Anthropogenic aerosols (AAs) significantly impact regional and global climate by altering cloud properties and radiative forcing. The geographic distribution of AAs has changed dramatically, shifting from the western to the eastern hemisphere since the 1980s. This shift, along with emission policy changes, has led to a significant change in the spatial distribution of aerosols. South Asia (SA) and East Asia (EA) are major aerosol emission sources. While aerosol optical depth (AOD) was similar in SA and EA in the early 2000s, an AAD pattern emerged in the early 2010s, showing a continuous rise in SA and a concurrent decline in EA, primarily due to emission controls in China. This change is reflected in the shortwave (SW) instantaneous radiative forcing (IRF) at the top-of-atmosphere (TOA), exhibiting a clear dipole pattern. The climate impacts of this emerging AAD pattern, anticipated to persist in coming decades, are largely unknown, particularly given that current CMIP6 models don't accurately capture the observed AAD pattern. This study utilizes idealized radiative perturbations in a state-of-the-art climate model to investigate the climate impacts of the evolving AAs in Asia, focusing on the dominant impacts of scattering aerosol changes. Two coupled sensitivity experiments were performed by perturbing the solar constant over SA and EA to mimic the effects of aerosol changes. Atmospheric Model Intercomparison Project (AMIP) simulations were also conducted for comparison.

Numerous studies have investigated the climate impacts of increasing AOD in SA and EA, representing changes up to the early 2000s. These studies have highlighted the influence of aerosols on mean climate and extreme events, such as heavy precipitation, consecutive dry days, extreme fire weather, and heat extremes. The hemispheric-scale shift of emission sources since the 1980s has been linked to the weakening of the summer Eurasian westerly jet and changes in tropical cyclone activity. However, research on the climate impacts of the emerging AAD pattern remains limited due to the inability of current climate models to accurately represent the observed pattern.

The study employed a state-of-the-art Geophysical Fluid Dynamics Laboratory (GFDL) climate model to conduct two coupled sensitivity experiments. These experiments mimicked the observed changes in aerosol radiative forcing by perturbing the solar constant over SA and EA. Six ensemble members were used for each experiment, varying only initial conditions. For comparison, AMIP simulations were performed using the same atmospheric model component, forced with fixed climatological sea surface temperature (SST) and sea ice concentration from the coupled control simulation. Observed changes in AOD and SW IRF were diagnosed from a combination of datasets, including MODIS Terra and Aqua, CERES-EBAF, AIRS, and MERRA-2. The observed SW IRF was estimated using a radiative kernel technique. A dipolar SAT index was constructed using observed SAT data from 1900-2015 to represent the temperature contrast between EA and SA. The linear baroclinic model (LBM) was used to investigate the influences of convective heating on large-scale circulations. The LBM experiments were forced with patterns similar to the precipitation changes over the forced domains and surrounding regions. The study scaled the model results based on the observed SW IRF to quantify the AAD-induced warming.

The annual mean climate response to SA aerosol increases showed weak global mean surface air temperature (SAT) cooling, with a homogenous cooling pattern in the tropics except for strong SA cooling. EA aerosol decreases resulted in stronger global mean SAT warming, with a more pronounced land-sea and interhemispheric contrast. The EA aerosol decreases led to a much stronger interhemispheric asymmetry, with significantly more warming in the Northern Hemisphere. The combined effect of the AAD, scaled to match the observed SW IRF trends, indicated a pronounced poleward shift of the northern subtropical jet and marked surface warming north of 30°N, particularly over Europe. The AAD also induced a significant negative phase Pacific Decadal Oscillation (PDO) pattern and an intensified Walker circulation, contributing to a La Niña-like SST pattern in the Indo-western Pacific. Analysis of summertime climate responses showed that SA aerosol increases significantly weakened the SA summer monsoon, with suppressed rainfall and circulation, while EA aerosol decreases strengthened the EA summer monsoon. The SA forcing resulted in a 2.7 times stronger land precipitation response than the EA forcing. LBM experiments revealed that the local convective forcing alone could not explain the global circulation changes observed in the coupled model simulations. Air-sea coupling, and the resultant downstream SST warming in the western North Pacific, were crucial in amplifying and spreading the EA aerosol decrease's effect to the Northern Hemisphere. The study quantified the recent AAD-induced annual mean warming rate for the northern extratropics beyond 30°N (0.024 ±0.010 °C decade⁻¹) and for Europe (0.049 ± 0.009 °C decade⁻¹), significantly exacerbating the background greenhouse gases-induced warming effect.

The findings highlight the distinct climate responses to regional-scale AAD compared to the hemispheric-scale shift of AAs in the 1980s. The contrasting responses are attributed to distinct air-sea feedbacks and background mean states. The westerly jet near EA efficiently advects temperature anomalies and acts as a waveguide for circulation changes in the northern mid-latitudes. The study's results are relevant to decadal prediction and understanding climate change associated with AA variations. The observational effective radiative forcing due to AAs has become less negative globally since 2000, with the emerging AAD pattern exacerbating northern hemisphere warming, especially over Europe. The study emphasizes the pattern effect of forcing in regulating global climate. The EA aerosol decreases induce a zonal wave number-5 pattern, representing the circumglobal teleconnection (CGT), which is more susceptible to EA aerosol decreases than SA aerosol increases. Land-atmosphere interaction through soil moisture feedback may amplify this teleconnection pattern. Increased AAs over India do not alleviate heat risks locally, while EA AA reduction increases heat extreme risks globally.

This study demonstrates the significant impact of the emerging Asian aerosol dipole pattern on global climate, highlighting the importance of considering the spatial pattern of forcings. The AAD significantly exacerbates Northern Hemisphere warming, especially over Europe, despite having a relatively weak global mean radiative forcing. The contrasting responses to aerosol changes in South Asia and East Asia are due to different air-sea feedbacks and background atmospheric states. Future research should focus on improving the representation of aerosol processes in climate models and on further investigating the impacts of AAD on extreme weather events.

The study uses idealized radiative perturbations to represent the complex interactions of aerosols with clouds and radiation. The model may not perfectly capture all aspects of the climate system's response to aerosol changes. The study focuses primarily on the direct radiative effects of aerosols, neglecting potential indirect effects and other rapid adjustments. The analysis relies on the assumption of linearity when combining the effects of SA and EA aerosol changes.