Structural stability changes of the Atlantic Meridional Overturning Circulation

Abstract

The proximity of the states of climatic tipping components to bifurcation points, which could mark the emergence of rapid and irreversible transitions, is a subject of major concern in the context of anthropogenic influence on climate. Among such systems, the Atlantic Meridional Overturning Circulation has one of the largest climatic impact and it could induce a cascade of abrupt critical transitions, through its multiple connections with other components. However, there is considerable uncertainty regarding the location of the overturning circulation’s current state relative to its stability thresholds. We identify similarities between observational and simulated spatial patterns phase spaces and bifurcation diagrams linked with overturning circulation changes. Our results suggest that the AMOC already underwent a Hopf bifurcation and entered a bistable regime before 1870, that it suffered a rate-induced tipping around 1970, possibly linked with the Great Salinity Anomaly, and that it approached the attractor of its ‘off’ state. These changes in the overturning circulation dynamics are indicative of complex structural stability changes during the preindustrial revolution, which underline the need for a long-term temporal assessment of the overturning circulation stability on multi-centennial to millennial time-scales to set its contemporary and future evolution in a long-term context.