Impacts of Anthropogenic Emissions over South Asia on East Asian Spring Climate: Two Possible Dynamical Pathways

Xinyue Hao aChina Meteorological Administration Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China

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Yiquan Jiang aChina Meteorological Administration Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China

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Xiu-Qun Yang aChina Meteorological Administration Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China

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Xiaohong Liu bDepartment of Atmospheric Science, Texas A&M University, College Station, Texas

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Yang Zhang aChina Meteorological Administration Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China

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Minghuai Wang aChina Meteorological Administration Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China

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Yuan Liang aChina Meteorological Administration Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China

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Yong Wang cDepartment of Earth System Science, Tsinghua University, Beijing, China

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Abstract

Both South Asia and East Asia are the most polluted regions of the world. Unlike East Asia, the aerosol optical depth (AOD) over South Asia keeps increasing for all recent years, which calls for more attention. This study investigates the impacts of anthropogenic emissions over South Asia on the downstream regional climate during spring with the Community Earth System Model 2 (CESM2). The model results suggest that South Asian pollutants have significant impacts on East Asian spring climate, and the impacts could be even larger than locally emitted aerosols. Two possible dynamical pathways (i.e., the northern and the southern pathways) bridging South Asian aerosol forcing and East Asian climate are proposed, and both ways are associated with the black carbon (BC)-induced climate feedbacks surrounding the Tibetan Plateau (TP). The northern pathway is mainly due to the TP warming induced by the BC snow darkening effect (SDE), which significantly reduces the surface air temperature (SAT) over northern East Asia. BC-induced TP warming increases the meridional thermal gradient and accelerates the midlatitude jet stream, which favors the cold-air advection over northern East Asia. The southern pathway is associated with the BC “elevated heat pump” hypothesis, which mainly affects the precipitation in southern East Asia. BC from South Asia accumulates near the south slope of TP, inducing an abnormal ascending motion near the Bay of Bengal. A compensating anomalous sinking motion is then forced in South China, which suppresses the precipitation there. A primary observational analysis is also performed to verify both dynamical pathways.

Significance Statement

The intensified air pollution over South Asia and its impacts on local climate have been extensively investigated, but its impacts on the climate of remote regions have not been well recognized. Two possible dynamical pathways bridging South Asian air pollutants and East Asian spring climate are proposed, and the black carbon (BC)-induced climate feedbacks surrounding the Tibetan Plateau (TP) are emphasized for both pathways. The findings of this study favor the projection of East Asian future climate under the background of Third Pole/TP warming.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Yiquan Jiang, yqjiang@nju.edu.cn

Abstract

Both South Asia and East Asia are the most polluted regions of the world. Unlike East Asia, the aerosol optical depth (AOD) over South Asia keeps increasing for all recent years, which calls for more attention. This study investigates the impacts of anthropogenic emissions over South Asia on the downstream regional climate during spring with the Community Earth System Model 2 (CESM2). The model results suggest that South Asian pollutants have significant impacts on East Asian spring climate, and the impacts could be even larger than locally emitted aerosols. Two possible dynamical pathways (i.e., the northern and the southern pathways) bridging South Asian aerosol forcing and East Asian climate are proposed, and both ways are associated with the black carbon (BC)-induced climate feedbacks surrounding the Tibetan Plateau (TP). The northern pathway is mainly due to the TP warming induced by the BC snow darkening effect (SDE), which significantly reduces the surface air temperature (SAT) over northern East Asia. BC-induced TP warming increases the meridional thermal gradient and accelerates the midlatitude jet stream, which favors the cold-air advection over northern East Asia. The southern pathway is associated with the BC “elevated heat pump” hypothesis, which mainly affects the precipitation in southern East Asia. BC from South Asia accumulates near the south slope of TP, inducing an abnormal ascending motion near the Bay of Bengal. A compensating anomalous sinking motion is then forced in South China, which suppresses the precipitation there. A primary observational analysis is also performed to verify both dynamical pathways.

Significance Statement

The intensified air pollution over South Asia and its impacts on local climate have been extensively investigated, but its impacts on the climate of remote regions have not been well recognized. Two possible dynamical pathways bridging South Asian air pollutants and East Asian spring climate are proposed, and the black carbon (BC)-induced climate feedbacks surrounding the Tibetan Plateau (TP) are emphasized for both pathways. The findings of this study favor the projection of East Asian future climate under the background of Third Pole/TP warming.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Yiquan Jiang, yqjiang@nju.edu.cn

1. Introduction

East Asia and South Asia are the most polluted regions of Eurasian continent (Fig. 1) due to rapid industrialization, urbanization, and other human activities (Li et al. 2016). The air pollution variations of these two regions are quite different in recent decades (Samset et al. 2019; Ratnam et al. 2021). The aerosol emission keeps increasing over South Asia (Lu et al. 2011), but it has started to level off over East Asia recently due to the clean-air policy of China (Zheng et al. 2020). As a result, the aerosol optical depth (AOD) over South Asia has continued to rise for recent decades (Fig. 2a), but it started to decrease over East Asia around year 2010 (Fig. 2b). The intensified air pollution over South Asia calls for more attentions.

