Download Antarctic sea ice: variability, trends, drivers and 21st century

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.