2 Key Laboratory of Meteorological Disasters of Ministry of Education, Nanjing University of InformationScience & Technology, Nanjing 210044
The increase in surface air temperature induced by anthropogenic black carbon(BC)is significantlygreater in the Northern Hemisphere than in the SouthernHemisphere, and a northward shift of the intertropical convergence zone(ITCZ)is thus predicted(Chung and Seinfeld, 2005). However,some studiesfound that inter-hemispheric asymmetry cooling dueto aerosols brings a significant reduction in precipitationto the north of the equator and an increase tothe south,shifting the ITCZ southward(Ming and Ramaswamy, 2009; Wang et al., 2013). China,located in East Asia,is a pronounced monsoon region.East Asian monsoons have great impacts on the regionalclimate change. Meanwhile,China has recently experienced a rapid economic development, and it semissions of anthropogenic aerosols have greatly increasedwith the expansion of urban areas along theeast coast of China. These local aerosol emissions canplay an important role in regional climate(Li Z. et al., 2010).
Effects of anthropogenic aerosols on East Asian monsoons have been studied and simulated. A decrease in surface temperature induced by aerosols canweaken both summer and winter monsoons(Liu et al., 2009; Deng et al., 2014a). Menon et al.(2002)showed that the solar radiation absorbed by BC could heat the air,leading to strengthened convective activities over southern China,which is in accordance with the“southern flood and northern drought” phenomenon in summer over China during the last 50 years. However,other studies suggested that aerosols do not contributeto the observed anomalous summer rainfall over EastAsia,or even cause an opposite precipitation pattern(Gu et al., 2006; Wang et al., 2009a; Zhang H. et al., 2009; Zhang et al., 2012). Therefore,uncertaintiesstill exist in simulating and estimating the impacts ofaerosols on the summer precipitation over East Asia.
Previous studies mostly focused on East Asian summer or winter monsoons. There is a lack of research on the effect of aerosols on atmospheric circulationduring spring. Tian and Yasunari(1998)advanced the concept of spring persistent rain(SPR) and regarded the SPR as a climatic event. The SPR usuallyoccurs in the area between the middle and lowerreaches of the Yangtze River and the Nanling Mountains(25°–30°N)during March and April,which isa persistent and relatively steady pluvial period overeastern China before the onset of the South China Seasummer monsoon(SCSSM; Wan et al., 2008c). Wan et al.(2006,2008a)suggested that the formation ofthe SPR was the result of mechanical forcing and thermalforcing of the Tibetan Plateau(TP) and the location and intensity of the SPR were influenced bythe topography of the Nanling and Wuyi mountains.Moreover,the seasonal reverse of the zonal l and -seathermal contrast would precondition establishment ofthe East Asian subtropical summer monsoon(Qi et al., 2008),which may be crucial to the formation ofthe SPR as well(Li Chao et al., 2010; Zhang et al., 2011). Interdecadal variability of the SPR was studied by Jiang and Zhao(2012). They found a spring precipitation surplus over eastern China from the mid 1970s to the early 1980s, and a precipitation deficit after the early 1990s.
With less precipitation in the Northern Hemispherebefore summer monsoon,wet deposition is atits minimum in spring, and as a result,aerosols willrema in in the atmosphere for a longer time. Significant thermal responses of atmosphere to the radiative forcing of aerosols may induce changes in general circulation to influence the subsequent evolution of theEast Asian summer monsoon(Lau and Kim, 2006b).Using the Community Atmosphere Model version 3.0(CAM3.0),Wang et al.(2009b)simulated the effect of BC on summer monsoon over South Asia. They suggested that BC warmed the lower troposphere bystrongly absorbing solar radiation in South Asia in late spring,leading to an advance of the rainy periods overthe Bay of Bengal and its coast,together with the onset of the Indian summer monsoon. Using the same general circulation model,Hu et al.(2011) showed that the spring precipitation decreased in central to southern China but increased in northern China due to sulfate(SF) and BC over East Asia. It is noted that the response of atmosphere to the aerosols during spring is far from a simple pattern related to local direct aerosol forcing, and the structure and amplitude of such a regional response is determined by the balance of several factors,including solar heating,condensation and adiabatic heating, and turbulent diffusionin the boundary layer(Kim et al., 2006).
