  The present-day Atlantic Meridional Overturning Circulation (AMOC) is
considered to be in a bi-stable regime and hence it is important to determine
probabilities and pathways for noise-induced transitions between its
equilibrium states. Here, using Large Deviation Theory (LDT), the most probable
transition pathways for the collapse and recovery of the AMOC are computed in a
stochastic box model of the World Ocean. This allows us to determine the
physical mechanisms of noise-induced AMOC transitions. We show that the most
likely path of an AMOC collapse starts paradoxically with a strengthening of
the AMOC followed by an immediate drop within a couple of years due to a short
but relatively strong freshwater pulse. The recovery on the other hand is a
slow process, where the North Atlantic needs to be gradually salinified over a
course of 20 years. The proposed method provides several benefits, including an
estimate of probability ratios of collapse between various freshwater noise
scenarios, showing that the AMOC is most vulnerable to freshwater forcing into
the Atlantic thermocline region.
