Contrasting Monsoon-Driven Susceptibility of Marine Warm Clouds to Aerosols over the South China Sea

Previous studies have demonstrated that the susceptibility of clouds to aerosol loading, quantified by the aerosol–cloud interactions (ACI) index, is strongly modulated by environmental conditions. The South China Sea (SCS), alternately influenced by the southwest and northeast monsoons, provides a unique natural laboratory for examining ACI under contrasting thermodynamic and moisture conditions. Using long-term satellite observations and reanalysis datasets, we investigate ACI over the SCS with a focus on non-precipitating warm liquid clouds. Based on large-scale circulation patterns and moisture conditions, the SCS monsoon system is classified into three distinct phases: the southwest monsoon wet period (SWMW), the northeast monsoon wet period (NEMW), and the northeast monsoon dry period (NEMD). The robust Twomey effect was observed across all three periods. The ACI intensity strengthens progressively from SWMW to NEMW and further to NEMD, corresponding to the transition from moist, convectively active conditions to dry, stably stratified environments. This transition is governed by variations in water-vapor availability and lower-tropospheric stability (LTS), where stable conditions may enhance ACI through aerosol accumulation, while moist environments are likely to weaken it via enhanced condensational and coalescence growth.These findings demonstrate that thermodynamic stability and moisture availability play central roles in regulating ACI over the SCS.The coupled effects of aerosols, humidity, and atmospheric stability control marine warm-cloud microphysical processes in tropical monsoon regions, providing robust observational constraints for improving ACI parameterizations in climate models.