  The Atlantic Meridional Overturning Circulation (AMOC) is a much studied
component of the climate system, because its suspected multistability leads to
tipping behaviour with large regional and global climatic impacts. In this
paper we investigate the global stability properties of the system using an
ocean general circulation model. We construct an unstable AMOC state, i.e., an
unstable solution of the flow that resides between the stable regimes of a
vigorous and collapsed AMOC. Such a solution, also known as a Melancholia or
edge state, is a dynamical saddle embedded in the boundary separating the
competing basins of attraction. It is physically relevant since it lies on the
most probable path of a noise-induced transition between the two stable
regimes, and because tipping occurs when one of the attractors and the
Melancholia state collide. Its properties may thus give hints towards physical
mechanisms and predictability of the critical transition. We find that while
the AMOC Melancholia state as viewed from its upper ocean properties lies
between the vigorous and collapsed regimes, it is characterized by an Atlantic
deep ocean that is fresher and colder compared to both stable regimes. The
Melancholia state has higher dynamic enthalpy than either stable state,
representing a state of higher potential energy that is in good agreement with
the dynamical landscape view on metastability.