Fig. 1.
Fig. 1.

Spatial distribution of annual mean surface emission (10−12 kg m−2 s−1) change between year 2000 and year 1850 for (a) black carbon and (b) SO2. The blue and red rectangles denote the areas of South Asia (0°–40°N, 60°–100°E) and East Asia (10°–50°N, 100°–140°E), respectively.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

Fig. 2.
Fig. 2.

The year-to-year variations of annual mean aerosol optical depth (AOD) anomalies (relative to the 2000–19 mean) for (a) South Asia and (b) East Asia.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The impacts of South Asian aerosols on Indian summer monsoon (ISM) were the mostly extensively investigated, for the monsoon rainfall acts as a primary water source of India (Lau et al. 2008). Previous studies have demonstrated that anthropogenic aerosols have great impacts on ISM, and the effects of non-absorbing (sulfate) and absorbing (BC and dust) aerosols could be quite different. Sulfate aerosol induces negative radiative forcing (cooling), weakens Indian summer monsoon, and decreases the rainfall over India (e.g., Bollasina et al. 2011; Ramanathan et al. 2005; Ganguly et al. 2012). Absorbing aerosols are believed to enhance the monsoon rainfall (e.g., Lau et al. 2006; Lau and Kim 2006; Vinoj et al. 2014) without modulating the sea surface temperature (SST, fast response). But on longer time scales, absorbing aerosol forcing leads to decrease of precipitation over Indian monsoon regions (Meehl et al. 2008), when SST feedbacks are involved (slow response).

During the premonsoon season, the aerosol forcing over South Asia could be even larger due to the weak wet scavenging (Kovilakam and Mahajan 2016). The premonsoon aerosol forcing tends to increase rainfall over Indian (Meehl et al. 2008; Collier and Zhang 2009; Kim et al. 2015) and the impacts of absorbing aerosols (black carbon) could be dominant. BC forcing could affect premonsoon climate over South Asia through a variety of pathways. First, BC increases the absorbed solar energy over the snow-covered regions of Tibetan Plateau (TP) and accelerates the snow melting, which is known as the snow darkening effect (SDE, Qian et al. 2015). Moreover, BC intensifies the heating in the elevated atmospheric layers of south slope of TP, which is known as the “elevated heat pump” hypothesis (Lau et al. 2006). Both effects induce upward motions along the south slope of TP and draw in more warm moist air from Indian Ocean, which increase the rainfall in India during the premonsoon season (Lau and Kim 2006, 2010).

Over East Asia, the local aerosol effects on East Asian monsoon climate also has attracted lots of attention (Li et al. 2019; Liao et al. 2015). In summer, the local aerosol forcing decreases the land–sea thermal contrast of East Asia (Liu et al. 2011; Li et al. 2007), weakens the East Asian summer monsoon (Dong et al. 2019; Jiang et al. 2013), and suppresses the rainfall over North China (Jiang et al. 2013). During the premonsoon season, local anthropogenic aerosol forcing is the largest over South China (Jiang et al. 2015), which leads to a significant drought there (Jiang et al. 2015; Kim et al. 2007; Hu and Liu 2013; Lee and Kim 2010). The local aerosol effects on East Asian winter climate were less investigated. Recent studies have demonstrated that anthropogenic aerosols tend to intensify East Asian winter monsoon circulation in North China (Jiang et al. 2017; Liu et al. 2019), but weaken it in South China (Lou et al. 2019; Zhuang et al. 2018).

Recently, several studies highlighted the impacts of nonlocal aerosol effects on East Asian climate, especially during summer (e.g., Wang et al. 2020, 2017). For example, it is found that non-Asian aerosols have great impacts on East Asian summer monsoon, and the impacts could be larger than local aerosols (Cowan and Cai 2011; Dong et al. 2016). It is also suggested that absorbing aerosols (dust and BC) over South Asia could perturb the East Asian summer monsoon precipitation (Lau et al. 2006; Lau and Kim 2006; Tang et al. 2018). During the premonsoon season, however, the impacts of nonlocal aerosols on East Asian climate are still not well recognized. Considering the continuous intensification of the premonsoon air pollution over South Asia, understanding how premonsoon aerosol forcing over South Asia affecting East Asian climate could be a first and essential step to unravel this issue.

In this study, the impacts of anthropogenic emissions over South Asia on East Asian spring climate are investigated with Community Earth System Model version 2.1.0 (CESM 2.1.0). A comparison with local aerosol effects of East Asia will be also made. Two possible pathways connecting South Asian aerosol forcing and East Asian premonsoon climate will be proposed, which are not revealed in previous studies. The paper is organized as follows. The model and data used in this study are described in section 2. The main findings are proposed in section 3. The conclusions and discussion are summarized in section 4.

