Elisabeth Schlosser

A Review of Antarctic Surface Snow Isotopic Composition: Observations, Atmospheric Circulation, and Isotopic Modeling*

A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

Effects of seasonal variability of accumulation on yearly mean δ18O values in Antarctic snow

The annual mean oxygen-isotope content of Antarctic snow is strongly influenced by the seasonal variability ofaccumulation. Since the annual mean δ 18 O value is frequently used to derive mean annual temperatures from ice cores, changes in atmospheric circulation pattern can lead to large errors in the deduced temperature record. At the German Antarctic wintering base, Neumayer, accumulation measurements have been carried out continuously over the last 16 years. Weekly readings of accumulation stakes combined with snow pits and shallow firn cores are used to investigate the influence of the seasonal variability of accumulation on the annual mean δ 18 O values and to estimate the possible error in the determination ofannual mean temperatures from ice cores by using the oxygen-isotope record.

Atmospheric influence on the deuterium excess signal in polar firn: implications for ice-core interpretation

The seasonal deuterium excess signal of fresh snow samples from Neumayer station, coastal Dronning Maud Land, Antarctica, was studied to investigate the relationship between deuterium excess and precipitation origin. An isotope model was combined with a trajectory model to determine the relative influence of different moisture sources on the mean annual course of the deuterium excess, focusing on the phase lag between d 18 O and excess d. Whereas the annual course of d 18 O always shows an austral summer maximum, which clearly depends on local temperature and the annual course of moisture source-area parameters, the deuterium excess of the fresh snow samples shows maximum values already in spring. There can be many different reasons for the time lag between d 18 O and deuterium excess in an ice core, including post-depositional processes and changes in the moisture source of precipitation. The use of fresh snow samples enabled us to exclude post-depositional processes and study solely the influence of precipitation origin. Changes in the moisture source connected to systematic changes in the general atmospheric circulation can have a strong influence on the phase lag between deuterium excess and d 18 O, which has to be taken into account for climatic interpretation of stable-isotope profiles from ice cores.

Interaction between Antarctic sea ice and synoptic activity in the circumpolar trough: implications for ice-core interpretation

Abstract Interactions between Antarctic sea ice and synoptic activity in the circumpolar trough have been investigated using meteorological data from European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-analysis and sea-ice data from passive-microwave measurements. Total Antarctic sea-ice extent does not show large interannual variations. However, large differences are observed on a regional/monthly scale, depending on prevailing winds and currents, and thus on the prevailing synoptic situations. the sea-ice edge is also a preferred region for cyclogenesis due to the strong meridional temperature gradient (high baroclinicity) in that area. the motivation for this study was to gain a better understanding of the interaction between sea-ice extent and the general atmospheric flow, particularly the frequency of warm-air intrusions into the interior of the Antarctic continent, since this influences precipitation seasonality and must be taken into account for a correct climatic interpretation of ice cores. Two case studies of extraordinary sea-ice concentration anomalies in relation to the prevailing atmospheric conditions are presented. However, both strong positive and negative anomalies can be related to warm biases in ice cores (indicated by stable-isotope ratios), especially in connection with the negative phase of the Southern Annular Mode.

Shallow firn cores from Neumayer, Ekströmisen, Antarctica: a comparison of accumulation rates and stable-isotope ratios

Abstract Since 1979/80, glaciological studies have been carried out at Ekströmisen, Antarctica, including accumulation-stake measurements, snow-pit and shallow-firn-core studies. Snow stratigraphy, chemical properties and stable-isotope ratios (δ18O) were investigated. This study focuses on three cores taken between 1982 and 1998. the 1998 core was dated using dielectric profiling, δ18O profiles and stake measurements. Accumulation rates show high interannual and spatial variability due to the extreme wind influence. No significant trend was found for the last 50 years; during the first half of the 20th century, accumulation decreased. the high spatial and interannual variability, however, means that trends must be interpreted with care. In spite of the highly irregular accumulation distribution, stable-isotope ratios show little spatial variability. the mean annual δ18O values of cores B04 and FB0198 agree fairly well for the time period 1955–82 covered by both cores. δ18O values have increased during most of the 20th century; since the late 1980s a decrease is observed. This change is not related to air temperature, since mean annual air temperatures at Neumayer show no significant trend over the last two decades.

