Interhemispheric coupling, the West Antarctic Ice Sheet and warm Antarctic interglacials

13 Pages·2010·5 MB·English

Clim. Past,6,431–443,2010
Climate
www.clim-past.net/6/431/2010/
of the Past
doi:10.5194/cp-6-431-2010
©Author(s)2010. CCAttribution3.0License.
Interhemispheric coupling, the West Antarctic Ice Sheet and warm
Antarctic interglacials
P.B.Holden1,N.R.Edwards1,E.W.Wolff2,N.J.Lang2,J.S.Singarayer3,P.J.Valdes3,andT.F.Stocker4
1EarthandEnvironmentalSciences,OpenUniversity,MiltonKeynes,UK
2BritishAntarcticSurvey,Cambridge,UK
3SchoolofGeographicalSciences,UniversityofBristol,Bristol,UK
4ClimateandEnvironmentalPhysics,UniversityofBern,Bern,Switzerland
Received: 27November2009–PublishedinClim. PastDiscuss.: 17December2009
Revised: 6July2010–Accepted: 7July2010–Published: 16July2010
Abstract. Ice core evidence indicates that even though at- 1 Introduction
mosphericCO concentrationsdidnotexceed∼300ppmat
2
anypointduringthelast800000years,EastAntarcticawas The AMOC transports a substantial amount of heat from
atleast∼3–4◦Cwarmerthanpreindustrial(CO ∼280ppm) south to north. If the AMOC is weakened through changes
2
in each of the last four interglacials. During the previous in surface buoyancy, Northern Atlantic cooling and South-
three interglacials, this anomalous warming was short lived ern Atlantic warming is simulated in ocean models of var-
(∼3000 years) and apparently occurred before the comple- ious complexities (Stouffer et al., 2006). This effect, the
tion of Northern Hemisphere deglaciation. Hereafter, we bipolar seesaw, has been proposed (Stocker and Johnsen,
refer to these periods as “Warmer than Present Transients” 2003) to explain the interhemispheric teleconnection of
(WPTs). We present a series of experiments to investigate abrupt millennial-scale shifts in glacial climate known as
theimpactofdeglacialmeltwaterontheAtlanticMeridional Dansgaard-Oeschger (DO) events (Dansgaard et al., 1993).
OverturningCirculation(AMOC)andAntarctictemperature. We here address the proposed role of the bipolar seesaw in
ItiswellknownthataslowedAMOCwouldincreasesouth- definingthecharacteristicsofglacialterminationsandthein-
ernseasurfacetemperature(SST)throughthebipolarseesaw terglacialswhichfollowthem(GanopolskiandRoche,2009;
and observational data suggests that the AMOC remained Masson-Delmotteetal.,2010).
weakthroughouttheterminationsprecedingWPTs,strength- Antarctic temperature (Jouzel et al., 2007) and atmo-
ening rapidly at a time which coincides closely with peak spheric methane (Loulergue et al., 2008) are illustrated in
Antarctictemperature.Wepresenttwo800kyrtransientsim- Fig.1a. Numerouspaleorecords(Carlson, 2008)revealthat
ulationsusingtheIntermediateComplexitymodelGENIE-1 the most recent termination (TI) exhibited rapid transitions
whichdemonstratethatmeltwaterforcinggeneratestransient in Northern Hemisphere climate, in approximate anti-phase
southernwarmingthatisconsistentwiththetimingofWPTs, with change in Antarctica (Blunier et al., 1998). Antarc-
butisnotsufficient(inthissingleparameterisation)torepro- ticawarmedslowlyfrom∼18kyrBPtotemperaturessimilar
duce the magnitude of observed warmth. In order to inves- to preindustrial at ∼12kyrBP, interrupted by the Antarctic
tigatemodelandboundaryconditionuncertainty,wepresent Cold Reversal (ACR) at ∼14kyrBP, a cooling event which
three ensembles of transient GENIE-1 simulations across hasbeeninterpretedasthesouthernanalogueoftheBølling-
Termination II (135000 to 124000BP) and three snapshot AllerødNorthernHemispherewarming,bothmanifestations
HadCM3simulationsat130000BP.Onlywithconsideration of a temporary resumption of Atlantic overturning (Blunier
ofthepossiblefeedbackofWestAntarcticIceSheet(WAIS) etal.,1998;Barkeretal.,2009). Rapidincreasesinmethane
retreatdoesitbecomepossibletosimulatethemagnitudeof provideaproxyforrapidincreasesinGreenlandtemperature
observedwarming. (Delmotteetal.,2004)andhence,accordingtothisinterpre-
tation,fortheresumptionofoverturning.
Correspondenceto: P.B.Holden
([email protected])
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
432 P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials
aa
bb
cc
kkyyrr BBPP
Fig.1.(a)DOMECδD-inferredAntarctictemperatureanomaly(Jouzeletal.,2007)(grey)andatmosphericCH4concentration(Loulergue
etal.,2008)(green)attFheigluasrtef 1ourterminations.(b)800kyrrecordsofDOMECAntarctictemperatureanomaly(grey)andplanktonicδ18O
anomaly (blue) at ODP site 982 (57.3◦N, 15.5◦W) (Venz et al., 1999), compared respectively with the GENIE-1 (GFW) Antarctic SAT
anomalyandtheSSTanomalyatthegridcellcentredon59◦N,15◦W.(c)350kyrrecordsofDOMEFδD-inferredAntarctictemperature
anomaly(Kawamuraetal.,2007)(grey),GENIE-1meltwater-induced(GFW-GNFW)AntarcticSATandSSTanomalies. SATisaveraged
◦ ◦
overAntarcticasouthof71 S.SSTisaveragedoveralltheSouthernOceansouthof62 S.Note: thesimulationin(b)includesmeltwater
forcing,buttheeffectsinAntarctica(upto∼1.5◦Cwarming)arelargelyobscuredbythelargerchangesacrosstheterminations.
