A Review of Antarctic Surface Snow Isotopic Composition : Observations, Atmospheric Circulation, and Isotopic Modeling
Summary (2 min read)
1. Introduction
- Regional Antarctic temperature reconstructions are also essential for the comparison between observed past climatic changes and simulations performed by AGCMs, conducted only for the inland East Antarctic plateau (Masson-Delmotte et al. 2006).
- To analyze the stable isotopic composition of snowfall in a model framework that is compatible with the observed climatology, several methods must be combined.
2. A database of Antarctic snow isotopic composition
- A. Sampling sites and related documentation Table 2 presents the list and references of the various sources of information compiled to produce the full Antarctic database (available as an Excel file online at http://www.lsce.ipsl.fr/Pisp/24/valerie.masson-delmotte. html).
- Annual mean surface air or firn temperatures are available for only 811 sites; the situation is even more restricted for annual mean accumulation data, available only for 322 sites (Fig. 2).
- The first two criteria are expected to reflect the quality of the isotopic measurements and sample preservation; the last three criteria have been defined with respect to the temporal scale and resolution of the samples, with the purpose of building “climatologies” of surface snow isotopic composition.
- The authors now describe the range of variability of D, 18O, and deuterium excess data, both spatially (from site to site) and temporally (within one site when several measurements have been averaged to produce the local average value).
- The amplitude of local D range (difference between maximum and minimum values of individual sample measurements at one location) varies between 4.9‰ and 262.3‰, with a mean range of 74.1‰.
D and 88% of the 18O spatial variance:
- The observations show isotopic values that are less depleted than the calculation (positive anomalies) on the flanks of the ice sheet (at elevations from 1000 to 2000 m) and inland West Antarctica, whereas they show isotopic values that are more depleted than the calculation (negative anomalies) in the central Antarctic Peninsula and the central East Antarctic plateau.
- To assess the spatial variations of this slope, the authors have developed a methodology to estimate local slopes.
- The distribution of deuterium excess as a function of D (Fig. 6d) now relies on 789 data points, including new traverse data available from the coast to the interior of East Antarctica and 269 data points from the Taylor Valley in the Dry Valleys (with many negative deuterium excess values).
In the Lambert Glacier area, deuterium excess values
- Atmospheric models can be used to analyze the vertical moisture advection to Antarctica (see section 3) and test this hypothesis.
- The observed deuterium excess spatial distribution also reflects changes in the D– 18O slope depending on the range of isotopic values (Fig. 4).
- These slope uncertainties have been obtained from a Monte Carlo method using 1000 random subsets.
7.30‰ (‰) 1 are observed in central East Antarctic ice cores (Vimeux et al. 1999; Stenni et al. 2001).
- Different moisture origins at coastal versus inland locations should influence the distribution of deuterium excess, but also 18O, D, and their relationships to local climatic parameters.
- Changes in isotope– temperature slopes between locations may be related to atmospheric transport paths.
- In fact, such spatial slopes very likely include the combined effects of distillation, including temperature gradients between source and site temperatures, and equilibrium fractionation effects along different ranges of temperatures.
- Isotopic models are used in the next section to assess the relative weight and role of these different physical processes on the Antarctic snow isotopic composition.
3. Model–data comparisons
- The authors also analyze the capability of AGCMs to simulate this observed distribution.
- The isotopic AGCMs offer the advantage of consistent simulations of climate and isotopic processes, but make it difficult to isolate the impact of each process on the isotopic composition of precipitation (Table 1).
- Diagnostic of the fraction of simulated fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) annual precipitation that is removed from the surface by sublimation (Bromwich et al. 2004). rence) condensation temperature, estimated by the temperature at the vertical level of the maximum condensed moisture (Helsen et al. 2007).
- The model–data comparison therefore points to the following two systematic model biases: (i) a lack of isotopic depletion, even in AGCMs simulating a correct range of Antarctic surface temperature, and (ii) an underestimation of moisture supply to inland Antarctica (specifically at temperatures below 30°C).
4. Conclusions and perspectives
- The authors compilation of surface Antarctic snow composition provides better spatial coverage than earlier studies, although it is still strongly biased toward East Antarctic locations.
- Systematic measurements of water vapor and snow isotopic composition should allow us to disentangle the effect of depositional and postdepositional processes.
- Intensive efforts based on accumulation histories derived from ice cores suggest that, despite a warming detected in winter tropospheric temperature in Antarctica during the past decades (Turner et al. 2006), there is no significant change in Antarctic accumulation since the International Geophysical Year in 1957–58 (Monaghan et al. 2006).
- This database confirms earlier findings regarding the spatial variability of the isotope distribution in relation to geographical parameters (latitude, distance from the coast, and elevation).
- It shows regional signatures, with variations mostly within 20%–.
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"A Review of Antarctic Surface Snow ..." refers background in this paper
...AGCMs equipped with the explicit representation of water-stable isotopes (Joussaume et al. 1984; Jouzel et al. 1987b, 1991; Hoffmann et al. 2000) allow us to disentangle the different factors involved in the spatial (Brown and Simmonds 2004; Schmidt et al....
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"A Review of Antarctic Surface Snow ..." refers background in this paper
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"A Review of Antarctic Surface Snow ..." refers background in this paper
...This result suggests that the accumulation–isotope relationships observed today for the cold conditions of the central plateau may not be extrapolated to warmer conditions, a result that has implications for the mod- eling of the accumulation used for ice core dating (Monnin et al. 2004)....
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