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Genthon, C.; Cosme, E. (2003). Intermittent signature of ENSO in west-Antarctic precipitation. Geophysical research letters, 30.
Abstract: Precipitation data from the new ERA40 reanalyses and from a 200-year simulation confirm a robust main mode of precipitation variability in west Antarctica. An intermittently strong ENSO signature is found in this mode. However, high correlation with ENSO indices appears infrequent. Thus, the high correlation found in ERA40, and previously in other chronologically realistic data, in the late 1980s and the 1990s may not be expected to last. Unlike previously suggested by others, the sign of the correlation between ENSO indices and west Antarctic precipitation, when significant, does not appear to change in time: Precipitation variability at the ENSO pace in the Bellingshausen-Weddell (Ross-Amunsden) region is consistently in phase (phase opposition, respectively) with the Southern Oscillation Index. This is consistent with a tropospheric wave train connecting the tropical Pacific and west Antarctic regions, which modulates in phase opposition the advection of air and moisture in the 2 regions.
Keywords: 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 3354 Meteorology and Atmospheric Dynamics: Precipitation; 1620 Global Change: Climate dynamics; 1655 Global Change: Water cycles; 9310 Information Related to Geographic Region: Antarctica
Programme: 411
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Domine, F.; Taillandier, A.S.; Simpson, W.R.; Severin, K. (2005). Specific surface area, density and microstructure of frost flowers. Geophysical research letters, 32.
Abstract: Frost flowers often grow on new sea ice. They are thought to have a high specific surface area (SSA) that provides sites for heterogeneous reactions. We have measured the SSA of frost flowers using CH4 adsorption at 77 K and obtained a value of 185 (+80 ?50) cm2/g, much lower than inferred by others. Their density is 0.02 g/cm3. We calculate that the total surface area of frost flowers is 1.4 m2 per m2 of ice surface, so that they do not increase the ice surface area significantly. Their role as sites for enhanced heterogeneous reactions should be reconsidered. Frost flowers also commonly grow on fresh water and the saline brine seen on young sea ice is not necessary for their growth. Photo- and electro-micrographs reveal hollow and concave structures, typical of very fast growing crystals. The brine that wicks up frost flowers considerably perturbs their growth.
Keywords: 0736 Cryosphere: Snow; 0750 Cryosphere: Sea ice; 0754 Cryosphere: Leads
Programme: 437
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Reijmer, C.H.; Oerlemans, J. (2002). Temporal and spatial variability of the surface energy balance in Dronning Maud Land, East Antarctica. J. Geophys. Res., 107.
Abstract: We present data of nine Automatic Weather Stations (AWS), which are located in Dronning Maud Land (DML), East Antarctica, since the austral summer of 1997. Potential temperature and wind speed are maximum at the sites with the steepest surface slope, i.e., at the edge of the Antarctic plateau. Specific humidity and accumulation decrease with elevation and distance from the coast. The annual average energy gain at the surface from the downward sensible heat flux varies between ?3 W m?2 and ?25 W m?2, with the highest values at the sites with the largest surface inclination and wind speeds. The net radiative flux is negative and largely balances this sensible heat flux and ranges from ??2 W m?2 to ??28 W m?2; maximum values can be linked to maxima in surface slope and wind speed, and suggest a strong connection between the heat budget and the katabatic flow in DML. The average latent heat flux is generally small and negative (??1 W m?2) indicating a slight net mass loss through sublimation.
Keywords: 1827 Hydrology: Glaciology; 1863 Hydrology: Snow and ice; 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 9310 Information Related to Geographic Region: Antarctica
Programme: 960
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Priestley, K.; Debayle, E. (2003). Seismic evidence for a moderately thick lithosphere beneath the Siberian Platform. Geophysical research letters, 30.
