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). 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 an AMOC slowdown; however, crossing the threshold of AMOC shutdown results in non-linear and more complex atmospheric circulation and climate responses. The atmosphere partially compensates for the significantly reduced oceanic energy transport due to AMOC shutdown through alterations in the cross-equatorial Hadley Cell (HC), with pronounced changes in boreal winter season. The northern winter HC is enhanced and expanded, while the southern winter HC is weakened but increased in width due to a northward shift of the ascending branch resulting from the AMOC shutdown. This drives seasonal climate variability in the tropical and subtropical regions via changes in the subtropical high pressure systems, subtropical jet, Southern Hemisphere mid-latitude westerly winds and other climate features such as the monsoon systems, with significant impacts on Australasian hydroclimate. The study demonstrates the potential location of a threshold in the climate system between linear slowdown and nonlinear shutdown of the AMOC, with differing climate impacts being broadly consistent with available proxy records for Heinrich and D–O stadials. This further highlights the importance of not crossing the threshold of AMOC shutdown in the future.