2. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044;
3. Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing 100081
There has been a rapid increase in the emission of anthropogenic aerosols—such as sulfate, organic carbon (OC), and black carbon (BC)—from the combustion of fossil fuels and biomass during recent decades. Aerosols can affect the global and regional radiation budget and the hydrological cycle (IPCC, 2013) by the direct absorption and scattering of solar radiation (Haywood and Ramaswamy, 1998), by heating the cloud layers (Koch and Del Genio, 2010), and by modifying cloud microphysics (e.g., the cloud albedo and lifetime) (Twomey, 1977; Albrecht, 1989).
The East Asian summer monsoon (EASM) is substantially induced by the sea–land thermal contrast. Changes in the EASM have important effects on the weather and climate of East Asia (Ding and Chan, 2005). The EASM is influenced by a range of elements, including variations in the sea surface temperature over the Pacific and Indian oceans (Li et al., 2010; Qian and Zhou, 2014), snow cover on the Tibetan Plateau (Ding and Chan, 2005), land use (Fu, 2003; Xu et al., 2006), and the emission of greenhouse gases and atmospheric aerosols (Li et al., 2010). East Asia, particularly eastern China, is one of the most polluted areas on earth. The interaction between the East Asian monsoon and anthropogenic aerosols is an important topic in research on the atmospheric environment and climate change (Li et al., 2016; Wu et al., 2016). The simulations reported by Liu et al. (2011) indicated that the radiative effects of aerosols reduced the surface sea–land thermal contrast in East Asia, weakening the circulation of the EASM and, in turn, reducing precipitation. Similar results were obtained by Jiang et al. (2013) based on the Community Atmospheric Model version 5. Zhang et al. (2012) used an online aerosol–climate model to show that the direct radiative effects of total anthropogenic sulfate, OC, and BC weakened the EASM and reduced precipitation. Multi-model ensemble simulations also indicated that aerosol forcing had an important impact on the weakening of the EASM during the period 1958–2001 (Salzmann et al., 2014; Song et al., 2014).
Despite much previous research, there is still some disagreement about how aerosols affect the monsoon system in East Asia. Two main factors contribute to the argument. The first factor is the complex chemical composition of aerosols, which affects their optical scattering and absorption properties. Sulfate and OC scatter so-lar radiation, whereas BC strongly absorbs solar radiation (Stocker, 2013). The effects of various types of aerosol on the regional climate may be significantly different. Lau et al. (2006) found that the absorbing aerosols on the north and south sides of the Tibetan Plateau absorbed significant amounts of solar radiation and heated the atmosphere over these regions, enhancing the southwesterly flow of air and rainfall in the Bay of Bengal from late May to early June. Guo et al. (2013) suggested that an increase in BC emission had a weaker impact on the circulation of the EASM and precipitation than an increase in sulfate aerosol. Jiang et al. (2013) reported that anthropogenic sulfate and OC aerosols reduced the surface temperature and suppressed precipitation in northern China by the direct scattering of solar radiation and by indirect effects on cloud microphysics. However, BC aerosol strengthened the southwesterly winds in South China and caused stronger convection between 25° and 30°N. Guo et al. (2016) claimed that the effect of changes in sulfur dioxide emissions on precipitation during the Indian summer monsoon were stronger than those of BC and led to a reduction in precipitation, particularly in northern India. However, the changes induced by BC aerosol were not significant until its emission was increased by five times, resulting in an increase in precipitation in northern India.
The second factor is the slow feedback of the ocean to aerosol forcing. On much longer timescales, the response of the monsoon to aerosol forcing consists of two components: (1) the fast adjustment of the atmosphere and land surface and (2) the slow feedback of the ocean. Many previous studies have only considered the impact of the fast aerosol-induced response on the EASM. However, Ganguly et al. (2012a) investigated the response of the South Asian monsoon system to aerosol forcing and found that slow ocean feedback might play a more important part in aerosol–monsoon interactions. We therefore investigated how the EASM system responds to changes in various anthropogenic aerosol species when taking these two factors into account.
