Proximity to a Bifurcation Threshold Enables Noise-Induced Collapse in a Simple AMOC Model: A Stochastic Perspective on the Younger Dryas

The Younger Dryas (YD) represents one of the most dramatic examples of abrupt climate change during the last deglaciation, characterized by a rapid return to near-glacial conditions in the North Atlantic region. The leading hypothesis attributes this event to a weakening or collapse of the Atlantic Meridional Overturning Circulation (AMOC) triggered by a massive influx of freshwater. However, the exact mechanism and the role of internal climate variability remain open questions. In this study, we adopt a nonlinear dynamical systems framework and treat the YD as a possible noise-induced tipping event of the AMOC. Using the reduced Stommel-Cessi model with a quadratic exchange law, we first characterize the deterministic bifurcation structure and identify the bistable regime. We then introduce an Ornstein-Uhlenbeck (OU) stochastic process to represent the observed salinity variability in the North Atlantic. The mean first passage time (MFPT) is computed via the backward Fokker-Planck equation in two dimensions, providing a rigorous quantification of the transition timescale as a function of freshwater forcing, noise intensity, and noise correlation time. Our results indicate that, for physically plausible parameter ranges, the system can escape the warm state on timescales compatible with the YD duration (~10^3 years) even below the deterministic bifurcation threshold. This finding suggests that internal stochastic fluctuations may have played a central role in triggering the YD abrupt transition, and that the AMOC is susceptible to noise-induced tipping near critical thresholds.