Susceptibility of Marine Warm Clouds to Aerosols in Different Monsoon Periods over the South China Sea

Abstract. Understanding the susceptibility of warm clouds to aerosol loading, quantified by the aerosol–cloud interactions (ACI) index, is essential for assessing ACI and their climate impacts. Previous studies have demonstrated that this susceptibility is strongly modulated by environmental conditions. The South China Sea (SCS), influenced alternately by the southwest and northeast monsoons, provides a unique natural laboratory for examining ACI under contrasting thermodynamic and moisture conditions. We utilized long-term satellite observations and reanalysis datasets to investigate ACI over the SCS, with a focus on non-raining warm liquid clouds. Based on large-scale circulation patterns and moisture conditions, the monsoon system over the SCS is categorized into three 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 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 reveal the dominant roles of thermodynamic stability and moisture in regulating ACI over the SCS. The interplay among aerosols, humidity, and stability governs marine warm-cloud microphysics in tropical monsoon environments, providing observational constraints for improving the representation of ACI in climate models.