Download Tropical Belt WG

ProspectusforaUSCLIVARWorkingGroup:
ChangingWidthoftheTropicalBelt
1.Motivation
ThetropicsareafundamentalclimatezoneonEarth,characterizedbyabandof
deepconvectivestormsalongtheEquator,knownastheIntertropicalConvergenceZone
(ITCZ),andboundedbyaridregionsinthesubtropicsofeachhemisphere,suchasthe
AfricanandAustraliandeserts.Together,themoistanddryregionsofthetropicsrepresent
surfacemanifestationsoftheHadleycirculation,themeanmeridionalcirculationinthe
tropicaltropospherewithascentneartheEquator,polewardflowinthetropicalupper
troposphere,anddescentinthesubtropicsofeachhemisphere.Theboundariesofthe
Hadleycirculationarenotstationarybutrathervaryintime.Infact,recentevidencehas
shownthatthetropicshaveexpandedoverthelastseveraldecades,pushingtheboundaries
oftheHadleycirculationandtheassociatedsubtropicaldryzonesfurtherpolewardineach
hemisphere(e.g.,Seideletal.,2008;FungandFu,2013;Birneretal.,2014).Ifsucha
climateshiftweretocontinueoverthe21stcenturyasprojectedbyclimatemodels(e.g.,Lu
etal.,2007;JohansonandFu,2009;Huetal.,2013),itcouldhavesubstantialimpactsonthe
waterresourcesofecosystemsandhumanpopulationcentersatthepolewardedgesofthe
currentsubtropicaldryzones(SherwoodandFu,2014),includingregionsofthesouthern
UnitedStates.
CurrenttheoriesexplainthelocationofthepolewardboundariesoftheHadley
circulationusingeitheracombinationofangularmomentumandenergyconservation
arguments(Schneider,1977;HeldandHou,1980;LindzenandHou,1988),orinstability
argumentsinvolvingbarocliniceddies(Held,2000;WalkerandSchneider,2006).These
theoriessuggestthattheHadleycellislikelytowideninawarmerclimate,asthetropics
becomemoremoist,andthetropicalatmospherebecomesaccordinglymorestable
(Friersonetal.,2007).Idealizednumericalsimulationsspanningarangeofclimatesagree
that,inclimatessimilartothatofthepresentday,awarmingclimateshouldresultina
widertropicalbelt(KortyandSchneider,2008;LevineandSchneider,2011;Levineand
Schneider,2015).Moresophisticatedandcomprehensivegeneralcirculationmodelsand
earthsystemmodelsalsogenerallyprojectawideningoftheglobalHadleycirculationin
globalwarmingscenarios(Luetal.,2007;Huetal.,2013).
Anumberofpapersfromthelastdecadehaveprovidedobservationalevidencethat
thetropicshaveexpandedinthelast~3decades(e.g.,Hudsonetal.,2006;Seideland
Randel,2007;HuandFu,2007;Seideletal.,2008),possiblyatahigherratethanthat
predictedbyclimatemodels(JohansonandFu,2009;Quanetal.,2013).However,the
interpretationoftheseresultsiscomplicatedbythefactthatindividualstudiesusedifferent
metricsforthewidthofthetropics,whichresultsintrendsrangingfromanear-zerochange
inthewidthofthetropicstouptoa2degreelatitudetropicalexpansionperdecade(e.g.,
Birner,2010;DavisandRosenlof,2012;Birneretal.,2014).Additionally,itremainsunclear
whethertheobservedtrendsinthewidthofthetropicsareduetoincreasinggreenhouse
gaslevels,oraremerelyasignatureofnaturalvariabilityintheocean-atmospheregeneral
circulation,suchastheElNiño-SouthernOscillation(ENSO)andthePacificDecadal
Oscillation(PDO).
Toaddresstheseissues,aswellasthesocietalandecologicalimpactsofan
expandingtropicalbelt,anAmericanGeophysicalUnion(AGU)ChapmanConferencewas
heldinSanteFe,NewMexicoon27-31July2015.Thisconferencebroughttogether
atmosphericandoceanicdynamicists,aswellasobservationalscientistsandhydrologists.
