Isotopic composition and origin of polar precipitation in present and glacial climate simulations

The Hamburg atmospheric general circulation model(AGCM) ECHAM-4 is used to identify the main source regions of precipitation falling on Greenland and Antarctica. Both water isotopes (H2O)-O-18 and HDO are explicitly built int o the water cycle of the AGCM, and in addition the capability to trace wate r from different source regions was added to the model. Present and LGM cli mate simulations show that water from the most important source regions has an isotopic signature similar to the mean isotope values of the total prec ipitation amount. But water from other source regions (with very different isotopic signatures) contributes an additional, non-negligible part of the total precipitation amount on both Greenland and Antarctica. Analyses of th e temperature-isotope-relations For both polar regions reveal a solely bias of the glacial isotope signal on Greenland, which is caused by a strong ch ange in the seasonal deposition of precipitation originating from nearby po lar seas and the northern Atlantic. Although the performed simulations unde r LGM boundary conditions show a decrease of the delta O-18 values in preci pitation in agreement with ice core measurements, the AGCM fails to reprodu ce the observed simultaneous decrease of the deuterium excess signal.