Rapid decadal weakening of the South Asian summer monsoon circulation within the slow response to abrupt CO2 quadrupling

Under greenhouse-gas-induced warming, the South Asian summer monsoon (SASM) circulation measured by large-scale circulation indices such as the Webster–Yang and Monsoon Hadley indices is projected to weaken, involving fast and slow processes. Previous studies have operationally defined the fast response as the first-year atmospheric adjustment, while attributing the circulation weakening primarily to the slow, ocean-mediated response over multi-decadal timescales. However, whether the weakening within the slow response proceeds uniformly or varies with time has received little attention. Using Coupled Model Intercomparison Project Phase 6 abrupt-4 × CO 2 simulations, this study reveals that the SASM circulation weakens nonlinearly. The SASM weakening, concentrated in the first decade (years 2–10) exhibits an early trend far exceeding the later one, with the early-to-later ratio declining by ∼90% from its peak to a steady level. This rapid decadal weakening is driven by reductions in the meridional upper-tropospheric thermal contrast between Eurasia and the tropical Indian Ocean (EUTT–IUTT) and the zonal upper-tropospheric thermal contrast between Eurasia and the northwestern Pacific (EUTT–NUTT). A climate feedback–response method attribution shows that the reduction in EUTT–IUTT is dominated by atmospheric dynamical processes, followed by ocean heat storage, and partially offset by latent heat flux. The weakening of EUTT–NUTT is controlled by atmospheric dynamical processes, with ocean heat storage and latent heat flux acting in opposition. A heat budget diagnosis further reveals that the dominant atmospheric dynamical contribution arises from horizontal temperature advection associated with the South Asian high and from diabatic heating associated with latent heat release in the Intertropical Convergence Zone.