We investigated the effect of changes in the emission of three major anthropogenic aerosols (scattering sulfate, OC aerosol, and absorbing BC aerosol) on the EASM since the start of the industrial era in 1850 to the year 2000 using an online global aerosol–climate model incorporating a slab ocean model. We determined which type of aerosol forcing had the greatest impact on the EASM. Unlike previous studies (e.g., Liu et al., 2011; Zhang et al., 2012; Jiang et al., 2013; Wang T. J. et al., 2015; Guo et al., 2016), this work simultaneously considered the impacts of the total direct, semi-direct, and indirect effects of aerosols and the feedback of the ocean to aerosol forcing on the EASM. This work will help us better understand the mechanism of the impacts of aerosols on the EASM.
The model and simulations are introduced in Section 2. The response of the EASM to changes in the emission of both scattering and absorbing aerosols and the nonlinearity of the climate response to different components of the aerosol forcing are reported in Section 3. Section 4 summarizes and discusses the results. The East Asian monsoon region (EAMR) is defined as the region of 0°–50°N, 100°–140°E. The summer months are defined as June, July, and August.2 Methodology 2.1 Model
We used the Atmospheric General Circulation Model of the Beijing Climate Center (BCC_AGCM2.0.1) coupled with the China Meteorological Administration’s Unified Atmospheric Chemistry Environment for Aerosols (CUACE/Aero) model (BCC_AGCM2.0.1_CUACE/Aero). The model was developed by Zhang et al. (2012, 2014) and Wang et al. (2014, 2016). The model includes the direct, semi-direct, and indirect effects of aerosols on water clouds. The horizontal resolution of BCC_AGCM2.0.1 is about 2.8° latitude × 2.8° longitude (T42) and there are 26 levels in the vertical direction, topping out at 2.9 hPa (Wu et al., 2010). The model also includes the Monte Carlo independent column approximation cloud overlap scheme, the new BCC radiative transfer model (Zhang et al., 2014), and a two-moment bulk cloud microphysical parameterization incorporating a physics-based aerosol activation scheme developed by Morrison and Gettelman (2008). CUACE/Aero is a size-separated, multi-component aerosol module including the processes of emission, transport, deposition, chemical transformation, and interactions of atmospheric aerosols with clouds (Zhou et al., 2012). All the aerosols are assumed to be mixed externally. The size spectrum of each aerosol is partitioned into 12 bins as a geometric series for a range of radii from 0.005 to 20.48 μm. The optical properties of the aerosols are calculated on the basis of Mie theory (Wei and Zhang, 2011; Zhang et al., 2012). A slab ocean model is coupled with BCC_AGCM2.0.1_CUACE/Aero to take the exchange of energy between the ocean and the atmosphere into account.
The BCC_AGCM2.0.1_CUACE/Aero has been used previously to evaluate the radiative forcing of aerosols and their resulting effects on the global and regional climate (Zhang et al., 2012, 2016; Wang et al., 2013a, b, 2015, 2016; Zhao et al., 2015; Wang et al., 2017). The model gives a good simulation of the spatial distribution of the aerosol optical depth, especially over regions affected by anthropogenic aerosols. However, it underestimates the aerosol optical depth over eastern China, South Asia, and the tropical oceans of the Northern Hemisphere, possibly as a result of the absence of secondary organic, nitrate, and ammonium aerosols (Wang et al., 2014). The model also gives a good simulation of the geographical patterns of summer winds and precipitation over the Asian monsoon regions (Wang et al., 2017).2.2 Experimental design
We performed five simulations (Table 1) to explore the equilibrium response of climate to changes in aerosol emission, and all other parameters were kept constant in the simulations. In the first simulation (PI), all aerosol emissions were kept at the year 1850 levels and in simulations 2–4, global sulfate, OC, and BC emissions were increased instantaneously from 1850 to 2000 levels (simulations PDSF, PDOC, and PDBC, respectively). In the fifth simulation (PDBC × 5), the global BC emission was scaled up by five times the 2000 levels, while the other aerosol species were kept at 1850 levels. Earlier work has suggested that the levels of emission of BC used in current climate models could be considerably underestimated (Cohen and Wang, 2014; Xu and Xie, 2015). The AeroCom Phase II multi-model comparison indicates that BCC_AGCM2.0.1_CUACE/Aero has smaller radiative forcing by BC than other models (Myhre et al., 2013). This may lead to weaker climate responses to forcing by BC in our model. Therefore, similar to Guo et al. (2016), we performed an additional PDBC × 5 experiment to obtain a stronger climate response to forcing by BC. All the anthropogenic aerosol emissions were from Lamarque et al. (2010). The emissions of natural aerosols, such as sea salt and soil dust, were calculated online in the model. The greenhouse gas concentrations in all simulations were kept at year 2000 levels. All the simulations were run for 80 yr and the last 50 years were used for analysis. The differences between the PDSF, PDOC, PDBC, and PDBC × 5 simulations and the PI simulation represent the effects of changes in sulfate, OC, BC, and PDBC × 5 (hereafter abbreviated as 5 × BC) aerosols, respectively.
