Ensemble simulation of the AMOC collapse in a conceptual climate model

The Atlantic Meridional Overturning Circulation (AMOC) is a mechanism of great importance, as its possible collapse would constitute a dramatic response to Earth’s changing climate. The AMOC is particularly important for Northern Europe, as it plays a central role in regulating the region’s climate, and a slowdown or collapse would lead to a significant cooling of the region. This critical transition has been the subject of many studies over the years, both from the aspects of climate modeling and dynamical systems theory. In the context of the latter, climate change is nothing but a complex, chaotic-like system, which possesses a time-dependent parameter, in the shape of e.g. the growing CO2 concentration. It has been known for some time now, that such systems not only have a chaotic attractor, but one which is also time-dependent, a so-called snapshot attractor. Such objects, and thus the systems they describe, can only be faithfully represented by statistics over an ensemble of trajectories, a single one does not suffice. We perform such ensemble simulations on a conceptual climate model of the AMOC, constructed by coupling the Lorenz84 and the Stommel box models. We find that the difference between the ensemble members in the point when the collapse occurs can be up to hundreds of years, and that some trajectories can even survive with the AMOC remaining in the “on” state.  This highlights the fact that that a single trajectory is unreliable, however, with the proper ensemble statistics (e.g. standard deviations, time-dependent Lyapunov exponents, etc), a probabilistic description of the collapse can be given.