Slowed Response of Atlantic Meridional Overturning Circulation Not a Robust Signal of Collapse

Abstract

Using an idealized model of the Atlantic meridional overturning circulation (AMOC), we test whether changes in the statistical properties of an AMOC time series can reveal Critical Slowing Down (CSD) and serve as early warnings of an upcoming critical transition. We calculate CSD indicators for simulations across varying parameter regimes, investigating the system's steady-state dynamical structure and its evolution under gradual climate forcing. We find that the modeled AMOC features bistability for relatively weak gyre salinity exchange, but no bistability when the gyres are sufficiently strong. However, CSD indicators consistently warn of a collapse across the gyre strength parameter space, even when no bifurcations occur, thus raising false alarms. We argue that CSD should be applied cautiously in systems where the dynamical structure and physical response to forcing are not fully known (such as the AMOC), specifically where it is not a priori clear whether the system is in a multistable regime.