Optimal Transition Paths for AMOC Collapse and Recovery in a Stochastic Box Model

There is strong evidence that the present-day Atlantic meridional overturning circulation (AMOC) is in a bistable 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 noiseinduced 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 the 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. Moreover, a comparison with a quasiequilibrium approach reveals the contrasts in the behavior of a bifurcation-and noise-induced collapse of the AMOC.