Stability of the Atlantic meridional overturning circulation: A model intercomparison

Abstract

[1] The evolution of the Atlantic Meridional Overturning Circulation (MOC) in 30 models of varying complexity is examined under four distinct Representative Concentration Pathways. The models include 25 Atmosphere-Ocean General Circulation Models (AOGCMs) or Earth System Models (ESMs) that submitted simulations in support of the 5th phase of the Coupled Model Intercomparison Project (CMIP5) and 5 Earth System Models of Intermediate Complexity (EMICs). While none of the models incorporated the additional effects of ice sheet melting, they all projected very similar behaviour during the 21st century. Over this period the strength of MOC reduced by a best estimate of 22% (18%-25%; 5%-95% confidence limits) for RCP2.6, 26% (23%-30%) for RCP4.5, 29% (23%-35%) for RCP6.0 and 40% (36%-44%) for RCP8.5. Two of the models eventually realized a slow shutdown of the MOC under RCP8.5, although no model exhibited an abrupt change of the MOC. Through analysis of the freshwater flux across 30 -32 S into the Atlantic, it was found that 40% of the CMIP5 models were in a bistable regime of the MOC for the duration of their RCP integrations. The results support previous assessments that it is very unlikely that the MOC will undergo an abrupt change to an off state as a consequence of global warming.

Ocean circulation response to anthropogenic aerosol and greenhouse gas forcing in the CSIRO-Mk3.6 climate model

“…2005, Weaver et al 2012. It is noteworthy that the AAinduced effects are larger than the GHG effects in Mk36.…”

An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010

“…Our analysis based on the long-term TG data and the AMOC-SLR relationship indicates that the 2009-2010 event is very unusual. In addition to internal variability, anthropogenic forcing could be another impact factor, as most climate models project a weakening of the AMOC during the twenty-first century in response to the increase in the atmospheric greenhouse gas concentrations 40 . Continuing observations of the AMOC is essential to confirm these modelling results.…”

The effect of model bias on Atlantic freshwater transport and implications for AMOC bi-stability

“…We would like to acknowledge Jeff Blundell and Adam Blaker for their technical help and useful conversations. And finally, we acknowledge Jan Sedáček and Michael Eby for discussions about the computations of M ov at 34°S in Weaver et al (2012).…”

“…However, 10 of the 18 values are between 0 Sv and -0.05 Sv, implying that these models are close to the boundary between mono-and bi-stability. For the models that overlap with (Weaver et al 2012 Fig. 1a).…”

“…Weaver et al (2012) have previously examined M ov in 30 CMIP5 and EMIC models however, that study only focused on the values at the southern boundary of the Atlantic. Here we investigate M ov over the entire Atlantic and how it is affected by model bias over 43 CMIP5 models, two renanalysis products and a current generation coupled climate model.…”