The purpose of this study is to investigate the impact of anthropogenic aerosols on the SPR over easternChina using a high-resolution global climate model(CAM5.1)from the National Center for AtmosphericResearch(NCAR). Section 2 describes the model,experiment aldesign, and observational data used in this study. In Section 3,the corresponding results in spring responses of atmosphere to the radiative forcing ofaerosols over eastern China are presented, and a possiblem echanism is discussed. Finally,the conclusions and discussion are given in Section 4. 2. Model,experimental design, and data 2.1 General circulation model and aerosol module
An earlier version of the Community AtmosphereModel(CAM3.0)has been widely used to study theeffect of aerosols on the climate over East Asia(Liu et al., 2009; Wang et al., 2009a; Hu et al., 2011; Zhang et al., 2011). CAM5.1 is the latest version of the at mospheremodel(Neale et al., 2010)in the CommunityEarth System Model version 1(CESM1)released in 2011 by the NCAR. It could simulate the at mosphericcirculation st and alone or be coupled with other models in the CESM1. Moreover,CAM5.1 providesa three-mode version of the modal aerosol module(MAM3)for long-term climate simulations,includingthe Atiken mode,accumulation mode, and coarsemode(Liu et al., 2012). The aerosols involved in eachmode are internally mixed,which is closer to the existentstate of aerosols in the observation. Therefore,the optical property of aerosols could be well described by MAM3 in CAM5.1, and the particle distribution and number concentration are accurately calculated for better simulation of the direct and indirect effects of aerosols. This aerosol module contains several main kinds of aerosols,such as SF,BC,organic carbon(OC),dust(Dst), and sea salt(SS). CAM5.1 used in this study has a high resolution of 0.9°× 1.25° and ahybrid vertical coordinate with 30 levels,including arigid lid at 3.643 hPa. 2.2 Experimental design
To investigate the effect of anthropogenic aer osolson the SPR over eastern China,we perform twonumerical experiments,i.e.,a control experiment(CTRL)with all anthropogenic aerosols forcing(for year 2000) and a sensitivity experiment(NOAERO).In NOAERO,anthropogenic aerosols(BC,SF, and OC)linked to human activities at the surface or in the upper air over eastern China(20°–45°N,100°–125°E)are all prescribed at the pre-industrial level(for year1850),but natural aerosols(Dst and SS)produced by natural processes such as volcanic eruptions are unadjusted(Fig. 1). The emission sources involved in both experiments are those used in the IntergovernmentalPanel on Climate Change(IPCC)fifth Assessment Report(AR5)(Lamarque et al., 2010). All greenhousegases and interactive l and -atmospheric forcing are the same in both simulations. Thus,the difference betweenCTRL and NOAERO can be regarded as theimpact of anthropogenic aerosols over eastern Chinaon the East Asian climate.
