Emissions of anthropogenic aerosols are rapidly changing, in amounts, composition and geographical distribution. In East and South Asia in particular, strong aerosol trends combined with high population densities imply high potential vulnerability to climate change. Improved knowledge of how near-term climate and weather influences these changes is urgently needed, to allow for better-informed adaptation strategies. To understand and decompose the local and remote climate impacts of regional aerosol emission changes, we perform a set of Systematic Regional Aerosol Perturbations (SyRAP) using the reduced-complexity climate model FORTE 2. Absorbing and scattering aerosols are perturbed separately, over East Asia and South Asia, to assess their distinct influences on climate. In this paper, we first present an updated version of FORTE2, which includes treatment of aerosol-cloud interactions. We then document and validate the local responses over a range of parameters, showing for instance that removing emissions of absorbing aerosols over both East Asia and South Asia is projected to cause a local drying, alongside a range of more widespread effects. We find that SyRAP-FORTE2 is able to reproduce the responses to Asian aerosol changes documented in the literature, and that it can help us decompose regional climate impacts of aerosols from the two regions. Finally, we show how SyRAP-FORTE2 has regionally linear responses in temperature and precipitation and can be used as input to emulators and tunable simple climate models, and as a ready-made tool for projecting the local and remote effects of near-term changes in Asian aerosol emissions.

The study investigates how sea surface temperature (SST) anomalies surrounding the Maritime Continent (MC) modulate the impact of developing El Niño events on Indian Summer Monsoon (ISM) rainfall. Using a climate model we find that the ISM rainfall response to tropical Pacific SST anomalies of eastern and central Pacific El Niño events is sensitive to the details of cold SST anomalies surrounding the MC. Furthermore, the remote rainfall responses to regions of SST anomalies do not combine linearly and depend strongly on gradients in the SST anomaly patterns. The cold SST anomalies around the MC have a significantly larger impact on the ISM response to eastern Pacific events than to central Pacific events. These results show the usefulness of idealised modelling experiments, which offer insights into the complex interactions of the ISM with modes of climate variability.

Now published: https://doi.org/10.1088/1748-9326/ac3b19 Anthropogenic aerosols over South and East Asia currently have a stronger impact on the Asian Summer Monsoon (ASM) than greenhouse gas emissions, yet projected aerosol emission changes in these regions are subject to considerable uncertainty in timescale, location, emission type, and even the sign of the change, implying large uncertainties in future ASM change. In addition, aerosol changes in either South or East Asia cause circulation anomalies that affect both countries and neighbouring regions. We use a circulation/climate model to demonstrate that the sum of ASM responses to individual aerosol emission reductions in each region is very different to the response to simultaneous reductions in both regions, implying the ASM response to aerosol emissions reductions is highly nonlinear. The phenomenon is independent of whether aerosols are scattering or absorbing, and is driven by large-scale teleconnections between the two regions. The nonlinearity represents a new source of uncertainty in projections of ASM changes over the next 30-40 years, and limits the utility of country-dependent aerosol trajectories when considering their Asia-wide effects. To understand likely changes in the ASM due to aerosol reductions, countries will need to accurately take account of emissions reductions from across the wider region, rather than approximating them using simple scenarios and emulators. The nonlinearity in the response to forcing therefore presents a regional public goods issue for countries affected by the ASM, as the costs and benefits of aerosol emissions reductions are not internalised; in fact, forcings from different countries work jointly to determine outcomes across the region.

Changes in climate are usually considered in terms of trends or differences over time. However, for many impacts requiring adaptation, it is the amplitude of the change relative to the local amplitude of climate variability which is more relevant. Here, we develop the concept of ‘signal-to-noise’ in observations of local temperature, highlighting that many regions are already experiencing a climate which would be ‘unknown’ by late 19century standards. The emergence of observed temperature changes over both land and ocean is clearest in tropical regions, in contrast to the regions of largest change which are in the northern extra-tropics - broadly consistent with climate model simulations. Significant increases and decreases in rainfall have also already emerged in different regions with the UK experiencing a shift towards more extreme rainfall events, a signal which is emerging more clearly in some places than the changes in mean rainfall.