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121.
Ola M Johannessen 《大气和海洋科学快报》2008,1(1):51-56
Arctic sea ice is a keystone indicator of greenhouse-gas induced global climate change, which is expected to be amplified in the Arctic. Here we directly compare observed variations in arctic sea-ice extent and CO2 since the beginning of the 20th century, identifying a strengthening linkage, such that in recent decades the rate of sea-ice decrease mirrors the increase in CO2, with r ~ –0.95 over the last four decades, thereby indicating that 90% (r2 ~ 0.90) of the decreasing sea-ice extent is empirically “accounted for” by the increasing CO2 in the atmosphere. The author presents an empirical relation between annual sea-ice extent and global atmospheric CO2 concentrations, in which sea-ice reductions are linearly, inversely proportional to the magnitude of increase of CO2 over the last few decades. This approximates sea-ice changes during the most recent four decades, with a proportionality constant of 0.030 million km2 per ppmv CO2. When applied to future emission scenarios of the Intergovernmental Panel on Climate Change (IPCC), this relationship results in substantially faster ice decreases up to 2050 than predicted by IPCC models. However, departures from this projection may arise from non-linear feedback effects and/or temporary natural variations on interannual timescales, such as the record minimum of sea-ice extent observed in September 2007.Citation: Johannessen, O. M., 2008: Decreasing arctic sea ice mirrors increasing CO2 on decadal time scale, Atmos. Oceanic Sci. Lett., 1, 51-56 相似文献
122.
Onset and end of the summer melt season over sea ice: thermal structure and surface energy perspective from SHEBA 总被引:1,自引:0,他引:1
P. Ola G. Persson 《Climate Dynamics》2012,39(6):1349-1371
Various measurements from the Surface Heat Flux of the Arctic Ocean (SHEBA) experiment have been combined to study structures and processes producing the onset and end of summer melt over Arctic sea ice. The analysis links the surface energy budget to free-troposphere synoptic variables, clouds, precipitation, and in-ice temperatures. The key results are (1) SHEBA melt-season transitions are associated with atmospheric synoptic events (2) onset of melt clearly occurs on May 28, while the end of melt is produced by a sequence of three atmospheric storm events over a 28-day period producing step-like reductions in the net surface energy flux. The last one occurs on August 22.; (3) melt onset is primarily due to large increases in the downwelling longwave radiation and modest decreases in the surface albedo; (4) decreases in the downwelling longwave radiation occur for all end-of-melt transition steps, while increases in surface albedo occur for the first two; (5) decreases in downwelling shortwave radiation contribute only to the first end-of-melt transition step; (6) springtime free-tropospheric warming preconditions the atmosphere–ice system for the subsequent melt onset; and (7) melt-season transitions also mark transitions in system responses to radiative energy flux changes because of invariant melt-season surface temperatures. The extensive SHEBA observations enable an understanding of the complex processes not available from other field program data. The analysis provides a basis for future testing of the generality of the results, and contributes to better physical understanding of multi-year analyses of melt-season trends from less extensive data sets. 相似文献
123.
Twenty-eight CS molecular clouds toward HII regions with Galactocentric distances from ~ 4 to 20 kpc have been studied based on observations obtained in the J=2→1 lines of CS and C34S on the 20-meter radio telescope of the Onsala Space Observatory (Sweden) in March 2001. All 28 clouds have been mapped with an angular resolution of ~40″. The peak intensity in the C34S line has been measured for 20 objects. An LTE analysis has been performed and the parameters of the molecular cloud cores derived. The core sizes are dA=0.3–4.8 pc, with a median value of ~1.6 pc. The mean hydrogen densities in the cloud cores are nH2=3.5×102–3.7 × 104 cm?3, with a median value of ~7.2×103 cm?3. The value of nH2 ends to decrease with increasing Galactocentric distance of the cloud. The masses of most clouds are 102?6×103M⊙, with the most probable value being MCS~103M⊙. The data follow the dependence MCS ∝d A (2.4–3.2) . As a rule, the cloud masses are lower than the virial masses for MCS<103M⊙. 相似文献
124.
Fault induced permanent ground deformations—an experimental comparison of wet and dry soil and implications for buried structures 总被引:1,自引:1,他引:1
Fault surface ruptures constitute a great risk to human lives, buildings and infrastructure. Despite this few building codes contain risk reducing provisions. Experiments provided results for establishing design rules in terms of possible dislocation and failure strains. Dry and wet soil behave substantially different. A brief discussion on counteracting effects of the wet soil on the interaction between a buried structure and the surrounding ground is included. 相似文献
125.
