Simulating NAO-driven AMOC collapse in the PlaSim-LSG Climate Model

The Atlantic Meridional Overturning Circulation (AMOC) is a critical component of the global climate system and its potential for abrupt collapse represents a significant tipping point. Our project investigates whether a persistent negative phase of the North Atlantic Oscillation (NAO), a dominant mode of atmospheric variability, can induce an AMOC collapse in the absence of external perturbations within the coupled PlaSim-LSG climate model of intermediate complexity. A control simulation establishes a baseline climatology, confirming that NAO variability leads AMOC fluctuations by approximately one year. To overcome the computational limitation of simulating rare events, we implement a rare event algorithm (GKLT) that efficiently biases the model toward trajectories with negative NAO conditions over 125-year simulations. The results reveal a fundamental bistability in the system. While persistent negative NAO forcing can trigger an AMOC collapse, the outcome is probabilistic: out of six independent ensemble simulations, four evolved entirely into a collapsed state (∼ 12 Sv), one remained entirely vigorous (∼ 23 Sv) and one split into both outcomes. A cluster-based analysis traces this divergence to the early amplification of small differences in North Atlantic heat fluxes, convection and sea-ice cover. These findings show that internal atmospheric variability alone can force the AMOC across a tipping point, highlighting the role of internal climate dynamics in shaping climate transitions.