Modelling Greenland precipitation sources
The isotopic composition of the water molecules in the ice cores does not only tell us about the local temperature back in time at the drill site. It also tells us about the conditions where the water initially evaporated. When water evaporates from an ocean or land surface, it is typically transported aloft in the atmosphere after which it follows the winds. Eventually it condenses and precipitates out as rain or snow a completely different place from where it evaporated. Because the history of the precipitation influences the isotopic composition, the isotopes can reveal past changes both at the source areas far from the drill site and in atmospheric circulation patterns.
Studying moisture sources
Analyses indicate that, although the NGRIP core was drilled only about 320 km from the GRIP core, there are significant differences in the sources of precipitation for the two sites both now and during the glacial. To contribute to an understanding of such differences, the Centre for Ice and Climate conducts a series of computer simulations of the transport of water vapor from the evaporation areas to the Greenland Ice Sheet. We use a general circulation model (GCM) where we have divided the surface of the Earth into a number of source areas such as North America, Greenland, the Pacific and Atlantic Oceans etc. The water evaporated from each of these areas is tracked in the model until it forms precipitation. For any given spot we can thus determine which percentages of the precipitation that originate from each of the source areas.
Shift toward Pacific source influence
The results from these computer experiments are still under way, but we have, for instance, seen how the much lower temperatures and the massive ice sheet that covered the whole northern part of North America (the Laurentide Ice Sheet) conspired to change the atmospheric circulation during the Last Glacial Maximum (LGM). The results show that Greenland encountered a shift towards greater Pacific influence than today. This shift is larger toward the northwest and will therefore be more pronounced at NGRIP than at GRIP, partially explaining the observed difference between ice core records of NGRIP and GRIP.
Langen PL, Vinther BM
Response in atmospheric circulation and sources of Greenland precipitation to glacial boundary conditions
Climate Dynamics, vol. 32, no. 7-8, pp 1035-1054, 2009
Look up abstract/reprint using doi: 10.1007/s00382-008-0438-y