2. Model and data

a. Model and experiments

CESM2.1.0 (Danabasoglu et al. 2020), developed by the National Center for Atmospheric Research (NCAR), is used for this study. Its atmospheric component, Community Atmosphere Model version 6 (CAM6), has several notable improvements compared to previous versions (Danabasoglu et al. 2020). A unified turbulence scheme, Cloud Layers Unified By Binormals (CLUBB), is included in CAM6, which unifies the description of cloudy turbulent layers by directly replacing the separate shallow-convection, boundary layer, and gridscale condensation schemes present in CAM5 (Bogenschutz et al. 2013). A new four-mode version of the modal aerosol module (MAM4) is included in CESM2, which significantly improves the simulated POM (particulate organic matter) and BC concentration of remote regions (Liu et al. 2016; Jiang et al. 2016). MAM4 predicts mass mixing ratios of different aerosol types and the number of mixing ratios for each mode. Aerosol species are assumed to be internally mixed within each mode and externally mixed among different modes. With MAM4, the model is able to simulate interactively aerosol–radiation interaction (ARI) processes. The cloud microphysics scheme of CAM6 has been updated to a new version (MG2; Gettelman and Morrison 2015), which is able to forecast, instead of diagnose, the mass and number concentration of falling condensed species (rain and snow). The aerosol–cloud interaction (ACI) is explicitly treated in CAM6, which includes the activation of the process for aerosol particles to affect cloud microphysical properties, and the process of slowing down the autoconversion of cloud water to rain. The land component of CESM2 is updated to Community Land Model version 5 (CLM5), which includes an extensive suite of new and updated processes and parameterizations (Lawrence et al. 2019).

In total four experiments (Table 1) are designed to investigate the respective impacts of anthropogenic emissions over East Asia and South Asia (Fig. 1). All experiments differ only in the aerosol and precursor gas emissions (Lamarque et al. 2010). The present-day (PD) experiment (control run) is run with aerosol and precursor gas emissions of year 2000. The PIEA run is the same as the PD run, but the aerosol emissions of the East Asia (EA) region are switched to year 1850 (preindustrial time; PI). The difference between PD and PIEA experiments (i.e., PD − PIEA) represents the impacts of anthropogenic emissions over East Asia (Fig. 1, red rectangle). Similarly, the PISA run is the same as the PD run, but the aerosol emissions of South Asia (SA) region are switched to year 1850. The impacts of anthropogenic emissions over South Asia (Fig. 1, blue rectangle) could be estimated by the difference between PD and PISA experiments (i.e., PD − PISA). A comparison between local (i.e., East Asian) and remote (i.e., South Asia) aerosol effects on East Asian climate could be made through these three experiments. The PD4xSABC run is the same as the PD run but the BC emission over South Asia is scaled by a factor of 4, considering the possible underestimation of BC emission over South Asia (Xu et al. 2016). It permits a stronger BC forcing over South Asia (compared to the PD run), which could further verify the role of BC forcing over South Asia.

Table 1

Numerical experiments and associated aerosol emissions in each experiment.

Table 1

All experiments are run in a horizontal resolution of 1.9° latitude × 2.5° longitude, and with 30 vertical levels. Each experiment is running for 21 years and only the last 20 years’ results are used for analysis. All experiments are run with prescribed monthly sea surface temperature (SST) and sea ice, for this study aims to investigate the fast response to the anthropogenic emissions over East Asia and South Asia. The slow response (mediated by changes in SST) to the aerosol forcing will be investigated in our future works.

b. Observational data

The monthly mean aerosol optical depth (AOD) and aerosol deposition fluxes from Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA2) are used for this study. The MERRA2 is the latest atmospheric reanalysis of the modern satellite era produced by NASA, with a high resolution (0.5° × 0.625°) available from 1980 to the present (Gelaro et al. 2017). In MERRA2, the aerosol products are assimilated from the Advanced Very High Resolution Radiometer (AVHRR) over the oceans, the Moderate Resolution Imaging Spectroradiometer (MODIS), the Multiangle Imagine SpectroRadiometer (MISR) over desert regions, and the ground-based Aerosol Robotic Network (AERONET).

The surface air temperature is from Climate Research Unit gridded Time Series (CRU TS) version 4.0 dataset, with a spatial resolution of 0.5° latitude × 0.5° longitude (Harris et al. 2020). The precipitation data used in this study is from Global Precipitation Climatology Project (GPCP), which combines estimates from surface and satellite measurements. The observational/reanalysis datasets are used to examine the observed long-term trend of aerosols over South Asia and climate over East Asia.

