Borehole versus isotope temperatures on Greenland: Seasonality does matter



New simulation results obtained with the Hamburg Atmosphere General Circulation Model ECHAM-4 under maximum glacial boundary (LGM) conditions confirm the paleotemperatures on Greenland determined by borehole thermometry. The disagreement between δ18O isotope based temperatures and the borehole temperatures of the LGM is not only reproduced by the model, but the simulation results provide a plausible explanation: Paleotemperatures inferred from δ18O measurements in ice cores are biased by a substantially increased seasonality of precipitation over Greenland during the LGM. During the glacial winter a much more zonal circulation prevents the effective transport of moisture to the Greenland ice sheet, and therefore reduces the contribution of isotopically strongly depleted winter snow to the annual mean isotope signal.