Rate-Dependent Hysteresis of the Atlantic Meridional Overturning Circulation System and Its Asymmetric Loop

Abstract

Abrupt climate changes have occurred repeatedly in the past when the Earth climate system was forced across a threshold. A hysteresis with saddle nodes is the best means to explain the abruptness of a climate system. Here, we study the dynamic hysteresis of the Atlantic Meridional Overturning Circulation (AMOC) with a Stommel's box model and an Earth system model subject to a freshwater flux forcing (FWF). In Stommel's box model, the area of the hysteresis loop increases with increasing FWF frequency, and thus an abrupt transition from the off-state to the on-state of the AMOC (opposite) occurs at lower (higher) FWF than in static hysteresis, referring a “lagged tipping.” In the Earth system model, the expansion/contraction of the hysteresis loop area as a function of FWF frequency is also observed. However, abruptness in the hysteresis is highly distinct in the off-to-on state than the on-to-off state due to asymmetric salt-advection feedback.