Newinsightsemergedthatcalledintoquestiontheprevailingviewaboutthenatureand
causesofchangesinthewidthofthetropics.Keyconclusionsofthismeeting(inour
interpretation)wereasfollows:
·Recentobservedestimatesoftheexpansionofthetropicsvarywidely,rangingfrom
0.25to3degreeslatitudeperdecade(DavisandRosenlof,2012),dependingonthetime
period,theinstrumentordataset,andthemetricusedtomeasureedgeofthetropics(Davis
andBirner,2013;Lucasetal.,2014).Thesheernumberofmetricsisnotable.Itwas
recommendedthatagroupbeconvenedtowriteapeer-reviewedjournalarticleto
documentwhateachmetricphysicallyrepresentsabouttheedgeofthetropics,toexplore
howthemetricsrelatetooneanother,andtorecommendasmallersubsetofmetricstobe
used,consistentlyandreproducibly,infuturestudies.
·Thecausesofvariabilityinthewidthofthetropicsondecadaltimescalesremainhighly
uncertain.Priortothismeeting,theprevailingviewpointwasthatrecentvariabilityinthe
widthofthetropicswasthedirectresultofanthropogenicforcing.Whiletherolesof
stratosphericozonedepletionintheSouthernHemisphere(Waughetal.,2015)and
aerosolsintheNorthernHemisphere(Allenetal.,2014)werenoted,theoverallconsensus
ofthismeetingwasthatnaturalvariabilitydominatedtropicalwideningoverthepast
severaldecades,primarilythroughmodesofcoupledatmosphere-oceanvariabilitysuchas
ENSO(Monteiroetal.,2015)orthePDO(Allenetal.,2014).Asaresultofthisshiftin
viewpoint,futureresearchwasrecommendedtoaddresstheroleofcoupledatmosphereoceanvariabilityinthewidthofthetropicalbelt,andhowchangesintheHadleycirculation
mightfeedbackonthegeneralcirculationoftheocean.
·ResultsfromCook(2003),KarnauskasandUmmenhofer(2014),andLucasandNguyen
(2015)havecalledintoquestionthemeritofexaminingthetraditionalzonal-meanviewof
theHadleycirculationforlocalimpacts.KarnauskasandUmmenhofer(2014)noted,for
example,thataHadleycell-likeoverturningcirculationisnottypicallypresentatall
longitudesinthetropics,butratherisconfinedtolongitudebandsineasternsubtropical
oceanbasins,wheretherearelargeland-seatemperaturecontrasts.Itremainsunclear
whethertherecentobservedwideningofthetropicsisfocusedintheseparticularlongitude
bands,orismoreglobalinitsextent.Itwasrecommendedthatabetterconnectionbe
madebetweenthezonal-meanexpansionoftheHadleycirculationandchangesinthe
regionalcirculationsofthetropicalandsubtropicalzones.
2.
Objectives,Tasks,Timeline
ToimplementtherecommendationsfromtherecentAGUChapmanconference,wepropose
toestablishathree-yearUSCLIVARWorkingGroupontheChangingWidthoftheTropical
Belt.Theobjectivesfortheproposedworkinggroupareasfollows:
Objective#1:Provideguidanceonwhichmetricsaremostappropriatetoquantifykey
impactsofthechangingwidthofthetropicalbelt.Todothis,wewillassessandsynthesize
knowledgefromnumerousstudiesthathaveuseddifferentmetrics,timeperiods,and
modelforcingscenariostodocumenttheexpansionofthetropics.Ourrationaleforthis
objectivearisesfromthelackofconsensusachievedduringdiscussionsattheChapman
Conference.Weexpectthataselectionofstandardoraccepteddefinitionsandmetricsmay
assistinthecommunicationoftropicalwidthchangestothebroaderscientificcommunity.
ThisobjectivedirectlyaddressestheUSCLIVARsciencegoalto“betterquantifyuncertainty
intheobservations,simulations,predictions,andprojectionsofclimatevariabilityand
change”,andthemissionoftheUSCLIVARPOSpanelto“developsynthesesofcritical
climateparameters.”
Objective#2:Identifyhowanthropogenicforcingandnaturalatmosphere-oceanvariability
contributeuniquelytodecadaltimescalechangesinthewidthofthetropicalbelt.Akey
themeoftherecentChapmanConferencewasthedifficultyofseparatingtherolesof
naturalvariability(particularlyfromcoupledatmosphere-oceanvariability,suchasENSO
andthePDO)andanthropogenicforcinginrecenttrendsinthewidthofthetropicalbelt.
