Contrasting Effects of Aerosols and Greenhouse Gases on Subseasonal Variability of the Indian Summer Monsoon

Subseasonal variability strongly influences the seasonal mean and spatial distribution of rainfall in the Indian Summer Monsoon (ISM). While the late-twentieth-century weakening of ISM precipitation has been widely attributed to anthropogenic aerosols, their effects on subseasonal variability remain less well understood. Using single-forcing and all-forcing simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) Detection and Attribution Model Intercomparison Project (DAMIP), this study investigates how aerosol and greenhouse gas (GHG) forcings modify monsoon variability across synoptic and intraseasonal timescales. Results show that aerosols and GHGs exert opposing influences: aerosol forcing suppresses convection, reduces low pressure system (LPS) rainfall intensity by about eight percent, and weakens the 25–90-day monsoon intraseasonal oscillation (MISO), whereas GHG forcing enhances moisture availability and amplifies both LPS-related and intraseasonal rainfall by roughly six percent. These contrasting effects are consistent with associated changes in vertically integrated moisture flux convergence, with aerosols diminishing oceanic moisture inflow and GHGs strengthening it. The combined historical forcing produces a nonlinear response, indicating interactions between radiative and dynamic feedback that cannot be explained by a linear superposition of individual forcings. The findings suggest that aerosols suppress subseasonal rainfall variability, while GHGs amplify it through thermodynamic and moisture feedback. Understanding these competing influences is critical for interpreting past monsoon trends and projecting future variability under evolving aerosol mitigation and greenhouse gas emission pathways.