J. Meteor. Res.  2016, Vol. 30 Issue (1): 1-11   PDF    
The Chinese Meteorological Society

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William K. M. Lau. 2016.
The Aerosol-Monsoon Climate System of Asia: A New Paradigm
J. Meteor. Res., 30(1): 1-11

Article History

Received September 1, 2015;
in final form September 28, 2015
The Aerosol-Monsoon Climate System of Asia: A New Paradigm
William K. M.Lau     
Earth Science Interdisciplinary Center, University of Maryland, College Park, MD 20740, USA
ABSTRACT: This commentary is based on a series of recent lectures on aerosol-monsoon interactions I gave at the Beijing Normal University in August 2015. A main theme of the lectures is on a new paradigm of “An Aerosol-Monsoon-Climate-System”, which posits that aerosol, like rainfall, cloud, and wind, is an integral component of the monsoon climate system, influencing monsoon weather and climate on all timescales. Here, salient issues discussed in my lectures and my personal perspective regarding interactions between atmospheric dynamics and aerosols from both natural and anthropogenic sources are summarized. My hope is that under this new paradigm, we can break down traditional disciplinary barriers, advance a deeper understanding of weather and climate in monsoon regions, as well as entrain a new generation of geoscientists to strive for a sustainable future for one of the most complex and challenging human-natural climate sub-system of the earth.
Keywords: aerosol     monsoon climate system     interaction     new paradigm    
1. Perspectives: Past and present

Monsoon climates are characterized by distinctivewet and dry seasons,a reversal of prevailing seasonalwinds, and an abrupt onset of the rainy season,followedby subsequent dry and pluvial spells. Since thefirst paper on monsoon published in the 17th century(Halley,1686),monsoon studies have been mainly concernedwith the meteorology of temperature,winds,water vapor,clouds, and precipitation governed byatmospheric dynamics and diabatic heating processesdriven variously by l and -sea thermal contrast,moiststatic stability,dynamical instability,surface topography,tropical-extratropical interactions,as well asregional and global sea surface temperature(SST)variations. The interactions of monsoon forcing and responses are extremely complex,involving coupledatmosphere-ocean-l and processes,as have been documentedin many books and review papers(Ramage,1971; Lau and Li, 1984; Chang and Krishnamurti, 1987; Ding 1994; Webster et al., 1998; Wang,2006;Lau and Waliser, 2012; Wu et al., 2014).

On the other h and ,studies of aerosols havetraditionally been confined to urban,environmental,health,safety, and air-quality issues. During the pastfew decades,aerosols,for the most part,are studiedunder the sub-disciplines of atmospheric chemistry and cloud physics,with emphases on primary and secondary emissions,chemical reactions, and cloudcondensation nucleation processes. Since the 2000s,aerosol studies in the context of climate change and inrelationship to a changing monsoon water cycle havegrown rapidly(Ramanathan et al., 2001; Menon et al., 2002; Lau and Kim, 2006; Lau et al., 2006,2008;Meehl et al., 2008; Ramanathan and Carmichael, 2008;Wang et al., 2009; Zhang et al., 2009). This newresearch direction stemmed in part from the adventof the earth observing satellite era(2000–present),in conjunction with increased field observations, and advanced climate modeling,revealing the ubiquitouspresence of natural and anthropogenic aerosols in theearth’s atmosphere, and their possible impacts on climate.

