lags in wave amplification.  相似文献   

Lunar daily geomagnetic variations in New Zealand   总被引：1，自引：0，他引：1  

J. D. McKnight 《Geophysical Journal International》1995,122(3):889-898

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Variations in some physico-chemical parameters in a hypersaline coastal lagoon of Baja California,Mexico     

Gabriela Montaño Moctezuma  L. Fernando Bückle Ramírez 《International Journal of Salt Lake Research》1995,4(3):265-280

San Jose lagoon is a hypersaline body of water located in Mexico in the Baja California Peninsula. The lagoon belongs to a system that lies between the fault ridge known as San Jose Creek. Because of its marine origin, it can be considered as thalassohaline, but its isolation from the ocean has brought about changes in its salt composition. It has an area of 13,500 m², a mean depth of 80 cm and a total volume of 10,000 m³. It does not desiccate and can be considered as a permanent lagoon. Seasonal variations are small. TheArtemia population in San Jose produces cysts all year. To determine the physico-chemical conditions inducing permanent production of cysts, temperature, salinity, dissolved oxygen, and pH of the lagoon were monitored, as well as relative humidity and wind conditions in the region in different seasons of the year. From spring to summer, differences of 1 mg L^–1 of O₂, 1°C in water temperature, and 8 g L^–1 in salinity were observed, and from summer to winter differences of 3.3 mg L^–1, 6.5°C, and 14 g L^–1, respectively. Despite small seasonal variations, the lagoon exhibits strong spatial and daily changes that are important for cyst production.  相似文献