Observational constraints imply limited future Atlantic meridional overturning circulation weakening

The Atlantic Meridional Overturning Circulation (AMOC), a critical component of the global ocean circulation system, is projected to experience limited weakening in the future, according to a study published in Nature Geoscience. The study finds that the AMOC will weaken by about 3–6 Sv (18–43%) by the end of the 21st century, regardless of the emissions scenario. This conclusion is based on the incorporation of observational constraints that significantly reduce the uncertainty associated with climate model projections. 1. **Incorporating Observational Constraints**: The study uses a thermal-wind expression to relate 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. By incorporating observational constraints, the study concludes that the AMOC will experience limited weakening of about 3–6 Sv by the end of the century, regardless of emissions scenario. 2. **Present-Day Ocean Stratification**: 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. 3. **Future Predictions**: The study's findings 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. In conclusion, the AMOC is expected to experience limited weakening in the future, with a weakening of about 3–6 Sv (18–43%) by the end of the 21st century. This conclusion is based on the incorporation of observational constraints that significantly reduce the uncertainty associated with climate model projections.