In both experiments,sea surface temperature(SST)is prescribed with monthly mean SST from1850 to 2010,which is a blended SST dataset withHadISST1 from the UK Met Office Hadley Center and OI SST from the National Oceanic and AtmosphericAdministration(NOAA)(Hurrell et al., 2008). Consequently,the seasonal and interannual variations ofthe SST are present during the model integrations, and as a result the influence of the intensity of EastAsian monsoons on the spatial distribution of aerosolsis considered; that is,the interaction between aerosols and atmosphere is included in the experiments. Bothsimulations are integrated from January 1988 to December2007, and the results from the last 15 years(1993–2007)are analyzed. Tian and Yasunari(1998)suggested that the duration of the SPR is in pentads12–16(March to early May),while Wan et al.(2006)noted the SPR for the period of March and April. Asthe pre-rainy season in South China generally occursin mid May,the period of March to April is chosen asthe SPR period in this paper as in Wan et al.(2006).2.3 Data
The daily and monthly reanalysis from the EuropeanCentre for Medium-Range Weather Forecasts(ECMWF),namely the ERA-Interim,is used to validatethe simulated atmospheric circulation. The ERAInterimputs emphasis on improving its earlier reanalysisof ERA-40,including the representation of thehydrological cycle,the quality of the stratospheric circulation, and the consistency in time of the reanalyzedfields(Dee et al., 2011). We use the pentad precipitationdataset from the Climate Prediction CenterMerged Analysis of Precipitation(CMAP; Xie and Arkin, 1997)provided by the NOAA to compare withthe simulated annual cycle of precipitation over easternChina. The CMAP dataset contains precipitationdistributions with a full global coverage and improveddata quality compared to individual data sources(Xie and Arkin, 1997). 3. Results 3.1 Model validation
Numerous studies have simulated the features ofaerosols and investigated the impacts of aerosols onregional and global climate by using CAM5.1(Gettelman et al., 2010; Hu and Liu, 2013; Jiang et al., 2013).Liu et al.(2012)showed that CAM5.1 is able to qualitativelycapture the observed geographical and temporalvariation of aerosols,including aerosol mass,numberconcentration,size distribution, and aerosol opticalproperties. To examine the capability of CAM5.1in simulating atmospheric circulation and precipitationfields over East Asia during the SPR period,Fig. 2 compares the simulated March–April mean circulationat 850 hPa and surface precipitation with observations.The simulated spatial distributions of 850-hPawind and surface rainfall are consistent with the observations.In boreal spring,the southwesterly wind prevailsover southern China(south of 30°N)with a heavyrainfall b and (rainfall exceeding 3 mm day−1)extendingfrom the north side of the subtropical high over thePacific Ocean to the east of the Philippines. There is aflow on the southeast side of the TP due to the topography,forming an approximately cyclonic circulationpattern. Meanwhile,the northwesterly wind prevailsin the mid and high latitudes of East Asia,which convergeswith the southwesterly wind over the middle and lower reaches of the Yangtze River. However,thesimulated 1500-gpm isoheight at 850 hPa shifts northwardin comparison with the observation,togetherwith the SPR rain belt,leading to an underestimationof the spring rainfall over the southern coastal regionsof China. Meanwhile,too much precipitation is simulatedover the northwestern part of the Indo-ChinaPeninsula and southwestern China,due to a westwardshift of the subtropical high and an enhanced southwesterlyin CTRL. Therefore,the spatial distribution and intensity of simulated precipitation over East Asiaby CAM5.1 show some systematic errors,as seen inmany other global climate models. In terms of theSPR,however,CAM5.1 still well captures the characteristicsof atmospheric circulation and precipitationover eastern China.3.2 Onset of the SPR
Figure 3 shows the latitude-time cross-section ofprecipitation along 110°–122.5°E. The SPR with precipitationexceeding 4 mm day−1 appears in earlyMarch(pentad 15)when averaged from 1993 to 2007, and is located between 25° and 30°N over easternChina. The SPR indicates the seasonal transition ofthe East Asian circulation from winter to spring and the weakening of the East Asian winter monsoon systemdue to the shrunken l and -sea thermal contrast,together with the occurrence of strong southwesterlywind and the water vapor convergence there. The rainbelt remains until the end of April,but with a relativelyweaker and interrupted heavy rainfall center(≥ 6 mm day−1). The precipitation over the northernSCS exceeds 4 mm day−1 in pentad 26,declaringthe onset of the SCSSM and the termination of theSPR. By contrast,the spring rainfall over 25°–30°N isslightly overestimated in CAM5.1, and a northwardshiftedrain belt is present in the model,which is stationary(Fig. 3b). It is clearly depicted that the springrainfall begins in pentad 15 in CTRL with a magnitudecontinuously exceeding 4 mm day−1,which is incoincidence with the observation.
