Using Water Vapor Isotope Observations from above the Greenland Ice Sheet to improve the Interpretation of Ice Core Water Stable Isotope Records

Abstract:

Water stable isotope data from Greenland ice cores provide key paleoclimatic information. For the purpose of improving the climatic interpretation from ice core records, a monitoring of the isotopic composition δ¹⁸O and δD at several height levels (up to 13 meter) of near-surface water vapor, precipitation and snow in the first 0.5 cm surface layer has been conducted during three summers (2010-2012) at NEEM, NW Greenland.

We compare the observed water vapor isotopic composition with model outputs from three isotope-enabled general circulation models: LMDZiso, isoGSM, ECHAM-wiso. This allows us to benchmark the models and address effect of model resolution, effect of transport, effect of isotope parameterization, and representation of significant source region contributions.

We find for all models that the simulated isotopic value δD are significantly biased towards too enriched values. A bias, which is only partly explained by the air temperature. The simulated amplitude in d-excess variations is ~50% smaller than observed and the simulated average summer level is ~10‰ lower than in observations. Using back trajectories we observe water vapor of Arctic origin to have a high d-excess fingerprint. This fingerprint is not observed in the GCMiso simulations indicating a problem of simulating accurately the Arctic hydrological cycle.

 The bias in the simulated δD and d-excess water vapor is similar to the already-documented bias in the simulated δD and d-excess of Greenland ice core records. This suggests that if we improve the simulation of the water vapor isotopic composition we might also improve the simulation of the ice core isotope record.

 During periods between precipitation events, our data demonstrate parallel changes of δ¹⁸O and d-excess in surface snow and near-surface vapor. The changes in δ¹⁸O of the vapor are similar or larger than those of the snow δ¹⁸O. It is estimated using the CROCUS snow model that 6 to 20% of the surface snow mass is exchanged with the atmosphere. We suggest that, in-between precipitation events, changes in the surface snow isotopic composition are driven by synoptic changes in near-surface vapor isotopic composition. This suggests that the ice core isotope climate signal might be driven by both the water vapor and precipitation isotope signal.