Precise Interhemispheric Phasing of the Bipolar Seesaw during Abrupt Dansgaard-Oeschger Events

Abstract:

Late Pleistocene glacial periods exhibit abrupt Dansgaard-Oeschger (DO) climatic oscillations, evidence of which is preserved in a variety of northern hemisphere (NH) palaeoclimatic archives. Ice cores show Antarctica is cooling during the warm phases of the Greenland DO cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism dubbed the bipolar seesaw. While it is generally accepted that variations in the Atlantic meridional overturning circulation (AMOC) strength play an important role, great uncertainty remains regarding the dynamics and trigger of the abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large and poorly constrained ice age-gas age difference (Dage) in Antarctic ice cores has precluded methane-based synchronization at the required sub-centennial precision. Here we present a new high accumulation deep Antarctic ice core, the West Antarctic Ice Sheet (WAIS)-Divide core, that is used to resolve the timing of the bipolar seesaw at unprecedented temporal resolution. We find that the abrupt Greenland warming phase leads the corresponding Antarctic cooling by 195 ± 59 years for DO-events, including the Bølling period; Greenland cooling leads the corresponding Antarctic warming by 179 ± 61 years. The centennial NH lead time shows that the abrupt phases of the DO cycle are initiated in the NH, after which the temperature anomaly is propagated to the southern hemisphere (SH) high latitudes via an oceanic teleconnection. The similar phasing of warming and cooling events suggests that to first order the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm the central role ocean circulation plays in the seesaw, and provide a clear criterion for testing hypotheses and model simulations of DO dynamics.