Redistribution of riverine and rainfall freshwater by the Bay of Bengal circulation

Ocean Dynamics - We use satellite-derived currents and a Lagrangian approach to investigate the redistribution of the precipitation minus evaporation (P-E) and river freshwater inputs into Bay of...     

Dating the Tethyan Ocean in the Western Alps with radiolarite pebbles from synorogenic Oligocene molasse basins (southeast France)

For the first time in the Western Alps, radiolarite pebbles collected from Tertiary foreland molasse conglomerates are treated for microfossil extraction and dated. Among forty pebbles collected in the field, seven of them released diagnostic radiolarian assemblages ranging in age from Late Bajocian?CEarly Callovian to Middle Oxfordian?CEarly Tithonian. These ages overlap previous biochronological data obtained from in situ localities of the Schistes Lustrés Piemont zone of the French-Italian Alps and triple the number of diagnostic radiolarite samples known so far in this segment of the chain. The diagnostic pebbles are characterized by low grade metamorphism, showing that some eroded thrust-sheets from the oldest parts of the ocean escaped any tectonic burial during the Alpine convergence. Mixing of low and high-grade radiolarites, mafics and ultramafics pebbles implies that various ocean-derived units were exposed. This tectonic scenario involves tight refolding and severe uplift of the Eocene subduction wedge in the Early Oligocene.     

Interactions climat-eustatisme-tectonique. Les enseignements et perspectives du Crétacé supérieur (Cénomanien-Coniacien)

Résumé

Une analyse de séries sédimentaires d’âge Crétacé supérieur dans le Sud-Est de la France a été entreprise et intégrée dans un cadre de stratigraphie séquentielle. Les limites Cénomanien–Turonien et la limite Turonien–Coniacien sont marquées par des variations rapides et de fortes amplitudes du niveau marin relatif. La comparaison effectuée avec d’autres bassins mondiaux nous amène à envisager le problème dans un cadre global. On constate : (1) le synchronisme d’événements à haute fréquence dans des contextes géodynamiques différents. (2) les effets de la superposition de cycles eustatiques hiérarchiquement différents, c’est-à-dire la superposition d’oscillations à haute fréquence sur une composante de 3^e ordre. Ces observations sont confrontées à deux hypothèses : le glacio-eustatisme et la tectonique à haute fréquence. © Elsevier, Paris.     

Development and decay of a lithalsa in Northern Québec: A geomorphological history

The complete life cycle of a permafrost mound is reconstructed from its growth until its degradation. The study site is a lithalsa, which is the subject of a long-term monitoring that includes geocryological observations, measurements of permafrost properties on cores, ground temperature measurements, and observations of landform changes. The landform likely grew as a palsa under cold climatic conditions in the past. The peat cover was subsequently eroded. Early stages of degradation are witnessed since 2003 as a new thermokarst pond is starting to form though mound collapse. Settlement of the structure has been observed, and a rim ridge has begun to form. Ultimately, the lithalsa shall disappear and be replaced by a circular thermokarst pond surrounded by a rampart, similar to many other ones in the study area. The monitoring of the thermal regime of the lithalsa illustrates the pattern of internal warming and points to the causes of its degradation.     

Impact of the configuration of stretching and ocean–atmosphere coupling on tropical cyclone activity in the variable-resolution GCM ARPEGE

This study starts by investigating the impact of the configuration of the variable-resolution atmospheric grid on tropical cyclone (TC) activity. The French atmospheric general circulation model ARPEGE, the grid of which is rotated and stretched over the North Atlantic basin, was used with prescribed sea surface temperatures. The study clearly shows that changing the position of the stretching pole strongly modifies the representation of TC activity over the North Atlantic basin. A pole in the centre of the North Atlantic basin provides the best representation of the TC activity for this region. In a second part, the variable-resolution climate model ARPEGE is coupled with the European oceanic global climate model NEMO in order to study the impact of ocean–atmosphere coupling on TC activity over the North Atlantic basin. Two pre-industrial runs, a coupled simulation and a simulation forced by the sea surface temperatures from the coupled one, are compared. The results show that the coupled simulation is more active in the Caribbean Sea and the Gulf of Mexico while the forced simulation is more active over eastern Florida and the eastern Atlantic. The difference in the distribution of TC activity is certainly linked with the location of TC genesis. In the forced simulation, tropical cyclogenesis is closer to the west African coast than in the coupled simulation. Moreover, the difference in TC activity over the eastern Atlantic seems to be related to two different mechanisms: the difference in African easterly wave activity over the west of Africa and the cooling produced, in the coupled simulation, by African easterly waves over the eastern Atlantic. Finally, the last part studies the impact of changing the frequency of ocean–atmosphere coupling on Atlantic TC activity. Increasing the frequency of coupling decreases the density of TC activity over the North Atlantic basin. However, it does not modify the spatial distribution of the TC activity. TC rainfalls are decreased by 8?% in the high frequency coupled run.     

Characterization of contact properties in biocemented sand using 3D X-ray micro-tomography

The mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented sand, such as the volume fraction of calcite, its distribution within the pore space (localized at the contact between grains, over the grain surfaces) and the contact properties: coordination number, contact surface area, contacts orientation and types of contact. In the present work, all these micromechanical properties are computed, for the first time, from 3D images obtained by X-ray tomography of intact biocemented sand samples. The evolution of all these properties with respect to the volume fraction of calcite is analyzed and compared between each other (from untreated sand to highly cemented sand). Whatever the volume fraction of calcite, it is shown that the precipitation of the calcite is localized at the contacts between grains. These results are confirmed by comparing our numerical results with analytical estimates assuming that the granular medium is made of periodic simple cubic arrangements of grains and by considering two extreme cases of precipitation: (1) The calcite is localized at the contact, and (2) the grains are covered by a uniform layer of calcite. In overall, the obtained results show that a small percentage of calcite is sufficient to get a large amount of cohesive contacts.

