Nonlinear response of Asian summer monsoon precipitation to emission reductions in South and East Asia

Ross Herbert,Laura J Wilcox,Manoj Joshi,Ellie Highwood,Dave Frame

doi:10.1088/1748-9326/ac3b19

Abstract

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 uncertainties such as timescale, location, or emission type. We use a circulation/climate model with idealised aerosol distributions to demonstrate that the sum of ASM responses to 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 results from interaction of induced atmospheric circulation changes. The nonlinearity from interactions between aerosol forcing from different regions represents a new source of uncertainty in projections of ASM changes over the next 30–40 years, and may limit the utility of country-dependent aerosol trajectories when considering their Asia-wide effects, though we recommend further work to establish whether the nonlinearity is buffered by other drivers. 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 such as India and China work jointly to determine outcomes across the region.

Highlights

Almost half of the world’s population rely on the Asian summer monsoon (ASM) precipitation for agriculture, energy, industry, and local water resources
By considering only aerosol-radiation interactions (ARI) we avoid the uncertainties associated with aerosol emissions and microphysics (e.g., [40,41]), which are reflected in large differences in the magnitude and pattern of anthropogenic aerosol radiative forcing in CMIP6 [6], and instead we focus on the potential for nonlinearities in the response to regional forcing
The removal of anthropogenic aerosol throughout Asia (RAsia) primarily increases precipitation over the Tibetan Plateau (TP); BC removal produces a stronger response than Sulphate aerosol (SU) and causes robust drying over NE Asia and the NW Pacific Ocean by -20%

Summary

Introduction

Almost half of the world’s population rely on the Asian summer monsoon (ASM) precipitation for agriculture, energy, industry, and local water resources. The ASM is driven by the large-scale meridional temperature gradient that occurs during boreal spring as a result of intense solar heating of the land surface in contrast to the slower ocean heating. An observed drying trend in ASM precipitation in the latter half of the 20th century opposed the impacts of GHG emissions, and studies have largely attributed this trend to anthropogenic aerosol emissions [1,4,7,8,9,10], though other factors such as warming of the surrounding oceans [11,12] and changes to land-surface properties [13] may play a secondary role. Recent years have seen a revival of the South Asian branch of the ASM occurring alongside increasing local aerosol emissions [14,15], which highlights remaining uncertainty in the drivers of ASM changes and a potential role for non-local aerosol emissions in the northern hemisphere (e.g., [16])

Methods

Results

Conclusion