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71.
Cristian A. Vargas Diego A. Narvez Andrea Piones Roberto M. Venegas Sergio A. Navarrete 《Estuarine, Coastal and Shelf Science》2004,61(4):603-612
We studied the occurrence of large high frequency temperature fluctuations and their potential association with settlement of intertidal invertebrates during the spring/summer period 1999/2000 at Las Cruces, on the coast of central Chile. Our results showed the existence of internal tidal bores, characterized by sharp drops in water temperature at the surface and near the bottom, and subsequent temperature increases, which occurred with a semidiurnal periodicity. Measured currents support the hypothesis of alternating onshore and offshore movement of warm-water fronts. The frequency of the events varied through the summer and their amplitude seems to be modulated by onshore winds. The strongest events were observed when strong onshore winds occurred in late afternoon hours and the entire water column showed a semidiurnal temperature signal. Highest values of chlorophyll concentration in the intertidal zone and daily settlement of bivalves, gastropods and crustaceans were observed at times when conditions were favorable for occurrence of internal tidal bores. Results suggest that internal tidal bore warm fronts could play an important role in the transport of neustonic larval invertebrates and the delivery of phytoplankton to at least some intertidal sites along the coast of Chile. 相似文献
72.
The helicity, electromotive force and α-effect produced in a homogeneous, rapidly rotating, electrically conducting fluid by an isolated source of buoyancy at small Elsasser number are calculated, visualized and analyzed. Due to physical symmetries of the system, the integrals of helicity and electromotive force over all space are zero. However, each has a significant non-zero value when integrated over the cross section of the Taylor column. The local α-effect is found to be significantly anisotropic; it is strongest when the applied magnetic field is toroidal and the resulting EMF is parallel to the applied field. 相似文献
73.
Michel Magny Gilles Bossuet Pascale Ruffaldi Aurélie Leroux Jacques Mouthon 《第四纪科学杂志》2011,26(2):171-177
This paper presents a lake‐level record for the Holocene at Lake Cerin (Jura Mountains, eastern France). It is based on a range of sedimentological techniques validated in previous studies, with a combination of systematic lithostratigraphic investigations of the infillings accumulated in the lacustrine basin, and sediment analyses of two selected cores. The chronology is based on 10 radiocarbon dates and pollen stratigraphy. On a millennial scale, the Cerin lake‐level record shows three distinct successive phases characterised by higher lake‐level conditions until ca. 9000 cal. a BP, followed by a maximal lowering at ca. 9000–8500 cal. a BP, and a progressive rise until the present. This rise was punctuated by centennial‐scale fluctuations, with major events around 4000, 2800 and after 1500 cal. a BP. Considered on a multimillennial scale, the general pattern of palaeohydrological changes reconstructed at Cerin reflects the impact of orbitally driven summer insolation. This is in agreement with other regional and extra‐regional palaeoclimatic records, although every record shows peculiarities in timing and shape depending on the proxy used for reconstruction. In this general context, centennial to multicentennial oscillations appear to have been second‐order events in comparison with the major influence of the orbital factor. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
74.
The fluctuation of the water table east of La Pampa province and northwest of Buenos Aires province, Argentina, influences
agricultural production in the region because it is closely related to the alternation of dry and wet periods. Sea-surface
temperature (SST) anomalies have been used as predictors to forecast atmospheric variables in different regions of the world.
The objective of this work is to present a simple model to forecast seasonal rainfall using SST distribution in the Pacific
Ocean as a predictor. Once the relationship between precipitation and water-table fluctuations was established, a methodology
for the prediction of water-table fluctuations was developed. A good agreement between observed and predicted water-table
fluctuations was found when estimating water-table fluctuations in the summer and autumn seasons.
Electronic Publication 相似文献
75.
A periodic vertical movement of the groundwater table results in a subsequent cyclic response of the water content and pressure profiles in the vadose zone. The sequence of periodic wetting and drying processes can be affected by hysteresis effects in this zone. A one-dimensional saturated/unsaturated flow model based on Richards’ equation and the Mualem (Soil Sci. 137 (1984) 283) hysteresis model is formulated which can take into account multi-cycle hysteresis effects in the relation between capillary pressure and water content. The numerical integration of the unsaturated flow equation is based on a Galerkin-type finite element method. The flow domain is discretised by finite elements with linear shape functions. Simulations start with static water content and pressure profiles, which correspond to either a boundary drying or wetting retention curve. To facilitate the numerical solution of the hysteretic case an implicit non-iterative procedure was chosen for the solution of the nonlinear differential equation. Laboratory experiments were performed with a vertical sand column by imposing a high frequency periodic pressure head at the lower end of the column. The total water volume in the column, and the periodic water content profile averaged over time were measured. The boundary drying and wetting curves of the relation between water content and capillary pressure were determined by independent experiments. The simulations of the experimental conditions show a clear effect of the hysteresis phenomenon on the water content profile. The simulations with hysteresis agree well with the measurements. Computed dimensionless water content profiles are presented for different oscillation frequencies with and without consideration of hysteresis. 相似文献
76.
