  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 Sv) is unlikely for pre-industrial CMIP6
variability unless external forcing shifts the AMOC closer to a threshold.
However, CMIP6 models seem to underestimate present-day Atlantic Ocean
variability, and stronger variability substantially increases the likelihood of
noise-induced collapse, especially if forcing brings the AMOC close to a
stability threshold. 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.