3. Results

a. Local versus remote aerosol effects

Figure 3 shows the spring aerosol optical depth (AOD) change induced by anthropogenic emissions over East Asia (Fig. 3a) and South Asia (Fig. 3b). As expected, the AOD changes are the largest over the source regions for both East Asian (Fig. 3a) and South Asian (Fig. 3b) emissions. The AOD change over East Asia is primarily due to local aerosol emissions (Fig. 3a, ∼0.18), while the emissions over South Asia increases the AOD over East Asia slightly by 0.04 (Fig. 3b). It is because the prevailing westerly wind over South Asia could transport the air pollutants to its downwind regions (e.g., Liang et al. 2019) and increase the AOD there. The response of East Asian spring climate to local (i.e., East Asian, Fig. 3a) and remote (i.e., South Asian, Fig. 3b) emitted aerosols will be examined in following paragraphs.

Fig. 3.
Fig. 3.

Spatial distribution of the spring [March–May (MAM)] aerosol optical depth (AOD) change induced by anthropogenic emissions over (a) East Asia and (b) South Asia. The AOD change induced by East (South) Asian emissions is estimated with the difference between PD and PIEA (PISA) simulations. The cross signs denote where the changes are statistically significant at the 0.1 level. The red and blue rectangles in (a) and (b) denote the areas of East Asia (10°–50°N, 100°–140°E) and South Asia (0°–40°N, 60°–100°E), respectively.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

Figure 4 demonstrates the response of East Asian spring climate to anthropogenic emissions over East Asia (i.e., locally emitted aerosols, Fig. 3a). The local aerosol forcing decreases the surface air temperature (SAT) of East Asia by up to 0.9 K (Fig. 4a). The maximum cooling is around 30°N (statistically significant near 105°E), which is consistent with the AOD change (Fig. 3a). The cooling is due to a decrease of downward solar flux at the surface (Fig. S1 in the online supplemental material, −12 W m−2), which is primarily induced by the aerosol indirect effects [i.e., aerosol–cloud interaction (ACI) −10 W m−2]. Both the cloud droplet number concentration (CDNC, by 80% compared to PIEA) and cloud liquid water path (LWP, by 20 g m−2) significantly increase over South China, which intensifies the shortwave cloud forcing (SWFC, more negative, −10 W m−2) and reduces the solar flux at surface. The precipitation is significantly decreased (by 1.2 mm day−1) over South China (25°N, Fig. 4b), which could be due to the slowing-down autoconversion of cloud water to rainwater by increased cloud condensation nuclei (CCN). The precipitation reduction could weaken the wet removal and further increase the AOD over East Asia (Fig. 3a). Associated with the rainfall decrease, an anticyclone anomaly is induced over the adjacent ocean regions of South China (Figs. 4b,d). The correlated southerly wind anomaly over South China (Fig. 4b) transports more water vapor to central East China (around 30°N), which acts to decrease the rainfall in South China but increases it near Yangtze River valley (Fig. 4b). Besides, the aerosol-induced downward motion (Fig. S2) tends to stabilize the lower troposphere and further suppresses the rainfall in South China. In the upper troposphere, the atmospheric response is the largest to the south of the rainfall anomaly (around 20°N), which is characterized as upper-level easterly anomaly near 20°N (Fig. 4c) and intensification of the western Pacific subtropical high (Fig. 4d). The upper-level tropospheric anomalies are mainly due to the anomalous atmospheric cooling and downward motion associated with the rainfall anomaly.

Fig. 4.
Fig. 4.

Spatial distribution of spring mean changes for (a) surface air temperature (K), (b) precipitation (mm day−1) and 850-hPa winds (vectors, m s−1), (c) 300-hPa zonal winds (m s−1) and (d) 500-hPa geopotential height (gpm) induced by anthropogenic emissions over East Asia. The changes are estimated with the difference between PD and PIEA simulations. The cross signs denote where the changes are statistically significant at the 0.1 level. The red contour in (a) denotes the position of Tibetan Plateau. The contours in (c) denote the climatological distribution 300-hPa zonal winds.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The responses of East Asian spring climate to anthropogenic emissions over South Asia (i.e., remote emitted aerosols, Fig. 3b) are shown in Fig. 5. Surprisingly, the SAT significantly decreases over northern East Asia (Fig. 5a) where the AOD change (Fig. 3b) is very small (less than 0.01). The precipitation significantly decreases (by 1.0 mm day−1) over South China (Fig. 5b) and the change is comparable to that induced by the local aerosol emissions (Fig. 4b). The upper-troposphere response to the South Asian aerosol forcing could be even larger (Figs. 5c,d). The upper-level zonal wind intensifies near 40°N (Fig. 5c), which indicates an acceleration of the midlatitude jet stream. A deceleration of zonal wind is located over high-latitude regions (60°N) and western Pacific Ocean (20°N). In the middle troposphere, negative geopotential height anomaly is found around 50°N, implying a deepening of the trough over northern East Asia (Fig. 5d). In short, aerosols from remote South Asian have significant impacts on both northern and southern East Asian climate during the premonsoon season, and the effects are comparable to locally emitted aerosols. How remote South Asian aerosol forcing affects the East Asian premonsoon climate still remains unclear, but it will be analyzed in following sections.