Eric A. Colhoun 《The Australian geographer》1993,24(1):3-11
Observational studies on Cainozoic environmental changes in both Tasmania and East Antarctica have led to important modifications in the accepted interpretation of the history of glaciation for each area. Many contributors have played an important role in the processes of investigation leading to the present state of knowledge. A crucial role has been played by the integration of the methods of glacial geomorphology, stratigraphy, sedimentology and dating methods. From studies based on combinations of these approaches, a model of multiple glaciations has been developed to explain observed features in Tasmania. This model is complemented by studies of vegetation history, largely through pollen analysis. A less complete picture of the history of ice‐sheet fluctuations in East Antarctica is beginning to emerge from similar applications of cross‐disciplinary studies. 相似文献
77.
N. Andresen J. J. G. Reijmer A. W. Droxler 《International Journal of Earth Sciences》2003,92(4):573-592
The sedimentological study of thirteen sediment cores from the periplatform setting surrounding Pedro Bank (Northern Nicaragua Rise, Caribbean Sea) shows that during the last 300 ka turbidite deposition is controlled by at least four factors: (1) late Quaternary sea level fluctuations, (2) prolific fine-grained sediment production and export resulting in oversteepening of the upper slope environment, (3) the proximity to the bank margin, and (4) local slope and seafloor morphology. The most intriguing finding of this study is the paucity of turbidites, with only 101 turbidites in 13 cores in this tectonically active setting near the Caribbean plate boundary. Throughout the last 300 ka, the frequency of turbidite input during interglacial stages is three times higher than during glacial stages. Also it is obvious that changes in sea level influence the timing of turbidite deposition. This is especially evident during the transgressions resulting in rapid renewed bank-top flooding, subsequent neritic sediment overproduction, and offbank export. The flooding event during each transgression is usually recorded by the onset of turbidite deposition at various sites along several platform-to-basin transects in down- and upcurrent slope settings. Overall, however, more turbidites are deposited during the regressive rather than the transgressive phases in sea level, probably as a result of sediment reorganisation on the slope resulting in slope failure. Five cores show "highstand bundling" of calciturbidites, i.e. higher number of turbidites during highstands than during lowstands in sea level. 相似文献
78.
This study presents scenario models for historical variations of climate and slope stability. A model for historical annual patterns of temperature and rainfall was established on the basis of seasonal proxies. A process-based, spatio-temporal model for groundwater variations and slope stability was developed using the GIS environment of the software PCRaster. We applied the slope stability model to study the effects of the different climate scenarios on slope stability for three different hillslopes in the area around Bonn (Germany). The findings indicate three climatic phases with different annual temperature and precipitation patterns over the historic period. The modeling results show that a climatic scenario representing unstable conditions of a transition from the more humid Little Ice Age to dryer recent climate produces the highest slope instabilities. The intensity of this impact, however, varies with the sensitivity of the geomorphic system, i.e. local landforms and lithology, and cannot be generally related to the stability of a specific hillslope. More unstable areas are not necessarily more sensitive to climatic changes: the location of permeable layers (prone to groundwater rise) in relation to sensitive layers (lower strength) and higher gradients (higher stress) influences the sensitivity of a site with respect to climate changes. The presented method is capable of modeling landscape sensitivity to climate change with respect to groundwater-controlled landslides. 相似文献
79.
A. E. Shlezinger Yu. B. Gladenkov V. A. Zakharov 《Stratigraphy and Geological Correlation》2006,14(6):690-692
Despite great practical achievements, sequence stratigraphy failed to elaborate until now the reliable technique for establishing relative sea-level fluctuations (RSLF). The method proposed recently by Yu.A. Volozh with colleagues for determining the RSLF represents one of interesting but unconvincing attempts in this respect. Use of their method for fundamental and practical purposes requires prudence because of potential errors. 相似文献
80.
Short-duration fluctuations in the concentration of airborne substances can be important in a variety of atmospheric dispersion
problems, especially when assessing the risks posed by harmful materials. This paper discusses a simulation technique for
generating individual realisations of fluctuating concentration time series in dispersing plumes based on target probability
distributions and spectral functions. The scheme uses a correlation-distortion approach to simulate these time series. Gaussian
processes with modified spectral characteristics are generated and then transformed to yield non-Gaussian processes with the
desired spectral characteristics. The simulation scheme is initially developed for a single receptor, and is then extended
to model pairs of correlated time series at two receptors. In fact, the modelling technique can be generalised to an arbitrary
number of receptors and this provides, in principal, an approach that is applicable to a wide class of similar problems (such
as the modelling of instantaneous puff releases or the response of line-of-sight detection systems). The simulation technique
is illustrated using observations made during recent field experiments, conducted both in the United Kingdom and in the U.S.A.,
investigating the short-range dispersion of a passive tracer. 相似文献