Speaker: Reyk Borner (Utrecht Univ)
Topic: Explaining divergent AMOC behavior in the NASA GISS model via chaotic edge states

The Atlantic Meridional Overturning Circulation (AMOC), a crucial ocean current system, could transition to a weak state under anthropogenic climate change. Ensemble simulations of the NASA GISS earth system model, forced with the intermediate SSP2-4.5 emissions scenario, demonstrate that AMOC tipping could depend sensitively on the initial condition. Here we take a dynamical systems perspective to explain this behavior, viewing Earth as a multiscale chaotic system. Using an intermediate-complexity climate model, we explore the stability landscape of the AMOC for different atmospheric CO2 concentrations. We explicitly compute the edge state (or Melancholia state), a chaotic saddle on the basin boundary separating the strong and weak AMOC attractors found in the model. While being unstable, the edge state can govern the transient climate for centuries, supporting centennial AMOC oscillations driven by atmosphere-ice-ocean interactions in the North Atlantic. At increased CO2 levels, we reveal a boundary crisis where the current AMOC attractor disappears by colliding with the edge state. Under crisis overshoot, long chaotic transients due to "ghost states" lead to diverging ensemble trajectories, akin to those observed in the GISS model.