Aerosol–cloud interactions influence the climate response to AMOC weakening

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) strongly influences regional climate, yet the response of atmospheric aerosols and aerosol-cloud interactions to its weakening remains largely unexplored. Using the ICON-HAM model, we investigate how a 60 % AMOC weakening affects aerosol distributions, cloud microphysics, and radiative budgets. The weakening of the AMOC drives a hemispheric aerosol redistribution through purely dynamical pathways, increasing the Northern Hemisphere aerosol burden by 5 % through enhanced Saharan dust emissions and extended aerosol lifetimes under suppressed wet deposition. Averaged over 40–90° N, these perturbations propagate into cloud properties via both liquid and ice-phase pathways. In-cloud droplet number concentrations increase by 8 % in warm clouds and 13 % in the mixed-phase regime. In the ice phase, enhanced dust ice-nucleating particles produce a 37 % increase in mixed-phase ice crystal number concentrations through multiple heterogeneous freezing pathways, promoting the Wegener-Bergeron-Findeisen process and reducing mixed-phase total water path by 8 %. The global-mean net cloud radiative effect (CRE) anomaly is +0.84 W m^-2, acting as a negative feedback that partially offsets AMOC-induced cooling. A linear decomposition reveals that this positive CRE arises not from cloud loss, but from a reduction in the cooling efficiency of existing clouds, which more than offsets the enhanced cooling from increased cloud cover. Our findings demonstrate that aerosol-cloud interactions form an active component of the climate response to AMOC weakening, exposing a critical gap in simulations that rely on prescribed aerosol fields.