Understanding Metastability of the Atlantic Ocean Circulation Via Edge States

Earth's climate is a metastable complex system: triggered by random fluctuations or changes in external forcing, climate subsystems may switch abruptly between competing equilibrium states. For example, past climate records indicate that the Atlantic Meridional Overturning Circulation (AMOC), a major ocean current, underwent transitions between two different flow regimes. There is growing concern that the AMOC could tip into a much weaker flow under global warming, with severe societal impacts. In this talk, we explore the global stability and resilience of the AMOC through the study of edge states - unstable chaotic sets located on the boundary between coexisting basins of attraction. Following large deviation theory, edge states may offer insight into the pathway and probability of noise-induced transitions; often they dominate the dynamics near a bifurcation. In a high-dimensional coupled climate model, we compute the edge state separating a strong and weak AMOC regime at two levels of greenhouse gas forcing. Even well below critical forcing, the dynamics on the edge state captures unstable oscillations that shed light on the transition mechanism. Near criticality, the edge state highlights how long transients challenge the predictability of long-term climate variability. Our results offer a physics-based angle on anticipating critical transitions that could help overcome key limitations of methods based on timeseries statistics.