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. ...
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 the 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, 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.