30
Incontrast, thethreepreviousterminations(TII,TIIIand displayed transient warming during each of these termina-
TIV) exhibit a behaviour that is quite distinct from TI, al- tions,lastingfor5–9kyrsandwithtemperatures2to3.5◦C
though they display remarkable similarities to one another. warmerthanpresent(inadditiontoa∼2◦Ctransientwarm-
IneachcaseatransientspikeinAntarctictemperature(WPT) inglastingfor7kyrduringTI).GCMsimulationshavethus
of up to ∼4◦C lasting ∼3kyr is apparent at the start of the farfailedtoproduceawarmerAntarcticaduringthelastin-
interglacial. Recentanalysis(Simeetal.,2009)suggeststhat terglacial(e.g.Montoyaetal.,2000;Grolletal.,2005;Otto-
the isotopic composition of East Antarctic ice is less sensi- Bliesneretal.,2006;Masson-Delmotteetal.,2010).
tivetotemperaturechangeinawarmclimate,consistentwith
The second striking similarity between these earlier ter-
even higher peak Antarctic temperatures during these inter-
minations is that the methane record does not exhibit the
glacials (at least ∼6◦C warmer than present day), though
oscillatory behaviour that is apparent in TI, but rather dis-
this work did not consider the warming mechanism of the
plays a single rapid jump (vertical tie-bars in Fig. 1a) late
bipolar seesaw that is addressed here. Observational evi-
in the termination (which we associate with a resumption
dence for WPTs in Antarctica is not confined to ice-core
of overturning). In each case this methane jump coincides
records. Notably,a430kyrSouthernOceanSSTreconstruc-
closelywiththepeakinAntarctictemperature. Furthermore,
tion (Cortese et al., 2007) at ODP site 1089 (41◦S 10◦E)
eachmethanejumpcorrespondstoanabruptshiftinChinese
Clim. Past,6,431–443,2010 www.clim-past.net/6/431/2010/
P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials 433
speleothem δ18O (Wang et al., 2001; Cheng et al., 2006, ofsuchamodel. Furthermore,wesupplementtheGENIE-1
2009), a proxy for Asian Monsoon strength, which further ensembleswithsimulationsusingtheHadleycentrecoupled
suggests a role of the bipolar seesaw during terminations modelHadCM3(Gordonetal.,2000)inordertoinvestigate
(Chengetal.,2009).The“WeakMonsoonIntervals”(Cheng robustnesswithrespecttospecificstructurallimitations, es-
et al., 2009) which precede each of the methane shifts ap- pecially related to the lack of a dynamic atmosphere. The
proximately coincide with peaks in ice rafted debris in the threeexperimentsare:
North Atlantic ODP 980 core (McManus et al., 1999), sup-
i) Two transient 800kyr simulations with GENIE-1, one
portingtheviewthatthesechangeswereassociatedwithdis-
whichincludestheeffectsofglacialmeltwateronocean
integratingNorthernHemisphereicesheets.
circulationandonewhichneglectsthisfeedback.These
Recentclimatemodelsimulationshaveaddressedthispo-
simulations provide a long time-series comparison be-
tential mechanism for early interglacial warmth. A simu-
tweenobservationsandmodelresultsandallowaqual-
lation with the IPSL GCM which included a parameterisa-
itativeassessmentoftheroleofmeltwaterindetermin-
tion of Greenland melt at 126kyrBP predicted year-round
warming of 0.5◦C in Antarctica, thus implicating the bipo- ing transient Northern Atlantic and Antarctic tempera-
tures.
lar seesaw as a possible driver of transient warmth in MIS
5.5 (Masson-Delmotte et al., 2010). Ganopolski and Roche
ii) ThreeensemblesofGENIE-1transientsimulationsover
(2009) performed idealised transient experiments across a
glacial termination TII, applying the LGM Plausibil-
glacialterminationwithCLIMBER-2,anIntermediateCom-
ity Constrained (LPC) parameter set which covers the
plexity model which includes a 2.5-D statistical-dynamical
range of large-scale feedback strength displayed by
atmosphere model coupled to a zonally-averaged, three-
multi-model GCM ensembles (Holden et al., 2010).
basinoceanmodel. Thesesimulationsproduced∼2◦Ctran-
These ensembles enable quantification of both mod-
sientAntarcticwarminginresponseto0.2Svfreshwaterflux
elling and boundary condition uncertainties, including
intotheNorthAtlanticandsuggestedthatthequalitativedif-
anevaluationofpotentialWAISretreatfeedbacks.
ferencesbetweenrecentterminationsmayresultfrommod-
estdifferencesintherateofdeglaciation. iii) Three equilibrium simulations with HadCM3 at
IthasrecentlybeensuggestedthatthesuppressionofDO 130000BP, performed to investigate the robustness of
events (which would cool Antarctica) enables terminations the conclusions derived from GENIE-1, in particular
to progress unchecked (Wolff et al., 2009). Here we pro- with respect to its simplified atmosphere and snow
pose, following Ganopolski and Roche (2009), that if the models.
AMOC remains weakened throughout the termination the
system will progress to a WPT state. Furthermore, we pro-
posetheWPTmayeventuallyleadtopartialcollapseofthe
2 Methods
WAIS, leading to further warming. Thus although the ra-
diative forcing due to Northern Hemisphere ice sheets and 2.1 GENIE-1
greenhousegaseswassimilartopreindustrialintheeachof
the last three interglacials, weakened overturning together The intermediate complexity model GENIE-1 has been ex-
withpotentialWAISretreatleadtoconditionsinAntarctica tensively applied to investigations of the thermohaline cir-
duringtheearlystagesoftheseinterglacialswhichweresig- culation (Marsh et al., 2004) and provides the computa-
nificantlywarmerthanmodern. Theresumptionofoverturn- tional efficiency required to perform large ensembles and
ing,associatedwiththecessationofdeglaciationmeltwater, quantify model uncertainty. The physical model comprises
subsequentlycoolsAntarcticatoconditionsthatarecompa- a 3-D frictional geostrophic ocean with eddy-induced and
rabletopresentday. isopycnalmixingcoupledtoa2-Dfixedwind-fieldEnergy-
In order to examine the temporal history of Northern Moisture Balance Model (EMBM) atmosphere and a dy-
Hemisphere meltwater feedbacks on Antarctic climate, and namicandthermodynamicsea-icecomponent(Edwardsand
evaluate the modelling and boundary condition uncertainty, Marsh, 2005). These are coupled to a minimum spatial
wehaveperformedthreesetsofexperiments. Theapproach, modelofvegetationcarbon, soilcarbonandsoilwaterstor-
buildingonHoldenetal.(2010),isdesignedtoallowforthe age (Williamson et al., 2006). The model configuration is
uncertainty that arises from structural error, the irreducible asdescribedinLentonetal.(2006),withadjustmentstothe
errorthatremainswhenthe“best”parametersareappliedto parameterisationofoutgoinglongwaveradiation(OLR)ap-
amodel(Rougier,2007).Themulti-millennialtransientsim- pliedintheensembleexperimentsandtheinclusionoforog-
ulations required here can only be performed with an inter- raphyfeedbacksforsurfaceprocesses(appliedthroughout).
