Investigating the possibility of rare spontaneous AMOC transitions in the intermediate complexity climate model FAMOUS.

Earth System Models of Intermediate Complexity (EMICs) allow for fast exploration of large-scale climate dynamics. These models thus enable the development and testing of large-ensemble-based techniques that would be too costly with more realistic climate models.

In this ongoing study we develop a rare event simulation setup to explore the possibility of a spontaneous collapse of the Atlantic Meridional Overturning Circulation (AMOC) in the FAMOUS model. FAMOUS is a low-resolution, coupled atmosphere-ocean general circulation model derived from the UK Met Office’s Unified Model specifically designed for efficient, long-duration and ensemble climate simulations. FAMOUS has previously been used to investigate the hysteresis of the Atlantic Meridional Overturning Circulation under freshwater hosing.

We apply a genealogical particle analysis (GPA) algorithm that is designed to probe the possibility of spontaneous AMOC transitions. The method initiates an ensemble of realisations in the "on"-state of the AMOC and clones ensemble members at regular intervals  that are showing a low AMOC.

Contrary to recent results in another EMIC, a straightforward sampling based on the AMOC indicator does not result in any spontaneous transitions to the AMOC "off"-state. To improve the selection of potentially exceedingly rare trajectories, we therefore investigate statistical methods to identify physical variables that correlate with the state of the AMOC ahead of time, to be used as selection criteria in the GPA algorithm.