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:
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*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
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