Kangetal.(2013)havepreviouslyidentifiedcoupledatmosphere-oceanvariabilityasakey
sourceofuncertaintyinfuturetropicalcirculationtrends.Furthermore,thetropical
oceanicoverturningcirculationisintegrallylinkedwiththeatmosphericHadleycirculation
(Clement,2006),asthecirculationsmirroreachotherthroughanEkmanforcebalance
(Schneideretal.,2014).Understandingtheinterplaybetweenatmosphere-oceancoupling
andthevaryingwidthofthetropicsrequiresajointteamofatmosphericscientistsand
oceanographerstoprovidenovelinsightsintopastobservedclimatevariabilityandfuture
climatemodelprojections.ThisobjectivedirectlyaddressestheUSCLIVARsciencegoalsto
1)“understandtheroleoftheoceansinobservedclimatevariabilityondifferent
timescales”and2)“improvethedevelopmentandevaluationofclimatesimulationsand
predictions.”ThisobjectivealsoservesthemissionoftheUSCLIVARPSMIpanelto“reduce
uncertaintiesinthegeneralcirculationmodelsusedforclimatevariabilitypredictionand
climatechangeprojectionsthroughanimprovedunderstandingandrepresentationofthe
physicalprocessesgoverningclimateanditsvariation.”Finally,thisobjectiveisdirectly
relevanttotheUSCLIVARResearchChallengeconcerningdecadalvariabilityand
predictability.
Objective#3:Addresshowtheglobal-scalewideningofthetropicsismanifestedthrough
regional-scaleimpacts.DuringtherecentChapmanConference,ageneralconsensuswas
reachedthattheimpactsofclimatevariabilityandchangeareexperiencedlocally,butit
wasnotgenerallyagreedthatthetraditionalzonal-meanviewoftheHadleycirculationwas
atallmeaningfulintermsofimpacts.Understandingtherelevanceofglobal-scaleHadley
circulationchangestoregional-scaleimpactsinthesubtropicsiscriticaltocommunicating
theimpactsoftheexpandingtropicstopolicymakers.Thisobjectivedirectlyaddressesthe
USCLIVARsciencegoalto“understandtheprocessesthatcontributetoclimatevariability
andchangeinthepast,present,andfuture,”andthemissionoftheUSCLIVARPOSpanelto
“improveunderstandingofclimatevariationsinthepast,present,andfuture.”This
objectivealsoservesthemissionoftheUSCLIVARPPAIpanel,withitsfocusonpredictions
andinterfacingwiththeapplications,operations,anddecision-makingcommunity.
Objective#4:Coordinateeffortwithotherinternationalprograms(e.g.,SPARCDynVar,
WCRPGrandChallengeonClouds,Circulation,andClimateSensitivity,GEWEX
HydroclimatologyPanel),andinformfundingagenciesofwhereresearchinitiativesare
neededtoadvanceunderstanding.Werecognizethattheproposedworkinggroupisoneof
manygroupswithstronginterestinpastandfuturechangesinthetropicalatmospheric
andoceanicgeneralcirculation,andthatoureffortswillbemosteffectiveinasmuchasthey
leverageinterconnectionswithexistingresearchcommunities.Assuch,wehavesuggested
membersofthisworkinggroupwithadiverserangeofexpertise,tohelpbuildbridges
acrossdistinctresearchcommunitiesinterestedintheimpactsofglobal-scaletropical
circulationchanges.ThisobjectivedirectlyaddressestheUSCLIVARsciencegoalto
“collaboratewithresearchandoperationalcommunitiesthatdevelopanduseclimate
information,”andthemissionoftheUSCLIVARPPAIpanelto“fosterimprovedpracticesin
theprovision,validationandusesofclimateinformationandforecaststhroughcoordinated
participationwithintheU.S.andinternationalclimatescienceandapplications
communities.”
Toaccomplishtheaboveobjectives,theworkinggroupwillcompletethefollowingtasks:
1)Publishapeer-reviewedjournalarticletoprovidethecommunitywitha
comprehensiveassessmentoftropicalwidthmetricsandtorecommendasubsetof
metricstobeusedbysubsequentstudies(tobetteraidincomparisonamong
studiesacrossthepublishedliterature).
2)Compareandcontrasttheimpactsofanthropogenicforcingandcoupled
atmosphere-oceanvariabilityondecadalvariabilityintropicalwidth.Synthesize
ourunderstandingofcouplingbetweentropicalwidthandseasurface
temperatures.
