Dust Suppresses Aerosol First Indirect Effects in Marine Warm Clouds over the North Atlantic Ocean

Mineral dust is one of the most abundant aerosols, yet its influence on aerosol–cloud interactions (ACI) is not well constrained. Even though ageing during transport allows otherwise weakly hygroscopic dust to act as cloud condensation nuclei (CCN) and modify droplet size and albedo, how dust affects the first indirect effect (FIE) for low-level warm clouds remains unclear. Using satellite observations and reanalysis datasets, we quantify how Saharan dust modulates cloud effective radius (CER) and FIE for single-layer warm marine clouds between March and May (2003–2024) when dust-cloud co-occurrence is maximum. We find in dusty environments, CER increases (by 9%), and FIE decreases (by 1.25 times), changing from positive to negative for thin clouds, contrasting traditional ACI. This counterintuitive cloud response likely arises through two mechanisms: a microphysical pathway in which coarse dust, as giant CCN, produces fewer but larger droplets under limited water availability and a radiative pathway in which dust-induced warming enhances evaporation of smaller droplets and suppresses supersaturation, leading to larger droplets. Overall, we find a domain-mean positive top-of-atmosphere forcing from the dust-induced FIE of 0.38 W m⁻², reflecting dust-induced weakening of ACI in the marine environment, with implications for climate forcing uncertainties.