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41.
Jucundus Jacobeit Peter Jönsson Lars Bärring Christoph Beck Marie Ekström 《Climatic change》2001,48(1):219-241
Zonal circulation indices with monthly and seasonal resolutions are calculated based on gridded monthly mean sea-level pressure (SLP) reconstructed back to 1780 by Jones et al. (1999): an overall zonal index for the whole European area between 30°W and 40°E, a normalized index for the North Atlantic Oscillation (NAO), and a similar index for Central Europe. For most of the early time up to the mid-nineteenth century we get preferred negative anomalies in the NAO index for winter and preferred positive ones for summer. The turning points in cumulative anomalies - during the 1850s for winter and during the 1870s for summer - indicate a transition period in circulation modes from the "Little Ice Age" to the recent climate in Europe. Running correlations (time windows of 21 years with time steps of one year) between zonal indices and regional temperature time series from Central England, Stockholm and two Central European regions are all indicating major instationarities in these relationships with a particular decline in winter correlations around the turn from the nineteenth to the twentieth centuries. Aspects of different circulation patterns linked with these variabilities are discussed. 相似文献
42.
—?Measurements of group velocity are derived from phase-velocity dispersion curves and modeled with global laterally-varying isotropic structure. Maps for both Love and Rayleigh waves are created in the period range 35?s to 175?s. The data set of group-velocity measurements includes over 50,000 minor-arc observations and 5,000 major-arc observations. The errors in the measurements are estimated by an empirical method of comparing pairwise-similar paths, resulting in uncertainties which are 20% to 40% of the size of the typical measurement. The models are determined by least-squares inversion for spherical harmonic maps expanded up to degree 40. This parameterization allows for resolution of structures as small as 500?km. The models explain 70–98% of the variance relative to the Preliminary Reference Earth Model (PREM). For the area of Eurasia, the group-velocity maps from this study are compared with those of Ritzwoller and Levshin (1998). The results of the two studies are in very good agreement, particularly in terms of spatial correlation. The models also agree in amplitude at wavelengths longer than 30?degrees. For shorter wavelengths, the agreement is good only for models at short periods. The global maps are useful for prediction of group arrival times, for revealing tectonic structures, for determination of seismic event locations and source parameters, and as a basis for regional group-velocity studies. 相似文献
43.
A method for sampling estuarine waters and seawater by scuba diving is described. Samples taken from the Krka River Estuary on the eastern Adriatic coast were analysed for mercury content using cold vapour atomic absorption spectrometry (CVAAS). Mercury levels as low as 1 ng dm−3 were determined and these are among the lowest values reported to date. A simple contamination free procedure for the preconditioning of sample containers in the sea was applied. Sampling of water by divers is particularly useful when the intermediate layer in an estuary has to be identified and sampled, as well as for obtaining detailed vertical concentration profiles for mercury and other trace elements. 相似文献
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45.
Jianliang Huang Goran Pavlic Alfonso Rivera Dan Palombi Brian Smerdon 《Hydrogeology Journal》2016,24(7):1663-1680
The applicability of the Gravity Recovery and Climate Experiment (GRACE) to adequately represent broad-scale patterns of groundwater storage (GWS) variations and observed trends in groundwater-monitoring well levels (GWWL) is examined in the Canadian province of Alberta. GWS variations are derived over Alberta for the period 2002–2014 using the Release 05 (RL05) monthly GRACE gravity models and the Global Land Data Assimilation System (GLDAS) land-surface models. Twelve mean monthly GWS variation maps are generated from the 139 monthly GWS variation grids to characterize the annual GWS variation pattern. These maps show that, overall, GWS increases from February to June, and decreases from July to October, and slightly increases from November to December. For 2002–2014, the GWS showed a positive trend which increases from west to east with a mean value of 12 mm/year over the province. The resulting GWS variations are validated using GWWLs in the province. For the purpose of validation, a GRACE total water storage (TWS)-based correlation criterion is introduced to identify groundwater wells which adequately represent the regional GWS variations. GWWLs at 36 wells were found to correlate with both the GRACE TWS and GWS variations. A factor f is defined to up-scale the GWWL variations at the identified wells to the GRACE-scale GWS variations. It is concluded that the GWS variations can be mapped by GRACE and the GLDAS models in some situations, thus demonstrating the conditions where GWS variations can be detected by GRACE in Alberta. 相似文献
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47.
The paleo-evolution of the Black Sea level during the Lateglacial–Holocene transition is still unclear, which motivates us
to provide new estimates for that period based on the analysis of water budget. Hydrological conditions in the Black Sea catchment
area are reconstructed here using water balance equation, available data, and constraints based on simple theory relating
the runoff ratio with climatic characteristics. In order to estimate the impact of the aridity of climate we consider two
alternative scenarios: (1) climate change under constant in time gradient in precipitation and evaporation over land and sea,
and (2) climate change accounting for changes in the horizontal distribution of precipitation and evaporation. Hydrological
data are compiled from available present-day data and paleo-observations. A number of sensitivity experiments is carried out
revealing that the hydrological conditions in the Black Sea watershed should have evolved towards a very arid climate (similar
to the present-day climate in the Caspian Sea area) in order to initiate a drop of sea level of ∼100 m below the sill depth
of the Bosporus Straits, as speculated in some recent research. Estimates of sea level changes reveal a qualitative agreement
with the coast-line evolution inferred from paleo-observations. The Younger Dryas draw down of the Black Sea starts at about
13.3 to 13 kyr BP, with a maximum low-stand of 104 m at 11.5 kyr BP. In an arid climate scenario the sea level reaches the
outlet at about 8.8 to 7.4 kyr BP. Approximately at that time, Mediterranean sea level was about 10 m higher, making flooding
events of the Black Sea possible. However, the nature and exact timing of the Holocene reconnection depends also on other
(not well known) factors; one of them is the Bosporus sill depth, motivating further research in this field. Estimates of
the water transport through the Bosporus Straits are also provided for the time of Lateglacial–Holocene transition. 相似文献
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49.
Anne Hormes Maarten Blaauw Svein Olaf Dahl Atle Nesje Goran Possnert 《Quaternary Geochronology》2009,4(4):267-277
The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the 14C calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event. 相似文献
50.