Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Antarcticseaice: variability,trends,driversand21stcenturyprojections. Marilyn Raphael University of California, Los Angeles Department of Geography SeaIceChallengesWorkshop– Hobart,Tasmania12– 14th May,2015 Acknowledgements: • Thispresentationisasynthesisofworkpublishedandunpublished.Iwantto acknowledgenumerouscolleagues,manycitedhereforthecontributionthattheir publishedworkhasmadetothispresentation. • IalsowanttoacknowledgeWillHobbsforthegenerousprovisionofoneofthekey Figuresinthispresentation. • Mypresenceherewouldnotbepossiblewithoutthekindinvitationandsupportof COMNAP. PresentationStructure: 1. Antarcticseaice–climatology,variability,trends 2.Antarcticseaicetrenddrivers 3.ModelsimulationofandprojectionsforAntarcticseaice Antarctic Sea Ice from 1979 to 2009 Summer and Winter Maximum Sea ice extent increasing slightly on average. (NSIDC, 2010) DailyAntarcticseaiceextentshowingindividualannualcyclesfrom 1978‐ 2014 Trendintotalseaiceextent Annualspatialtrendinseaice StandardDeviationOfSeaIceExtentAnomaliesByLongitude. Boundariesoffiveregionsofseaicevariabilitydefinedbythe minimainstandarddeviation(RaphaelandHobbs,2014) AnnualcycleofeachseaiceregionbasedonSSMIaveragedailyseaicefraction Therearedistinctdifferencesintimingofadvanceandretreatandthelength oftimethattheyremainatmaximumextent,thatmayunderliethe differencesinvariabilitythattheregionsexhibit.(RaphaelandHobbs,2014) Lineartrendsinseasonal‐meanSICcalculatedover1980–2008for(firstrow)DJF, (secondrow)MAM,(thirdrow)JJA,and(fourthrow)SON.(Simpkinsetal,2012) TheDriversofAntarcticSeaIceTrends: Thesearenumerous,ameasureofthecomplexityoftheAntarctic seaicesystem. SouthernHemisphereatmosphericcirculation– SAM,ASL, ZW3 Tropicalcirculation‐ ENSO,AMO(Lietal,2014,Schneideretal, 2012;Dingetal,2011) Freshwaterinfluxfrombasalmeltingoficeshelves– Bintanja etal,2013 WindsonIceMotionandDrift– HollandandKwok,2012 Ice‐Oceanfeedbacks‐ Goosse andZunz,2014 Atmosphere‐OceanFeedbacks‐ Powelletal2005;Zhang,2007 -5.0 -3.0 -1.0 1.0 3.0 5.0 7.0 TheSAMisalargelyzonallysymmetricstructurerepresentinganalternationin massbetweenthemiddleandhighlatitudesoftheSH.AnindexoftheSAM measuresthepressuregradientbetweenthemiddleandhighlatitudesofthe SH. S e a so n a l S A M • ApositiveSAMinducesanoveralltransientcoolingthroughtheenhanced EkmantransportofcoldsurfacewatersnorthwardfromAntarctica promotingsea‐icegrowth. • ThereisagreementthatthecurrentpositivetrendintheSAM,particularly insummerandautumnisduetostratosphericozonedepletion. Year -7.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 SouthernAnnularMode(SAM) ZonalWaveThree(ZW3) ZW3inducespreferredregionsofequatorward andpoleward flowthereby influencingthemeridional transportofheatintoandoutoftheAntarcticwith resultingimpactontemperatureandseaiceextent. • ResearchsuggeststhatZW3hasthepotentialtoinfluencetheregionality of Antarcticseaicetrends. AmundsenSeaLowandassociatedwinds ASLvariabilityinfluencestheclimateofWestAntarcticabycontrollingthemeridional componentofthelarge‐scaleatmosphericcirculation,withconsequencesmeridional windvelocity,surfaceairtemperature,precipitation,andseaiceconcentration. • EnhancednortherlyairflowacrossthewesternAntarcticPeninsulasector,resultsin highersurfaceairtemperature,andreducedseaiceextentintheBellingshausenand easternAmundsenSeas. • EnhancedsoutherlyflowofcoldcontinentalairalongthewesternflankoftheASL willresultinincreasedseaiceextentinthewesternAmundsenandRossSeas. PotentialoftheASLtoexplain someintraseasonal variability inAntarcticseaice. DothecharacteristicsoftheASL(its centralpressure,longitude,latitude) duringiceadvancehaveanyrelationship withtheseaiceduringtheiceretreat immediatelyfollowing(e.g.,advancein 1979influencingretreatin1979/1980)? Raphael,HollandandLandrum,2015in prep. WindsonicemotionandDrift: Largeandstatisticallysignificantchangesin icemotionaredrivenbychangesinthe winds.(a),Ice‐motiontrendvectorsoverlaid on19‐yearchangeinmeridional icespeed (changeislineartrendmultipliedbyperiod, positivenorthwards;blackvectorshave meridional ice‐motiontrendssignificantat >90%).