Do Differences in Future Sulfate Emission Pathways Matter for Near-term Climate? A Case Study for the Asian Monsoon

The impacts of aerosols upon regional hydroclimate could dominate over greenhouse gas impacts in the near future, especially in regions with high aerosol loading such as Asia. However, future emissions of aerosols and their precursors (e.g. sulfur dioxide (SO2)) are not well constrained. Here, we investigate the sensitivity of the Asian monsoon to future East Asian aerosol emissions using fully-coupled climate modelling experiments with HadGEM2-ES. We use Representative Concentration Pathway 2.6 and two sensitivity scenarios with the same greenhouse gas forcing, but spanning the range of plausible global SO2 emissions. The largest difference in emissions is over China, which analysis shows leads to regional cooling and surface dimming during the summer monsoon season, due to increased aerosol optical depth (AOD) when considering the difference of high SO2 minus low SO2 simulations. Further cooling is seen over southern China and Indochina where cloudiness and precipitation increase. The precipitation response over China is similar to the observed southern-flood-northern-drought pattern, which is explained by higher pressure to the north and weakening of the West Pacific Subtropical High preventing northward progression of the monsoon. In addition, drying occurs over India, linked via closed cells of three-dimensional circulation. We find anomalies in upper-level streamfunction and velocity potential, exciting a northern hemisphere Rossby wave response, therefore leading to the the potential for aerosols to drive climate anomalies. We conclude that climate is sensitive to the difference in a plausible range of aerosol emissions and, consequently, it is important that different aerosol emission pathways are considered when studying future climate.