European sulphate aerosols were a key driver of the early twentieth-century intensification of the Asian summer monsoon

Abstract. Observations show that the Asian summer monsoon experienced substantial multi-decadal changes during the early 20^th century, including a wetting trend over South Asia and a southward rainfall shift over East Asia. Despite their significance, these variations have received limited attention, and the underlying mechanisms remain poorly understood. This study investigates the role of increased European sulphate aerosol emissions in shaping these monsoon changes using ensemble experiments with the Community Earth System Model. The aerosol-driven rainfall patterns over South and East Asia resemble observations, suggesting that European aerosols played an important role in modulating the monsoon. These changes are linked to large-scale anomalies in surface climate and three-dimensional atmospheric circulation across the Indo-Pacific, which alter moisture transport to the continent, the main driver of the rainfall anomalies. Regional circulation anomalies form part of a hemispheric upper-tropospheric wave train originating over central Europe and extending through the Middle East to the Pacific. The wave train arises as a thermodynamic adjustment to the aerosol-induced surface cooling and related anticyclone over Europe, extends to the upper troposphere, and, while propagating eastward, induces three-dimensional circulation anomalies across Asia that affect the monsoon. These findings provide compelling evidence for the influence of European sulphate aerosols on the early 20^th-century monsoon variability, which is relevant for improving current understanding of the regional-scale impacts of anthropogenic aerosols. As European SO₂ emissions continue to decline, this study sheds light upon a possible ongoing and future pathway which may significantly modulate the monsoon response to Asian aerosol changes.