|Experiment||Sulfate||Black carbon||Organic carbon|
|PDBC × 5||1850||2000 × 5||1850|
Figure 1 shows the global spatial distributions of the simulated changes in the column burdens of anthropogenic aerosols from 1850 to 2000. These changes are consistent with the changes in the emissions of these aerosols over the same time period. Increases in sulfate primarily appear in mid–high latitudes of the Northern Hemisphere, particularly over eastern North America, western Europe, South Africa, eastern China, and Southeast Asia, with peak values > 18 mg m –2 (Fig. 1a). The burdens of OC have increased markedly over central and eastern China, Southeast Asia, South America, and central Africa, with the increase generally > 2 mg m –2 (Fig. 1b). The distribution of the change in the column burden of BC is generally consistent with that of OC, but with a smaller magnitude (Fig. 1c). The simulated concentration of BC increases when the emission of BC is scaled up by five times, although there is no obvious change in its distribution (Fig. 1d). The simulated global annual mean loadings of sulfate, OC, and BC are increased by 2.9, 0.47, and 0.09 mg m–2, respectively, in 2000 relative to 1850. These results are close to those reported previously by Lamarque et al. (2010) and Ganguly et al. (2012b).3.2 Effect of increased levels of scattering aerosols (sulfate and OC) on the EASM
Aerosols of sulfate and OC can affect the net shortwave radiation flux at the earth’s surface either directly by scattering solar radiation or indirectly by changing the microphysical properties of clouds. Figure 2 shows the effects of changes in two types of scattering aerosol on the summer mean concentration of surface cloud condensation nuclei and the net all-sky solar flux over the EAMR. The greatest reduction in the net all-sky solar flux at the earth’s surface due to the increase in sulfate (Fig. 2b) is > 5 W m –2 to the north of 20°N. An increase in sulfate (Fig. 1a) can scatter solar radiation, thus reducing the solar flux arriving at the earth’s surface, and highly hygroscopic sulfates can be activated into cloud droplets by acting as cloud condensation nuclei. An increase in sulfate concentration (Fig. 2a) can increase the cloud droplet number concentration, leading to a pronounced increase in cloud liquid water path and albedo. However, the surface net solar flux is increased south of 20°N over most of the EAMR (Fig. 2b). This is mainly caused by the reduction in low cloud cover (Fig. 3a) as a result of the marked vertical descending motion (see Fig. 5a) and weakening of the moisture transport over these regions. The surface net all-sky solar flux is decreased in most of the EAMR due to the increase in OC, with significant reductions in North China and the surrounding oceans (maximum up to –8 W m–2), whereas the surface net all-sky solar flux is slightly increased over central and northeastern China (Fig. 2d). This is closely related to changes in the fraction of cloud cover (Fig. 3d). The effect of sulfate on clouds and the radiation flux are significantly larger than the effects of OC. This is because the increased sulfate emissions result in larger increases in the aerosol optical depth and the cloud droplet number concentration due to the higher activation efficiency of sulfate (Table 2; Figs. 2a, c). The increase in sulfate and OC decreases the surface net all-sky solar fluxes averaged over the EAMR in summer by 3.6 and 2.3 W m–2, respectively (Table 2). These changes in the solar flux reflect not only the aerosol forcing, but also the feedback and response of the climate system to these forcing, which can be very different from the diagnostic forcing.
