| Summary: | Aerosols have an inhomogeneous distribution across the globe because of their short lifetimes in the atmosphere. They impose a localised radiative forcing close to their emission source that is effective at driving circulation changes and influencing the hydrological cycle. One such type of circulation that has recently been suggested to be affected by aerosols is the South Asian summer monsoon, which provides 80% of annual rainfall to over a billion people during the summer months. I used the UK Met Office HadGEM3 coupled global climate model to investigate, for the first time, the centennial-scale South Asian precipitation responses to removing regional anthropogenic sulphur dioxide emissions. This study is unique since these experiments have never been attempted on a scale that is multi-regional, centennial and in a fully coupled ocean-atmosphere model setup. Despite the imposed localised heating in my experiments resulting from emissions removals over the United States, Europe, East Asia, South Asia or the northern mid-latitudes as a whole, the large-scale spatial pattern and magnitude of the precipitation response over South Asia was similar for all experiments. This was due to the ocean responding to the atmosphere enabling the full response to be realised and the ocean dynamics driving a structurally similar global climate response. The sum of the responses from the experiments where the emissions are removed from the United States, Europe and East Asia resemble the response seen in the experiment where emissions are removed in the entire northern mid-latitude band, suggesting that the response is roughly linear, with East Asia being the largest contributor to the response. East Asia’s proximity to South Asia meant that it was more effective at influencing the land-sea thermal contrast, pressure gradients, and therefore the local monsoon circulation itself. On the other hand, local emissions removals, i.e. over South Asia itself, led to decreases in precipitation over central and northern India, a result not seen in any other experiment. Moreover, the response of the circulation was still found to be the main driver of the response in this case, despite changes to the clouds making them thinner and less reflective. In the future, further work is warranted to allow for comparisons with other coupled ocean-atmosphere models performing similar multi-regional experiments, to enhance confidence in our results and to further our understanding of the processes involved. Similar experiments can also be performed to explore the role of other short-lived pollutants, such as tropospheric ozone. The insight provided could prove invaluable for informing emissions reduction policies and for understanding the implications for short-term regional climate prediction.
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