Numerical study of localization in soil systems 总被引:1,自引:0,他引:1
A numerical study of the mechanical behavior of heterogeneous soil systems, consisting of a bulk of sand with embedded stiff gravel inclusions or soft clay inclusions, is performed. A solution scheme using parallel computing is employed when analyzing two different categories of problems. First, a homogenization problem is studied, where use of a single representative volume element subjected to plane strain compression offers the possibility to investigate the coupling between the response at a local scale and at a global scale. Second, a plane strain footing problem with different heterogeneous soil systems is analyzed using a traditional finite element formulation. The material model utilized for the soil is a large deformation formulation of non-associated elasto-plasticity with an isotropic hardening law, able to represent dilation. It was found that the shape of the gravel or clay inclusions in the systems had no significant effect on the global responses, whereas the strain localizations in the two different soil systems, sand–gravel and sand–clay, were found to have different character. The effect of the initial density on the response was clearly observed in the localization patterns. 相似文献
126.
Tatjana Rehfeldt Karsten Obst Leif Johansson 《International Journal of Earth Sciences》2007,96(3):433-450
Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites)
and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region.
The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing
orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fetot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C)
than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the
spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy
and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic
peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites
and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes
that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower
Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites
show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites
with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature
ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within
the Fennoscandian crust. 相似文献
127.
An improved and extended GPS-derived 3D velocity field of the glacial isostatic adjustment (GIA) in Fennoscandia 总被引:1,自引:1,他引:1
Martin Lidberg Jan M. Johansson Hans-Georg Scherneck James L. Davis 《Journal of Geodesy》2007,81(3):213-230
We present a new GPS-derived 3D velocity field for the Fennoscandia glacial isostatic adjustment (GIA) area. This new solution
is based upon ∼3,000 days of continuous GPS observations obtained from the permanent networks in Fennoscandia. The period
encompasses a prolongated phase of stable observation conditions after the northern autumn of 1996. Several significant improvements
have led to smaller uncertainties and lower systematic errors in the new solutions compared to our previous results. The GPS
satellite elevation cut-off angle was lowered to 10°, we fixed ambiguities to integers where possible, and only a few hardware
changes occurred over the entire network. The GAMIT/GLOBK software package was used for the GPS analysis and reference frame realization. Our new results confirmed earlier findings
of maximum discrepancies between GIA models and observations in northern Finland. The reason may be related to overestimated
ice-sheet thickness and glaciation period in the north. In general, the new solutions are more coherent in the velocity field,
as some of the perturbations are now avoided. We compared GPS-derived GIA rates with sea-level rates from tide-gauge observations,
repeated precise leveling, and with GIA model computations, which showed consistency. 相似文献
128.
129.
Mesoscale modeling study of the oceanographic conditions off the southwest coast of India 总被引:1,自引:0,他引:1
Vibeke E. Haugen Ola M. Johannessen Geir Evensen 《Journal of Earth System Science》2002,111(3):321-337
A high resolution model, using the Miami Isopycnic Coordinate Ocean Model (MICOM), has been implemented for the first time
to study the seasonal circulation and coastal upwelling off the southwest Indian coast during 1974. This model is part of
a model and data assimilation system capable of describing the ocean circulation and variability in the Indian Ocean and its
predictability in response to the monsoon system.
Along the southwest coast of India the dominant coastal current is the reversing West Indian Coastal Current which is well
simulated and described, in addition to the weaker undercurrent of the opposite direction. Upwelling of cold water, 4‡C lower
than offshore temperatures appear in April. The upwelling intensifies with the southwest monsoon and is simulated in accordance
within situ observations. Upwelling appears to be strongest off Cochin and Quilon, and the upwelling of cold water is seen together with
a decrease in salinity in the model simulation. 相似文献
130.
S. V. Kalenskii V. I. Slysh I. E. Val’tts A. Winnberg L. E. Johansson 《Astronomy Reports》2001,45(1):26-33
Forty-eight objects were detected in the 5?1–40 E methanol line at 84.5 GHz during a survey of Class I maser sources. Narrow maser features were found in 14 of these. Broad quasi-thermal lines were detected toward other sources. One of the objects with narrow features at 84.5 GHz, the young bipolar outflow L1157, was also observed in the 80–71 A + line at 95.2 GHz; a narrow line was detected at this frequency. Analysis showed that the broad lines are usually inverted. The quasi-thermal profiles imply that there are no more than a few line opacities. These results confirm the plausibility of models in which compact Class I masers appear in extended sources as a result of a preferential velocity field. 相似文献