Based on precipitation and low-level circulation,Wan et al.(2008b)defined the onset of the SPR asthe first pentad after pentad 7 that satisfies the followingcriteria: 1)area-averaged precipitation mustbe greater than 4 mm day−1 over the SPR region(RegionA: 23°–30°N,110°–120°E); 2)mean southwesterlyvelocity at 850 hPa must be greater than 4 m s−1over its upstream region(Region B: 20°–25°N,110°–115°E); 3)in the subsequent three pentads,at leastone pentad satisfies the above two criteria. Figure 4shows the temporal evolutions of the aforementionedprecipitation and southwesterly velocity at 850 hPa.As shown in Fig. 4a,under the influence of anthropogenicaerosols,a decrease in precipitation is obviousover Region A in pentads 14–15. Therefore,the onsettime of precipitation exceeding 4 mm day−1 in RegionA is pentad 16 in CTRL,two pentads later than thatin NOAERO. Similarly,the southwesterly velocityat 850 hPa is weakened over Region B during pentads 13–15,showing that the onset of southwesterly velocitygreater than 4 m s−1 in CTRL is delayed by threepentads compared to that in NOAERO(Fig. 4b). Asa result,the SPR is established in pentads 16 and 14in CTRL and NOAERO,respectively,meaning a delayedonset of the SPR due to anthropogenic aerosolsover eastern China. Moreover,Deng et al.(2014b)found an advanced onset of the SCSSM induced byanthropogenic aerosols using CAM5.1,indicating thatthe SPR is established later but also ends earlier witha shortened duration.3.3 Precipitation and 850-hPa winds
Figure 5 shows the time series of March–Aprilmean precipitation over Region A and 850-hPa southwesterlyvelocity over Region B. In most years,theSPR is reduced due to anthropogenic aerosols as amarked reduction in precipitation by 0.94 mm day−1 and reduced southwesterly velocity by 0.92 m s−1. Figure 6a shows the mean changes of geopotential height and wind at 850 hPa in the SPR period. A positivegeopotential height anomaly appears over southernChina with a maximum greater than 5 gpm on thesoutheast side of the TP,while a negative anomalyappears over the western Pacific Ocean due to theweakening of the subtropical high. This positive and negative geopotential height anomalies form an east–west dipole pattern. Correspondingly,some anticyclonicanomaly flow shows up on the southeast sideof the TP,which weakens the flow there. The northeasterlywind anomaly dominates southern China,thenorthern SCS, and the Indo-China Peninsula,whileobvious northwesterly wind anomaly prevails north ofthe Yangtze River. Besides,there is westerly windanomaly over the ocean east of the Philippines. Asa result,the gradient of geopotential height is attenuatedbetween the southeast side of the TP and the western Pacific Ocean under the effect of anthropogenicaerosols,leading to weakened southwesterlyon the north side of the subtropical high and reducednorthward transport of water vapor from the tropics.Eventually,precipitation over southern China willbe suppressed associated with the circulation change.To further verify the rainfall change,Fig. 6b givesthe March–April mean rainfall anomaly field. A significantlysuppressed rain belt is clearly seen oversoutheastern China,the East China Sea, and southernJapan, and precipitation drops obviously by morethan 2 mm day−1 over the middle and lower reachesof the Yangtze River.
