Slow and soft passage through tipping point of the Atlantic Meridional Overturning Circulation in a changing climate

Abstract

Paleo-proxy records suggest that the Atlantic Meridional Overturning Circulation (AMOC) exhibits a threshold for an abrupt change, a so-called tipping point. A classical bifurcation theory, a basis of the tipping dynamics of AMOC implicitly assumes that the tipping point is fixed. However, when a system is subjected to time-varying forcing (e.g., AMOC exposed to ice meltwater) an actual tipping point can be overshot due to delayed tipping, referred to as the slow passage effect. Here, using an Earth system model of intermediate complexity and a low-order model with freshwater forcing, we show that the tipping point of AMOC is largely delayed by the slow passage effect. It causes a large tipping lag of up to 1300 years, and strongly relaxes the abruptness of tipping as well. We further demonstrate that the tipping modulation can actively occur in past, present, and future climates by quantifying the effect during Dansgaard-Oeschger events, meltwater pulse 1A (MWP-1A), and current Greenland ice sheet melting. The suggested slow passage effect may explain the observed lagged AMOC collapse to MWP-1A of about 1000 years and provides implications tipping risk in the future.