Absence of Aerosol Indirect Effect Dependence on Background Climate State in NCAR CESM2

Abstract The aerosol indirect effect (AIE) dominates uncertainty in total anthropogenic aerosol forcing in phase 6 of the Coupled Model Intercomparison Project (CMIP6) models. AIE strength depends on meteorological conditions that have been shown to change between preindustrial (PI) and present-day (PD) climates, such as cloud cover and atmospheric moisture. Hence, AIE strength may depend on background climate state, impacting the dependence of model-based AIE estimates on experiment design or the evolution of AIE strength with intensifying climate change, which has not previously been explicitly evaluated. Using atmosphere-only simulations with prescribed observed sea surface temperatures (SSTs) and sea ice in the National Center for Atmospheric Research (NCAR) Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), model, we impose a PD (2000) aerosol perturbation onto a PI (1850), PD, and PD with a uniform 4 K increase in the SST (PD + 4 K) background climate to assess the dependence of the total aerosol effective radiative forcing (ERF) and AIE on background climate. We find statistically insignificant increases in aerosol ERF when estimated in the different background climates, almost entirely from increases in direct ERF but with some regionally significant compensating signals in PD + 4 K. The absence of an AIE dependence on background climate in our PD simulation may be tied to documented differences in cloud responses to the observed SSTs used in our simulations versus SSTs produced by the fully coupled models from which most cloud feedback studies are derived, known as the “pattern effect.” Our findings indicate that AIE and aerosol forcing overall may not have a strong dependence on the background climate state in the near future but could regionally under extreme climate change. Significance Statement Diverse model representations of aerosol–cloud interactions strongly contribute to uncertainty in historical anthropogenic aerosol forcing and are associated with uncertainty in climate sensitivity. This study aims to highlight the dependence of aerosol indirect effects on the background climate state in Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), by identifying microphysical and meteorological changes between aerosol-driven atmospheric responses in present-day and preindustrial climate states to understand anthropogenic aerosol-driven forcing more thoroughly.