Abstract: We have built a Sv-wavespeed tomographic model for the upper mantle beneath the Siberian platform and surrounding region derived from the analysis of more than 13,000 fundamental and higher mode regional waveforms. The dense path coverage and rich higher mode content of the data allow building an upper mantle image with an horizontal resolution of a few hundred kilometers extending to ?400 km depth. The high velocity, upper mantle lid or seismic lithosphere is ?200 km thick beneath most of the Siberian platform but may extend to ?250 km depth beneath small areas. A high velocity seismic lid also underlies a large region west of the Siberian platform. Our observation of a ?200 thick seismic lithosphere beneath the Siberian platform on the slow-moving Eurasian plate, similar to the thickness of the seismic lithosphere beneath Precambrian terrains on the fast-moving Australian plate, suggests that a moderately thick seismic lithosphere beneath Precambrian terrains may be more common than previously supposed.
Keywords: 7207 Seismology: Core and mantle; 7218 Seismology: Lithosphere and upper mantle; 7255 Seismology: Surface waves and free oscillations
Programme: 133;906
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Legrand, M.; Preunkert, S.; Galy-Lacaux, C.; Liousse, C.; Wagenbach, D. (2005). Atmospheric year-round records of dicarboxylic acids and sulfate at three French sites located between 630 and 4360 m elevation. J. Geophys. Res., 110.
Abstract: An atmospheric year-round study of C2–C5 dicarboxylic acids (oxalic, malonic, succinic, malic, and glutaric) and sulfate was conducted in 2002 and 2003 at three remote western Europe continental sites located at different elevations (from 630 to 4360 m asl). Whatever the site and the season, oxalic acid is always the dominant diacid (average 64% of total dicarboxylic acids) followed by malonic acid (15% of total dicarboxylic acids). High correlation coefficients are observed between C3 (malonic), C4 (malic and succinic), and C5 (glutaric) acids and oxalic acid. These strong relationships between C2–C5 diacids support the hypothesis of a common production of these diacids through the aqueous phase chemistry of glutaric acid. Data gained at different elevations are here useful to compare the mass formation rates of sulfate and dicarboxylic acids. It is shown that in summer the decrease of the sum of dicarboxylic acids with height is far less pronounced than the decrease of sulfate (a factor of 2 instead of 6.8 from 630 to 4360 m asl). That demonstrates that the production of dicarboxylic acids occurs at up to 4300 m elevation while the production of sulfate from SO2 mainly takes place between the boundary layer and 3000 m elevation. With respect to summer 2002 the sum of dicarboxylic acids was enhanced in summer 2003 (from 136 to 331 ng m?3 STP at 2870 m asl, for instance) whereas a weaker increase is observed for sulfate (from 1700 to 2500 ng m?3 STP at 2870 m asl). These changes are attributed to the particular summer 2003 conditions which led to enhanced level of oxidants (strengthened secondary productions) and warmer temperatures (enhanced emissions of biogenic precursors of diacids).
Keywords: dicarboxylates; secondary production; high-elevated sites; 0365 Atmospheric Composition and Structure: Troposphere: composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere: constituent transport and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks
Programme: 414
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Barnes, P.R.F.; Wolff, E.W.; Mader, H.M.; Udisti, R.; Castellano, E.; Röthlisberger, R. (2003). Evolution of chemical peak shapes in the Dome C, Antarctica, ice core. J. Geophys. Res., 108.
Abstract: Interpretation of the chemical layers measured in ice cores requires knowledge of processes occurring after their deposition on the ice sheet. We present evidence for the diffusion of soluble ions in the top 350 m of the Dome C ice core, Antarctica, that helps in explaining the unexpectedly broad volcanic peaks observed at depth. A windowed-differencing operation applied to chemical time series indicates a damping of the signals over the past 11,000 years, independent of minor climatic variation, for sulfate and chloride, but not sodium. This implies a diffusive process is transporting both sulfate and chloride ions while the sodium ions remain fixed. We estimate the effective diffusivity in the core to be 4.7 × 10?8 m2 yr?1 for sulfate and 2.0 × 10?7 m2 yr?1 for chloride. These values are not high enough to significantly disrupt chemical interpretation in this section of core, but could be significant for older ice. The temperature of this section of ice (?53°C) implies that the predominantly acidic sulfate (and possibly chloride ions) will exist in the liquid phase while the sodium may be solid. We propose and develop two new mechanisms that could explain the observed solute movement. One involves the diffusion of solute through a connected vein network driven by liquid concentration imbalances instigated by the process of grain growth. The other considers a system of discontinuous veins where grain growth increases connectivity between isolated vein clusters allowing the spread of solute. In both mechanisms, the effective diffusivity is governed indirectly by grain growth rate; this may be a significant factor controlling effective diffusion in other cores.