mediatecomplexitymodel. WeuseGENIE-1(Lentonetal., The adjustment to the OLR parameterisation is designed to
2006),builtaroundalowresolution3Dfrictionalgeostrophic allow for uncertainties in cloud and lapse rate feedbacks.
oceanmodel. Anensembleapproachisessentialtoquantify BothchangesaredetailedinHoldenetal.(2010).
modelling uncertainty that arises from structural limitations
www.clim-past.net/6/431/2010/ Clim. Past,6,431–443,2010
434 P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials
aa:: LLPPCC aavveerraaggee pprreeiinndduussttrriiaall SSAATT bb:: SSttaannddaarrdd ddeevviiaattiioonn LLPPCC pprreeiinndduussttrriiaall SSAATT
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LLoonnggiittuuddee [[EE]] LLoonnggiittuuddee [[EE]]
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((ººCC)) ººCC
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--22 --22
--33 --33
--44 --44
--55 --55
--2200 00 2200 4400 6600 8800 --2200 00 2200 4400 6600 8800
--55 00 55 1100 1155 2200 00 11 22 33 44 55 66
SSvv SSvv
◦
Fig.2.Ensembleaverageandstandarddeviationofpreindustrial(a,b)SAT( C)and(c,d)Atlanticoverturningstreamfunction(Sv).Data
arederivedfromtheLPC(LGMPlausibilityConstrained)parameterset(Holdenetal.,2010).
Figure 2
The 800kyr simulations (experimental set-up described (Lisiecki and Raymo, 2005). We derive three variables for
in Sect. 2.1.1) are performed with the traceable parameter each grid cell representing (a) the threshold value δ18O at
th
set (Lenton et al., 2006). The TII ensembles (Sect. 2.1.2) which the grid cell becomes ice covered, (b) the present-
are performed using the LPC parameter set (Holden et al., day orography h and (c) the incremental orography h at
0 1
2010). TheLPCaveragedpreindustrialsurfaceairtempera- maximumattainableicethickness. Whenδ18O>δ18O ,the
th
ture (SAT) and Atlantic overturning stream function are re- heightoftheicesurfacehateachcellisgivenbythesaturat-
produced in Fig. 2, together with ensemble standard devi- ingrelationship
ations. The LPC ensemble exhibits a cold Antarctic bias,
withAntarcticSAT−31±10◦C(∼10◦CcoolerthanNCEP h=h +h (δ18O−δ18Oth) (1)
data), likely a result of enforcing modern plausible sea-ice 0 1(δ18O−δ18Oth+k)
coverage; GENIE is known to underestimate Antarctic sea-
where h is derived at each cell to give the Ice-4G height
1
ice (Lenton et al., 2006). The major shortcoming of ocean
h=h when δ18O=δ18O . The threshold for ice
circulation in this configuration of GENIE is the failure of 31 LGM LGM
coverδ18O isderivedfromIce-4Greconstructionsat1kyr
th
AABW to penetrate into the Atlantic sector (Lunt et al.,
intervals from 21kyrBP to present. The constant k is fitted
2006). However, the LPC parameter set has been designed
toapproximatealinearrelationshipbetweenglobalicevol-
to exhibit a wide range large-scale feedback strengths (see
umeandδ18O.TherelationshipinEq.(1)isassumedtohold
Sect.2.1.2)thatgenerallyencompasstherangeofGCMre-
throughoutthe800kyrrecordsothatthevalueofδ18Oatany
sponses to both LGM and 2×CO forcing (Holden et al.,
2 pointintimedefinesthespatialdistributionandorographyof
2010).
the ice sheets globally. As the Greenland and Antarctic ice
InbothGENIE-1experiments,changingatmosphericCO sheets are fixed at their preindustrial distributions in these
2
is prescribed from ice core records (Luethi et al., 2008). simulations, the preindustrial ice sheet configuration arises
Othergreenhousegasesareneglected. Weapplytheorbital whenever δ18O≤3.37‰ (c.f. preindustrial δ18O∼3.2‰).
forcingofBerger(1978). TransientLaurentideandEurasian Thisarisesbecause3.37‰is thelowestthresholdvalueas-
IceSheetsarerepresentedbyinterpolatingthespatialdistri- cribedtoanygridcell,beingthevalueofδ18Ointhebenthic
butionofIce-4G(Peltier,1994)ontothebenthicδ18Orecord stack at 7kyrBP (the most recent time when no Laurentide
Clim. Past,6,431–443,2010 www.clim-past.net/6/431/2010/
P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials 435
or Eurasian Ice-4G ice remains). When δ18O>δ18O , sphere. Surface air temperature is thus calculated (and pre-
LGM
ice sheets are allowed to thicken (but not expand laterally) sented throughout) in terms of a sea-level equivalent. Two
beyond the Ice-4G LGM configuration. This assumption simulationsareperformed: G ,whichincludestheeffects
FW
leadstominimumsea-levelsat433kyrBPthatare6mlower of glacial meltwater on ocean circulation (FFX=1.5), and
than at the LGM. We note that the only effects of sea-level G whichneglectsthisfeedback(FFX=0).