|PI||PDSF–PI||PDOC–PI||PDBC–PI||5 × PDBC–PI|
|Aerosol optical depth at 550 nm||0.06||0.08||0.02||0.004||0.013|
|Column cloud droplet number concentration (1010 m–2)||4.3||1.6||0.32||0.04||0.02|
|Liquid water path (g m–2)||115.2||10.7||4.0||1.0||1.7|
|FSNT (W m–2)||303.1||–4.8||–1.9||–0.5||0.2|
|FSNTC (W m–2)||386.2||–4.4||–1.2||0.04||0.9|
|FSNS (W m–2)||203.1||–3.6||–2.3||–1.8||–6.9|
|FSNSC (W m–2)||289.5||–2.8||–1.5||–1.4||–7.3|
|Surface temperature (K)||299.2||–1.9||–0.2||0.01||0.4|
|Surface pressure (hPa)||970.1||0.34||0.12||–0.03||0.04|
|V850 (m s–1)||1.8||–0.2||0.03||0.01||0.08|
|Precipitation rate (mm day–1)||7.6||–0.72||–0.04||0.07||0.17|
|CLDtot is total cloud cover; FSNT and FSNTC are the net solar flux at the top of the atmosphere under all-sky and clear-sky conditions, respectively; FSNS and FSNSC are the net solar flux at the surface under all-sky and clear-sky conditions, respectively; and V850 is the 850-hPa meridional wind.|
The zonally averaged cloud cover in summer decreases significantly in the troposphere between 10° and 20°N and increases over most of the rest of the study area due to increased concentrations of sulfate (Fig. 3a). The change in cloud cover caused by the increase in OC is almost opposite to that caused by the increase in sulfate (Fig. 3b). The change in cloud cover is primarily due to the change in relative humidity (Wang et al., 2017), which is determined by changes in atmospheric temperature and the transport of moisture. For example, the surface cooling between 10° and 20°N induced by sulfate forcing (Fig. 4a) decreases evaporation at the surface. The anomalous descending motion (Fig. 5a) further suppresses the transport of moisture. The relative humidity of the atmosphere (figure omitted) and the cloud co-ver both decrease. The reduction in cloud cover results, in turn, in an increase in the surface radiation flux (Fig. 2b).
The changes in aerosol emissions cause changes in the radiation field, which, in turn, alter the thermal and dynamic structures. Different degrees of surface cooling are seen throughout the EAMR as a result of the increase in the two types of scattering aerosol, but there are some differences in their distributions (Fig. 4). The surface cooling induced by the increase in sulfate aerosols increases markedly with latitude and is, in general, more than –2 K north of 20°N (Fig. 4a). The surface temperature is reduced between 10° and 20°N, while the surface net all-sky solar flux increases (Fig. 2b). This is due to the substantial weakening of the meridional transport of water and heat over the region caused by aerosol forcing (Wang et al., 2017). Significant cooling over the EAMR due to the increase in OC aerosol is seen in southern China and the Sea of Japan, with a maximum cooling of up to –0.7 K (Fig. 4c). This is generally in agreement with the induced changes in the surface net all-sky solar flux (Fig. 2d).
The anomalies in the vertical descending motion induced by cooling at the earth’s surface due to the increase in sulfate aerosol leads to an increase in surface pressure, mainly in southern China (Fig. 4b). The changes in surface temperature and pressure caused by the increase in sulfate emission are smaller over oceans than over land in the EAMR, leading to a decrease in the surface sea–land thermal contrast (Figs. 4a, b) and marked northwesterly and northerly wind anomalies in southern and eastern China and the adjacent oceans (Fig. 4b). The sea–land thermal contrast in the troposphere also decreases (Figs. 5b, c). All these changes indicate an obvious weakening of the EASM circulation. However, an anomalous cooling center is produced and an anomalous anticyclone is formed over the Sea of Japan and Northwest (NW) Pacific as a result of the change in OC emission (Fig. 4d). These lead to pronounced southerly and southeasterly wind anomalies to the west of the anticyclonic center, thereby strengthening the northern EASM circulation (Figs. 4c, d). The reduction in the sea–land thermal contrast at the surface (Figs. 4c, d) and in the troposphere (Figs. 5e, f) over southern China and the northerly and northeasterly wind anomalies due to the increase in OC aerosol indicate weakening of the southern EASM circulation (Figs. 4c, d). These results are in contrast with those reported by Jiang et al. (2013), who only considered the fast response of climate to forcing by OC. Jiang et al. (2013) suggested that significant northeasterly and southwesterly wind anomalies appeared north and south of 30°N, respectively, due to the increase in OC aerosol, weakening the EASM. The forcing by OC aerosol has a much smaller impact on the surface temperature, pressure, and 850-hPa winds than forcing by sulfate aerosols (Table 2). Figure 5 shows that forcing by sulfate and OC aerosols weakens the EASM primarily through decreasing the meridional sea–land thermal contrast.
