Impact of Asian aerosols on the summer monsoon strongly modulated by regional precipitation biases

Abstract. Reliable attribution of Asian summer monsoon variations to aerosol forcing is critical to reducing uncertainties in future projections of regional water availability, which is of utmost importance for risk management and adaptation planning in this densely populated region. Yet, simulating the monsoon remains a challenge for climate models which suffer from long-standing biases, undermining their reliability in attributing anthropogenically-forced changes. We analyse a suite of climate model experiments to identify a link between model biases and monsoon responses to Asian aerosols, and the physical mechanism underpinning this link, including the role of large-scale circulation changes. The aerosol impact on monsoon precipitation and circulation is strongly influenced by a model’s ability to simulate the spatial distribution and temporal variability of the climatological monsoon winds, clouds and precipitation across Asia, which critically modulates the magnitude and efficacy of aerosol-cloud-precipitation interactions, the predominant driver of the total aerosol response. There is a strong interplay between South and East Asia monsoon precipitation biases and their relative predominance in driving the overall monsoon response. We found a striking contrast between the early and late summer aerosol-driven changes ascribable to opposite signs and seasonal evolution of the biases in the two regions. A realistic simulation of the evolution of the large-scale atmospheric circulation is crucial to realise the full extent of the aerosol impact over Asia. These findings provide important implications to better understand and constrain the diversity and inconsistencies of model responses to aerosol changes over Asia in historical simulations and future projections.