Keywords: 1863 Hydrology: Snow and ice; 9310 Information Related to Geographic Region: Antarctica; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3999 Mineral Physics: General or miscellaneous; 0325 Atmospheric Composition and Structure: Evolution of the atmosphere
Programme: 960
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Masson-Delmotte, V.; Landais, A.; Stievenard, M.; Cattani, O.; Falourd, S.; Jouzel, J.; Johnsen, S.J.; Dahl-Jensen, D.; Sveinsbjornsdottir, A.; White, J.W.C.; Popp, T.; Fischer, H. (2005). Holocene climatic changes in Greenland: Different deuterium excess signals at Greenland Ice Core Project (GRIP) and NorthGRIP. J. Geophys. Res., 110.
Abstract: Water stable isotope measurements (?D and ?18O) have been conducted on the Holocene part of two deep Greenland ice cores (Greenland Ice Core Project (GRIP) and NorthGRIP), located ?320 km apart. These combined measurements provide the first two continuous Greenland Holocene deuterium excess profiles (d = ?D ? 8?18O), a parameter strongly influenced by changes in moisture sources. We discuss here temporal and regional fluctuations of the deuterium excess within central to north Greenland, with a mean temporal resolution of ?4 years. Although GRIP and NorthGRIP exhibit similar annual mean surface temperatures and ?18O levels, a significant offset of modern deuterium excess is observed between the two sites. We attribute this offset to a different mix of modern moisture sources, pointing to regional-scale differences in moisture advection toward Greenland. The common long-term deuterium excess Holocene increasing trend is probably related to the increased relative contribution of low-latitude moisture to Greenland snowfall, in response to the change in the Earth obliquity, as symmetrically observed in Antarctica. Three abrupt declines punctuate the GRIP excess record (8.2, 4.5, and 0.35 ka BP), suggesting associated reorganizations of the northern high latitudes hydrological cycle. The 8.2 ka BP event is characterized by (1) a rapid cooling followed by a progressive warming and (2) a deuterium excess cooling restricted to GRIP, therefore totally different from rapid events during glacial times. By contrast, the NorthGRIP deuterium excess record is more stable. We propose that a slightly larger proportion of moisture supplied by local storm tracks to GRIP induces an isotopic compensation mechanism between simultaneous site and source temperature coolings, resulting in a rather temperature-insensitive ?18O profile, together with well-marked deuterium excess amplitudes. NorthGRIP ?18O seems less biased by isotopic processes and should provide a more reliable past temperature record.
Keywords: paleoclimate; polar ice sheet; water cycle; 1041 Geochemistry: Stable isotope geochemistry; 0724 Cryosphere: Ice cores; 1620 Global Change: Climate dynamics; 1655 Global Change: Water cycles; 3344 Atmospheric Processes: Paleoclimatology
Programme: 458
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Lambotte, S.; Rivera, L.; Hinderer, J. (2006). Rupture length and duration of the 2004 Aceh-Sumatra earthquake from the phases of the Earth's gravest free oscillations. Geophysical research letters, 33.
Abstract: The Aceh-Sumatra 2004 earthquake strongly excited the Earth's free oscillations. Well separated split multiplets provide useful information on the earthquake source. Particularly, the phases of split singlets constrain the duration, rupture length and mean rupture velocity. We analyze the initial phases of some of the Earth's gravest free oscillations (0
S
2, 0
S
3, 0
S
0 and 1
S
0) in order to constrain the space-time finiteness of the source. We use recordings of vertical broadband seismometers and superconducting gravimeters from several worldwide geophysical networks. We estimate a rupture length of about 1220 km, a source time duration of about 500 s, and a mean rupture velocity of 2.4 km/s.