NFW
change represented in GENIE-1 are on ocean salinity (and
otheroceantracerswhenapplicable). 2.1.2 TransientGENIE-1TerminationIIensembles
Changing ice volume is translated into meltwater fluxes
Three ensembles of transient simulations over TII (provid-
at each grid cell, routed to the Atlantic, Pacific and Arctic
ingapproximateanaloguesforTIIIandTIV)areperformed,
Oceans assuming modern topography. Accumulating ice is
differing in the boundary conditions applied in order to in-
representedbyreducedrun-off. Thisensuresthatfreshwater
vestigate uncertainties in forcing. Ensemble members are
fluxesarespatiallyandtemporallyconsistentwiththerepre-
weakly constrained to produce plausible preindustrial and
sentation of the ice sheets, and avoids the problems associ-
LGM climate states by applying the 480 member LPC pa-
atedwithanunrealistictreatmentofsaltcompensationwhich
rameter set, which varies 26 parameters over wide ranges
isresponsibleformuchoftheuncertaintyinthefar-fieldre-
(see Table 1 of Holden et al., 2010). The LPC parameters
sponse to freshwater forcing (Stocker et al., 2007). Only
producemodern-plausibleglobalaverageSAT,Atlanticover-
the Laurentide and Eurasian Ice Sheets, which account for
turning,Antarcticsea-icecoverageandlandcarbonstorage,
∼80%ofglobalice-sheetchange(Peltier,2004),areallowed
withdistributionsthatareapproximatelycentredonobserva-
to change. This eliminates the potentially confounding ef-
tions, and are additionally constrained to simulate plausible
fectsofassumingsynchronousAntarcticmeltwateronocean
LGM Antarctic cooling (of 6–12◦C), though we note that
circulation; it is well known that millennial-scale Southern
glacialcoolingofAntarcticamaybeoverstatedduetodiffu-
and Northern Hemisphere changes are likely to be out of
siveheattransportthatisdrivenbycoolingduetoNorthern
phase(Blunieretal.,1998). Ascalingofthefreshwaterflux
Hemisphereicesheets. Theapproachisdesignedtoquantify
(parameter “FFX”, default value 1.5) is applied to correct
modelerrorbyallowingparametricuncertaintytodominate
for isostatic depression at the ice-bedrock interface (which
overstructuralerror. Theensemblemembersexhibitawide
wedonotmodel)andfortheassumptionofafixedland-sea
rangeofocean,atmospheric,sea-iceandvegetationfeedback
mask;eachofthesesimplificationswouldotherwiseproduce
strengthswhichgenerallyencompasstherangeoflarge-scale
a∼20%underestimationofice-sheetvolume.
GCMresponsesto2×CO andLGMforcing, withadistri-
The simulations apply an Atlantic-Pacific freshwater flux 2
bution for climate sensitivity of 3.8±0.6◦C and for LGM
adjustment (parameter “APM”) to correct for the ∼0.29Sv
globally averaged cooling of 5.9±1.2◦C (c.f. 5.8±1.4◦C,
underestimationofatmosphericmoisturetransport,required
SchneidervonDeimlingetal.,2006).
to maintain a stable Atlantic overturning (Edwards and
Thethreeensemblesare:
Marsh, 2005). The flux adjustment is held constant during
eachtransientsimulation. GCMsimulationssuggestanun- i) EFW whichincludestheimpactofmeltwaterrunoffon
certaintyof ±0.15Svinthisflux,especiallyrelevanttocli- oceancirculation(Sect.2.1)andallowsforuncertainty
mate states that differ from modern (Zaucker and Broecker, inthestrengthofthisfeedbackbyvaryingFFX(inad-
1992). However, althoughtheneglectofatmospherictrans- ditionto25otherparameters),uniformlydistributedin
port feedbacks quantitatively alters the modelled sensitivity therange1to2,
totransientmeltwaterfluxes(Marshetal.,2004),theensem-
ii) E whichneglectstheroleofmeltwater(FFX=0in
NFW
ble is designed to cover the range of possible sensitivities,
allensemblemembers),and
varying APM in the range 0.05 to 0.64Sv across the LPC
ensemblemembers. iii) EWAIS which is identical to EFW except that the WAIS
is replaced with land at sea level. Ice-sheet mod-
2.1.1 TransientGENIE-1800kyrsimulations els (Pollard and DeConto, 2009) have simulated sub-
stantial WAIS retreat driven by sea-level rise at termi-
The climatology of the traceable parameter set used in the nations (neglecting the additional bipolar forcing ad-
800kyrsimulationsisdiscussedindetailelsewhere(Lenton dressed here). Several lines of indirect evidence sug-
etal., 2006). Theequilibriumclimatesensitivityofthispa- gest that WAIS retreat contributed to the observed el-
rameterisationtoadoublingofCO is3.4◦C,slightlyhigher evated sea levels during the last interglacial (Overpeck
2
than the value of 3.2◦C in the configuration of Lenton et et al., 2006; Kopp et al., 2009). We capture the un-
al. (2006) due to the increased snow albedo feedback that certainty associated with the degree and timing of po-
arises from the inclusion of a lapse rate-related orography tentialWAISretreatthroughthetwoextremeboundary
effectfortemperature-drivensurfaceprocesses. Alapserate conditionsofE andE . ThemodernWAIScom-
FW WAIS
adjustmentisnotappliedforatmosphericprocessesasthese prises both marine-based and land-based ice. For con-
represent averages through the depth of the 1-layer atmo- sistency with the HadCM3 simulation (see Sect. 2.2),
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436 P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials
weassumetheWAISisremovedandreplacedentirely
0.20 80
by ice-free land in the E ensemble. We note that
WAIS
70
rtoepsliamciunlgateWsAliIgShtwlyithhigohceeranpeianksteAandtaorfctliacnSdAwTasanfoomunad- 0.15 60valent)