Nowadays,it is well recognized that monsoondroughts and floods, and worsening air qualities aretwo of the gravest environmental hazards afflictingover 60% of the world population living in monsoonregions. In the last decade,there has been a rapidlygrowing body of observational and modeling studies onthe possible impacts of monsoon circulation on aerosoltransport, and impacts of aerosol on monsoon precipitation and climate,mostly by aerosol scientists.By comparison,effort by monsoon scientists in investigatingthe impacts of aerosols on monsoon hasbeen somewhat lacking. Even today,the interactionsof these two largest atmospheric science communitieshave been underwhelming,given the urgency requiredto better underst and aerosol-monsoon interaction and climate change. For the monsoon community,themost pressing issues today are arguably still the intraseasonal and seasonal-to-interannual prediction ofmonsoon variability,as they are critically importantfor agricultural productivity,water resource management,floods and droughts, and disaster relief planning.However,despite many decades of research,predictingmonsoon rainfall on the above timescales is stilla major challenge. One of the most common refrainsfor not including aerosol in monsoon studies is that themonsoon climate system is already extremely complex.The thinking is that as long as the focus is not on longtermtrend,such as climate change,aerosol effects onshort timescales are likely to be small compared to dynamicaleffects, and therefore could be ignored. This,however,is a faux premise,as demonstrated from recentwork that the aerosol impacts on monsoon naturalvariability,i.e.,daily,intraseasonal,seasonal-tointerannualtimescales could be substantial. Underst and ingthe dynamical feedback induced by aerosolmonsooninteractions on these natural timescales isessential for a better and deeper underst and ing of theroles of aerosols(natural and anthropogenic)on monsoonclimate change.

2. A revelation

In the early 2000s,ideas and studies showinglarge aerosol radiative forcing and possible impactsof aerosols on monsoon were emerging. I was,likemany other monsoon scientists,skeptical of the ideathat these tiny particles suspended in the atmospherecalled aerosols can actually affect the mighty monsoonclimate system. In 2005,while working at the NASAGoddard Space Flight Center,I came across the firstNASA satellite image of global aerosols(Fig. 1)thatcompletely changed my perception regarding the impactsof aerosol on the monsoon climate system. Firstof all,as shown in Fig. 1,the global distribution ofaerosol “hotspots”,i.e.,regions of high aerosol concentrationsall year around speaks volumes regardingthe possible global impacts of aerosols,both natural and anthropogenic. East Asia and the Indo-GangeticPlain,South Africa,the Sahara desert, and the Amazonsare clearly aerosol hotspots,linked seasonally and geographically to major monsoon regions. The all-yearround aerosol hotspot over the East Asian monsoon regioncoincides with the industrial mega-city complexof China—a major source of anthropogenic aerosol.This region also happens to be located downwind ofthe Taklimakan desert and arid regions of westernChina, and well known to be affected by dust storms,especially during the pre-monsoon season of March–May(Fig. 2). Similarly,the Indo-Gangetic Plain isa monsoon mega-city complex situated downwind ofthe Thar and Middle East deserts. The West Africanmonsoon is affected not only by moist monsoon flowfrom the south and east,but also by dry air and desertdusts transported from the vast Saharan desert. Themaximum total aerosol radiative forcing over the monsoonhotspots during the boreal summer months JJA(June–July–August)for the atmospheric column hasbeen estimated to be positive 10–30 W m−2,i.e.,atmosphericheating,with comparable amount of coolingat the l and surface(Ramanathan and Carmichael, 2008; Li et al., 2010; Lau and Kim, 2015a). Hence,a first order effect of aerosols is to increase heating ofthe atmospheric column, and cooling of the earth surface.The magnitude of the aerosol heating and coolingis comparable to or even larger than the latent heatingin the atmosphere during the monsoon season. Thelarge atmospheric aerosol radiative forcing over the aerosol hotspots in monsoon regions also evinces theimportance of absorbing aerosols(black carbon(BC) and dust)in these regions. During the peak monsoon(JJA),the aerosol signal is most pronounced over theSouth Asian monsoon region including the northernArabian Sea,compared to the rest of the year. This isdue to the transport of large amount of dust aerosolsfrom the Middle East and West Asian deserts, and seasalt aerosols from the Arabian Sea,by the prevailingwinds as the monsoon season progresses.(Satheesh and Srinivasan, 2002). This means that aerosols arenot all washed out by the monsoon rain, and their impacts are likely to continue into the peak monsoonseason.

