Oscillatory Melancholia state of the Atlantic Meridional Overturning Circulation in an intermediate-complexity climate model

The Atlantic Meridional Overturning Circulation (AMOC) is considered a tipping element of the earth system featuring bistability: for a given external forcing, a strong and a weak circulation state coexist as competing attracting states of the system. In the presence of random fluctuations, noise-induced transitions between the competing states are possible, posing a risk of abrupt AMOC tipping even without crossing a critical forcing threshold. It is thus crucial to better understand the stability landscape of the earth system with a multistable AMOC, particularly the properties of the boundary separating the basins of attraction of the strong and weak AMOC states. For weak noise, transitions are expected to cross the basin boundary at so-called edge states or "Melancholia states", typically chaotic saddles which are attracting on the boundary but asymptotically unstable. Here we find an edge state between the two stable AMOC states in an earth system model of intermediate complexity, PlaSim-LSG. Our approach is based on an edge-tracking technique that allows to construct a pseudo-trajectory on the chaotic saddle. We characterize the climatic and dynamical properties of this edge state and map out its location in different projections of state space. Near the edge state, the AMOC strength exhibits strong transient oscillations which we link to the ongoing physical processes. We relate our findings to the theory of unstable chaotic sets and discuss implications for the predictability of potential AMOC tipping in the future.