waMrmelGiilnetEisgnN,g(2IwEo.8of-1◦utChlsdeimcWr.fue.lAlea2aIt.iS2soe,◦ntCsmri)ngeoiglnttewdreeaexdtspeclrbroiyrtbaoestdoetahrhy-eele6rsve5oe.0ultkrhyiesrerntroaronclsoeieacnnalt, Meltwater flux (Sv)0.10 345000me (m sea level equi
potentially reducing convection and hence reducing local 0.05 20e volu
warming (Weaver et al., 2003; Swingedouw et al., 2009). 10Ic
Giventhesubstantialuncertaintiesinvolved,however,wedo 0.00 0
not attempt to model this feedback in the present study. A 124,000 126,000 128,000 130,000 132,000 134,000
years BP
hosing flux of 0.1Sv into the Southern Ocean applied for
1000years(equivalentinmagnitudetothecompletelossof
Fig. 3. Representation of ice sheets in the TII ensembles. The
WAISicein650years)simulatedAntarcticSATcoolingof globaldistributionoficesheetsisgivenbyEquation1, assuming
∼0.5◦Cinthepreindustrialclimatestate(Swingedouwetal., aconstantreductionofδ18Ofrom4.86to3.16‰from135000to
2009).TransientSouthernOceancoolingeventsofthismag- 126000BP.Meltwateriscalculatedateachgridcellandroutedto
nitudewouldnotbeinconsistentwith“coolingrebounds”of theoceanassumingmoderndaytopography. Thedashedlineillus-
up to ∼1◦C that have been observed in several high south- tratestotalLaurentideandEurasianicesheetvolume,expressedin
ern latitude locations during the later stages of each of the Feigquurei v3alentmetersofsea-level.Thesolidlinesillustratestheprofile
ofensemblemeanmeltwaterflux(bold, FFX=1.5)andtherange
last five terminations (Cortese et al., 2007, and references
ofmeltwaterfluxes(FFX=1.0toFFX=2.0). Thetemporaldevel-
therein).
opmentofmeltwaterisfixedacrossallensemblemembers.
Transient CO , orbit and ice sheets are applied as for the
2
longsimulations, exceptthatthebenthicδ18Orecordislin-
earised across the termination to produce an approximately
entirely by ice-free land (at an elevation of 200m), in or-
constant (±20%) meltwater pulse, eliminating the oscilla-
dertoavoidoceantime-steppinginstabilitieswhicharisedue
torybehavioursimulatedduringTIIinthe800kyrsimulation to converging meridians ne32ar the pole (which are resolved
(see Fig. 1c). This oscillatory behaviour is not apparent in
with Fourier filtering at the North Pole). As noted above,
observations and is largely a consequence of translating the
inGENIE-1simulations, replacingWAISwithoceanrather
temporal signal in the gradient of benthic δ18O into global
thanlandresultedinamodestincreaseinwarming. Simula-
ice-sheet change. The temporal development of ice sheet
tionsarerunfor200years(fromthemodernspun-upstate)
volume (only Laurentide and Eurasian ice sheets contribu-
andaveragedoverthelast30years.
tions are considered here) and the resulting meltwater flux
is illustrated in Fig. 3. As in the 800kyr simulation, melt-
waterisroutedintotheoceanassumingmoderntopography. 3 Results
Themeltwaterpulsecommencesat135000BPandlastsfor
∼7600yearswithanaverageof∼0.11Sv,equivalentto76m 3.1 800kyrGENIE-1TransientSimulations
sealevel,butvaryingbetweenensemblemembers(∼0.07to
Figure 1b compares 800kyr records of i) modelled Antarc-
0.15Sv)throughthefreshwaterscalingparameterFFX.Sim-
tic SAT anomaly with DOME C (Jouzel et al., 2007) and
ulationsarespun-uptoequilibriumat135000BPandrunfor
ii)modelledSSTanomaly(withaNorthAtlanticplanktonic
11000years.
δ18Orecord(Venzetal.,1999)whichwehavemappedonto
2.2 HadCM3snapshotsimulations theLR04timescale(LisieckiandRaymo,2005). Modeldata
arefromthesimulationG (whichincludesmeltwaterforc-
FW
The snapshot simulations are performed using the Hadley ing). We note the generally good agreement with both sets
Centre coupled model HadCM3 (Gordon et al., 2000), a ofobservationaldata,withtheexceptionofthemarkedfail-
coupledatmosphere(2.5◦×3.75◦×19verticallevels)/ocean ure to predict the existence of Antarctic WPTs. A similar
(1.25◦×1.25◦×20 vertical levels) model which does not discrepancyisnotapparentintheNorthAtlanticrecordand
require flux adjustments to prevent climate drifts. Bound- we inferthat WPTsare likely drivenby an absent Southern
ary conditions are (H ) preindustrial, (H ) 130000BP Hemisphereforcing. MIS7isparticularlyinterestinginthis
PI NFW
orbit and greenhouse gases (CO 256ppm, CH 506ppb, regard as the δ18O record suggest that substantial Northern
2 4
N O 239ppb) with modern ice sheets and no meltwater Hemisphere ice sheets remained in place during this inter-
2
flux,(H )asH with1SvNorthAtlantichosing(large glacial,sothattemperatures∼2◦Chigherthanpreindustrial
FW NFW
enoughtoensurecollapseontimescaleswhichcanbeprac- againsuggesttheWPTwarmingwaslocalisedtotheSouth-
ticallysimulated)and(H )asH withWAISreplaced ern Hemisphere. For completeness we note that orbitally-
WAIS FW
Clim. Past,6,431–443,2010 www.clim-past.net/6/431/2010/
P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials 437
forced summer warming is simulated at high northern lat- giventheabruptcessationoftheforcing(Fig.3). Thecool-
itudes during the Eemian, with maximum summer warm- ingdoesnotariseintheTIIensembleaverage(c.f.blueand
ing (averaged over all grid points north of 62◦N) peaking pinkcurvesinFig.4e),inpartduetodifferencesinthetim-
at3.5◦Cin126000BP. ing of these cooling events and in part due to the masking
The first plot in Fig. 1c is the 350kyr DOME F tem- effect of the 19 simulations in which the AMOC does not
perature record, reconstructed from δ18O and δD isotopic recoverandAntarcticaremainswarm.
