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Alexandra Touzeau, Amaëlle Landais, Barbara Stenni, Ryu Uemura, Kotaro Fukui, Shuji Fujita, Sarah Guilbaud, Alexey Ekaykin, Mathieu Casado, Eugeni Barkan, Boaz Luz, Olivier Magand, Grégory Teste, Emmanuel Le Meur, Mélanie Baroni, Joël Savarino, Ilann Bourgeois, Camille Risi. (2016). Acquisition of isotopic composition for surface snow in East Antarctica and the links to climatic parameters (Vol. 10).
Abstract:
Abstract. The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture. However, recent analyses of snow from shallow pits indicate that the climatic signal can become erased in very low accumulation regions, due to local processes of snow reworking. The signal-to-noise ratio decreases and the climatic signal can then only be retrieved using stacks of several snow pits. Obviously, the signal is not completely lost at this stage, otherwise it would be impossible to extract valuable climate information from ice cores as has been done, for instance, for the last glaciation. To better understand how the climatic signal is passed from the precipitation to the snow, we present here results from varied snow samples from East Antarctica. First, we look at the relationship between isotopes and temperature from a geographical point of view, using results from three traverses across Antarctica, to see how the relationship is built up through the distillation process. We also take advantage of these measures to see how second-order parameters (d-excess and 17O-excess) are related to δ18O and how they are controlled. d-excess increases in the interior of the continent (i.e., when δ18O decreases), due to the distillation process, whereas 17O-excess decreases in remote areas, due to kinetic fractionation at low temperature. In both cases, these changes are associated with the loss of original information regarding the source. Then, we look at the same relationships in precipitation samples collected over 1 year at Dome C and Vostok, as well as in surface snow at Dome C. We note that the slope of the δ18O vs. temperature (T) relationship decreases in these samples compared to those from the traverses, and thus caution is advocated when using spatial slopes for past climate reconstruction. The second-order parameters behave in the same way in the precipitation as in the surface snow from traverses, indicating that similar processes are active and that their interpretation in terms of source climatic parameters is strongly complicated by local temperature effects in East Antarctica. Finally we check if the same relationships between δ18O and second-order parameters are also found in the snow from four snow pits. While the d-excess remains opposed to δ18O in most snow pits, the 17O-excess is no longer positively correlated to δ18O and even shows anti-correlation to δ18O at Vostok. This may be due to a stratospheric influence at this site and/or to post-deposition processes. Programme: 1177
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Fifi Ibrahime Adodo, Frédérique Remy, Ghislain Picard. (2018). Seasonal variations of the backscattering coefficient measured by radar altimeters over the Antarctic Ice Sheet (Vol. 12).
Abstract:
Abstract. Spaceborne radar altimeters are a valuable tool for observing the Antarctic Ice Sheet. The radar wave interaction with the snow provides information on both the surface and the subsurface of the snowpack due to its dependence on the snow properties. However, the penetration of the radar wave within the snowpack also induces a negative bias on the estimated surface elevation. Empirical corrections of this space- and time-varying bias are usually based on the backscattering coefficient variability. We investigate the spatial and seasonal variations of the backscattering coefficient at the S (3.2GHz∼9.4cm), Ku (13.6GHz∼2.3cm) and Ka (37GHz∼0.8cm) bands. We identified that the backscattering coefficient at Ku band reaches a maximum in winter in part of the continent (Region 1) and in the summer in the remaining (Region 2), while the evolution at other frequencies is relatively uniform over the whole continent. To explain this contrasting behavior between frequencies and between regions, we studied the sensitivity of the backscattering coefficient at three frequencies to several parameters (surface snow density, snow temperature and snow grain size) using an electromagnetic model. The results show that the seasonal cycle of the backscattering coefficient at Ka frequency is dominated by the volume echo and is mainly driven by snow temperature evolution everywhere. In contrast, at S band, the cycle is dominated by the surface echo. At Ku band, the seasonal cycle is dominated by the volume echo in Region 1 and by the surface echo in Region 2. This investigation provides new information on the seasonal dynamics of the Antarctic Ice Sheet surface and provides new clues to build more accurate corrections of the radar altimeter surface elevation signal in the future. Programme: 1110
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Mathieu Casado, Amaelle Landais, Ghislain Picard, Thomas Münch, Thomas Laepple, Barbara Stenni, Giuliano Dreossi, Alexey Ekaykin, Laurent Arnaud, Christophe Genthon, Alexandra Touzeau, Valerie Masson-Delmotte, Jean Jouzel. (2018). Archival processes of the water stable isotope signal in East Antarctic ice cores (Vol. 12).