Keywords: 3255 Mathematical Geophysics: Spectral analysis; 7209 Seismology: Earthquake dynamics; 7215 Seismology: Earthquake source observations; 7255 Seismology: Surface waves and free oscillations; 7260 Seismology: Theory
Programme: 133;906
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Thébault, E.; Gaya-Piqué, L. (2008). Applied comparisons between SCHA and R-SCHA regional modeling techniques. Geochem. Geophys. Geosyst., 9.
Abstract: Spherical cap harmonic analysis (SCHA) has become a common tool for the regional modeling of potential fields since its introduction by Haines (1985). The fact that SCHA satisfies Laplace equation and the possibility of representing high-frequency fields with a small number of coefficients (compared to the global spherical harmonic analysis) made SCHA the preferred choice for the development, for example, of magnetic field models at national scale. However, Thébault et al. (2006a) demonstrated that the traditional SCHA presented some deficiencies, in particular related to the inversion of multilevel data sets. The authors presented the R-SCHA technique as an alternative method in which the introduction of a new set of basis functions and boundary conditions solved this issue. In this paper we present some numerical comparisons between the SCHA and R-SCHA techniques applied with different synthetic vector data sets, from near-surface main field, main difference, and crustal field data simulating a World Digital Magnetic Anomaly Map subset. Other analyses are carried out with synthetic vector data set that mimics the expected data distribution from a multisatellite mission like the forthcoming European Swarm mission. No regularization, weighting, or ad hoc procedures are applied to the synthetic vector data, and a cap of 7° aperture is considered. The numerical analyses show that SCHA is a satisfying approximation in a band-limited spectral region that depends on the cap's size. It does not work correctly either for main field or for the short-scale crustal field modeling. These aspects are supported by equations illustrating why SCHA may fail. On the contrary, R-SCHA converges more slowly than SCHA but is valid in all cases. It gives a consistent set of regional coefficients and fits the radial variation of the field in a realistic way. At last, the special case of data incompatibility shows that R-SCHA does not fit incompatible data while SCHA assimilates most of them. These results should help the scientific community to evaluate the level of approximation needed for the development of regional magnetic field models in the era of the European Space Agency Swarm mission.
Keywords: regional modeling; SCHA; R-SCHA; 1517 Geomagnetism and Paleomagnetism: Magnetic anomalies: modeling and interpretation; 1541 Geomagnetism and Paleomagnetism: Satellite magnetics: main field, crustal field, external field
Programme: 139
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Ferrari, C.P.; Dommergue, A.; Boutron, C.F.; Jitaru, P.; Adams, F.C. (2004). Profiles of Mercury in the snow pack at Station Nord, Greenland shortly after polar sunrise. Geophysical research letters, 31.
Abstract: Mercury (Hg) species have been measured in the snow pack at Station Nord, Greenland both in the snow and in the air of snow from February 25 to March 15, 2002, during twilight and low solar irradiation periods. More than 99% of Hg is in the snow itself (?94–97% as Hg2+ and ?5% as MeHg+) while less than 1% is in the interstitial air of snow as Hg°. Gaseous Elemental Mercury (Hg°) concentrations, decreased exponentially with depth from ?1.5 ng/m3 outside to ?0.1 ng/m3 at 120 cm depth in the snow air. Hg° incorporation flux to the snow pack has been evaluated to ?5.8–7.0 pg/m2/h which is weak, indicating that this process does not change significantly the Hg content in the snow. We believe that this decrease in the air of snow is the result of fast oxidation processes of Hg° rather than adsorption of Hg° onto snow surfaces.
Keywords: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0330 Atmospheric Composition and Structure: Geochemical cycles
Programme: 399
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