Fig. 1. The first estimate of global aerosol optical depth(AOD)over ocean and l and including bright desert surfaces from MODIS(moderate resolution imaging spectroradiometer)at 0.55-μm wavelength in 2005.(a)MAM(March-April-May),(b)JJA(June-July-August),(c)SON(September-October-November), and (d)DJF(December-January-February). Adopted from Lau et al.(2009a).
Fig. 2. A satellite image showing anthropogenic aerosols,haze,and clouds over the industrial regions in northern and central China,and the desert regions to its west and north.Abundant dust aerosols are transported into the Beijing area during the pre-monsoon months of March-May.
3. A primer on aerosol-monsoon interaction

Based on their optical properties,aerosols canbe classified into two types: those that absorb solarradiation(BC and dust), and those that do not absorbsolar radiation(sulfate and sea salt). Yet,both typesof aerosols scatter sunlight and reduce the amountof solar radiation from reaching the earth’s surface,causing it to cool—the so-called solar dimming effect(SDM). The surface cooling by aerosol is strongerover l and ,where the sources of aerosols are located,compared to the oceans. Thus,as a first order effect,aerosols reduce the l and -sea thermal contrast and weaken the monsoon. In addition to cooling the surface,absorbing aerosol can heat the atmosphere. Theheating of the atmosphere may reduce the amount oflow clouds by increased evaporation in cloud dropletsthrough the so-called semi-direct effect. By warmingthe atmosphere relative to the surface below,the semidirecteffect also stabilizes the atmosphere,suppressingmonsoon convection and rainfall.

Aerosols indirect(microphysics)effects can increasethe concentration of cloud condensation nuclei(CCN),increase cloud amount, and decrease coalescence and collision rates,leading to reduced precipitation.However,in the presence of increasing moist and warm air in monsoon regions,the reduced coalescence/collision may lead to supercooled drops athigher altitudes where ice precipitation falls and melts.The latent heat release from freezing aloft and meltingbelow favors greater upward heat transport in pollutedclouds and invigorates deep convection(Rosenfeld et al., 2008; Li et al., 2011). Additionally,microphysicaleffects of aerosols may lead to more extensive and prolongedanvil clouds associated with deep convection,modulating the SDM effect,especially under conditionsof weak wind shear(Fan et al., 2013). Thus,depending on the ambient large-scale conditions,theaerosol optical and nucleation properties and inducedfeedback,the impacts of aerosol on monsoon precipitation and climate can be positive,negative,or neutral.This also means that the net aerosol impact on monsoondepends on the type,composition, and mixingstates of ambient aerosols(Zhang et al., 2012). However,the impacts of different composition of aerosolson monsoon are outside the scope of this commentary.In the following,the effects of absorbing aerosols ininducing dynamical feedbacks in the Asian monsoonregion and how these feedbacks can be important ininfluencing monsoon climate change,are discussed.

In monsoon regions,both direct and indirect effectsmay take place locally and simultaneously,interactingwith each other. Local effects are most predominantduring the boreal winter monsoon(DJF),whenthe monsoon atmosphere is generally more stable,dusttransport is weak or absent, and most aerosols are confinedto the shallow boundary layer. Under these conditions,aerosol local effects are manifest in the frequentoccurrence of haze and fog(Niu et al., 2010),reduced precipitation, and cold surface air over citiesin the Asian monsoon region,such as New Delhi,Beijing, and Hong Kong. During the winter monsoon,becauseof increased atmospheric stability due to coldertemperature and lack of moisture,aerosol induced dynamicalfeedback is generally weaker compared to thatduring the summer monsoon, and the aerosol effectsare strongly controlled by the large-scale circulation, and less so the other way round.