Figure 5 shows the changes in the zonal-mean meridional circulation and atmospheric temperature in the EAMR in summer due to the changes in sulfate and OC aerosols. The increase in both sulfate and OC aerosols cools the troposphere in summer, but the degrees and regions of cooling are different for the two types of aerosol. This differs significantly from the conclusions reported by Jiang et al. (2013). The difference is because the scattering aerosols do not directly change the atmosphe-ric radiation energy (fast response), but can greatly affect the temperature of the troposphere by changing the inter-hemispheric gradient in sea surface temperature (slow response) (Xu and Xie, 2015). The marked anomalies in vertical descending motion from 10° to 20°N and from 35° to 45°N and the resulting northerly anomalies in the low troposphere in the EAMR in summer caused by the increase in sulfate aerosols weaken the transport of warm moisture from the low to upper troposphere and from low to high latitudes, further cooling the troposphere and forming an anomalous cooling center in the upper troposphere (300 hPa) near 40°N (Fig. 5a). During summer, the East Asian subtropical jet (EASJ) is normally located around 40°N at 200 hPa. Changes in the position of the jet are strongly associated with the intensity of the EASM (Song et al., 2014). The anomalous cooling center resulting from the increase in sulfate aerosols decreases the pressure in the uppermost troposphere, thus increasing the northward pressure gradient force in the south of the cooling areas and leading to a southward shift of the EASJ (Fig. 6a). These changes signify weakening of the EASM (Yu et al., 2004).
The prominent descending motion anomalies at about 30° and 55°N caused by the increase in OC aerosol lead to the formation of two anomalous cooling centers in the upper troposphere around these two latitudes (Fig. 5d). However, the degrees of cooling are much smaller than those due to the sulfate aerosols. The change in atmospheric temperature due to the increase in OC aerosol increases the poleward pressure gradient force at 40°N, causing a northward shift in the EASJ (Fig. 6b) and a slight enhancement in the northern EASM circulation. The descending flow anomalies at 30°N induced by OC aerosol diverge at the surface, leading to an increase in southward (northward) movement to the south (north) of 30°N (Fig. 5d). This is consistent with the changes in winds in the low troposphere shown in Fig. 4d. The changes in the intensity of the EASM due to the increase in sulfate and OC emissions are generally in agreement with the corresponding changes in the sea–land thermal contrast both at the earth’s surface (Fig. 4) and in the troposphere (Fig. 5).
The amount of rainfall decreases over most of the EAMR due to the significant weakening of the EASM induced by the increase in sulfate aerosol (Fig. 7a). A marked reduction occurs between 10° and 20°N and over central and Northeast (NE) China, with a maximum reduction of up to 2.5 mm day–1. The increase in OC emission significantly reduces the amount of rainfall over central, northeastern, and East China, but increases the amount of rainfall over the Huang–Huai valleys of eastern China and the southeast coast of China (Fig. 7b). The changes in rainfall are strongly linked to local changes in circulation (Figs. 4b, d). Our results are the opposite of those reported by Jiang et al. (2013) using the Community Atmospheric Model version 5 with a fixed sea surface temperature. Jiang et al. (2013) reported changes in rainfall over East China between 25° and 30°N due to an increase in sulfate emission and over central and East China and the Huang–Huai valleys of eastern China due to an increase in OC emission. The increase in sulfate and OC decreases the average summer rainfall over the EAMR by 0.72 and 0.04 mm day–1, respectively (Table 2). Our results also indicate that the changes in convective rainfall caused by these scattering aerosols dominate the changes induced in the total rainfall. This agrees with the earlier work by Wang Z. L. et al. (2016) and Wang et al. (2017).