3)Informlinkagesbetweenglobal-scalecirculationchangesandtheregional-scale
circulationchangesmostrelevantforimpacts,settingthestageforasessionatthe
AGUFallMeetingonglobalandregionalimpactsoftropicalcirculationchange.
Thetaskswillbecompletedaccordingtothefollowingtimeline:
● Year1:Planandscheduleworkinggroupactivities,reviewpreviousresearch,and
identifystrategiesforworkinggroupresearchinitiatives.
● Year2:Research,assessandsynthesizeresearchfindings,andbegintodraftpeerreviewedjournalarticlestosummarizeworkinggroupfindings.
● Year3:Finalizeresearchandjournalarticles,holdsessionatAGUFallMeeting,
reportresultstoCLIVAR,andproposefuturework.
3.PublicationsandOutreach
Thepublicationsresultingfromtheactivitiesoftheproposedworkinggroupwillinclude:
1)Apaperthatprovidesacomprehensiveassessmentoftropicalwidthmetricsandthat
recommendsasubsetofmetricstobeusedbysubsequentstudies
2)Apaperthatreviewshowvarioustypesofcoupledatmosphere-oceanvariabilityare
linkedtothewidthofthetropicalbelt
3)Apaperthatinvestigateslinkagesbetweenglobal-scaletropicalcirculationchangesand
regional-scaleimpacts(summarizingtheresultsoftheAGUsessionheldonthistopic)
TheoutreachoftheworkinggroupwillincludepresentationstoUSCLIVAR,articlesinthe
USCLIVARVariationsnewsletter,andspecialsessionsattheAGUorAmerican
MeteorologicalSocietyannualmeetings.
4.ReportingPlan
Aslistedabove,theobjectivesoftheproposedworkinggrouparehighlyrelevanttothe
sciencegoalsofUSCLIVAR,aswellastothemissionsofthethreeUSCLIVARpanels(POS,
PSMI,andPPAI).SeveralmembersofthePSMIpanelrecommendedtheideaforthis
workinggroup,andweproposetoreportourprogresstoboththePOSandPSMIpanelsand
seektheiradviceandsupport.WealsoproposetoreportonourprogressattheannualUS
CLIVARsummitandpanelmeetings,asappropriate.
5.LeadershipandSuggestedMembership
PaulStaten(IndianaUniversity)andKevinGrise(UniversityofVirginia)willserveascochairsoftheproposedworkinggroupandwereresponsibleforthedevelopmentofthis
prospectus.Theco-chairshaveexpertiseinatmosphericandclimatedynamics,anduse
bothobservationsandglobalclimatemodelexperimentstounderstandtheatmospheric
generalcirculation,itsvariability,anditsresponsetoanthropogenicforcing.
Weproposetohaveaworkinggroupmembershipwithdiverseexpertise,including
memberswithspecialtiesinatmosphericdynamics,oceandynamics,atmosphere-ocean
interaction,globalclimate,regionalclimate,monsoons,observationalanalysis,andclimate
modeling.Wealsoproposeahealthymixofscientistsatdifferentcareerstages,to
maximizecommunityinvolvementandnetworking/mentoringopportunities.Suggested
membersalongwiththeiraffiliationsandareasofexpertisearelistedbelow:
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
*OriAdam(ETHZurich):Energeticconstraintsofthetropicalcirculation
RobertAllen(California-Riverside):Aerosolsandoceanicvariability
ThomasBirner(ColoradoState):Troposphere-stratosphereinteractions
GangChen(UCLA):Atmosphericdynamics,regionalimpacts
KerryCook(UniversityofTexas):Regionalclimatechangeimpacts
SeanDavis(NOAA):Stratosphericozoneandwatervaporvariability
QiangFu(UniversityofWashington):Remotesensingandradiativetransfer
KevinGrise(UniversityofVirginia):Atmosphericcirculationvariabilityandchange
KristopherKarnauskas(Univ.ofColorado):SimulatedregionalHadleycirculations
*JamesKossin(NOAA,Univ.ofWisconsin):Hurricanesandclimatevariability
*ChrisLucas(BureauofMeteorology,Australia):Tropicalwidthobservations
*AmandaMaycock(UniversityofLeeds):Troposphere-stratosphereinteractions,
chemistrycoupling
*TimothyMerlis(McGillUniversity):Ocean-atmospheregeneralcirculation,tropical
meteorology
*Xiao-WeiQuan(Univ.ofColorado,NOAA):Rainfallvariabilityandhydrological
impacts
*KarenRosenlof(NOAA):Stratosphericdynamicsandcomposition
IslaSimpson(NCAR):Linkageofglobal-scaleandregional-scaleclimatechange
PaulStaten(IndianaUniversity):Atmosphericdynamics
CarolineUmmenhofer(WoodsHole):Rainfallvariabilityandhydrologicalimpacts
DarrynWaugh(JohnsHopkins):Circulationwideningandstratosphericozone
*Indicatesacontributingmember
6.ResourceRequirements
Theproposedworkinggroupwillmeetonceperyearfor2-3daysandwillholdmonthly
teleconferences.Specifically,werequestfundingfor:
-Travelsupportfortwoannualworkinggroupmeetings
-Publicationchargesforthethreejournalarticlesdetailedabove
References
AllenR.J.,J.R.Norris,andM.Kovilakam,2014:Influenceofanthropogenicaerosolsandthe
PacificDecadalOscillationontropicalbeltwidth.NatureGeosci.,7,270–274,
doi:10.1038/ngeo2091.