The boreal summer monsoon is a very differentstory. Here,aerosol-monsoon interactions and dynamicalfeedbacks are strong and occur over all timescales.For the Indian monsoon,during the pre-summer monsoon(April through mid June),because of strongsouthwesterly low-level flow,abundance of moisture, and strong transport of desert dust into the monsoonregion,non-local effects are likely more important.Non-local responses and atmospheric feedbackinduced by aerosol radiative effects are key features ofthe Elevated Heat Pump(EHP)hypothesis(Lau and Kim, 2006; Lau et al., 2006,2008). Through the EHPeffect,aerosol heating from accumulation of absorbingaerosols(dust and BC)over the Indo-Gangetic Plain and central India during the pre-monsoon period canshift monsoon rainfall over central India to the Himalayanfoothills,strengthening the Indian monsoon during May–June(Fig. 3). The associated increasedlatent heating enhances the monsoon overturning circulation,i.e.,strong anomalous ascending motion overthe foothill region,coupled with descending motionover the Tibetan Plateau, and the northern IndianOcean. Because of the strong dynamical effect,themaximum response in temperature is found,not inthe region of maximum concentration of aerosol in thelower troposphere of the Indo-Gangetic Plain and centralIndia,but rather in the pristine upper troposphereover the Tibetan Plateau,due to adiabatic warmingfrom the anomalous subsidence over that region(Lau et al., 2008).

Fig. 3. Schematics showing the effects of absorbing aerosols (dust and BC) built up during the pre-monsoon period (April-May) over the Indo-Gangetic Plain,in altering the Indian monsoon evolution through the EHP-induced atmospheric feedback from (a) a "clean" monsoon to (b) a "dirty" monsoon during May-June.
4. Coupled aerosol-monsoon variability

In this subsection,representative examples showingthe importance of aerosol-monsoon interactionson intraseasonal and interannual variability and onextreme precipitation events in the Asian monsoonregions are presented. For a comprehensive review ofaerosol and monsoon variability,the readers are referred to three recent articles(Rosenfeld et al., 2014;Lau and Kim, 2015a,b).

First,absorbing aerosols can amplify the effectsof ENSO on the Indian monsoon. Since the seminalwork of Rasmusson and Carpenter(1983),it has beenfirmly established by thous and s of published worksthat anomalous SST from ENSO is the single most importantforcing agent and a major source of monsoonrainfall seasonal-to-interannual predictability. However,a recent study(Kim et al., 2015)changes thisparadigm,suggesting that strong ENSO SST influenceon the Indian monsoon may have large contributionsfrom the feedback induced via atmospheric heating bydust aerosols. Using multi-decadal years of satellitedata and the NCAR/NCEP reanalysis data,Kim et al.(2015)found a significant correlation between ENSO and AOD of absorbing aerosols(dust and BC)overnorthern India,with La Ni˜na(El Ni˜no)favoring high(low)AOD. Their composite analyses show that LaNi˜na years with high AOD(dusty ENSO)substantiallyamplify the ENSO signal than La Ni˜na yearswith “normal” AOD(pure ENSO). The monsoon responses,featuring enhanced rainfall over the southernArabian Sea,the Bay of Bengal, and the Himalayanfoothills,coupled with strong rising motion over theHimalayan foothills and sinking motion over the TibetanPlateau and the northern Indian Ocean,aremuch stronger during “dusty” compared with “pure”La Ni˜na,with areas of the maximum upper troposphericwarming(> 2℃)over the Tibetan Plateaumore than doubled(Fig. 4). These results have beencorroborated by an independent study(D’Errico et al., 2015),using the ECHAM-HAM coupled ocean-l and atmospheremodel,which includes both state-of-the-art radiative and microphysical effects with fully interactiveaerosols, and internal and external mixing processes.Desert dust aerosols are internally generatedby coupled l and -atmosphere processes in the model.Here,the authors analyzed model outputs for the last80 years from a thous and -year simulation with fixedpresent-day greenhouse gas(GHG) and anthropogenicaerosol emissions. Composite analyses with respect tohigh and low AOD over northern India clearly showthe much enhanced vertical motion, and warmer uppertroposphere over the Tibetan Plateau from Aprilthrough July,in agreement with Kim et al.(2015).These two studies strongly support the EHP hypothesisregarding the roles of aerosol-induced atmosphericfeedbacks in altering the trajectory of monsoon evolutionon interannual timescales.

