Non-linear Climatic Response to the Weakening of the Atlantic Meridional Overturning Circulation During Glacial Times

Abstract. The climatic response to the weakening of the Atlantic Meridional Overturning Circulation (AMOC) is investigated under glacial conditions representative of Heinrich Stadial 5 using a fully coupled Earth System Model (ACCESS-ESM1.5), with a focus on Southern Hemisphere and Australian hydroclimate. We find that the climatic response to an AMOC slowdown or shutdown, respectively representing Dansgaard-Oeschger (D-O) and Heinrich stadials, is non-linear. Global mean temperature and precipitation anomalies increase linearly with AMOC weakening; however, crossing the threshold of AMOC shutdown results in non-linear and more complex atmospheric circulation and climate responses. A shutdown of the AMOC in the simulations leads to an enhanced and expanded northern winter Hadley Cell (HC), with a southward shift of its ascending branch. The southern winter HC is weaker but increased in width due to a northward shift of the ascending branch due to AMOC shutdown. This change in the HC drives seasonal variations in the Northern and Southern Hemispheres subtropical high pressure systems and subsequently, changes in the cross-equatorial atmospheric circulation, as well as the Southern Hemisphere mid-latitude westerly winds and other climate features such the monsoon systems. The simulation results are broadly consistent with available proxy records for Heinrich and D-O stadials as well as previous model simulations. The study demonstrates the potential location of a threshold in the climate system between linear weakening and nonlinear shutdown of AMOC with differing climate impacts, further highlighting the importance of not crossing the threshold of AMOC shutdown in the future.