3.4 Underst and ing the reduced SPR
Aerosols can influence the radiative balance betweenl and and atmosphere by scattering and absorbingsolar radiation and long wave radiation,which is the direct effect; so the thermal forcing of aerosols isregarded as an essential way to affecting the climate.Figure 7a shows the vertical structure of air temperaturedifference between CTRL and NOAERO averagedover eastern China. We can see that the air temperaturedecreases in the lower and upper tropospheresouth of 35°N with an amplitude of 0.4℃,but it increasesat 200 hPa. However,anthropogenic aerosolsheat the air below 250 hPa over the mid-high latitudes,with a warming amplitude greater than 1℃ inthe mid-lower troposphere. Accordingly,the meridionaltemperature gradient averaged between 850 and 200 hPa is significantly reduced over southern China,but enhanced over the region 30°–50°N(solid line inFig. 7b). Therefore,this north–south asymmetricalthermal effect reduces the mean meridional temperaturegradient in spring over East Asia. The thermalwind relationship is as follows:Fig. 7b,the axis of the subtropicalwesterly jet(dash-dotted line)is located near30°N in NOAERO. In CTRL,zonal wind(dashed line)is enhanced south of 30°N and weakened within 30°–50°N with a minimum near 40°N,suggesting that thewesterly jet is weaker and moves southward. Therefore,the circulation change in the upper troposphere isindeed the thermal response of atmosphere to anthropogenicaerosols,associated with the out-of-phase relationshipbetween upper-layer zonal wind and meridionaltemperature gradient between the upper and lower troposphere. Meanwhile,air mass is accumulatedover southern China due to aerosols’ cooling effect,which explains the increase in 850-hPa geopotentialheight and northeasterly wind anomaly there(Fig. 6a). Eventually,the southwesterly on the north sideof the subtropical high is weakened. Besides,500-hPageopotential height decreases over the western equatorialPacific Ocean,the SCS, and the East Asian Continent,yielding a southward displacement of the weakersubtropical high(figure omitted). Zhang Jie et al.(2009)suggested that the main moisture of the SPRcomes from the western equatorial Pacific Ocean and both the subtropical high and the upper-level westerlyjet move northward to their normal positions. Thus,after considering the effect of anthropogenic aerosols,circulation change provides adverse conditions for themaintenance of the SPR.
To further reveal the relationship between circulationchange and precipitation anomaly,Fig. 8 showsthe differences of wind divergence and vertical meridionalcirculation. Because of the weakened southwesterlyover southern China(Fig. 6a),a positive divergenceanomaly(solid line in Fig. 8a)occurs at 850hPa over 25°–35°N. Meanwhile,a significant negativeanomaly(dashed line in Fig. 8a)is found in the uppertroposphere south of the Yangtze River due to theweaker and southward-shifted upper-level westerly jet.Both divergence anomaly centers in the lower and uppertroposphere are near the axis of the westerly jet(30°N). This aerosol-induced circulation results in asignificant anomalous downward motion over southernChina through weakening the pumping below theaxis of the westerly jet(Fig. 8b),acting to suppressthe spring precipitation there. However,the fact that the condensation latent heating decreases due to aweaker upward motion(figure omitted)will further decreasethe meridional temperature gradient by coolingthe air in the low latitudes,forming a positive feedbackmechanism. Therefore,the response of air temperatureto anthropogenic aerosols is determined by theradiative forcing of the aerosols and the positive feedbackinvolved, and the thermal effect of anthropogenicaerosols is very likely to cause a primary perturbationfor the adjustments of air temperature and circulation.
In addition,aerosols can serve as cloud condensationnuclei(CCN), and thus change the cloud dropleteffective radius(Reff),giving rise to higher cloudalbedo(Twomey,1997) and reducing precipitation efficiencydue to longer cloud lifetime(Albrecht,1989).This is the so-called indirect effect. Numerous studieshave verified the impact of anthropogenic aerosolson the micro-properties of clouds through in-clouddroplet number concentration(CDNC)(Han et al., 1994; Wetzel and Stowe, 1999; Pwlowska and Brenguier, 2000; Bréon et al., 2002). After consideringthe effect of anthropogenic aerosols,the aerosol opticaldepth increases markedly by 0.09 over the SPRregion,together with CCN at supersaturation of 0.1%(CCN0.1)by 219.9 cm−3 at 850 hPa(158.1%; Table 1). Figure 9 shows the changes of CDNC and Reff at850 hPa. With the increase in CCN0.1 induced by anthropogenicaerosols,CDNC rises in southern China(Fig. 9a),especially in the SPR region by a ratio of48.6%. The reduction of Reff is clear in the SPR region(–13.5%)(Fig. 9b and Table 1),suppressing thecoalescence of the cloud droplets and precipitationthere. As a result,the indirect effect of anthropogenicaerosols plays an important role in reducing the SPR.However,it is noted that the impact of aerosols onprecipitation is a complex matter, and it is difficult to find the exact relationship between CCN and rainfall.Nonlinearity between in-cloud microphysical parameters and surface rainfall remains uncertain. Furthermore,the cloud-aerosol interaction is also influencedby the background circulation,such as atmosphericstability and wind shear(Yang et al., 2011).
