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

Abstract

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 mean 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%. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a circulation pattern in the Atlantic Ocean that is an important component of the climate system, due to its ability to redistribute and sequester heat and carbon. An accurate representation of the AMOC is a critical test for climate models and essential for building confidence in their projections. Here we investigate the AMOC in 27 climate models that contributed simulations to the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that many models reproduce the observed AMOC quite well, but there are still several models in which the AMOC is too weak or too strong. Most models suggest a slight upward trend in the AMOC from 1850 to the 1980s. Simulations representing different scenarios for future socioeconomic development suggest a stronger AMOC decline compared to previous assessments. Using direct measurements of the AMOC since 2004 and an emerging across-model relationship between AMOC decline in the 21st century and their present-day mean state, we find that the majority of CMIP6 models point to an end of century AMOC weakening of 34-45% of its present-day strength. A smaller group of models projects much less weakening of only up to 30% of its present state.

Role of AMOC in Transient Climate Response to Greenhouse Gas Forcing in Two Coupled Models

“…The Atlantic meridional overturning circulation (AMOC) is a global-scale ocean circulation that plays a significant role in modulating the global and regional climate via changes in the redistribution of oceanic heat, salt, and biogeochemical tracers. Studies based on both idealized models and coupled general circulation models suggest that the AMOC may have multiple equilibrium states and changes from one equilibrium state to another may be abrupt, and thus capable of inducing abrupt climate changes around the North Atlantic region and the globe [e.g., Rahmstorf 1996;Stocker and Wright 1991a,b;Stocker et al 1992;Stouffer et al 2006;Hu et al 2010Hu et al , 2012; for more details see Weijer et al (2019) and the references therein]. Because of the potential abrupt transition of the AMOC from one state to another, it has been a central focus of many studies on both past and future climate changes (e.g., Hu et al 2004Hu et al , 2008Hu et al , 2010Hu et al , 2012Hu et al , 2015Gregory et al 2005;Stouffer et al 2006).…”

Anthropogenic aerosol forcing of the AMOC and the associated mechanisms in CMIP6 models

“…Although climate models disagree on the precise magnitude of the AMOC weakening−and differ substantially in their representation of the strength and depth of the AMOC−model simulations predict AMOC weakening in response to increasing greenhouse gases (Gregory et al, 2005;Solomon et al, 2007;Drijfhout and Hazeleger, 2007;Cheng et al, 2013;Kirtman et al, 2013;Kostov et al, 2014). By the end of the 21st century, for example, models estimate a 24-39% decline in the AMOC, with larger weakening under larger increases in future GHG emissions (Weijer et al, 2020). This has been related to reduced ocean heat loss, and secondarily through increased freshwater input at high latitudes, both of which decrease the density of sea water in the subpolar North Atlantic (i.e., the sinking region) (Thorpe et al, 2001).…”

Accelerated Greenland Ice Sheet Mass Loss Under High Greenhouse Gas Forcing as Simulated by the Coupled CESM2.1‐CISM2.1

“…By end of simulation, the location of the maximum overturning has migrated equatorward and towards the surface. Significant NAMOC reduction and shoaling of the overturning cell in elevated CO 2 scenarios are common in CMIP6 ocean models, which generally simulate greater NAMOC decline than CMIP5 models (Weijer et al, 2020). Figure 3a shows the simulated evolution of the 20-year mean January-March mixed layer depth (MLD) in the deep convection regions in the Labrador Sea, Irminger Sea, Iceland Basin, and Barents Sea.…”