The evolution and future stability of the Atlantic Meridional Overturning Circulation since the mid-20th century: a review and outlook

The Atlantic Meridional Overturning Circulation (AMOC), a key component of the global climate system, behaves as a typical climate tipping element with multi-stable states. Changes in AMOC instigate a series of cascading effects, profoundly affecting distributions of heat and moisture globally. Up till now, due to relatively short durations of direct observations from monitoring arrays (e.g., RAPID array (21 years) and OSNAP array (11 years)), our understanding of the evolution of the AMOC since the mid-20th century remains highly controversial. Fingerprint reconstructions based on sea-surface temperature and salinity proxies show a marked AMOC weakening since the mid-to-late 20th century, whereas estimates derived from the impacts of circulation intensity changes as well as those based on physical principles—such as ocean-atmosphere heat fluxes or sea surface height gradients—mostly show no significant trend. Additionally, although some metrics may also be able to characterize the AMOC variability, they are generally difficult to robustly assess the long-term trends due to data limitations (e.g., resolution of sedimentary records and measurement uncertainties) and specific processing methods (e.g., the detrending applied to sea-level gradient indices). These discrepancies underscore the inherent limitations of different reconstruction methods, and/or the potential masking effects of the natural variability.