Fig. 5.
Fig. 5.

As in Fig. 4, but for the changes induced by anthropogenic emissions over South Asia. The changes are estimated with the difference between PD and PISA simulations.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

b. Role of BC snow darkening effects

In this section, how aerosols emitted from South Asia (Fig. 3b) affecting northern East Asian climate will be analyzed. As shown in Fig. 5a, the aerosols from South Asia also increase the SAT over the TP by up to 1.5 K, and the maximum warming is over western and southern TP (Fig. 6a). The surface warming is associated with the positive radiative forcing over the TP (up to 15 W m−2), which is primarily induced by the surface albedo change (SAC, ∼10 W m−2) and aerosol–radiation interaction (ARI, ∼2 W m−2). The surface solar flux is also significantly increased over the TP (Fig. 6b, by up to 15 W m−2), which is associated with the increase of black carbon in snow (BCS, Fig. 6c). The vertically integrated BC concentration (BC burden) is significantly increased over South Asia (Fig. S3) due to intense human activities there. The increase could be even larger over the south slope of the TP, and the largest increase is near Bay of Bengal (90°E). It is because large amount of BC over South Asia is transported by prevailing westerly and southwesterly winds from source regions to the TP, and accumulates over its south slope. A small part of BC could be transported to the inner part of the TP, deposited on snow, and then increase the BCS there (Fig. 6c).

Fig. 6.
Fig. 6.

Spring mean changes for the (a) surface air temperature (K), (b) clear-sky surface solar flux (W m−2), (c) BC concentration in snow column (μg kg−1), and (d) surface albedo induced by anthropogenic emissions over South Asia. The changes are estimated with the difference between PD and PISA simulations. The cross signs denote where the changes are statistically significant at the 0.1 level. The red contours denote the position of Tibetan Plateau.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The BCS concentration over the TP regions simulated by CESM2 is 25% larger than that by CESM1, which is in general consistent with the observation in magnitude (10–100 μg kg−1, Ming et al. 2009). The BCS change (Fig. 6c) is also the largest (∼25 μg kg−1, by 50% compared to PISA simulations) over the southern TP, for it is directly exposed to South Asian emissions (Fig. 1). Associated with the increase of BCS over the TP, the snow albedo is decreased by 0.05 (darkening) over the western and southern TP (Fig. 6d). The reduced snow albedo could explain the increase of solar radiation and SAT over the TP. Associated with the TP warming, the snow water equivalent (SWE) is decreased over the TP (Fig. S4), implying an acceleration of snow melting. The decrease of SWE could further reduce the surface albedo, cause the land surface to absorb more solar radiation, and amplify the surface warming (i.e., snow albedo feedback, Qian et al. 2011; Yasunari et al. 2015).

The warming over the TP could alter the thermal structure of the surrounding regions and further affect the climate of its downstream regions (e.g., Yao et al. 2019). Associated with TP warming, the meridional temperature gradient increases (decreases) to the north (south) of the Tibetan Plateau. As a result, the upper-level zonal wind tends to accelerate (decelerate) to the north (south) of the TP according to the thermal wind relation. The acceleration of the jet stream around 40°N (north of TP) is much stronger, with the acceleration extending to downwind regions (Fig. 5c). An increase of the thermal gradient over midlatitude regions could affect the atmospheric baroclinicity and synoptic transient activities, which could further accelerate the midlatitude jet stream (e.g., Lau and Holopainen 1984; Fang and Yang 2016). The low-level maximum Eady growth rate is shown in Fig. 7a to measure the atmospheric baroclinicity change. The Eady growth rate is increased by 0.2–0.8 day−1 around 40°N, which favors more transient eddy activities and accelerates the eddy-driven jet (i.e., transient eddy–mean flow feedback, Fang et al. 2022). It could explain why the zonal wind acceleration north of the TP is stronger than the deceleration south of the TP (Fig. 5c). An anomalous equivalent barotropic low is observed around 60°N (Fig. 5d), which could be due to the positive transient-eddy vorticity forcing associated with the increased low-level baroclinicity (Fang and Yang 2016). In the upper troposphere, anomalous low exhibits as a deepened trough over northern East Asia (Fig. 5d), which favors the cold-air advection over northern East Asia (Fig. 7b), and induces a significant cooling there (Fig. 5a).

Fig. 7.
Fig. 7.