Birner,T.,S.M.Davis,andD.J.Seidel,2014:ThechangingwidthofEarth’stropical
belt.PhysicsToday,67,38-44,doi:10.1063/PT.3.2620.
Birner,T.,2010:Recentwideningofthetropicalbeltfromglobaltropopausestatistics:
Sensitivities.J.Geophys.Res.Atmos,115,D23109,doi:10.1029/2010JD014664.
Clement,A.C.,2006:TheRoleoftheOceanintheSeasonalCycleoftheHadleyCirculation.J.
Atmos.Sci.,63,3351–3365,doi:http://dx.doi.org/10.1175/JAS3811.1
Cook,K.H.,2003:RoleofContinentsinDrivingtheHadleyCells.J.Atmos.Sci.,60,957–976,
doi:10.1175/1520-0469(2003)060<0957:ROCIDT>2.0.CO;2.
Davis,N.A.,T.Birner,2013:Seasonaltomulti-decadalvariabilityofthewidthofthetropical
belt.J.Geophys.Res.,118,doi:10.1002/jgrd.50610,2013.
Davis,S.M.,andK.H.Rosenlof,2012:Amultidiagnosticintercomparisonoftropical-width
timeseriesusingreanalysesandsatelliteobservations.J.Climate,25,1061–1078,
doi:10.1175/JCLI-D-11-00127.1.
Feng,S.,andQ.Fu,2013:Expansionofglobaldrylandsunderawarmingclimate,Atmos.
Chem.Phys.,13,10081-10094,doi:10.5194/acp-13-10081-2013.
Frierson,D.M.W.,J.Lu,andG.Chen,2007:WidthoftheHadleycellinsimpleand
comprehensivegeneralcirculationmodels.Geophys.Res.Lett.,34,L18804,
doi:10.1029/2007GL031115.
Held,I.M.,2000:Thegeneralcirculationoftheatmosphere.GeophysicalFluid
DynamicsProgram,WoodsHoleOceanographicInstitute,70pp.
Held,I.M.,andA.Y.Hou,1980:Nonlinearaxiallysymmetriccirculationsinanearly
inviscidatmosphere.J.Atmos.Sci.,37,515–533,doi:10.1175/15200469(1980)037<0515:NASCIA>2.0.CO;2.
Hu,Y.,L.Tao,andJ.Liu,2013:PolewardexpansionoftheHadleycirculationin
CMIP5simulations.Adv.Atmos.Sci.,30,790–795,10.1007/s00376-012-2187-4.
Hu,Y.,andQ.Fu,2007:ObservedpolewardexpansionoftheHadleycirculationsince1979.
Atmos.Chem.Phys.,7,5229-5236,doi:10.5194/acp-7-5229-2007.
Hudson,R.D.,M.F.Andrade,M.B.Follette,andA.D.Frolov,2006:Thetotalozonefield
separatedintometeorologicalregimes–PartII:NorthernHemispheremid-latitudetotal
ozonetrends.Atmos.Chem.Phys.,6,5183–5191,doi:10.5194/acp-6-5183-2006.
Johanson,C.M.,andQ.Fu,2009:Hadleycellwidening:Modelsimulationsversus
observations.J.Climate,22,2713–2725,doi:10.1175/2008JCLI2620.1.