|Note: Italic and bold fonts represent values statistically significantat the 95% and 99% confidence levels,respectively.|
In general,the thermal forcing associated with anthropogenicaerosols induces circulation changes overEast Asia. On the other h and ,the indirect effect ofanthropogenic aerosols has a great impact on cloudsin southern China. Both effects of the aerosol forcingsuppress the SPR,which could be summarized asfollows. Under the impact of anthropogenic aerosols,the decrease in air temperature leads to increased 850-hPa geopotential height in the low latitudes over EastAsia,reducing the geopotential gradient between thesouth side of the TP and the western Pacific Ocean and weakening the southwesterly on the north side ofthe subtropical high in the lower troposphere,togetherwith convergence in southern China. In comparison,a marked increase in air temperature is found in themid-high latitudes,which reduces the meridional temperaturegradient between the upper and lower troposphere.The upper-level westerly jet is weaker and shifts southward,giving rise to reduced convergencein the upper troposphere. This anomalous circulationleads to weakened upward motion and suppressedspring rainfall with a shortened duration. Moreover,in the SPR region,850-hPa CDNC increases due toanthropogenic aerosols acting as CCN,leading to adecrease in Reff . As a result,the SPR is also suppressedby reduced coalescence of cloud droplets.
Hu and Liu(2013)noted that anthropogenicaerosols affected the decadal change of late springprecipitation in South China during 1950–2000. Themechanism of aerosols affecting spring rainfall by adjustmentof circulation in East Asia is in coincidencewith our results. Therefore,our results confirm thecrucial role of anthropogenic aerosols in spring climatechange in East Asia. However,the SPR usually occursin early spring(March–mid April),with pre-rainy seasonin southern China in late spring(late April–May).Although both rainfall periods are considered springprecipitation in southern China,the features of precipitation and the intensity of circulation are differentfor these periods. Thus,further studies are needed toinvestigate the effect of anthropogenic aerosols duringdifferent spring precipitation periods. 4. Conclusions and discussion
Based on a high-resolution model of CAM5.1,theimpact of anthropogenic aerosols on the SPR overeastern China is investigated in this study. The resultsare as follows.
(1)The SPR is established two pentads later dueto anthropogenic aerosols,together with a shortenedduration and reduced precipitation amount.
(2)Under the influence of anthropogenic aerosols,a significant decrease in air temperature leads to an increase in 850-hPa geopotential height in the lowlatitudes over East Asia,reducing the geopotentialgradient between the south side of the TP and thewestern Pacific Ocean and weakening the southwesterlyon the north side of the subtropical high in thelower troposphere,together with convergence oversouthern China. In comparison,a marked increasein air temperature is found in the mid-high latitudes.This north-south asymmetrical thermal effect acts toreduce the meridional temperature gradient so theupper-level westerly jet is weaker and shifts southward,giving rise to reduced convergence in the uppertroposphere. As a result,the SPR is suppressed witha shortened duration.
(3)The indirect effect of anthropogenic aerosolsalso plays an important role in reducing the SPR.CDNC in the lower troposphere is increased byaerosols acting as CCN in the SPR region,leadingto an evident decrease in Reff . As a result,the SPRis further suppressed due to reduced coalescence ofcloud droplets.
In this study,we only explored the effect of anthropogenicaerosols on the SPR over eastern China,though remote aerosols could also enhance the impactof local aerosols in the monsoon region(Cowan and Cai, 2011). Furthermore,because of the SST responseto aerosol forcing,the influence of aerosols onprecipitation presented in the air-sea coupled modeldiffers from that in the atmosphere model driven byprescribed SST(Chung et al., 2002; Wang et al., 2005;Lau and Kim, 2006a). Our future studies will use afully coupled model including the SST response toaerosol forcing to better underst and the impact of anthropogenicaerosols on spring climate in East Asia.
Acknowledgments. We thank three anonymousreviewers for their valuable comments and suggestions.Thanks also go to Dr. Zuojun Yu for hercareful review of the language and kindly help in improvingthe English.
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