Fig. 4. EHP induced structural changes in the Indian monsoon during May-June showing anomalies in (a,c) rainfall (mm day−1) and (b,d) temperature (℃) and circulation for a "clean" monsoon (a,b) compared to a "dirty" monsoon (c,d).Regions where the anomalies exceed the 1% and 5% statistical significances are indicated by large and small black circles.Adopted from Kim et al.(2015).

Second,absorbing aerosols can influence monsoonrainfall on fast(days to weeks)timescales,modulatingthe transitions of monsoon onsets,breaks, and activespells(Manoj et al., 2011; Hazra et al., 2013). Vinojet al.(2014)showed from satellite observations thatduring boreal summer,a strong positive correlationexists between daily rainfall over northern-central Indiawith AOD derived from MODIS and MISR overthe Arabian Sea. The authors found that the variabilityof AOD is mainly due to dust transported from theMiddle East deserts with a smaller contribution of seasalt from the Arabian Sea. They carried out a seriesof climate model simulations showing that anomalousaerosol heating,mainly due to absorption of solar radiationby the dust layer over the Arabian Sea,can leadto large-scale lowering of surface pressure,resultingin increased low-level southwesterly flow into northernIndia. The increased monsoon flow brings anomalouswater vapor,enhances moisture convergence,which further increases the southwesterly flow, and enhances rainfall over northern India. The change ofdust loading has to be sufficiently large in magnitude and coverage,for the atmospheric feedback to be excited.The timescale from a change in dust aerosols tothe changes rainfall is estimated to be of the order of5–10 days. This means that new atmospheric monsoonstates can be achieved quickly in equilibrium with theslow changing part,i.e.,the ocean,of the monsoonclimate system. The fast atmospheric feedback willchange the surface fluxes acting on the ocean,whichwill response to the accumulated fluxes changes overlonger timescales. Thus,the new equilibrium monsoonclimate will be quite different compared to theone without the fast feedback. Vinoj et al.(2014)didnot consider the impacts of aerosol on SST. Includingthe SST effect could prolong the influence of absorbingaerosols on monsoon,with important contributions tointerannual variability and long-term climate change(Lau,2014).

Third,previous studies have demonstrated thataerosols can affect strong dynamical systems such asstorm tracks over the North Pacific(Zhang et al., 2007),the Atlantic ITCZ(Lau et al., 2009a), and tropical cyclones via radiative,microphysical aerosoleffects,as well as modulating the large-scale verticalwind shear(Evans et al., 2011; Rosenfeld et al., 2012; Wang et al., 2012; Wang et al., 2014). Overthe Asian monsoon l and regions,a case study of theheavy precipitation events in June 2008 showed thatincreased heavy precipitation events in the Himalayanfoothills could be attributed to EHP induced nonlocaleffects,suppressing rainfall locally,but increasingrainfall downwind over the Himalayan foothills(Lau et al., 2009b). In a more recent study,Fan etal.(2015)using a convection-permitting high resolutionatmospheric model demonstrated that the disastrousflood event of 8–9 July 2013 over the mountainslopes of the Sichuan basin in southwestern Chinacould be caused by aerosols. They found that airpollutionaerosols trapped in the basin increased atmosphericstability and suppressed convection locally,but enhanced rainfall downstream over the mountainranges. The short-term responses found in Vinoj et al.(2014) and Fan et al.(2015)likely belong to the sameclass of aerosol induced atmospheric feedback as theEHP mechanism,whereby aerosol semi-direct effectsuppressed local rainfall,allowing the un-precipitatedmoisture to be advected by the prevailing winds and blocked by the downwind mountain ranges. As a result,moist static energy accumulates over the mountainslope. Forced orographic uplift enhances convectiveactivities, and eventually the precipitation inten sity is greatly enhanced through moisture convergencefeedback,over the mountain slope downwind fromthe aerosol source region. Extreme events with devastatingsocial and economic impacts appear to beoccurring more frequently and with increased intensityin Asian monsoon regions(Wang and Zhou, 2005;Goswami et al., 2006). The reasons are not clear,butcould be due to natural variability,global warming,aerosols, and most possibly combination of and interactionsamong these factors. The aforementionedexamples show that greater efforts need to be devotedto better underst and ing the mechanisms of aerosol impact, and both direct and indirect effects of aerosols onextreme heavy rain events and their predictions. For areview of a past major extreme heavy rain event overEast Asia,the readers are referred to Ding(2015).