Scattering aerosols therefore affect the EASM by altering the sea–land thermal contrast and the atmospheric circulation. The increase in sulfate aerosol weakens the EASM and significantly reduces the amount of rainfall over the EAMR by decreasing the local sea–land thermal contrast and shifting the EASJ southwards. However, the increase in OC strengthens the northern EASM by slightly increasing the sea–land thermal contrast north of 30°N and shifting the EASJ northwards, while weakening the southern EASM by decreasing the sea–land thermal contrast south of 30°N.3.3 Effect of increased amounts of absorbing aerosols (BC) on the EASM
Unlike the scattering aerosols, absorbing aerosols heat the air directly, modifying the cloud fraction and the atmospheric temperature profiles in addition to affecting the surface radiative flux directly. Figure 8 shows the effects of change in BC aerosol on the surface net all-sky solar flux and the zonal-mean cloud cover in the EAMR in summer. The net all-sky solar flux in eastern China and the surrounding oceans is reduced due to the increase in BC aerosol, with a maximum reduction of 10 W m–2 (Fig. 8a). However, the solar flux increases remarkably over NW and NE China (Fig. 8a), mainly due to the reduced cloud fraction over these regions (Fig. 8b). The increase in BC aerosol decreases the surface net all-sky solar flux averaged over the EAMR in summer by 1.8 W m–2 (Table 2). It is surprising that the change in the net all-sky flux at the top of the atmosphere caused by BC is also negative (Table 2); this may be a result of cloud feedback.
The increase in BC leads to a warming north of 35°N and a slight cooling between 15° and 35°N over the EAMR (Fig. 9a), which is different from the effects caused by the scattering aerosols. The slight cooling between 15° and 35°N is mainly due to the decrease in the surface net all-sky solar flux (Fig. 8a) over this region. The change in surface temperature is consistent with that in the surface net all-sky solar flux (Fig. 8a). The enhanced vertical ascending motion north of 40°N (Fig. 10a) induced by the warming at surface results in a reduction in surface pressure (Fig. 9b). The surface cools over Japan and the adjacent oceans, increasing the sea–land surface thermal contrast over these regions and inducing an anticyclonic anomaly over the oceans. The southerly and southeasterly wind anomalies appear to the west of the anticyclonic center (Fig. 9b). Figures 10b and 10c show that the zonal sea–land thermal contrast increases between 30° and 40°N in the middle and high troposphere. All these changes signify an enhanced circulation of the northern EASM. The enhanced vertical ascending motion south of 25°N caused by the increase in BC aerosol (Fig. 10a) leads to a surface cyclonic anomaly (Fig. 9b). Pronounced northeasterly and northerly anomalies to the west and NW of the cyclonic center and a slight decrease in the sea–land thermal difference at the surface (Figs. 9a, b) and in the troposphere (Figs. 10b, c) south of 30°N greatly weaken the southern EASM. This is obviously different from the results reported by Jiang et al. (2013) and Wang T. J. et al. (2015), who excluded the response of oceans to aerosol forcing in their models.
Figure 10 shows the changes in the zonal-mean meridional circulation and atmospheric temperature in the EAMR in summer due to the changes in emission of BC aerosol. Warming appears in most of the troposphere over the EAMR due to the increase in BC aerosol. Significant warming occurs in the upper troposphere (300 hPa) around 20°N and the whole troposphere north of 40°N, but a deep cooling occurs from 30° to 40°N in the upper troposphere (Fig. 10a). Such a change in tropospheric temperature results in the northward shift of the eastern EASJ and the southward shift of the western EASJ (Fig. 11a). This indicates enhancement of the northern EASM and weakening of the southern EASM.
The increase in BC aerosol leads to a pronounced reduction (maximum –0.6 mm day–1) in rainfall over some parts of China’s southeast coast and central, north, and northeastern China and a remarkable increase (maximum 0.6 mm day–1) in rainfall over the South China Sea, southern China, and the Huang–Huai valleys of eastern China (Fig. 12a). This is opposite to the results reported by Jiang et al. (2013), who suggested that the increase in BC aerosol led to an increase in rainfall in East China between 25° and 30°N and a decrease in rainfall on both sides of this band. It is also different from the phenomenon of “southern flood and northern drought” reported to be caused by BC in Menon et al. (2002).
