Towards a quantitative understanding of millennial-scale Antarctic Warming events

The interhemispheric temperature response to a meltwater-induced weakening of the Atlantic Meridional Overturning Circulation (AMOC) is characterized by a Northern Hemispheric cooling and a Southern Hemispheric warming. This so-called bipolar seesaw pattern explains many millennial-scale features identified in paleo-proxy records from both hemispheres. Here we present modeling evidence that suggests that the Southern Hemispheric response to a weakening of the AMOC includes elements that have previously been overlooked. Under present-day conditions, an AMOC weakening leads to an intensification of the negative phase of the Pacific South America (PSA) pattern with its southernmost pole north of the Ross Sea. An intensified PSA pattern may lead to a regional cooling of Marie Byrd Land, thereby favoring an in-phase temperature relationship between the Northern Hemisphere and the western side of the West Antarctic Ice Sheet on millennial timescales. However, under glacial conditions due to reduced tropical diabatic forcing and reduced tropical extratropical teleconnections, the weakening of the AMOC induces a different austral winter-time atmospheric response pattern that is characterized by a wavenumber 2 structure and different air-temperature anomalies. We furthermore demonstrate that the observed CO2 changes that accompanied major disruptions of the AMOC may have helped to amplify Antarctic warming events (A-events) due to radiative forcing and the polar amplification mechanism. Our modeling results provide a new framework to explain a considerable fraction of the observed millennial-scale variance in Southern Hemispheric climate records during Marine Isotope Stage 3 (MIS3).