Inconsistent Fast and Slow Responses of East Asian Summer Monsoon Precipitation Forced by Black Carbon

Abstract The East Asian summer monsoon (EASM) region, teeming with anthropogenic black carbon (BC), lacks clarity on the fast and slow responses of EASM precipitation and other atmospheric water cycle processes to BC forcings. Through Community Earth System Model, version 1, simulations with an atmospheric water tracking (AWT) method, fast atmospheric adjustments and slow sea surface temperature (SST)-meditated responses of EASM to BC are found inconsistent, and the summertime precipitation anomaly exhibits a “positive–negative” and “negative–positive” meridional pattern, respectively. Quantitative AWT results highlight the dominant influences of BC-induced atmospheric dynamics adjustments in shaping the EASM precipitation anomaly pattern. The impacts of moisture transport by the southwesterlies, primarily from tropical Indian Ocean, overweigh those of surface evaporation changes, governing both fast and slow precipitation responses. Tropospheric warming from high BC concentrations over northern East Asia in the fast response creates a positive meridional temperature gradient anomaly at the lower level of the East Asian subtropical westerly jet (EASWJ) center. This weakens EASWJ, leading to significant upper-level convergence at 200 hPa and anomalous descending motion north of 30°N, reducing precipitation over North China. Compensatory ascending motion and low-level convergence south of 30°N enhance precipitation over South China. Conversely, BC-induced SST warming in the Indo-Pacific warm pool generates a negative meridional temperature gradient anomaly and strengthens EASWJ, leading to an opposite slow precipitation response pattern. Overall, the total EASM responses manifests as a “wetter–drier–wetter” meridional precipitation pattern and slightly strengthened circulation, aligning with the main structures of interdecadal summer precipitation anomalies over East Asia in recent years reported in previous observational studies. Significance Statement Black carbon (BC), the most influential absorbing aerosol with warming effects second only to greenhouse gases, poses significant risks to human health and the climate. East Asia, a major contributor to global BC loading, is recognized as one of the largest monsoon regions, with its summer hydrological processes being sensitive to BC forcings. Our study utilizes an Earth system model coupled with an atmospheric water tracking method and reveals the opposing effects of BC-induced fast atmospheric adjustments and slow SST-meditated climatic responses on East Asian summer monsoon precipitation. Emphasis is placed on the dominant role of moisture transport related to the BC-induced atmospheric circulation adjustments in governing both fast and slow summer precipitation responses over East Asia.