On the enigmatic similarity in Greenland δ¹⁸O between the Oldest and Younger Dryas

The last deglaciation (20.0-10.0 kyr B.P.) was punctuated by two major cooling events affecting the Northern Hemisphere: the Oldest Dryas (OD; 18.0-14.7 kyr B.P.) and the Younger Dryas (YD; 12.8-11.5 kyr B.P.). Greenland ice core δ¹⁸O temperature reconstructions suggest that the YD was as cold as the OD, despite a 50 ppmv increase in atmospheric CO₂, while modeling studies suggest that the YD was approximately 4-5°C warmer than the OD. This discrepancy has been surmised to result from changes in the origin of the water vapor delivered to Greenland; however, this hypothesis has not been hitherto tested. Here we use an atmospheric circulation model with an embedded moisture-tracing module to investigate atmospheric processes that may have been responsible for the similar δ¹⁸O values during the OD and YD. Our results show that the summer-to-winter precipitation ratio over central Greenland in the OD is twice as high as in the YD experiment, which shifts the δ¹⁸O signal toward warmer (summer) temperatures (enriched δ¹⁸O values and it accounts for ~45% of the expected YD-OD δ¹⁸O difference). A change in the inversion (cloud) temperature relationship between the two climate states further contributes (~20%) to altering the δ¹⁸O-temperature-relation model. Our experiments also show a 7% decrease of δ¹⁸O-depleted precipitation from distant regions (e.g., the Pacific Ocean) in the OD, hence further contributing (15-20%) in masking the actual temperature difference. All together, these changes provide a physical explanation for the ostensible similarity in the ice core δ¹⁸O temperature reconstructions in Greenland during OD and YD.