Kang,S.M.,C.Deser,andL.M.Polvani,2013:Uncertaintyinclimatechangeprojectionsof
theHadleycirculation:Theroleofinternalvariability.J.Climate,26,7541–7554,doi:
10.1175/JCLI-D-12-00788.1.
Karnauskas,K.B.,andC.C.Ummenhofer,2014:OnthedynamicsoftheHadleycirculation
andsubtropicaldrying.ClimateDyn.,42,2259-2269,doi:10.1007/s00382-014-2129-1.
Korty,R.L.,andT.Schneider,2008:ExtentofHadleycirculationsindryatmospheres.
Geophys.Res.Lett.,35,L23803,doi:10.1029/2008GL035847.
Levine,X.J.,andT.Schneider,2015:BarocliniceddiesandtheextentoftheHadley
circulation:AnidealizedGCMstudy.J.Atmos.Sci.,72,2744-2761,doi:10.1175/JAS-D-140152.1.
Levine,X.J.,andT.Schneider,2011:ResponseoftheHadleycirculationtoclimatechangein
anaquaplanetGCMcoupledtoasimplerepresentationofoceanheattransport.J.Atmos.
Sci.,68,769-783,doi:10.1175/2010JAS3553.1.
Lindzen,R.S.,andA.Y.Hou,1988:HadleyCirculationsforZonallyAveragedHeating
CenteredOfftheEquator.J.Atmos.Sci,45,2416-2427,10.1175/15200469(1988)045<2416:HCFZAH>2.0.CO;2.
Lu,J.,G.A.Vecchi,andT.Reichler,2007:ExpansionoftheHadleycellunderglobalwarming.
Geophys.Res.Lett.,34,L06805,doi:10.1029/2006GL028443.
Lucas,C.,andH.Nguyen,2015:Regionalcharacteristicsoftropicalexpansionandtheroleof
climatevariability.J.Geophys.Res.Atmos.,120,6809-6824.
Lucas,C.,B.Timbal,andH.Nguyen,2014:Theexpandingtropics:acriticalassessmentof
theobservationalandmodelingstudies,WIREsClimChange,5,89-112,
doi:10.1002/wcc.251.
Monteiro,J.,J.M.Wallace,J.Sukhatme,andR.G.Murtugudde,2015:Thecontributionof
ENSOvariabilitytotherecentexpansionofthetropicalbelt.PresentedatAGUChapman
ConferenceonTheWidthoftheTropics:ClimateVariationsandTheirImpacts.July29,2015.
Quan,X.-W.,M.P.Hoerling,J.Perlwitz,H.F.Diaz,andT.Xu,2014:HowFastAretheTropics
Expanding?,J.Climate,27,1999–2013,doi:http://dx.doi.org/10.1175/JCLI-D-13-00287.1.
Schneider,E.K.,1977:Axiallysymmetricsteady-statemodelsofthebasicstatefor
instabilityandclimatestudies.PartII.Nonlinearcalculations.J.Atmos.Sci.,34,280–296,
doi:10.1175/1520-0469(1977)034<0280:ASSSMO>2.0.CO;2.
Schneider,T.,T.Bischoff,andG.H.Haug,2014:MigrationsanddynamicsoftheIntertropical
ConvergenceZone.Nature,513,45-53,doi:10.1038/nature13636.
Seidel,D.J.,Q.Fu,W.J.Randel,andT.J.Reichler,2008:Wideningofthetropicalbelt
inachangingclimate.Nat.Geosci.,1,21–24,doi:10.1038/ngeo.2007.38.
Seidel,D.J.,andW.J.Randel,2007:Recentwideningofthetropicalbelt:Evidencefrom
tropopauseobservations.J.Geophys.Res.,112,D20113,doi:10.1029/2007JD008861.
Sherwood,S.,andQ.Fu,2014:,ADrierFuture?,Science,343,737-739,
doi:10.1126/science.1247620.
Walker,C.C.,andT.Schneider2006:EddyinfluencesonHadleycirculations:Simulations
withanidealizedGCM.J.Atmos.Sci.,63,3333-3350,doi:10.1175/JAS3821.1.
Waugh,D.W.,C.I.Garfinkel,andL.M.Polvani,2015:Driversoftherecenttropical
expansionintheSouthernHemisphere:ChangingSSTsorozonedepletion?J.Climate,in
press.