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61.
Analysis of the projected regional sea-ice changes in the Southern Ocean during the twenty-first century 总被引:1,自引:1,他引:1
Using the set of simulations performed with atmosphere-ocean general circulation models (AOGCMs) for the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change (IPCC AR4), the projected regional distribution of sea ice for the
twenty-first century has been investigated. Averaged over all those model simulations, the current climate is reasonably well
reproduced. However, this averaging procedure hides the errors from individual models. Over the twentieth century, the multimodel
average simulates a larger sea-ice concentration decrease around the Antarctic Peninsula compared to other regions, which
is in qualitative agreement with observations. This is likely related to the positive trend in the Southern Annular Mode (SAM)
index over the twentieth century, in both observations and in the multimodel average. Despite the simulated positive future
trend in SAM, such a regional feature around the Antarctic Peninsula is absent in the projected sea-ice change for the end
of the twenty-first century. The maximum decrease is indeed located over the central Weddell Sea and the Amundsen–Bellingshausen
Seas. In most models, changes in the oceanic currents could play a role in the regional distribution of the sea ice, especially
in the Ross Sea, where stronger southward currents could be responsible for a smaller sea-ice decrease during the twenty-first
century. Finally, changes in the mixed layer depth can be found in some models, inducing locally strong changes in the sea-ice
concentration.
相似文献
W. LefebvreEmail: |
62.
Two new campaigns devoted to the observation of the solar limb distortions were made at the Pic-du-Midi Observatory, in September 2000 and September 2001, by means of the scanning heliometer. This apparatus can be used now routinely to accurately determine solar limb profiles (at two wavelengths), at any heliographic latitudes. Each measurement is made within 44 milliseconds (of time) which permits to record a limb profile together with the seeing. Scans are automatically rejected for seeing larger than 1.3 arc sec. Such conditions are essential to perform high-quality observations necessary to obtain the quadrupole term (l=2) in the polynomial expansion of the radius contour R() = constant = R
0
left(1+l
c
l
P
l()right). Exceptional meteorological conditions in September 2001 (seeing of the order of 18 cm, for a 50 cm clear aperture of the refractor) enabled us to determine c
2 and c
4 (see Table I) with an accuracy of a few milli-arc-sec. Results indicate a distorted solar shape, the departures from a pure spherical body not exceeding 20 milli-arc-sec. We propose a model to interpret such results (the combination of a nearly uniform rotating core with a prolate solar tachocline and an oblate surface), which is briefly discussed. Our results are confronted to those obtained from space. We conclude that measurements of the quadrupole term from the ground are possible, but of high difficulty and can be obtained only during excellent weather conditions. The hexadecapole term should be only obtained from space. We show that an astrometric satellite would be required, whose mission would be also to accurately determine the solar rotation profiles (both surface and in depth) in order to unambiguously determine the inertia moments of the Sun through the J
n terms. Such values are also briefly discussed. 相似文献
63.
Nina Stark Hendrik Hanff Christian Svenson Verner B. Ernstsen Alice Lefebvre Christian Winter Achim Kopf 《Geo-Marine Letters》2011,31(4):249-258
In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal
inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification
pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below
(quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the
top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared
to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based
on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated
from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion
of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport
and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase
in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers
can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface
layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such
data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes
in highly dynamic coastal environments. 相似文献
64.
Andrea De Stefano Nathalie Lefebvre Maya Kopylova 《Contributions to Mineralogy and Petrology》2006,151(2):158-173
A suite of 80 macrodiamonds recovered from volcaniclastic breccia of Wawa (southern Ontario) was characterized on the basis
of morphology, nitrogen content and aggregation, cathodoluminescence (CL), and mineral inclusions. The host calc-alkaline
lamprophyric breccias were emplaced at 2.68–2.74 Ga, contemporaneously with voluminous bimodal volcanism of the Michipicoten
greenstone belt. The studied suite of diamonds differs from the vast majority of diamond suites found worldwide. First, the
suite is hosted by calc-alkaline lamprophyric volcanics rather than by kimberlite or lamproite. Second, the host volcanic
rock is amongst the oldest known diamondiferous rocks on Earth, and has experienced regional metamorphism and deformation.
Finally, most diamonds show yellow-orange-red CL and contain mineral inclusions not in equilibrium with each other or their
host diamond. The majority of the diamonds in the Wawa suite are colorless, weakly resorbed, octahedral single crystals and
aggregates. The diamonds contain 0–740 ppm N and show two modes of N aggregation at 0–30 and 60–95% B-centers suggesting mantle
storage at 1,100–1,170°C. Cathodoluminescence and FTIR spectroscopy shows that emission peaks present in orange CL stones
do not likely result from irradiation or single substitutional N, in contrast to other diamonds with red CL. The diamonds
contain primary inclusions of olivine (Fo92 and Fo89), omphacite, orthopyroxene (En93), pentlandite, albite, and An-rich plagioclase. These peridotitic and eclogitic minerals are commonly found within single
diamonds in a mixed paragenesis which also combines shallow and deep phases. This apparent disequilibrium can be explained
by effective small-scale mixing of subducted oceanic crust and mantle rocks in fast “cold” plumes ascending from the top of
the slabs in convergent margins. Alternatively, the diamonds could have formed in the pre-2.7–2.9 Ga cratonic mantle and experienced
subsequent alteration of syngenetic inclusions related to host magmatism and ensuing metamorphism. Neither orogenic nor cratonic
model of the diamond origin fully explains all of the observed characteristics of the diamonds and their host rocks.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献