The Impact of Anthropogenic Aerosol on the East Asian Summer Monsoon

Abstract:

The atmospheric component of HadGEM2-ES has been used to investigate the impacts of local and non-local emissions of anthropogenic sulphur dioxide on the East Asian summer monsoon (EASM). We focus on the very different fast responses to sudden changes in emissions from Asia and Europe. During days 1-40, Asian emissions have an impact on the sulphate burden itself over Asia, resulting in changes to the shortwave energy budget, cooling of East Asia and a weakening of the EASM. In contrast, European emissions have no significant impact on the sulphate burden over Asia, but do produce mid-tropospheric cold and dry anomaly over the European sector which is advected into Asia, where it induces atmospheric and surface feedbacks over Asia and the Western North Pacific, also weakening the EASM. The large scale pattern of changes in land–sea thermal contrast, atmospheric circulation and local precipitation over East Asia from day 40 onwards in both simulations exhibits similar structures, indicating a preferred response, and suggesting that emissions from both regions likely contributed to the observed weakening of the EASM.
A weakening of the EASM tends to lead to flooding in southern China and drought in the north. Northeast Asia experienced a severe drought in summer 2014 itself within the context of two decades of dry summers. We used HadGEM3-A simulations of summer 2014 to quantify the roles of greenhouse gases, anthropogenic aerosol, and sea surface temperature in the drought. These show reductions in precipitation over East Asia in response to recent changes in sea surface temperature, which are characteristic of the warm phase of the PDO. However, model biases meant that these experiments were unable to capture the observed pattern of precipitation anomalies, thus precluding definitive attribution.
These results show mechanisms by which anthropogenic aerosol could have contributed to the observed weakening of the EASM, but suggest that model biases in the Asian summer monsoon need to be reduced in order to enable successful attribution of individual extreme precipitation events.