Changes in External Forcings Drive Divergent AMOC Responses Across CESM Generations

Abstract

It has been suggested that the Atlantic meridional overturning circulation (AMOC) in many CMIP6 models is overly sensitive to anthropogenic aerosol forcing, and it has been proposed that this is due to the inclusion of aerosol indirect effects for the first time in many CMIP6 models. We analyze the AMOC response in a newly released ensemble of simulations performed with CESM2 forced by the CMIP5 input data sets (CESM2-CMIP5). This AMOC response is then compared to the CMIP5-generation CESM1 large ensemble (CESM1-LE) and the CMIP6-generation CESM2 large ensemble (CESM2-LE). A key conclusion, only made possible by this experimental setup, is that changes in aerosol-indirect effects cannot explain differences in AMOC response between CESM1-LE and CESM2-LE. Instead, we hypothesize that the difference is due to increased interannual variability of anthropogenic emissions. This forcing variability may act through a nonlinear relationship between the surface heat budget of the North Atlantic and the AMOC.