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.

Convective aggregation is an important atmospheric phenomenon which frequently occurs in idealised models in radiative-convective equilibrium (RCE), where the effects of land, rotation, sea surface temperature gradients, and the diurnal cycle are often removed. This aggregation is triggered and maintained by self-generated radiatively driven circulations, for which longwave feedbacks are essential. Many questions remain over how important the driving processes of aggregation in idealized models are in the real atmosphere. We approach this question by adding a continentally-sized, idealized tropical rainforest island into an RCE model to investigate how land-sea contrasts impact convective aggregation and its mechanisms. We show that convection preferentially forms over the island persistently in our simulation. This is forced by a large-scale thermally driven circulation. First, a sea-breeze circulation is triggered by the land-sea thermal contrast, driven by surface sensible heating. This sea-breeze circulation triggers convection which then generates longwave heating anomalies. We find that these longwave heating anomalies are essential for maintaining the aggregation of convection over the island through mechanism denial tests. We also show, by varying the island size, that the aggregated convective cluster appears to have a maximum spatial extent of 10,000 km. These results highlight that the mechanisms of idealized aggregation remain relevant when land is included in the model, and therefore these mechanisms could help us understand convective organization in the real-world.

Absorbing aerosol from biomass burning impacts the hydrological cycle and fluxes of radiation both directly and indirectly via modifications to convective processes and cloud development. Using the ICON model in a regional configuration with convection-permitting resolution of 1500 m, we isolate the response of the Amazonian atmosphere to biomass burning smoke via enhanced cloud droplet number concentrations Nd (aerosol-cloud-interactions; ACI) and changes to radiative fluxes (aerosol-radiation-interactions; ARI). We decompose ARI into contributions from surface cooling (reduced surface shortwave flux) and localized heating of the smoke layer. We show that ARI influences the formation and development of convective cells: surface cooling below the smoke drives suppression of convection that increases with the smoke optical depth, whilst the elevated heating promotes initial suppression and subsequent intensification of convection overnight; a corresponding diurnal response from high precipitation rates is shown. Enhanced Nd (ACI) perturbs the intensive cloud properties and suppresses low-to-moderate precipitation rates. Both ACI and ARI result in enhanced high-altitude ice clouds that have a strong positive longwave radiative effect. Changes to low-cloud coverage (ARI) and albedo (ACI) drive an overall negative shortwave radiative effect, that slowly increases in magnitude due to a moistening of the boundary layer. The overall net radiative effect is dominated by the enhanced high-altitude clouds, and is sensitive to the plume longevity. The considerable diurnal responses that we simulate cannot be observed by polar orbiting satellites widely used in previous work, highlighting the potential of geostationary satellites to observe large-scale impacts of aerosols on clouds.