The role of aerosol process representation in uncertainty in the North Atlantic region in CMIP6 

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<p>Previous studies have shown that anthropogenic aerosol emissions drive a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) in CMIP6 historical simulations that was not simulated in the CMIP5 multi-model mean. The strength of the CMIP6 AMOC trend has been linked to the strength of the aerosol forcing, with the inclusion of aerosol-cloud interactions accounting for a large proportion of the difference between CMIP5 and CMIP6. However, there is large uncertainty in the magnitude and distribution of aerosol effective radiative forcing in CMIP6. Understanding this uncertainty is important for the interpretation of simulated AMOC variability.</p><p> </p><p>We present an evaluation of the atmospheric variables with the potential to influence AMOC changes in CMIP6 historical and AMIP simulations, including downwelling shortwave radiation, surface heat fluxes, surface air temperature, and precipitation. We examine the links between aerosol effective radiative forcing, the magnitude and pattern of biases in the mean and trends in modelled quantities, and the complexity of the representation of aerosol chemistry and aerosol-cloud interactions. Using these results, we highlight areas where model diversity in the representation of aerosol process may be particularly important for uncertainty in simulations of North Atlantic climate. </p>