     

Characterization of microstructural and physical properties changes in biocemented sand using 3D X-ray microtomography

An experimental study has been performed to investigate the effect of the biocalcification process on the microstructural and the physical properties of biocemented Fontainebleau sand samples. The microstructural properties (porosity, volume fraction of calcite, total specific surface area, specific surface area of calcite, etc.) and the physical properties (permeability, effective diffusion) of the biocemented samples were computed for the first time from 3D images with a high-resolution images obtained by X-ray synchrotron microtomography. The evolution of all these properties with respect to the volume fraction of calcite is analysed and compared with success to experimental data, when it is possible. In general, our results point out that all the properties are strongly affected by the biocalcification process. Finally, all these numerical results from 3D images and experimental data were compared to numerical values or analytical estimates computed on idealized microstructures constituted of periodic overlapping and random non-overlapping arrangements of coated spheres. These comparisons show that these simple microstructures are sufficient to capture and to predict the main evolution of both microstructural and physical properties of biocemented sands for the whole range of volume fraction of calcite investigated.     

Segregated ice structures in various heaved permafrost landforms through CT Scan

The growth of segregated ice lenses in frost susceptible sediments in the discontinuous permafrost zone is the dominant mechanism for the formation of permafrost mounds, such as palsas, lithalsas and permafrost plateaus. Thawing of these mounds creates thermokarst lakes, which are particularly abundant in Nunavik, east of the Hudson Bay area. The inception of the permafrost in mounds and their growth are regulated by climate conditions, by local Quaternary geology and by environmental factors such as topography, vegetation, snow cover and surface humidity. Variable sizes and morphology of the permafrost mounds can be attributed to local factors that affect the ice segregation process, particularly the supply of water needed for ice‐lens growth and grain‐size composition of the soil into which aggradation takes place. Computer image analysis of CT scans on high quality cores obtained from permafrost mounds and plateaus of various shapes reveal that the ice layer sequences and permafrost internal structure vary with landform types. A relationship therefore exists between different morphological type within a family of landforms and their microscale internal structure. Copyright © 2007 John Wiley & Sons, Ltd.     

Summer heat waves over western Europe 1880–2003, their relationship to large-scale forcings and predictability

We investigate the large-scale forcing and teleconnections between atmospheric circulation (sea level pressure, SLP), sea surface temperatures (SSTs), precipitation and heat wave events over western Europe using a new dataset of 54 daily maximum temperature time series. Forty four of these time series have been homogenised at the daily timescale to ensure that the presence of inhomogeneities has been minimised. The daily data have been used to create a seasonal index of the number of heat waves. Using canonical correlation analysis (CCA), heat waves over western Europe are shown to be related to anomalous high pressure over Scandinavia and central western Europe. Other forcing factors such as Atlantic SSTs and European precipitation, the later as a proxy for soil moisture, a known factor in strengthening land–atmosphere feedback processes, are also important. The strength of the relationship between summer SLP anomalies and heat waves is improved (from 35%) to account for around 46% of its variability when summer Atlantic and Mediterranean SSTs and summer European precipitation anomalies are included as predictors. This indicates that these predictors are not completely collinear rather that they each have some contribution to accounting for summer heat wave variability. However, the simplicity and scale of the statistical analysis masks this complex interaction between variables. There is some useful predictive skill of summer heat waves using multiple lagged predictors. A CCA using preceding winter North Atlantic SSTs and preceding January to May Mediterranean total precipitation results in significant hindcast (1972–2003) Spearman rank correlation skill scores up to 0.55 with an average skill score over the domain equal to 0.28 ± 0.28. In agreement with previous studies focused on mean summer temperature, there appears to be some predictability of heat wave events on the decadal scale from the Atlantic Multidecadal Oscillation (AMO), although the long-term global mean temperature is also well related to western European heat waves. Combining these results with the observed positive trends in summer continental European SLP, North Atlantic SSTs and indications of a decline in European summer precipitation then possibly these long-term changes are also related to increased heat wave occurrence and it is important that the physical processes controlling these changes be more fully understood.     

Methodological Development of the Conditional Sampling Method. Part I: Sensitivity to Statistical and Technical Characteristics

The relaxed eddy accumulation (REA), method based on the conditional sampling concept, has received increasing attention over the past few years as it can be used to measure surface fluxes of a wide variety of trace gases for which fast response analysers are not available. In the REA method, a turbulent flux is simply expressed as the product of the standard deviation of vertical wind velocity, the difference between mean scalar concentration in the updrafts and downdrafts and an empirical coefficient, (about 0.63 as based on simulations with a Gaussian distribution, and 0.58 as derived from experimental data). A simulation technique is developed here to evaluate the performance of a ground-based REA system. This analysis uses generated series whose internal structure can be controlled to a large extent. They are stationary and their characteristics are similar to those of physical turbulence. In a first step the influence of some statistical characteristics of vertical velocity and scalar concentration series is investigated. The effect of the third- and fourth-order moments can explain to some degree the difference between calculated and measured values. The impact of a threshold on the vertical velocity is then considered, and its effect on is quantified. The influence of the time lag between and the effective scalar sampling, and the consequences of lowpass filtering of the signal are also investigated. The simulation technique presented in this study can be used to develop elaborate algorithms for near real-time conditional sampling, based on the statistical characteristics of the previous sample.