(b),ERA‐Interimreanalysis10‐m windtrendvectorsoverlaidontrendinsea‐ levelpressure(whiteandgreycontours showpressuretrendssignificantat90%and 95%;blackvectorshavemeridional wind trendssignificantat>90%;magentacontour showsextentofmotiontrends)(Hollandand Kwok,2012.) Ice‐OceanFeedback SchematicrepresentationofthestabilizationoftheSouthernOceanbyseaiceprocesses(Goosse and Zunz,2014). • IncreaseinSICisassociatedwithdecreasedmixedlayerdepthandstabilizationof thewatercolumnduetoboththenetinflowofwaterandbrinerejection.Thewater columnbecomesverystratified,limitingtheverticaltransferoftheoceanicheat fluxandandmaintainingalargerseaiceextent. Tropicalcirculation‐ AMO ‐ SeasurfacewarmingrelatedtotheAtlanticMultidecadal Oscillation(AMO) reducesthesurfacepressureintheAmundsenSeaandcontributestotheobserved dipole‐likesea‐iceredistributionbetweentheRossandAmundsen–Bellingshausen– WeddellseasandtotheAntarcticPeninsulawarming(Lietal,2014). ENSO– someinfluenceonthereducedseaiceextentintheBellingshausenSea.But notsufficienttocauseallofthenegativetrend(Liuetal,2004) FreshwaterInfluxfrombasalmeltingoficeshelvesfresheningseawatertoallowice toformmoreeasily– Bintanja etal(2013)– thedegreeofinfluenceofthis mechanismisinquestionas,forexample,SwartandFyfe(2013)findthatthis freshwaterinputisnotsignificantenoughtoproduceseaicetomatchtheobserved increase. Atmosphere‐Oceanfeedbacks– warmerclimateleadingtomoreprecipitation (snow)andmoresnow‐iceformation(Powelletal2005,Zhang,2007) CMIP5multi‐modelmeanseaiceconcentration– February,September Multi‐modelmeanofseaiceconcentration,computedfromhistoricalsimulations overtheperiod1979–2005.White(black)linereferstotheseaiceedge,i.e.the15% concentrationlimitofthemulti‐modelensemblemean(observations)Zunz etal, 2013. (a)MonthlymeanofSouthernOceanseaiceextent,computedovertheperiod 1979–2005.(b)Standarddeviationofdetrended SouthernHemisphereseaice extent,computedovertheperiod1979–2005foreachmonthoftheyear.Colours correspondtotheensemblemeanofhistoricalsimulationsfrom24different models.Dottedlinesrefertomodelsthatprovidebothhistoricalandhindcast simulations,buthereresultsareonlyfromhistoricalsimulations.Orangeboldline isthemulti‐modelmean.Blackboldlinereferstoobservations.Zunz etal,2013. Whydon’tthecoupledclimatemodelsproducetheincreasedseaiceextent thatisobserved? ManyofthemodelshaveanannualSIEcyclethatdiffersmarkedlyfromthatobserved overthelast30years.AlargenumberofmodelshaveanSIEthatistoosmallatthe minimuminFebruarytheapproximatetimewhenthelargestpositivetrendinSIEhas beenobserved.Turneretal,2013suggeststhatthiswillmakeitdifficultforthemto simulatetheoverallincreaseinSIE. Modelsdonotsimulatetheice,ocean,andatmosphericprocessesresponsibleforthe observedtrendinseaice.Thiscouldbebecauseofmodelresolutionbeingtoolarge scalesothatsubgridscale processesintheiceandoceanhavetobeparameterised. Projectedseaicetrendsforthe21st century Projectedseaicetrendsbysectorfor8CMIP5modelsthatsuccessfullysimulated theobservedseaicetrendinautumn(MAM)(Hobbs,2015unpublished) SUMMARY 1. Antarcticseaice–climatology,variability,trends. Antarcticseaiceextentdisplaysaconsistentannualcycleandstrongspatialvariability. Observationsshowanetincreaseinseaiceextentovertherecord.Antarcticseaicecannot betreatedsolelyasaintegratedvariable,thereissignificanttemporalandspatialvariability intheicecover. 2.Antarcticseaicetrenddrivers Numerous,physicallyplausibleagentshavebeenproposedasdriversoftheobserved seaicetrendbutnonesofarhaveproventobeabletoexplainallofthetrend.Thisisan indicationofthecomplexityofthesystem.Theanswermaylieinacombinationofthe proposedfactors. 3.ModelprojectionsforAntarcticseaice. Climatemodelsproduceahemisphericdecreaseinseaicewhereasobservationsshowan overallincreaseinseaiceextentthatisdominatedbythemainareaofincreaseinthe RossSea.Projectionsofseaicesuggestacontinuationofthenegativetrendinseaice extentintothe21st century. Climatemodelsareunabletosimulatethekeyfeaturesoftheannualcycleofseaice. Thisfailuremayberelatedtomodelinabilitytosimulatefundamentalseaiceprocesses. Thisisthecasewhenmodelresolutionistoolargescalesothatsubgridscale processesin theiceandoceanhavetobeparameterised. Standarddeviationofseaiceextentanomaliesbylongitude(black)and decorrelation lengthscales(red).Newseaiceboundaries(dashedblack lines);Traditionalboundaries(ParkinsonandCavalieri,2012)– dashedblue lines.Thegreyshadedregionisazonewherethenewseaiceboundary betweenEastAntarcticaandtheRoss‐Amundsensectorisuncertain.