records,correctingforvapoursourcetemperatureandseawa-
terisotopiccompositionvariations(Kawamuraetal.,2007). 3.2 GENIE-1ensemblesoftransientsimulationsover
DOME F was chosen as it lies south of the Atlantic (77◦S, TerminationII
39◦E) and may be expected to be more strongly influenced
by AMOC changes than DOME C(75◦S, 123◦E). In addi- Although there is strong chronological similarity between
tiontotheWPTs,andthewarmspikeintheearlyHolocene, observedandsimulatedwarming,themagnitudeofAntarctic
three interstadials at DOME F are warmer than preindus- SAT spikes in the single 800kyr simulation GFW (∼1.5◦C,
trial at 317, 218 and 200kyrBP. These three interstadials Fig.1candseeninFig.1bassmalloscillationsnearthetop
areobservedtobeatleast∼1◦Ccoolerthanpreindustrialat ofeachsimulatedSATrise)isinsufficienttoexplaintheob-
DOMEC(Fig.1b),thoughwenotethattheseapparenttem- served warmings of ∼4◦C above present (Fig. 1b). Quan-
peraturedifferencesbetweenDOMECandDOMEFmayal- tificationofthediscrepancyrequiresanassessmentofmod-
ternativelyreflectspatialvariationsintheseasonalityofpre- ellinguncertainities; weapplyanensemblemethodologyto
cipitationinawarmAntarctica(Simeetal.,2009). ascertainthemostprobablemodelresponseandquantifyun-
AntarcticSATandSSTmeltwateranomalies(G -G , certainty.
FW NFW
i.e. the differences in temperature when meltwater is im- The480meltwater-forcedensemblemembersEFWuniver-
posed) are plotted in Fig. 1c and reflect the impact of melt- sallyexhibitedweakenedoverturningduringthetermination
wateronoceancirculationandheattransport.TheSATmelt- (average weakening at 128000BP of 9±5Sv with respect
wateranomalydisplaysthemillennialvariabilityapparentin to preindustrial). Antarctic SAT at 128000BP is >0.5◦C
observations; the SST meltwater anomaly exhibits a simi- warmerthanpreindustrialin110ofthesesimulations,allof
lar temporal behaviour, though variability is suppressed by whichexhibitedacollapseofoverturning(peakoverturning
high-latitude sea ice during glacial periods. The SST melt- <6Svnorthof44◦N),demonstratingthatAMOCcollapseis
water spikes coincide very closely with spikes in isotope- requiredforsignificantAntarcticwarminginGENIE-1. We
inferredtemperature. LatePleistoceneLR04age-modelun- confine analysis to the 174 parameter sets with maximum
certainty(theagemodelwhichdefinesthemeltwatertiming overturning<6Svtoquantifytherangeofresponse.
throughthegradientofthebenthicδ18O)isestimatedat4kyr Figure4aillustratesthemeltwater-forcedSSTanomalyat
(LisieckiandRaymo,2005). 128500BP (E -E , i.e. the ensemble-averaged change
FW NFW
The timing of large scale meltwater spikes reflects large in SST when meltwater is imposed) and displays the char-
changes in δ18O and is likely to be robust (though subject acteristic behaviour of the bipolar seesaw, although cer-
to the aforementioned age-model uncertainties). However, tain atmospheric feedbacks, such as the southward shift
this is not the case with respect to the detail and timing of of the ITCZ, cannot be captured by the fixed-wind field
the millennial variability, including that which is simulated EMBM.Southof62◦S,ensemble-averagedSSTwarmingof
duringterminations. Thesefeaturesarisefromvariabilityin 0.4±0.3◦C is simulated (uncertainties are expressed as 1σ
thegradientofthebenthicδ18Osignal,whichtranslatesinto ensemble standard deviations throughout). Modern obser-
the meltwater fluxes. Although variability in ice-sheet melt vationsindicatethatbasalmeltratesattheWAISgrounding
ratespresumablycontributestothissignal,contributionsalso lineincreaseby∼1myr−1fora0.1◦CincreaseinSST(Rig-
arise from a number of additional sources, notably includ- not and Jacobs, 2002), suggesting that the simulated warm-
ing uncertainty in observational values of δ18O; 1σ errors ingmaybesignificantforWAISinstability. Ice-sheetmod-
of up to ∼0.1‰ (Lisiecki and Raymo, 2005) are compara- elling has indicated that a 10myr−1 increase in Antarctic
ble to the average millennial decrease of ∼0.2‰ across a basal melt rates leads to sea level rise of ∼25cm per cen-
termination. The transient Antarctic SAT cooling of ∼1◦C tury (Huybrechts and de Wolde, 1999). The ensemble de-
that occurs at the end of each termination (Fig. 1c) arises sign allows a wide range of sea-ice responses, with annual
duetothepositiveAtlanticoverturningcellextendingfurther average Antarctic sea ice extent increasing by 9–15million
south during the reorganisation of the circulation at the end km2 underLGMforcinganddecreasingby5–9millionkm2
ofthemeltwaterpulse.Wenotethatintheensembleanalysis under 2×CO forcing (Holden et al., 2010). Meltwater-
2
