A saddle-node bifurcation may be causing the AMOC collapse in the Community Earth System Model

Abstract. Recently, a collapse of the Atlantic Meridional Overturning Circulation (AMOC) was found in the Community Earth System Model (CESM) under constant pre-industrial greenhouse gas forcing conditions. To determine the stability changes of the AMOC with changing (freshwater) parameters in models, it is important to determine the origin of the collapse behavior. In this paper, we argue that the classical picture of a saddle-node bifurcation holds for the AMOC collapse in the CESM. We provide specific supporting arguments by showing results of additional pre-industrial CESM simulations. The CESM results are compared with those of a five-box AMOC model, which is known to have saddle-node bifurcations, and with which many sensitivity experiments can be performed. Theoretical arguments are also provided showing that the essential dynamics of the CESM can be reduced to a low-dimensional model in which a saddle-node bifurcation causes the AMOC collapse. The underlying physical reason is that the AMOC behaviour in CESM is controlled by a small set of dominant feedback processes. This has important consequences for the value of conceptual AMOC models, for assessing the effect of model biases on the AMOC stability, and for the interpretation of AMOC behaviour under climate change scenarios.