Spatial distribution of spring mean changes for (a) 850-hPa maximum Eady growth rate (day−1) and (b) surface winds (vectors, m s−1) induced by anthropogenic emissions over South Asia. The changes are estimated with the difference between PD and PISA simulations. The cross signs in (a) and the shading in (b) denote where the changes are statistically significant at the 0.1 level, respectively.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The jet stream acceleration is even larger over the downstream regions, and the acceleration is the largest around the Korean Peninsula (125°E, Fig. 5c). Associated with the strong jet acceleration, a rising motion is found around 30°N of China (Fig. S5), implying a secondary circulation anomaly (Kim et al. 2007). The westerly acceleration requires a poleward ageostrophic wind component (40°N, Fig. S5) and induces a divergence around 30°N. The upper-level divergence produces a rising motion and intensifies the rainfall over Yangtze River Valley. Increased rainfall releases more latent heat (warming), intensifies the meridional temperature gradient, and further accelerates the jet stream near 125°E. Considering the underestimation of BC emission over South Asia (Xu et al. 2016), we would further verify the impacts of BC snow darkening effects (SDE) on East Asian climate with an additional BC simulation in section 3d.

c. Role of “elevated heat pump hypothesis”

How aerosols emitted from South Asia (Fig. 3b) affect the southern East Asian climate will be discussed in this section. The dominant feature of the climate change over the southern Asian continent (south of 30°N) is characterized as an increase of precipitation near the Bay of Bengal and a decrease of rainfall over South China (Fig. 5b). The rainfall anomaly over South Asia could be associated with aerosol-induced thermal structure change around the Tibetan Plateau, as suggested by previous studies (e.g., Lau et al. 2006; Rahimi et al. 2019). The solar heating rate change over the south slope of the TP is shown in Fig. 8a to explore its thermal structure change. The solar heating increase is the largest (0.2 K day−1) in the lower troposphere, which is induced by the BC accumulated over the south slope of the TP. The accumulated BC amplifies the warming induced by SDE, decreases the low-level atmospheric stability, and induces an anomalous ascending motion around 25°N (Fig. 8b). The anomalous ascent draws warmer and moister air from the Indian Ocean and intensifies the convection near the Bay of Bengal (Fig. 5b). The intensified convection releases more latent heat and further amplifies the warming over the south slope of the TP. This positive feedback process is consistent with the elevated heat pump hypothesis, which is first proposed by Lau et al. (2006).

Fig. 8.
Fig. 8.

Latitude–altitude distribution of spring mean changes for (a) clear-sky solar heating rate (K day−1) and (b) meridional circulation (vectors), together with vertical pressure velocity (shaded, unit: 1000 Pa s−1) averaged in the longitudinal band 80°–100°E induced by anthropogenic emissions over South Asia. The changes are estimated with the difference between PD and PISA simulations. The cross signs denote where the changes are statistically significant at the 0.1 level.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The tropical zonal circulation change is analyzed to understand how the anomalous ascending near the Bay of Bengal affecting the climate of downstream regions. An anomalous zonal overturning circulation is observed between South Asia and East Asia, which is characterized as an ascending motion around 90°E and a descending motion near 110°E (Fig. 9a). In the upper troposphere, a positive (negative) velocity potential anomaly (Fig. 9b) is located over the Bay of Bengal (South China). Corresponding to the velocity potential change, anomalous divergent westerlies are observed around 20°N (Fig. 9b). The divergent westerlies produce convergence over its downstream regions and further induce a descending motion there (Fig. 9a). In the lower troposphere, a correlated convergent easterly wind is forced around 100°E (Fig. 9a), which weakens the zonal water vapor transportation to southern East Asia (Fig. 9a). Both descending motion and decreased water vapor tend to suppress the rainfall over South China, which could explain the significant drought there (Fig. 5b). Considering the underestimation of the BC absorption over South Asia (Xu et al. 2016), we further examine the impacts of the elevated heat pump hypothesis on East Asian climate with a sensitive BC experiment in the next section.

Fig. 9.
Fig. 9.

(a) Longitude–altitude distribution of spring mean changes for vertical pressure velocity (shaded, in 1000 Pa s−1) and zonal circulation (vectors) averaged in the latitudinal band 15°–30°N induced by anthropogenic emissions over South Asia. The cross signs denote where the changes are statistically significant at the 0.1 level. (b) Spatial distribution of spring mean changes for 300-hPa velocity potential (shaded, m s−1) and divergent winds (vectors, m s−1) induced by anthropogenic emissions over South Asia. The changes are estimated with the difference between PD and PISA simulations.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

d. Sensitive BC experiments

In this section, we further verify the role of BC over South Asia on East Asian climate with an additional sensitivity simulation. The PD4xSABC run is the same as the PD run but the BC emission over South Asia is scaled by a factor of 4 to better meet the observed BC absorption (Xu et al. 2016). The responses to anthropogenic emissions over South Asia (scaled BC) are shown in Fig. 10. With the scaled BC, the SAT increase over TP could be up to 3 K (Fig. 10a) and the warming extends to the eastern TP (Fig. S6a). The stronger warming could be explained by the larger radiative forcing over TP (up to 18 W m−2, Fig. S6b), confirming the dominant role of BC forcing in the TP warming. Consistently, both the BCS and snow albedo changes (Figs. S6c,d) are larger with the scaled BC emission, implying an increase of BC could induce stronger SDE over the TP. The midlatitude jet stream acceleration (Fig. 10c) and anomalous low (Fig. 10d) in the upper troposphere are also stronger, which could explain the greater surface cooling over northern East Asia (Fig. 10a). The responses in the scaled BC simulation (i.e., PD4xSABC run) further confirm that BC from South Asia could significantly affect the northern East Asian climate with the snow darkening effects over the TP.