5. Concluding remarks

From the preceding discussions and countlessother present and past works in the literature,we nowknow that aerosol effects on clouds,precipitation, and climate are not limited to anthropogenic sources inlocal regions,but also include natural sources suchas desert dusts,black carbon from natural biomassburning,biogenic volative organic compounds,sea saltfrom the oceans,as well as sulfates from volcanic eruptionsfrom nearby as well as remote sources. It isworth noting that globally natural aerosols have beenestimated to be at least up to 6–7 times more abundantthan present anthropogenic aerosols(Satheesh and Moorthy, 2005). A recent study(Carslaw et al., 2014)has also suggested that the largest uncertaintiesof aerosol effects in climate models may be rootedin the way that natural aerosols and their interactionwith the environments are represented in climate models.Additionally,natural aerosols such as desert dust and biogenic aerosols could be important in inducingatmosphere-ocean-l and feedback processes that contributeto the state of the monsoon climate,in quasiequilibriumwith the long-term anthropogenic forcing.Dust emissions could also be changed due to the increasingarid and semi-arid regions around the world inrecent decades,possibly due to l and use and change,aswell as the changing large-scale circulation from GHGwarming(Dai,2011; Fu,2003; Lau and Kim, 2015b).More importantly,studies cited here and many othershave shown that pre-monsoon heating/cooling byaerosols is important in determining the subsequentevolution and intensity of the monsoon precipitation,especially over l and . Hence,pre-monsoon aerosolbuilding up could possibly be used as a potential predictorfor l and precipitation over the monsoon regionson seasonal-to-interannual timescales. This obviouslyneeds to be tested and confirmed by further research.

Given the abundance of natural and anthropogenicaerosols in monsoon regions, and their fundamentalcontributions to the distribution of heatsources and sinks, and as CCN for formation of clouds and precipitation,a deeper underst and ing of aerosol effects on monsoon climate must be built on a newframework that treats natural aerosols with the samestatus as temperature,moisture,clouds, and precipitation,i.e.,as an integral part of a natural aerosolmonsoonclimate system(inner circle in Fig. 5),governedby fundamental thermodynamic,dynamics, and diabatic heating controls. These controls togetherwith related regional processes and remote forcingfrom teleconnections(middle ring in Fig. 5)are likelyto be altered by humans through increased GHG and anthropogenic aerosol emissions,as well as l and use and change practices(outer ring in Fig. 5). Furtherwork should focus on how fundamental processesgoverning the natural aerosol-monsoon climate systemhave been, and will be altered by GHG,anthropogenicaerosols, and l and use and change jointly and separately.Under such a framework,observations and modeling strategies can be developed to better underst and the impacts of humans on monsoon climatevariability and change, and to inform sound policy foradaptation and mitigation.

Fig. 5. A schematic showing key components of an aerosol-monsoon climate system:Intrinsic observables of the natural system (inner circle),fundamental governing processes (middle ring),and anthropogenic forcing (outer ring).
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