To obtain a more obvious climate response to BC forcing, we performed another simulation with a five-fold perturbation in BC emission in the year 2000 (PDBC × 5). A marked warming appears throughout the EAMR due to the five-fold increase in BC emission and this is significantly enhanced with latitude (Fig. 9c). The effects of the five-fold increase in BC emission on the EASM circulation and precipitation are generally consistent with those of 1 × BC, although the former shows a stronger signal (Figs. 10, 12).3.4 Nonlinear effects of climate response to forcing by different aerosol components
To identify any potential nonlinearity among the response to different aerosol forcings, we used another simulation with all the aerosol emissions fixed at the year 2000 levels, i.e., the present day simulation (PD). This simulation has been performed previously by Wang et al. (2017). We then compared the difference between the responses of the EASM to total aerosol forcing from the PD – PI simulations and the addition of different components of aerosol forcing, i.e., (PDSF – PI) + (PDOC – PI) + (PDBC – PI). Figure 13 shows that there is a high similarity in the spatial distributions between the changes in summer mean surface temperature, 850-hPa winds, atmospheric temperature, and meridional circulation over the EAMR due to the total aerosol forcing and the addition of forcing from different aerosol components, despite some differences in magnitude over some regions. Thus, the addition of the responses of the EASM to forcing by different aerosol components may be a good reflection of its response to forcing by the sum of the different aerosol components.4 Summary and discussion
We investigated the impact of various anthropogenic aerosol species on the EASM system using the BCC_AGCM2.0.1_CUACE/Aero coupled aerosol–climate model. The simulated global annual mean loadings of sulfate, OC, and BC are increased by 2.9, 0.47, and 0.09 mg m–2, respectively, from 1850 to 2000, in agreement with previously reported results (e.g., Lamarque et al., 2010; Ganguly et al., 2012b).
The anthropogenic aerosols affect the EASM by changing the sea–land thermal contrast, not only at the surface, but also in the troposphere (Li et al., 2010; Dai et al., 2013; Qian and Zhou, 2014). Different degrees of cooling induced throughout the EAMR by the increase in sulfate aerosol cause a decrease in the sea–land thermal contrast and significant northerly and northwesterly wind anomalies in southern and eastern China and the adjacent oceans, weakening the EASM circulation. An anomalous cooling center appears in the upper troposphere (300 hPa) around 40°N and the pressure gradient forces to the south and north of the cooling center change due to the increase in sulfate aerosol. This results in a southward shift in the EASJ, weakening the EASM circulation further. Rainfall is reduced in most of the EAMR due to this weakening of the EASM, with a pronounced reduction between 10°N and 20°N. The increase in sulfate aerosol decreases the average rainfall over the EAMR in summer by 0.72 mm day–1.
The increase in the emission of OC aerosol also leads to a reduction in temperature over most of the EAMR, but the magnitude of the change is weaker than that caused by the increase in sulfate aerosol. This results in a slight increase in the sea–land thermal contrast north of 30°N and a northward shift in the EASJ, thus strengthening the northern EASM. However, it results in a decrease in the sea–land thermal contrast at the surface south of 30°N and weakening of the southern EASM. Therefore, the rainfall is greatly reduced over central, eastern, and northeastern China, while it is markedly increased over the southeast coast and the Huang–Huai valleys of eastern China. The increase in OC aerosol reduces the average rainfall over the EAMR in summer by 0.04 mm day–1.
The increase in the emission of BC aerosol leads to a warming at the surface north of 35°N and a slight cooling to the south of 35°N over the EAMR. The change in tropospheric temperature due to the increase in BC aerosol causes a northward shift in the eastern EASJ, thereby strengthening the northern EASM. However, it leads to a southward shift of the western EASJ, thus weakening the southern EASM. The average rainfall over the EAMR in summer increases by 0.07 mm day–1 due to the increase in the emission of BC aerosol. Our results suggest that the simulated EASM responses to BC forcing are not changed when the emission of BC increases dramati-cally, but that the intensity of the response is enhanced.
Our results highlight the differences in the impacts of various anthropogenic aerosol species on the EASM. It is indicated that the effects of different types of aerosol need to be taken into account when investigating the effects of aerosol forcing on the EASM. The results of this study also confirm the importance of the slow response of the ocean to aerosol forcing in the impact of aerosols on the EASM, as suggested by Ganguly et al. (2012b). The mechanism by which the oceanic response to aerosol forcing affects the EASM deserves further investigation. Our results are very different from those of previous research using climate models with a fixed sea surface temperature (e.g., Jiang et al., 2013). In addition to the response of the ocean, the different sensitivities to individual forcings among the various models make a contribution to the differences in the effects of aerosol emissions.
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