(Sect.3.2)thistransientcoolinggenerallyarisesinthosesim- forced reduction in Antarctic sea-ice area at 128500BP is
ulations in which the AMOC recovers. Amongst these 155 2.7±1.6million km2. The simulated retreat of Antarctic
simulations, ensemble averaged cooling of 0.9±0.9◦C oc- sea ice is consistent with the southerly shift of productiv-
curs1400±900yearsafterthecessationofmeltwater. This ityinferredfrombariumrainratesduringMIS5.5whichcan
may not be a robust feature of the simulations, especially onlybeexplainedbyareductioninAntarcticseaice(Nuern-
www.clim-past.net/6/431/2010/ Clim. Past,6,431–443,2010
438 P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials
aa MMeellttwwaatteerr iinndduucceedd SSSSTT aannoommaallyy bb SSAATT aannoommaallyy ttoo mmooddeerrnn ((nnoo mmeellttwwaatteerr))
9900 9900
4455 4455
Latitude [N]Latitude [N] 00 Latitude [N]Latitude [N] 00
--4455 --4455
--9900 --9900
--118800 --9900 00 9900 118800 --118800 --9900 00 9900 118800
LLoonnggiittuuddee [[EE]] ººCC LLoonnggiittuuddee [[EE]] ººCC
--44 --33 --22 --11 00 11 22 --1100 --55 00 55
cc SSAATT aannoommaallyy ttoo mmooddeerrnn ((mmeellttwwaatteerr iinncclluuddeedd)) dd SSAATT AAnnoommaallyy ttoo mmooddeerrnn ((mmeellttwwaatteerr iinncclluuddeedd && nnoo WWAAIISS))
9900 9900
4455 4455
N]N] N]N]
Latitude [Latitude [ 00 Latitude [Latitude [ 00
--4455 --4455
--9900 --9900
--118800 --9900 00 9900 118800 --118800 --9900 00 9900 118800
LLoonnggiittuuddee [[EE]] ººCC LLoonnggiittuuddee [[EE]] ººCC
--1100 --55 00 55 --1100 --55 00 55
66 2200 6600
ee ff
C)C) 1188 ss
T Anomaly (ºT Anomaly (º 2424 111111246246urning (Sv)urning (Sv) mble membermble member45450000
East Antarctic SAEast Antarctic SA ----424200 246812468100Atlantic OvertAtlantic Overt Number of enseNumber of ense123123000000
--66 00 00
112244,,000000 112266,,000000 112288,,000000 113300,,000000 113322,,000000 113344,,000000 --44 --33 --22 --11 00 11 22 33 44 55 66
yyeeaarrss BBPP AAnnttaarrccttiicc SSAATT aannoommaallyy rreellaattiivvee ttoo mmooddeerrnn ((CC))
Fig. 4. GENIE-1 ensemble-averaged SST and SAT anomalies. Fields (at 128500 BP) of (a) meltwater induced SST anomaly (EFW-
ENFW) and (b, c, d) SAT anomalies relative to an equilibrium preindustrial ensemble EPI with (b) no meltwater flux (ENFW-EPI), (c)
includingLaurentideandEurasianmeltwaterfluxes(EFW-EPI)and(d)includingLaurentideandEurasianmeltwaterfluxesandreplacing
WestAntarcticIceSheetwithlandatsea-level(EWAIS-EPI). (e)Ensemble-averagedtemporalbehaviourofEastAntarcticSATanomaly.
Thedashedgreenlineistheensemble-averaged(EFW)Atlanticoverturning(maximumstreamfunctionatdepthsbelow400m). Thegrey
lineistheDOMECdeuteriuminferredanomaly(Jouzeletal.,2007).SolidlinesareEastAntarcticSATanomalieswithblue:nomeltwater
forcing(ENFW-EPI)F, ipginukr:e m4eltwaterforcingincluded(EFW-EPI), brown: meltwaterforcingincludedandWAISremovedtolandatsea
level(EWAIS-EPI).(f)EnsembledistributionsofEastAntarcticSATanomalyat128500BP.
berg et al., 1997). The ensemble member with the largest Figure4b–dillustratesensemble-averagedSATanomalies
meltwater-induced loss of annually-averaged Antarctic sea at 128500BP with respect to an equilibrium ensemble E
PI
ice (7.9million km2) is associated with an East Antarctic 33 forced with preindustrial boundary conditions. In the ab-
SAT2.4◦Cwarmerthanpreindustrial,whilethegreatestEast sence of meltwater forcing (Fig. 4b), East Antarctic SAT is
Antarcticwarming,4.7◦Cabovepreindustrial,isassociated 1.0±0.6◦Ccoolerthanpreindustrial(despiteslightlyhigher
with the loss of 4.8million km2 of Antarctic sea-ice. Thus atmospheric CO concentrations of ∼285ppm). This re-
2
thepossibilitythatWPTscouldbeexplainedwithoutasub- flects a combination of global cooling due to the inferred
stantialWAISretreatfeedbackappearsunlikely(onlysixof remnant of the Laurentide ice sheet (which may be over-
the174E simulationsexhibitEastAntarctictemperatures stated in Antarctica due to the over-diffusive atmosphere)
FW
greaterthan2.5◦Cabovepreindustrial),butcannotberuled and lower Antarctic summer insolation; a single 800kyr
out. transient simulation forced only with orbital changes pre-
Clim. Past,6,431–443,2010 www.clim-past.net/6/431/2010/
P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials 439
dictsannuallyaveragedAntarctictemperatures0.6◦Cbelow sembleswiththeassumptionofsomeWAISretreat(oralter-
preindustrial at this time (ice sheets and atmospheric CO nativelywithanoverturningthatremainsweakenedthrough-
2
fixed at glacial conditions throughout). Meltwater forcing out the interglacial – the AMOC does not recover from its
(Fig. 4c) increases East Antarctic SAT by 1.6±1.0◦C (to collapsedstatein19ofthe174simulations).