Fig. 10.
Fig. 10.

Spatial distribution of spring mean changes for (a) surface air temperature (K), (b) precipitation (mm day−1) and 850-hPa winds (vectors, m s−1), (c) 300-hPa zonal winds (m s−1) and (d) 500-hPa geopotential height (gpm) induced by anthropogenic emissions over South Asia (4XBC). The changes are estimated with the difference between PD4xSABC and PISA simulations. The cross signs denote where the changes are statistically significant at the 0.1 level. The red contour in (a) denotes the position of Tibetan Plateau. The contours in (c) denote the climatological distribution 300-hPa zonal winds.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

Over the southern Asian continent, the solar heating rate change over the south slope of the TP (Fig. S7a) is larger in the PD4xSABC simulation as expected. Stronger anomalous ascending motion (Fig. S7b) and a rainfall increase (Fig. 10b) are found near the Bay of Bengal, implying stronger elevated heat pump effects with more BC aerosols. Accordingly, the compensating anomalous downward motion extends to downwind areas of South China and suppresses the rainfall there (Fig. 10b). Such responses further confirm that BC absorption over South Asia could perturb the rainfall over downstream regions (i.e., southern East Asia) through elevated heat pump effects.

e. Observed long-term trend

Our model results suggest that anthropogenic aerosols over South Asia have significant impacts on East Asian climate. In this section, the observed long-term trend of South Asian aerosols and East Asian climate are analyzed as a first step to verify such impacts. The air pollution over South Asia has continued an increasing trend during recent decades (Fig. 2a). Consistently, the BC over South Asia significantly increases since 1980 (Fig. 11a), which is due to the increase of anthropogenic BC emissions over India (Rana et al. 2019). More BC aerosols are transported to Tibetan Plateau and deposit on the snow of the TP (Zhang et al. 2015; Li et al. 2020), which increases the BC deposition over the TP region (Fig. 11b).

Fig. 11.
Fig. 11.

The year-to-year variations of (a) aerosol optical depth (AOD) of BC over South Asia, (b) total BC deposition (wet and dry, in 10−13 kg m−2 s−1) over the Tibetan Plateau (26°–40°N, 73°–105°E), (c) SAT over the Tibetan Plateau (K), (d) SAT over Northern East Asia (40°–60°N, 110°–140°E, in K), (e) precipitation over the Bay of Bengal (15°–30°N, 80°–100°E, in mm day−1), and (f) precipitation over South China (15°–30°N, 100°–120°E, in mm day−1) anomalies (relative to the 1980–2019 mean) during spring, together with their corresponding linear trends (black dashed lines). The linear trends in (c) and (d) are calculated for the periods between 1989 and 2013, and the others are calculated between 1980 and 2019. The AOD and aerosol deposition fluxes are from MERRA2. The SAT is from the Climate Research Unit gridded Time Series (CRU TS) version 4.0 dataset. The precipitation is from the Global Precipitation Climatology Project (GPCP) dataset.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The surface air temperature over the TP has significantly increased since 1980 (Fig. 11c, 0.4 K decade−1), which is primarily due to anthropogenic greenhouse gas emissions (You et al. 2021). The warming trend is the largest between 1989 and 2013 (0.59 K decade−1), which could be associated with BC-induced snow-cover retreat over the TP (Xu et al. 2016). Associated with the accelerated warming over the TP, a cooling over northern East Asia (−0.2 K decade−1) is observed during this period (Fig. 11d). In the upper troposphere, an acceleration of the midlatitude jet stream and an anomalous low over northern East Asia are also observed (figure not shown). Such an anomalous pattern favors cold-air advection over northern East Asia, which could partly explain the cooling there. During the whole period (1980–2019), the SAT over northern East Asia still exhibits a warming, implying the global warming signal still could be dominant.

We further examine the precipitation anomalies associated with the BC increase over South Asia. Both an intensified low-level solar heating and a tropospheric warming over the south slope of the TP are also observed as expected (Gautam et al. 2009). The precipitation has increased near the Bay of Bengal (0.1 mm day−1 decade−1) since 1980 (Fig. 11e), which is consistent with the elevated heat pump hypothesis. Correspondingly, a decrease of rainfall in South China (−0.1 mm day−1 decade−1; Fig. 11f) is observed. The rainfall anomalies over East Asia generally support our model results.

4. Conclusions and discussion

In this study, the impacts of anthropogenic aerosols from South Asia on East Asian spring climate are investigated with CESM2, and a comparison with local aerosol effects of East Asia is also made. The AOD change in East Asia is primarily due to local emissions (∼0.18), and the change due to South Asian anthropogenic emissions (∼0.04) is much smaller. The response of East Asian climate to remote South Asian emissions, however, is comparable to or even larger than that due to local aerosol emissions.