Abstract:
Abstract. The oldest ice core records are obtained from the East Antarctic Plateau. Water isotopes are key proxies to reconstructing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions depends on knowledge of all processes affecting water vapour, precipitation and snow isotopic compositions. Fractionation processes are well understood and can be integrated in trajectory-based Rayleigh distillation and isotope-enabled climate models. However, a quantitative understanding of processes potentially altering snow isotopic composition after deposition is still missing. In low-accumulation sites, such as those found in East Antarctica, these poorly constrained processes are likely to play a significant role and limit the interpretability of an ice core's isotopic composition. By combining observations of isotopic composition in vapour, precipitation, surface snow and buried snow from Dome C, a deep ice core site on the East Antarctic Plateau, we found indications of a seasonal impact of metamorphism on the surface snow isotopic signal when compared to the initial precipitation. Particularly in summer, exchanges of water molecules between vapour and snow are driven by the diurnal sublimation–condensation cycles. Overall, we observe in between precipitation events modification of the surface snow isotopic composition. Using high-resolution water isotopic composition profiles from snow pits at five Antarctic sites with different accumulation rates, we identified common patterns which cannot be attributed to the seasonal variability of precipitation. These differences in the precipitation, surface snow and buried snow isotopic composition provide evidence of post-deposition processes affecting ice core records in low-accumulation areas. Programme: 1110
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Emmanuel Le Meur, Olivier Magand, Laurent Arnaud, Michel Fily, Massimo Frezzotti, Marie Cavitte, Robert Mulvaney, Stefano Urbini. (2018). Spatial and temporal distributions of surface mass balance between Concordia and Vostok stations, Antarctica, from combined radar and ice core data: first results and detailed error analysis (Vol. 12). Bachelor's thesis, , .
Abstract:
Abstract. Results from ground-penetrating radar (GPR) measurements and shallow ice cores carried out during a scientific traverse between Dome Concordia (DC) and Vostok stations are presented in order to infer both spatial and temporal characteristics of snow accumulation over the East Antarctic Plateau. Spatially continuous accumulation rates along the traverse are computed from the identification of three equally spaced radar reflections spanning about the last
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Thomas Laepple, Thomas Münch, Mathieu Casado, Maria Hoerhold, Amaelle Landais, Sepp Kipfstuhl. (2018). On the similarity and apparent cycles of isotopic variations in East Antarctic snow pits (Vol. 12).
