Observational constraints imply limited future Atlantic meridional overturning circulation weakening

Abstract

The degree to which the Atlantic meridional overturning circulation (AMOC) weakens over the twenty-first century varies widely across climate models, with some predicting substantial weakening. Here we show that this uncertainty can be greatly reduced by using a thermal-wind expression that relates the AMOC strength to the meridional density difference and the overturning depth in the Atlantic. This expression captures the intermodel spread in AMOC weakening, with most of the spread arising from overturning depth changes. The overturning depth also establishes a crucial link between the present-day and future AMOC strength. Climate models with a stronger and deeper present-day overturning tend to predict larger weakening and shoaling under warming because the present-day North Atlantic is less stratified, allowing for a deeper penetration of surface buoyancy flux changes, larger density changes at depth and, consequently, larger AMOC weakening. By incorporating observational constraints, we conclude that the AMOC will experience limited weakening of about 3–6 Sv (about 18–43%) by the end of this century, regardless of emissions scenario. These results indicate that the uncertainty in twenty-first-century AMOC weakening and the propensity to predict substantial AMOC weakening can be attributed primarily to climate model biases in accurately simulating the present-day ocean stratification.