Precipitation mechanisms at Antarctic deep drilling sites

To assess the present and possible future climate changes a thorough understanding of the climate of the past is required. Considerable progress in paleoclimatology was made using information from ice cores drilled into the large ice sheets of Greenland and Antarctica. Of special interest is the ratio of stable oxygen and hydrogen isotopes, since they are used to derive the paleotemperature. However, for a correct interpretation of the ice data we need to understand the meteorological processes that are responsible for the precipitation that formed the ice in the cores. The annual mean stable isotope ratio is linearly related to annual mean air temperature, however, it depends in a complex way on moisture origin and transport, and evaporation and condensation processes during precipitation formation. This means that, for a correct interpretation of the data, we need to know the entire precipitation history. Another factor that influences the stable isotope ratio measured in an ice core is the seasonality of precipitation. The climate information is supposed to stem equally from all seasons. This is only true, if precipitation is distributed evenly over the year. If certain seasons are preferred or if this seasonality changes during a climate change, a strong bias can occur in the temperature derived from an ice core. Unfortunately, we do not have any direct information about precipitation history of the past. Therefore it is important to exactly know the processes involved under present conditions. At the deep drilling site Dome C, East Antarctica, an ice core has been obtained that yields the so far oldest ice of 800.000yr. Since 2006, samples of fresh snow have been taken at Dome C in the frame of the Italian Antarctic Research Programme. These new and in Antarctica unique data set enable us to identify precipitation events and correlate them to the corresponding meteorological conditions prevailing before and during the precipitation event. The stable isotope ratios of the samples are being measured and can be related to the meteorological data, too, in particular to the conditions at the moisture source, which is determined by calculating back-trajectories with a mesoscale atmospheric model. This model, organized in AMPS (Antarctic Mesoscale Prediction System), is also used for an analysis of the synoptic situation associated with the precipitation events. Two types of precipitation are distinguished: i) clear-sky precipitation ("diamond dust"), which forms in the cold air due to radiative cooling, producing very fine ice needles and leading to only very small precipitation amounts. ii) "synoptic precipitation", which is related to cyclone activity in the low pressure belt that encircles the Antarctic continent. Model data have to be used since observational data are still scarce in Antarctica, due to the harsh environmental conditions. AMPS employs a high-resolution mesoscale atmospheric model that was adapted for polar regions and is run by the National Center for Atmospheric Research, Boulder, Colorado, and the Polar Meteorology Group of The Ohio State University. In this study the precipitation mechanisms and origin at Dome C and two other Antarctic deep drilling sites, Dome Fuji and Kohnen Station, are investigated. A classification of precipitation-related weather situations combined with a trajectory classification will be compiled and the seasonality of the precipitation will be studied. The trajectory study will be combined with a simple isotope model to investigate the influence of precipitation origin on stable isotopes, particularly on the so-called deuterium excess, a combination of oxygen and hydrogen isotope ratios, which depends on sea surface temperature, wind speed, and humidity at the oceanic moisture source. Recently, improvements of the analytical devices made it possible to measure also the much less abundant isotope 17O and the corresponding 17O-excess, which is not dependent on source temperature. However, so far 17O excess is poorly understood, and this study under well-known meteorological conditions will yield a deeper insight into the relationship between 17O excess of Antarctic snow/ice and moisture source conditions. This will ultimately lead to a better quantitative interpretation of deep ice core data.

Considerable progress in paleoclimatology was made using information from ice cores drilled into the large ice sheets of Antarctica. Of special interest is the ratio of stable oxygen and hydrogen isotopes (i.e. different molecule types), since they are used to derive the paleo temperature. However, for a correct interpretation of the ice data we need to understand the meteorological processes that are responsible for the precipitation that formed the ice in the cores. The annual mean stable isotope ratio is linearly related to annual mean air temperature, however, it depends in a complex way on moisture origin and transport, and evaporation and condensation processes during precipitation formation. This means that, for a correct interpretation of the data, we need to know the entire precipitation history. At the deep drilling site Dome C, East Antarctica, an ice core has been obtained that yields the so far oldest ice of 800.000yr. Since 2006, samples of fresh snow have been taken at Dome C in the frame of the Italian Antarctic Research Programme. These new and in Antarctica unique data set enable us to identify precipitation events and correlate them to the corresponding meteorological conditions prevailing before and during the precipitation event. The stable isotope ratios of the samples are being measured and can be related to the meteorological data, too, in particular to the conditions at the moisture source, which is determined by calculating back-trajectories with a mesoscale atmospheric model. This model, organized in AMPS (Antarctic Mesoscale Prediction System), is also used for an analysis of the synoptic situation associated with the precipitation events. Two types of precipitation are distinguished: i) diamond dust, which forms in the cold air due to radiative cooling, producing very fine ice needles and leading to only very small precipitation amounts. ii) synoptic precipitation, which is related to cyclone activity in the low pressure belt that encircles the Antarctic continent. Model data have to be used since observational data are still scarce in Antarctica, due to the harsh environmental conditions. AMPS employs a high-resolution mesoscale atmospheric model that was adapted for polar regions and is run by the National Center for Atmospheric Research, Boulder, Colorado, and the Polar Meteorology Group of The Ohio State University. In this study the precipitation mechanisms and origin at Dome C and at Dome Fuji, two Antarctic deep drilling sites, Dome Fuji and Kohnen Station, are investigated. It was found that larger precipitation events occur preferably when the westerly wind belt around Antarctica starts meandering. This leads to a transport of relatively warm and moist air southwards. The air is forced to rise above the continent, thus is cooled and precipitation is formed. In those case unusually high temperatures are observed in the interior of the continent, even in winter. Since the isotope ratio depends on the temperature difference between the oceanic moisture source area and the drilling location on the continent, this mechanism is very important for deriving former temperatures from the ice core stable isotope ratios. It was shown in the project that several basic assumptions that have been used for decades in ice core science, are not valid. Further investigations, in particular the study of interactions between atmosphere and the snow cover between precipitation events, are required in order to get a complete understanding of the processes that determine the stable isotope ratios of the ice.