Abstract: Stable isotope ratios δ18O and δD in polar ice provide a wealth of information about past climate evolution. Snow-pit studies allow us to relate observed weather and climate conditions to the measured isotope variations in the snow. They therefore offer the possibility to test our understanding of how isotope signals are formed and stored in firn and ice. As δ18O and δD in the snowfall are strongly correlated to air temperature, isotopes in the near-surface snow are thought to record the seasonal cycle at a given site. Accordingly, the number of seasonal cycles observed over a given depth should depend on the accumulation rate of snow. However, snow-pit studies from different accumulation conditions in East Antarctica reported similar isotopic variability and comparable apparent cycles in the δ18O and δD profiles with typical wavelengths of ∼ 20cm. These observations are unexpected as the accumulation rates strongly differ between the sites, ranging from 20 to 80mm w. e. yr−1 ( ∼ 6–21cm of snow per year). Various mechanisms have been proposed to explain the isotopic variations individually at each site; however, none of these are consistent with the similarity of the different profiles independent of the local accumulation conditions.Here, we systematically analyse the properties and origins of δ18O and δD variations in high-resolution firn profiles from eight East Antarctic sites. First, we confirm the suggested cycle length (mean distance between peaks) of ∼ 20cm by counting the isotopic maxima. Spectral analysis further shows a strong similarity between the sites but indicates no dominant periodic features. Furthermore, the apparent cycle length increases with depth for most East Antarctic sites, which is inconsistent with burial and compression of a regular seasonal cycle. We show that these results can be explained by isotopic diffusion acting on a noise-dominated isotope signal. The firn diffusion length is rather stable across the Antarctic Plateau and thus leads to similar power spectral densities of the isotopic variations. This in turn implies a similar distance between isotopic maxima in the firn profiles.Our results explain a large set of observations discussed in the literature, providing a simple explanation for the interpretation of apparent cycles in shallow isotope records, without invoking complex mechanisms. Finally, the results underline previous suggestions that isotope signals in single ice cores from low-accumulation regions have a small signal-to-noise ratio and thus likely do not allow the reconstruction of interannual to decadal climate variations.
Programme: 1110
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Christoph Kittel, Charles Amory, Cécile Agosta, Alison Delhasse, Sébastien Doutreloup, Pierre-Vincent Huot, Coraline Wyard, Thierry Fichefet, Xavier Fettweis. (2018). Sensitivity of the current Antarctic surface mass balance to sea surface conditions using MAR (Vol. 12).
Abstract:
Abstract. Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale boundary forcing fields prescribed by reanalyses or general circulation models (GCMs). Since the recent variability of sea surface conditions (SSCs, namely sea ice concentration, SIC, and sea surface temperature, SST) over the Southern Ocean is not reproduced by most GCMs from the 5th phase of the Coupled Model Intercomparison Project (CMIP5), RCMs are then subject to potential biases. We investigate here the direct sensitivity of the Antarctic SMB to SSC perturbations around the Antarctic. With the RCM “Modèle Atmosphérique Régional” (MAR), different sensitivity experiments are performed over 1979–2015 by modifying the ERA-Interim SSCs with (i) homogeneous perturbations and (ii) mean anomalies estimated from all CMIP5 models and two extreme ones, while atmospheric lateral boundary conditions remained unchanged. Results show increased (decreased) precipitation due to perturbations inducing warmer, i.e. higher SST and lower SIC (colder, i.e. lower SST and higher SIC), SSCs than ERA-Interim, significantly affecting the SMB of coastal areas, as precipitation is mainly related to cyclones that do not penetrate far into the continent. At the continental scale, significant SMB anomalies (i.e greater than the interannual variability) are found for the largest combined SST/SIC perturbations. This is notably due to moisture anomalies above the ocean, reaching sufficiently high atmospheric levels to influence accumulation rates further inland. Sensitivity experiments with warmer SSCs based on the CMIP5 biases reveal integrated SMB anomalies (+5% to +13%) over the present climate (1979–2015) in the lower range of the SMB increase projected for the end of the 21st century. Programme: 411
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Sentia Goursaud, Valérie Masson-Delmotte, Vincent Favier, Suzanne Preunkert, Michel Legrand, Bénédicte Minster, Martin Werner. (2019). Challenges associated with the climatic interpretation of water stable isotope records from a highly resolved firn core from Adélie Land, coastal Antarctica (Vol. 13). Bachelor's thesis, , .