The response of East Asian spring climate to local anthropogenic emissions is exhibited as cooling around the Yangtze River valley (30°N) and as drought over South China, which is primarily due to aerosol–cloud interaction (ACI). The aerosol indirect effect intensifies shortwave cloud forcing (SWFC, more negative) and reduces the surface solar flux, which could explain the cooling near 30°N. Besides, the aerosol–cloud interaction (ACI) slows down the autoconversion of cloud water to rainwater by increasing the CCN over East Asia, which suppresses the rainfall in South China.

The response of premonsoon East Asian climate to anthropogenic emissions over South Asia is characterized as cooling over northern East Asia and as drought over South China, which is comparable to the change due to local emissions. Two possible dynamical pathways bridging South Asian pollutants and East Asian spring climate are proposed (Fig. 12), and the northern (southern) pathway mainly affects the climate over northern (southern) East Asia. The role of anthropogenic BC forcing and associated climate feedbacks around the Tibetan Plateau are emphasized for both northern and southern pathways.

Fig. 12.
Fig. 12.

Schematic diagram for the impacts of anthropogenic aerosols over South Asia on East Asian climate during spring. Impacts are shown for (a) northern and (b) southern East Asian climate.

Citation: Journal of Climate 36, 10; 10.1175/JCLI-D-22-0049.1

The northern pathway (Fig. 12a) is associated with the BC snow darkening effect, which leads to a significant cooling over northern East Asia. BC is transported from South Asia to the TP and increases the BCS concentration over the TP. The increased BCS decreases the snow albedo and increases the surface solar flux, which increases the SAT over the TP. The TP warming accelerates the snow melting and further increases the SAT through a positive snow albedo feedback. The warming increases the meridional temperature gradient and low-level atmospheric baroclinicity to the north of the TP, which favors an acceleration of the midlatitude jet stream. An anomalous equivalent barotropic low is forced to the north of the jet stream (60°N) through a transient eddy–mean flow feedback. The deepened trough in the upper troposphere favors cold air-advection over northern East Asia and induces a significant cooling there.

The southern pathway (Fig. 12b) is associated with the BC’s “elevated heat pump” hypothesis, which mainly affects the rainfall over southern East Asia. BC from South Asia induces a warming over the south slope of the TP through both ARI and SDE and results in an anomalous ascending there. The anomalous ascending motion intensifies the convection near the Bay of Bengal, releases more latent heat, and further amplifies the warming over the south slope of the TP (i.e., elevated heat pump effects). An anomalous zonal overturning circulation is then forced between South Asia and East Asia, which is characterized as ascending motion near the Bay of Bengal and descending motion near South China. The descending motion suppresses the rainfall over South China, which could explain the drought there.

A primary observational analysis is performed to verify both dynamical pathways. The BC over South Asia significantly has increased since 1980, and more BC has been transported to Tibetan Plateau and has been deposited on the snow of the TP. An accelerated warming over the TP is observed between 1989 and 2013, which could be associated with BC-induced snow cover retreat. Associated with the accelerated warming over the TP during this period, a cooling over northern East Asia is observed. An acceleration of the jet stream and an anomalous low is found in the upper troposphere of northern East Asia, implying the accelerated TP warming could be a possible cause of the cooling over northern East Asia. Associated with the BC increase, an anomalous low-level tropospheric warming over the south slope of the TP is observed. The rainfall has increased near Bay of Bengal, which is consistent with the elevated heat pump hypothesis. Correspondingly, the rainfall has significantly decreased over South China, which is consistent with our model results.

The main findings of this study favor the climate prediction under a background of Third Pole warming (You et al. 2021). Both the increased greenhouse gases and absorbing aerosols could induce a warming over the TP, and the warming could be amplified by a positive snow albedo feedback. Such warming over the TP could affect the downstream East Asian spring climate through two possible dynamical pathways (i.e., the northern and the southern pathways) revealed in our study. Besides, two positive feedbacks (i.e., the midlatitude transient eddy–mean flow feedback and the elevated heat pump hypothesis) associated with TP warming also need be considered in the projection of East Asian future climate.

Acknowledgments.

This work is jointly supported by the National Key Research and Development Program of China under Grants 2022YFE0106600 and 2018YFC1506001, the National Natural Science Foundation of China (NSFC) under Grants 42175022 and 41621005, and the Jiangsu Collaborative Innovation Center of Climate Change. All the numerical experiments have been run on the computing facilities in the High Performance Computing Center (HPCC) of Nanjing University.

Data availability statement.

MERRA2 data are available at https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/. GPCP Precipitation data are available at https://psl.noaa.gov/data/gridded/data.gpcp.html/. The Climate Research Unit gridded Time Series (CRU TS) version 4.0 dataset is available at https://crudata.uea.ac.uk/cru/data/hrg/.

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