The study, led by Feifei Luo of Chengdu University of Information Technology with colleagues from institutions in the United Kingdom, Norway, the United States and China, used the Fast Ocean Rapid Troposphere Experiment version 2, or FORTE2, a reduced-complexity coupled atmosphere-ocean model, together with the Systematic Regional Aerosol Perturbations experimental framework. The researchers ran paired simulations of identical Asian sulfate aerosol optical depth forcing under two background climates: one with pre-industrial carbon dioxide concentrations of 280 parts per million by volume, and one with 500 parts per million that produces roughly 1 kelvin of global-mean warming relative to the pre-industrial state.

Each simulation was run for 200 model years, with the first 50 years discarded as spin-up and the remaining 150 years used for analysis. The sulfate forcing was prescribed as repeating monthly climatologies of aerosol optical depth derived from the Copernicus Atmosphere Monitoring Service Reanalysis for 2003 to 2021, with fixed vertical profiles. Because the aerosol fields do not evolve in response to the simulated climate, any difference in the precipitation response between the two background states can be attributed entirely to the climate itself rather than to changes in the strength or distribution of the aerosol forcing.

The team examined four precipitation indices following the Expert Team on Climate Change Detection and Indices framework: PRCPTOT, the total precipitation on wet days; R95p, the total precipitation falling on extremely wet days above the 95th percentile threshold; R95d, the number of extremely wet days; and R95int, the average intensity of those extreme events. All analyses focused on boreal summer, from June through August. In both background climates, the added Asian sulfate aerosols reduced summer mean and extreme precipitation over East and South Asia, while enhancing precipitation over northwestern India. Extreme precipitation responded more strongly than mean precipitation, a pattern consistent with earlier findings that extremes are more sensitive to sulfate forcing.

The crucial new result emerged when the researchers compared the two climate backgrounds directly. Over East Asia, the differences between the two states did not reach statistical significance. Over South Asia, however, the same sulfate forcing under the warmer climate produced a substantially stronger fractional reduction in precipitation, with differences of 14.5 percent for total wet-day precipitation, 20 percent for extreme precipitation amounts, and 12.1 percent for extreme precipitation intensity, all statistically significant at the 95 percent confidence level. The number of extreme wet days showed only a negligible and statistically insignificant difference, indicating that the amplified drying over northern India manifests primarily as a reduction in the intensity of extreme events rather than their frequency.

To understand the mechanism, the authors applied an atmospheric column energy budget framework that partitions precipitation-related energy changes into a thermodynamic component and a dynamic component. In this framework, the latent heat release associated with precipitation changes is balanced by changes in net atmospheric diabatic cooling and in the divergence of dry static energy flux. The analysis showed that the dynamic component dominated the precipitation reductions in both regions, driven by changes in vertical circulation. The resulting surface cooling generates high-pressure sea level pressure anomalies and anomalous descent, suppressing the upward motion that feeds convection and rainfall.

The key to the climate-state difference lay in a counteracting term. When aerosols were added to the pre-industrial climate, an amplified increase in the horizontal dry static energy gradient over northern India, associated with a north-south surface temperature contrast featuring warm anomalies over southern India and cold anomalies over the north, partially offset the aerosol-induced dynamic suppression of rainfall. Under the warmer climate, higher atmospheric static stability, specifically increased lower-tropospheric stability, weakened the local meridional circulation induced by the aerosol forcing. This diminished the north-south temperature gradient and the offsetting horizontal thermal advection term, allowing the dynamic vertical suppression to dominate more strongly. In other words, the warmer background does not make the aerosols intrinsically more powerful; it removes a natural brake on their drying influence.

The study also separated the two main aerosol pathways: aerosol-radiation interactions, in which particles directly scatter and absorb radiation, and aerosol-cloud interactions, in which particles modify cloud properties. In the radiation-only simulations, precipitation responses over India were limited. In the cloud-interaction-only simulations, precipitation over most of India was suppressed in both climates, but this suppression was significantly weaker over northern India under the warmer, wetter background, apparently because increased moisture and deep convection there can enhance collision-coalescence efficiency or invigorate convection, partially offsetting the suppression. The surface shortwave response to the sulfate forcing did not differ significantly between the two climates, confirming that the weakened suppression was not due to weaker radiative forcing but to the modulating effect of the background state.

Perhaps most strikingly, the two aerosol pathways combined approximately linearly under the pre-industrial climate but interacted nonlinearly under the warmer one. Under the warmer background, the sum of the individual radiation and cloud effects predicted increased rainfall over northern India, while the combined simulation showed a decrease, a significant discrepancy over northeastern India. The authors propose, as a hypothesis requiring further exploration, that aerosol-induced surface cooling reduces water vapor through weakened evaporation and weakens the Indian summer monsoon through a reduced land-sea thermal contrast, and that these drier conditions may in turn favor precipitation suppression through cloud interactions. This transition from linear to nonlinear behavior suggests that aerosol-climate interactions may change qualitatively as the background climate evolves, rather than simply scaling with the magnitude of the aerosol forcing.

The researchers supported their reduced-complexity model results with a qualitative comparison against the Precipitation Driver and Response Model Intercomparison Project, using five fully coupled earth system models in which Asian sulfate emissions or concentrations were raised to ten times year-2000 levels. Although that perturbation is roughly an order of magnitude stronger than the FORTE2 forcing, with a regional summer multi-model mean aerosol optical depth of about 2.5 over East and South Asia compared with about 0.23 in FORTE2, the sign and spatial pattern of the responses agreed, and the energy-budget analysis in the multi-model ensemble supported the dominance of dynamical vertical suppression. The authors caution several limitations: FORTE2 lacks interactive wet deposition feedbacks, which may somewhat amplify the precipitation response, and the model exhibits a dry bias in climatological summer precipitation over India. The PDRMIP experiments, conducted under a single climate state, cannot directly assess the influence of background warming. The authors also note that whether the climate-state dependence is reversible under aerosol reductions remains unresolved, so the implication that future aerosol emissions cuts may unmask or amplify extreme precipitation increases over northern India should be regarded as a hypothesis rather than a direct projection, pending dedicated aerosol-reduction experiments across different warming levels and tests in more comprehensive models.

Subject of Research: How the background climate state modulates the impact of Asian sulfate aerosols on regional summer extreme precipitation

Article Title: Contrasting impacts of Asian sulfate aerosols on regional summer extreme precipitation under different climate backgrounds

Article References: Luo, F., Joshi, M., Samset, B. H., Wilcox, L. J., Stjern, C. W., Allen, R. J., Kuo, Y.-N., Hua, W., & Li, S. (2026). Contrasting impacts of Asian sulfate aerosols on regional summer extreme precipitation under different climate backgrounds. Atmospheric Chemistry and Physics, 26(20), 15037-15053. https://doi.org/10.5194/acp-26-15037-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-15037-2026

Keywords: sulfate aerosols, extreme precipitation, Asian summer monsoon, northern India, aerosol-cloud interactions, aerosol-radiation interactions, climate modeling, atmospheric energy budget, global warming, FORTE2, PDRMIP, atmospheric stability