Abstract: A New 21.3 M Firn Core Was Drilled In 2015 At A Coastal Antarctic High-accumulation Site In Adélie Land (66.78∘ S; 139.56∘ E, 602 M A.s.l.), Named Terre Adélie 192a (Ta192a). The Mean Isotopic Values (-19.3‰±3.1 ‰ For δ18o And 5.4 ‰±2.2 ‰ For Deuterium Excess) Are Consistent With Other Coastal Antarctic Values. No Significant Isotope–temperature Relationship Can Be Evidenced At Any Timescale. This Rules Out A Simple Interpretation In Terms Of Local Temperature. An Observed Asymmetry In The δ18o Seasonal Cycle May Be Explained By The Precipitation Of Air Masses Coming From The Eastern And Western Sectors In Autumn And Winter, Recorded In The D-excess Signal Showing Outstanding Values In Austral Spring Versus Autumn. Significant Positive Trends Are Observed In The Annual D-excess Record And Local Sea Ice Extent (135–145∘ E) Over The Period 1998–2014. However, Process Studies Focusing On Resulting Isotopic Compositions And Particularly The Deuterium Excess–δ18o Relationship, Evidenced As A Potential Fingerprint Of Moisture Origins, As Well As The Collection Of More Isotopic Measurements In Adélie Land Are Needed For An Accurate Interpretation Of Our Signals.
Programme: 411,414
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Sentia Goursaud, Valérie Masson-delmotte, Vincent Favier, Susanne Preunkert, Michel Fily, Hubert Gallée, Bruno Jourdain, Michel Legrand, Olivier Magand, Bénédicte Minster, Martin Werner. (2017). A 60-year ice-core record of regional climate from Adélie Land, coastal Antarctica (Vol. 11).
Abstract: Abstract. A 22.4 M-long Shallow Firn Core Was Extracted During The 2006/2007 Field Season From Coastal Adélie Land. Annual Layer Counting Based On Subannual Analyses Of δ18o And Major Chemical Components Was Combined With 5 Reference Years Associated With Nuclear Tests And Non-retreat Of Summer Sea Ice To Build The Initial Ice-core Chronology (1946–2006), Stressing Uncertain Counting For 8 Years. We Focus Here On The Resulting δ18o And Accumulation Records. With An Average Value Of 21.8 ± 6.9 Cm W.e. Yr−1, Local Accumulation Shows Multi-decadal Variations Peaking In The 1980s, But No Long-term Trend. Similar Results Are Obtained For δ18o, Also Characterised By A Remarkably Low And Variable Amplitude Of The Seasonal Cycle. The Ice-core Records Are Compared With Regional Records Of Temperature, Stake Area Accumulation Measurements And Variations In Sea-ice Extent, And Outputs From Two Models Nudged To Era (European Reanalysis) Atmospheric Reanalyses: The High-resolution Atmospheric General Circulation Model (Agcm), Including Stable Water Isotopes Echam5-wiso (European Centre Hamburg Model), And The Regional Atmospheric Model Modèle Atmosphérique Régional (Ar). A Significant Linear Correlation Is Identified Between Decadal Variations In δ18o And Regional Temperature. No Significant Relationship Appears With Regional Sea-ice Extent. A Weak And Significant Correlation Appears With Dumont D'urville Wind Speed, Increasing After 1979. The Model-data Comparison Highlights The Inadequacy Of Echam5-wiso Simulations Prior To 1979, Possibly Due To The Lack Of Data Assimilation To Constrain Atmospheric Reanalyses. Systematic Biases Are Identified In The Echam5-wiso Simulation, Such As An Overestimation Of The Mean Accumulation Rate And Its Interannual Variability, A Strong Cold Bias And An Underestimation Of The Mean δ18o Value And Its Interannual Variability. As A Result, Relationships Between Simulated δ18o And Temperature Are Weaker Than Observed. Such Systematic Precipitation And Temperature Biases Are Not Displayed By Mar, Suggesting That The Model Resolution Plays A Key Role Along The Antarctic Ice Sheet Coastal Topography. Interannual Variations In Echam5-wiso Temperature And Precipitation Accurately Capture Signals From Meteorological Data And Stake Observations And Are Used To Refine The Initial Ice-core Chronology Within 2 Years. After This Adjustment, Remarkable Positive (Negative) δ18o Anomalies Are Identified In The Ice-core Record And The Echam5-wiso Simulation In 1986 And 2002 (1998–1999), Respectively. Despite Uncertainties Associated With Post-deposition Processes And Signal-to-noise Issues, In One Single Coastal Ice-core Record, We Conclude That The S1c1 Core Can Correctly Capture Major Annual Anomalies In δ18o As Well As Multi-decadal Variations. These Findings Highlight The Importance Of Improving The Network Of Coastal High-resolution Ice-core Records, And Stress The Skills And Limitations Of Atmospheric Models For Accumulation And δ18o In Coastal Antarctic Areas. This Is Particularly Important For The Overall East Antarctic Ice Sheet Mass Balance.
