What caused record-breaking aerosol loading over the South China Sea in April 2023

Abstract. In April 2023, the South China Sea (SCS) experienced an unprecedented surge in aerosol loading, reaching the highest levels recorded in the two-decade Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data period (2003–2023). Satellite observations revealed a 150 % increase in aerosol optical depth (AOD) from MODIS and a 50 % rise in carbon monoxide (CO) at 700 and 500 hPa from Measurements Of Pollution In The Troposphere (MOPITT) over SCS. Here, we investigate the drivers and atmospheric mechanisms responsible for this extreme event, identifying large-scale biomass burning (BB) across northern Peninsular Southeast Asia (PSEA), particularly Laos and Myanmar as the primary source. Our analysis indicates that anomalously high surface temperatures, low soil moisture, reduced precipitation, and a persistent upper-tropospheric anticyclone created favorable BB conditions over PSEA. Laos alone accounted for ~56 % of the BB activity in the region, recording its largest monthly burned area (1.08 million hectares) since 2002. Dynamical analysis of the large-scale atmospheric circulation patterns revealed a major shift in regional wind regimes: the climatological south-westerlies over the SCS were replaced by anomalous northerlies, driven by the eastward shift of the Bay of Bengal anticyclone and the development of a cyclone anomaly over the western North Pacific (WNP). These changes redirected smoke transport from the usual WNP pathway to the SCS, resulting in significant transboundary pollution. This study highlights the critical role of compound meteorological extremes and circulation anomalies in amplifying regional aerosol loading, with implications for air quality, climate feedbacks, and environmental monitoring across Southeast Asia.