Quantifying risk of a noise-induced AMOC collapse from northern and tropical Atlantic Ocean variability

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) exerts a major influence on global climate. There is much debate about whether the current strong AMOC may collapse as a result of anthropogenic forcing and/or internal variability. Increasing the noise in simple salt-advection models can change the apparent AMOC tipping threshold. However, it's not clear if `present-day' variability is strong enough to induce a collapse.
Here, we investigate how internal variability affects the likelihood of AMOC collapse. We examine internal variability of basin-scale salinities and temperatures in four CMIP6 pre-industrial simulations. We fit this to an empirical, process-based AMOC box model, and
find that noise-induced AMOC collapse (defined as a decade in which the mean AMOC strength falls below $5 \ \rm{Sv}$) is unlikely, however, if the AMOC is pushed closer to a bifurcation point due to external climate forcing, noise-induced tipping becomes more likely.
Surprisingly, we find a case where forcing temporarily overshoots a stability threshold but noise decreases the probability of collapse. Accurately modelling internal decadal variability is essential for understanding the increased uncertainty in AMOC projections.