Programme: 411
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Nicolas Champollion, Ghislain Picard, Laurent Arnaud, Éric Lefebvre, Giovanni Macelloni, Frédérique Rémy, Michel Fily. (2019). Marked decrease in the near-surface snow density retrieved by AMSR-E satellite at Dome C, Antarctica, between 2002 and 2011 (Vol. 13).
Abstract:
Abstract. Surface snow density is an important variable for the surface mass balance and energy budget. It evolves according to meteorological conditions, in particular, snowfall, wind, and temperature, but the physical processes governing atmospheric influence on snow are not fully understood. A reason is that no systematic observation is available on a continental scale. Here, we use the passive microwave observations from AMSR-E satellite to retrieve the surface snow density at Dome C on the East Antarctic Plateau. The retrieval method is based on the difference of surface reflections between horizontally and vertically polarized brightness temperatures at 37 GHz, highlighted by the computation of the polarization ratio, which is related to surface snow density. The relationship has been obtained with a microwave emission radiative transfer model (DMRT-ML). The retrieved density, approximately representative of the topmost 3 cm of the snowpack, compares well with in situ measurements. The difference between mean in situ measurements and mean retrieved density is 26.2 kg m−3, which is within typical in situ measurement uncertainties. We apply the retrieval method to derive the time series over the period 2002–2011. The results show a marked and persistent pluri-annual decrease of about 10 kg m−3 yr−1, in addition to atmosphere-related seasonal, weekly, and daily density variations. This trend is confirmed by independent active microwave observations from the ENVISAT and QuikSCAT satellites, though the link to the density is more difficult to establish. However, no related pluri-annual change in meteorological conditions has been found to explain such a trend in snow density. Further work will concern the extension of the method to the continental scale. Programme: 1110
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Ghislain Picard, Laurent Arnaud, Romain Caneill, Eric Lefebvre, Maxim Lamare. (2019). Observation of the process of snow accumulation on the Antarctic Plateau by time lapse laser scanning (Vol. 13).
Abstract:
Abstract. Snow accumulation is the main positive component of the mass balance in Antarctica. In contrast to the major efforts deployed to estimate its overall value on a continental scale – to assess the contribution of the ice sheet to sea level rise – knowledge about the accumulation process itself is relatively poor, although many complex phenomena occur between snowfall and the definitive settling of the snow particles on the snowpack. Here we exploit a dataset of near-daily surface elevation maps recorded over 3 years at Dome C using an automatic laser scanner sampling 40–100 m2 in area. We find that the averaged accumulation is relatively regular over the 3 years at a rate of +8.7 cm yr−1. Despite this overall regularity, the surface changes very frequently (every 3 d on average) due to snow erosion and heterogeneous snow deposition that we call accumulation by “patches”. Most of these patches (60 %–85 %) are ephemeral but can survive a few weeks before being eroded. As a result, the surface is continuously rough (6–8 cm root-mean-square height) featuring meter-scale dunes aligned along the wind and larger, decameter-scale undulations. Additionally, we deduce the age of the snow present at a given time on the surface from elevation time series and find that snow age spans over more than a year. Some of the patches ultimately settle, leading to a heterogeneous internal structure which reflects the surface heterogeneity, with many snowfall events missing at a given point, whilst many others are overrepresented. These findings have important consequences for several research topics including surface mass balance, surface energy budget, photochemistry, snowpack evolution, and the interpretation of the signals archived in ice cores. Programme: 1110
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