The influence of precipitation origin on the d18O-T relationship at Neumayer Station, Ekströmisen, Antarctica

Abstract The relationship between δ18O and air temperature at Neumayer station, Ekstrmisen, Antarctica, was investigated using fresh-snow samples from the time period 1981–2000. A trajectory model that calculated 5 day-backward trajectories was used to study the influence of different synoptic weather situations and thus of different moisture sources on this correlation. Generally a high correlation between air temperature and δ18O was found, but the quality of the δ18O–T relationship varied with the different trajectory classes. Additionally, the sea-ice coverage on the travel path of the moist air was considered. The amount of open ocean water underneath the trajectory has a large influence on the δ18O–T relationship. For trajectories that lead completely above open water, no significant correlation between δ18O and T was found, because mixing with air masses containing additionally evaporated water vapour from the ocean influences the isotope ratio of precipitation. A very high correlation, however, was found for transports over the completely ice-covered Weddell Sea.

A Comparison of Antarctic Ice Sheet Surface Mass Balance from Atmospheric Climate Models and In Situ Observations

AbstractIn this study, 3265 multiyear averaged in situ observations and 29 observational records at annual time scale are used to examine the performance of recent reanalysis and regional atmospheric climate model products [ERA-Interim, JRA-55, MERRA, the Polar version of MM5 (PMM5), RACMO2.1, and RACMO2.3] for their spatial and interannual variability of Antarctic surface mass balance (SMB), respectively. Simulated precipitation seasonality is also evaluated using three in situ observations and model intercomparison. All products qualitatively capture the macroscale spatial variability of observed SMB, but it is not possible to rank their relative performance because of the sparse observations at coastal regions with an elevation range from 200 to 1000 m. In terms of the absolute amount of observed snow accumulation in interior Antarctica, RACMO2.3 fits best, while the other models either underestimate (JRA-55, MERRA, ERA-Interim, and RACMO2.1) or overestimate (PMM5) the accumulation. Despite underestima...

Seasonal variations of accumulation and the isotope record in ice cores: a study with surface snow samples and firn cores from Neumayer station, Antarctica

Abstract At the German wintering base Neumayer, an intensive glacio-meteorological programme has been carried out during the last two decades. A complete meteorological dataset and data from surface snow samples, snow pits, firn cores and weekly accumulation measurements from a stake array are available. We first investigated the attenuation of the seasonal δ18O signal due to water-vapour diffusion in the snowpack. A comparison of surface snow samples and firn cores of different ages shows that only one-third of the seasonal δ18O signal of the surface snow samples remains in the cores after the first year. No further significant change in the amplitude of the seasonal δ18O signal is found later. Changes in the seasonal distribution of accumulation can lead to a bias in ice-core properties. This is studied on a short time-scale, using high-time-resolution data of accumulation, stable-isotope ratios and air temperature. Mean annual δ18O values from firn cores are not well correlated to annual mean air temperatures. However, the correlation is improved considerably by calculating an annual mean air temperature using monthly mean temperatures weighted by monthly accumulation. At Neumayer, it is mainly the cyclonic activity in late winter/early spring that determines whether and how the core data are biased.

Strong-wind events and their influence on the formation of snow dunes: observations from Kohnen station, Dronning Maud Land, Antarctica

Analyses of shallow cores obtained at the European Project for Ice Coring in Antarctica (EPICA) drilling site Kohnen station (75800 0 S, 00804 0 E; 2892 m a.s.l.) on the plateau of Dronning Maud Land reveal the presence of conserved snow dunes in the firn. In situ observations during three dune formation events in the 2005/06 austral summer at Kohnen station show that these periods were characterized by a phase of 2 or 3 days with snowdrift prior to dune formation which only occurred during high wind speeds of >10 m s -1 at 2 m height caused by the influence of a low-pressure system. The dune surface coverage after a formation event varied between 5% and 15%, with a typical dune size of (4 � 2) m � (8 � 3) m, a maximum height of 0.2 � 0.1 m and a periodicity length of about 30 m. The mean density within a snow dune varied between 380 and 500 kg m -3 , whereas the mean density at the surrounding surface was 330 � 5k g m -3 . The firn cores covering a time-span of 22 � 2 years reveal that approximately three to eight events per year occurred, during which snow dunes had been formed and were preserved in the firn.

Optical studies of Antarctic sea ice

Abstract During the austral winter 1986 the spectral albedo of both snow-covered and bare sea ice was measured over the wavelength range 400–1350 nm in the eastern Weddell Sea. The albedo shows high values and a week wave-length dependence in the visible part of the spectrum and a strong decrease with increasing wavelength in the near IR. Maxima in albedo correspond with minima in ice absorption. For relatively thin ice the albedo depends strongly on ice thickness. Additionally the bidirectional reflectance of sea ice was studied in the laboratory. For ice thicknesses of a few millimeters nearly specular reflectance is found. With growing ice thickness more scattering within the ice occurs and consequently the intensity of the sidewards scattered radiation increases. The reflectance distribution shows a peak in the forward direction which is less developed with thicker ice and smaller crystal size.