The ISOARC project: From source to sink – Monitoring the isotopic fingerprints of Arctic moisture

In order to constrain the isotopic fingerprint of the Arctic hydrological cycle, a new generation of Picarro water isotope analyzers using CRDS (cavity ringdown spectroscopy) is used to monitor the isotopic composition of atmospheric water vapour at different sites. Since July 2015, within the ISOARC project water vapour stable isotopes (HDO and H218O; d excess) have been continuously measured: (1) on-board Polarstern (as oceanic moisture source) and (2) at Samoylov Island in the Lena Delta in northern Siberia (72°22’ N, 126°29’E) as sink in the water cycle. On Polarstern the water vapour has been sampled during ship cruise at 30 meters above sea surface. In addition to these water vapour measurements, ocean surface waters have been sampled on a daily basis and were later analysed in the laboratory for its water isotope composition. Data comprises two summer Arctic campaigns in 2015 and 2016 covering a large region of the Arctic Ocean, including the North Pole in September 2015. In the central and eastern Arctic Ocean, a large area of complete sea ice cover revealed a strong impact on the moisture above the ice cap under very cold conditions. On Samoylov Island, water vapour has been sampled from an inlet situated at 5 m height above the Siberian tundra typical for the Lena Delta. Isotope measurements were compared with meteorological and surface data collected throughout the year in the Lena delta, as well as to data from other sites such as Svalbard or Iceland run by partner institutes. We were able to measure the isotope composition to low humidity levels in Arctic winter. This water vapour network is a new approach into the understanding of the Arctic hydrological cycle at the regional scale. A first model-data comparison of our measurements with simulation results by the isotope-enabled atmospheric general circulation model ECHAM5-wiso have depicted relevant model biases in the Arctic realm, particularly close to the sea ice covered areas

Moisture origin as a driver of temporal variabilities of the water vapour isotopic composition in the Lena River Delta, Siberia

In the context of the Arctic amplification of climate change affecting the regional atmospheric hydrological cycle, it is crucial to characterize the present-day moisture sources of the Arctic. The isotopic composition is an important tool to enhance our understanding of the drivers of the hydrological cycle due to the different molecular characteristics of water stable isotopes during phase change. This study introduces 2 years of continuous in situ water vapour and precipitation isotopic observations conducted since July 2015 in the eastern Siberian Lena delta at the research station on Samoylov Island. The vapour isotopic signals are dominated by variations at seasonal and synoptic timescales. Diurnal variations of the vapour isotopic signals are masked by synoptic variations, indicating low variations of the amplitude of local sources at the diurnal scale in winter, summer and autumn. Low-amplitude diurnal variations in spring may indicate exchange of moisture between the atmosphere and the snow-covered surface. Moisture source diagnostics based on semi-Lagrangian backward trajectories reveal that different air mass origins have contrasting contributions to the moisture budget of the Lena delta region. At the seasonal scale, the distance from the net moisture sources to the arrival site strongly varies. During the coldest months, no contribution from local secondary evaporation is observed. Variations of the vapour isotopic composition during the cold season on the synoptic timescale are strongly related to moisture source regions and variations in atmospheric transport: warm and isotopically enriched moist air is linked to fast transport from the Atlantic sector, while dry and cold air with isotopically depleted moisture is generally associated with air masses moving slowly over northern Eurasia

Resolving the controls of water vapour isotopes in the Atlantic sector

Water isotope modelling is an important tool in climate reconstructions, but there remain gaps in our understanding of the effects upon oxygen and hydrogen isotope fractionation, and thus the source of the deposited signal. Here, the authors present a dataset assembled over two years that shows deuterium excess is controlled by humidity and sea surface temperature, and oxygen and hydrogen isotopes as well as deuterium excess are controlled by sublimation of snow in sea-ice regions