Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
22 August 2020

CMIP6 Models Predict Significant 21st Century Decline of the Atlantic Meridional Overturning Circulation

Science

We explore the representation of the Atlantic Meridional Overturning Circulation (AMOC) in 27 models from the CMIP6 multimodel ensemble. Comparison with RAPID and SAMBA observations suggests that the ensemble mean represents the AMOC strength and vertical profile reasonably well. Linear trends over the entire historical period (1850–2014) are generally neutral, but many models exhibit an AMOC peak around the 1980s. Ensemble means AMOC decline in future (SSP) scenarios is stronger in CMIP6 than CMIP5 models. In fact, AMOC decline in CMIP6 is surprisingly insensitive to the scenario at least up to 2060. We find an emergent relationship among a majority of models between AMOC strength and 21st-century AMOC decline. Constraining this relationship with RAPID observations suggests that the AMOC might decline between 6 and 8 Sv (34–45%) by 2100. A smaller group of models projects much less AMOC weakening of only up to 30%.

Impact

Our results support the emerging body of literature that suggests relationships between AMOC mean strength and the role of the AMOC in the climate response to radiative forcing. The positive relationship between mean AMOC and AMOC decline under CO2 forcing is mediated by sea ice. Models with weaker AMOC have generally more Arctic sea ice than models with a stronger AMOC, as a result of the weaker northward heat transport. As the atmosphere warms under enhanced radiative forcing, models with weaker AMOC experience the strongest sea ice retreat, exposing more ocean to the atmosphere. The resulting ocean heat loss counteracts the AMOC weakening induced by atmospheric warming. At the same time, the models with stronger AMOC decline experience a stronger reduction in northward heat transport, cooling the subpolar North Atlantic, with implications for sea ice and cloud feedbacks. This provides negative feedback on Arctic warming and lowers ECS. It is not obvious why the AMOC decline in 4xCO2 and SSP simulations does not conform to this picture. However, we speculate that the framework describing the subtle interplay between AMOC and high-latitude feedbacks is no longer valid when large-amplitude forcing perturbations are applied and sea ice is no longer a dominant element in the subpolar North Atlantic and Nordic Seas.

 

Summary

The CMIP6 ensemble does not show a significant AMOC trend over the historical period from 1850 to 2014, but many models (and the ensemble mean) display a local maximum in the later part of the 20th century. This behavior reflects a combined effect of the accelerating increase in CO2 concentrations, and a sulfate aerosol load that peaks in the 1980s, before declining in the 1990s and 2000s. This is at odds with CMIP5 models, which generally show a weak decline over the 20th century, and with reconstructions based on proxies. All models show a decline in the AMOC in the 21st century, with a rate of decline that is independent of emission scenario at least through 2060; only a few models show appreciable divergence between the SSPs thereafter. On average, AMOC decline is 24%, 29%, 32%, and 39% for SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively. CMIP5 models on average displayed a 9% decline for the RCP2.6 scenario (comparable to SSP1-2.6); 21% for RCP4.5 (SSP2-4.5); and 36% for RCP8.5 (SSP5-8.5). The CMIP6 model generation hence projects a stronger decline than CMIP5, with the discrepancy being most pronounced for the scenarios with weakest radiative forcing.

 

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)
Publications
Weijer, W, W Cheng, OA Garuba, A Hu, and BT Nadiga.  2020.  "CMIP6 Models Predict Significant 21st Century Decline of the Atlantic Meridional Overturning Circulation."  Geophysical Research Letters.  https://doi.org/10.1029/2019GL086075.