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.

Continuer la lecture de Archival processes of the water stable isotope signal in East Antarctic ice cores →

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative.

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Climate trends in the Antarctic region remain poorly characterized, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the PAGES Antarctica2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records.

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Polar ice core water isotope records are commonly used to infer past changes in Antarctic temperature, motivating an improved understanding and quantification of the temporal relationship between δ¹⁸O and temperature. This can be achieved using simulations performed by atmospheric general circulation models equipped with water stable isotopes. Here, we evaluate the skills of the high resolution water-isotope-enabled atmospheric general circulation model ECHAM5-wiso (the European Centre Hamburg Model), nudged to European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis using simulations covering the period 1960–2013 over the Antarctic continent.

We compare model outputs with field data, first with a focus on regional climate variables and, second on water stable isotopes, using our updated dataset of water stable isotope measurements from precipitation, snow and firn/ice core samples.

Although some model-data mismatches are observed, the (precipitation minus evaporation) outputs are found to be realistic products for surface mass balance. A warm model bias over Central East Antarctica and a cold model bias over coastal regions explain first-order δ¹⁸O model biases by too strong isotopic depletion on coastal areas and underestimated depletion inland. At the second order, despite these biases, ECHAM5-wiso correctly captures the observed spatial and seasonal patterns of δ¹⁸O.

These results build trust in the use of ECHAM5-wiso outputs to investigate the spatial, seasonal and interannual δ¹⁸O-temperature relationships. We show that local spatial or seasonal slopes are not a correct surrogate for inter-annual temporal slopes, leading to the conclusion that a stationary isotope-temperature slope cannot be applied for the climatic interpretation of Antarctic ice core. We finally explore the phasing between the seasonal cycles of deuterium excess and δ¹⁸O, as a source of information on changes in moisture sources affecting the δ¹⁸O-temperature relationship. The few available records and ECHAM5-wiso show different phase relationships in coastal, intermediate and central regions.

Authors :  Goursaud, S., Masson-Delmotte, V., Favier, V., Orsi, A., and Werner, M.

Ref. :  Clim. Past Discuss., doi:10.5194/cp-2017-118

The Arctic is among the fastest warming regions on Earth, but it is also one with limited spatial coverage of multi-decadal instrumental surface air temperature measurements. Consequently, atmospheric reanalyses are relatively unconstrained in this region, resulting in a large spread of estimated 30-year recent warming trends, which limits their use to investigate the mechanisms responsible for this trend.

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