Background-state dependence of aerosol responses in CESM2 boreal-summer climate

Abstract. Anthropogenic aerosol forcing is commonly estimated using aerosol-only single-forcing experiments, but can also be inferred as a residual from all-forcing and fixed-aerosol simulations. Although previous studies have demonstrated nonlinear interactions among climate forcings, it remains unclear whether these approaches provide dynamically equivalent aerosol responses, particularly at regional scale. Here, we compare the directly simulated aerosol response (AAER) with the residual estimate (ALL−xAER) in Community Earth System Model version 2 (CESM2) across two contrasting periods: 1965–1985 relative to 1920–1940, representing the historical increase in anthropogenic aerosol loading, and 2030–2050 relative to 1965–1985, representing the subsequent decline and geographical redistribution of aerosol emissions. We focus on boreal summer, when aerosol forcing strongly interacts with monsoon circulation and large-scale atmospheric dynamics.

During the historical transition, AAER and ALL−xAER produce broadly similar Northern Hemisphere responses, particularly in near-surface temperature, although regional differences occur in precipitation and circulation. During the future transition, the two estimates diverge substantially, especially in precipitation over the Indo-Pacific and Asian monsoon regions. These differences are associated with reorganised low-level moisture transport and upper-tropospheric circulation, including changes in jet structure, Rossby-wave propagation, and tropical–extratropical teleconnections. The North Atlantic exhibits an additional coupled ocean–atmosphere component, with different sea surface temperature and Atlantic Meridional Overturning Circulation responses between AAER and ALL−xAER. These results demonstrate that the two approaches are not dynamically interchangeable in CESM2: the diagnosed aerosol response depends on the evolving background climate state in which aerosol forcing operates, with important implications for residual-based attribution of regional climate change under future greenhouse-gas forcing.