Susceptibility of marine warm clouds to aerosols in different monsoon periods over the South China Sea

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. Using long-term satellite observations and reanalysis data, we investigate ACI in non-raining warm liquid clouds over the SCS across three monsoon phases: the southwest monsoon wet (SWMW), northeast monsoon wet (NEMW), and northeast monsoon dry (NEMD) periods. The robust Twomey effect is observed across all periods. Shallow stratocumulus clouds show no significant differences in ACI across periods, whereas deeper cumulus clouds exhibit the strongest ACI during NEMD, with no clear separation between SWMW and NEMW. The enhanced ACI during NEMD is consistent with the relatively dry and stable lower-tropospheric environment (LTS), where stable conditions may enhance ACI through aerosol accumulation, while moist environments are likely to weaken it via enhanced condensational and coalescence growth. However, these differences likely reflect co-varying environmental conditions across monsoon periods rather than a single dominant controlling factor. Limitations of the aerosol index (AI) as a marine cloud condensation nuclei (CCN) proxy and satellite retrieval biases may affect these conclusions. These findings suggest that, within a monsoon-organized framework, the interplay among aerosols, humidity, and stability is associated with marine warm-cloud microphysics, providing observational constraints for climate model representation of ACI.