Late Summer Enhancement of East Asian Monsoon Rainfall under CO₂ Removal Scenario

The East Asian summer monsoon (EASM) exhibits complex intraseasonal variability under global warming, with critical implications for regional hydroclimate. Using simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and Carbon Dioxide Removal Model Intercomparison Project (CDRMIP), this study investigates EASM rainfall responses to symmetric CO₂ forcing under equivalent global mean warming of 3°C. The results reveal pronounced asymmetry: EASM rainfall over the Meiyu region (25°–40°N, 105°–150°E) is significantly enhanced during CO₂ ramp-down compared to ramp-up, primarily driven by intensified late summer (July–August) rainfall, while early-summer (May–June) rainfall shows slight decreases, resulting in a delayed annual cycle of EASM rainfall over the Meiyu region. Moisture budget analysis indicates that dynamic effects associated with monsoon circulation changes dominate this intraseasonal asymmetry, as thermodynamic moisture contributions are nearly identical between the two periods under equivalent global-mean warming. The distinct intraseasonal dynamics arise from contrasting sea surface temperature evolution in the Indo-western North Pacific region. In late summer, enhanced warming over the northern tropical Indian Ocean generates easterly wind changes across the western North Pacific, subsequently establishing an anticyclonic circulation. These circulation changes strengthen southwesterly moisture transport and ascending motion over the Meiyu region, amplifying late summer rainfall.