Strengthening of the East Asian Summer Monsoon in response to local and remote reductions in anthropogenic aerosol

The East Asian Summer Monsoon (EASM) has been shown to be sensitive to changes in local and remote aerosol emissions in multiple generations of climate models. Global increases in anthropogenic aerosol cause cooling, especially over Northern Hemisphere land, leading to a southward shift in the ITCZ, a weakened land-sea temperature gradient, and a weakened EASM. Local cooling from local aerosol increases act to weaken the land-sea temperature contrast, and thus the EASM, while the advection of cold air and circulation adjustments resulting from increases in European aerosol increases ultimately have the same effect. While increasing greenhouse gas emissions act to strengthen the EASM via enhanced moisture transport, the two effects do not cancel each other out. The predominantly dynamical response to aerosol increases resulted in a weakening of the EASM in the late 20th century, and determined the spatial pattern of the observed precipitation anomalies, with flooding in southern China and drying in the north.

 

Concerns about air quality have resulted in large, rapid reductions in aerosol emissions over East Asia since 2010. Similar reductions may occur in other regions in the near future. Here, we use data from 10 models that participated in the Regional Aerosol Model Intercomparison Project (RAMIP) to quantify the EASM response to recent reductions in aerosol emissions over East Asia, continuing reductions over North America and Europe, and potential future reductions over South Asia and Africa and the Middle East. In addition to considering seasonal mean changes, we show the impact of regional aerosol reductions on temperature and precipitation extremes. We present an analysis of the mechanisms for the response of the EASM to both local and remote aerosol changes, assessing the relative roles of thermodynamic and dynamic changes, and show a moisture budget decomposition. The RAMIP dataset includes 10 models, and 10-member ensembles for all experiments, which enables us to identify robust physical responses to aerosol emission changes, and to identify where structural differences between the participating models lead to differences in near-future projections. The EASM strengthens in response to all aerosol reductions, although it is most strongly influenced by local aerosol changes.