Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

Abstract

The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic ocean basin. These results reveal spatially consistent empirical evidence that in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.The Atlantic Meridional Overturning Circulation (AMOC) is the key circulation system of the Atlantic ocean, transporting water masses northward at the surface, and southward at the bottom of the ocean 1 . The AMOC is the archetypical example of potentially multi-stable Earth system components 2 . Early studies based on box models 3 have indicated that the AMOC has two different stable states of operation, corresponding to a strong and a weak circulation mode. The AMOC's bistability and corresponding hysteresis has henceforth been confirmed in a hierarchy of models, from Earth system models of intermediate complexity (EMICs) 4,5 to comprehensively coupled atmosphere-ocean general circulation models (AOGCMs) 6 . It should be noted, however, that AMOC bistability has not been identified in a considerable number of state-of-the-art Earth system models 7 (ESMs). It remains debated whether this should be interpreted as evidence for a monostable AMOC under current climate conditions, or rather as evidence for excessive AMOC stability in these models 8,9 : most comprehensive climate models likely underestimate the freshwater export from the northern Atlantic ocean basin and hence suppress the associated destabilizing feedback 6,9-12 . Moreover, it has been argued that in comprehensive models, very high spatial resolution is needed to obtain a good representation of the AMOC response to freshwater forcing 13 . Indeed, AMOC bistability has been revealed in recent simulations with a comprehensive model with Eddy-permitting ocean module 14 .The strong AMOC state is currently attained, while the weak state has arguably been occupied recurringly during previous glacial intervals. Different lines of evidence from paleoclimate proxy records indicate that northern hemisphere temperatures have varied abruptly at millennial time scales during previous glacial episodes, with corresponding changes of the AMOC between its weak and strong modes 1,15,16 . Speleothem and ocean sediment records from arou...

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