0.5±1.0◦C warmer than preindustrial). In these simula-
tions, the bipolar warming of Antarctica peaks in the At- 3.3 HadCM3Eemiansimulations
lantic sector (i.e. in the vicinity of the forcing), with maxi-
mumwarmingat∼15◦E(Fig.4c),intheapproximatevicin- InordertoinvestigatetherobustnessoftheGENIE-1ensem-
ityofDOMEF.Thisisqualitativelyconsistentwithobserva- bles, in particular with regard to the simplified atmosphere
tionswhichindicatethatinterstadialtemperaturesmayhave and snow models, we performed four equilibrium HadCM3
been higher at DOME F than at DOME C. The removal of simulations. HosinginducesAMOCcollapseandresultsin
WAIS(Fig.4d)introducesfurtherEastAntarcticwarmingof statistically significant warming of ∼0.2–0.5◦C in summer
1.2±0.6◦C(to1.6±1.3◦Cwarmerthanpreindustrial)aris- (DJF) SST in the Weddell and Ross Seas (Fig. 5a), accom-
ing from widespread loss of West Antarctic summer snow panied by a reduction in Antarctic summer sea-ice extent
coverandreducedalbedo. WithinensembleE ,39ofthe anddepth. Amulti-modelhosingensemble(0.1Sv,modern
WAIS
174 simulations exhibit East Antarctic temperatures greater boundary conditions), performed to investigate inter-model
than2.5◦Cabovepreindustrial. uncertaintyinresponsetohosing,alsosimulatedwarmingin
Figure 4e summarises the temporal development of theWeddellSea,apparentlyaconsequenceofenhanceddeep
ensemble-averagedEastAntarcticSATunderthethreeforc- convectionandreducedseaice(Stoufferetal.,2006).
ingscenarios.WedonotregardWAISretreatearlyintheter- Intheabsenceofmeltwaterandice-sheetforcing(Fig.5b)
mination as realistic; in the absence of a dynamic ice-sheet HadCM3failstopredictsignificantAntarcticSATincrease,
model we have simply assumed WAIS is absent throughout though orbitally-forced summer warming of 3.0◦C is sim-
the E run, so the temporal behaviour would more rea- ulated in Greenland. With freshwater hosing, precipitation-
WAIS
sonably be described by a transition from the E ensem- weightedSAT(Fig.5c)increasesby2.2◦CatDOMEFand
FW
ble towards the E ensemble (implying a warming rate 1.4◦C at DOME C, supporting GENIE-1 (annual-average
WAIS
greater than either ensemble and hence closer to observa- East Antarctic warming of 1.6±1.0◦C). This warming is
tions). Maximum overturning is also illustrated. In con- achieved after ∼100 years of the 200 year simulation.
trasttopaleoclimaticevidencesuggestingthatglacial(LGM) The removal of WAIS (Fig. 5d) increases the precipitation-
overturningwasweakerthantoday(McManusetal.,2004), weighted SAT anomaly to 4.9◦C at DOME F and to 5.0◦C
GENIE-1 ensemble-averaged overturning is stronger in the atDOMEC.Increasedsummerprecipitationcombineswith
glacialstate.However,thischangeisofunclearsign,with63 greatestwarminginsummerwhenwidespreadWestAntarc-
ofthe174simulationsdisplayingaweakenedoverturningat ticsnowmeltissimulated. TheH andH simulations
FW WAIS
135000BP(ensembleaverage0.9±2.6Svrelativetoprein- encompass observed WPT warming. As the GENIE-1 en-
dustrial). The modelled resumption of overturning (com- sembleparameterisationsweredesignedtoprovideanunbi-
mencing at 127500BP) is within ∼1500 years of the ob- ased estimate of the uncertainty associated with large-scale
served methane jump (commencing at 129000BP, Fig. 1a), processes,E andE EastAntarcticSATvariabilityof
FW WAIS
lessthanthe4kyragemodeluncertaintyofthebenthicδ18O ±1.0◦Cand ±1.3◦Cprovideindicativelower-boundmea-
stack (Lisiecki and Raymo, 2005). We note that coral reef suresoftheparametricuncertaintyinthesesingleHadCM3
evidence indicates that the onset of Northern Hemisphere simulations.
deglaciation occurred ∼4500 years earlier (Thomas et al.,
2009)thanissuggestedbyasea-levelreconstructionderived
fromthisbenthicδ18Ostack(Bintanjaetal.,2005). 4 Discussion
The E ensemble distribution of East Antarctic SAT
NFW
(Fig. 4f) illustrates that no point in our parameter space is i) The800kyrsimulationsdemonstratethatGENIE-1,to-
capable of reconciling GENIE-1 with observations in the getherwiththeimposedtransientboundaryconditions,
absence of substantial AMOC weakening (though we note is able to reproduce g-ig temperature change at the di-
that∼10%ofthesimulationsexhibitAMOCcollapseinthe verse locations of the Antarctic and the North Atlantic
absence of meltwater forcing). However, the two extreme over 800000 years, effectively providing an extensive
boundaryconditionsrepresentedbythedistributionsofE validationofmodelledclimatechangeforthefullrange
FW
and E encompass the observational estimate of ∼4◦C of climate states and the entire period of the record.
WAIS
warming. Although we do not rule out the possibility that However, these simulations fail to reproduce Antarctic
the bipolar seesaw could explain the discrepancy without a warmth that is significantly greater than present at any
substantial WAIS feedback, we note that the observed East pointintime, afailurewhichtakesonagreatersignif-
Antarcticwarmingof∼2◦Cwhichpersistsaftertheresump- icance in view of the success of the model elsewhere
tionofoverturningcanonlybeexplainedintheGENIE-1en- intherecord.Furthermore,thesimulationsdemonstrate
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440 P.B.Holdenetal.: Interhemisphericcoupling,theWestAntarcticIceSheetandwarmAntarcticinterglacials
DDoommee FF
DDoommee CC
ººCC ººCC
aab b
DDoommee FF DDoommee FF
DDoommee CC DDoommee CC
ººCC ººCC
cc dd
Fig.5. HadCM3SSTandSATanomaliesat130000BP.(a)Southernsummerfreshwater-inducedSSTanomalydueto1SvNorthAtlantic
hosingapplieduniformlybetween50–70◦N(HFW-HNFW).(b,c,d)Precipitation-weightedSATanomaliesrelativetopreindustrial(b)with
nohosing(HNFW-HPI),(c)includinghosing(HFW-HPI),(d)includinghosingandwithWAISremovedtolandat200m(HWAIS-HPI).
Figure 5
substantialmeltwater-forcedWPTs(∼1.5◦Cinthissin- ing for hosing fluxes of 0.15 and 0.2Sv respectively,
gle parameterisation) at times which coincide closely although our ensemble design (varying 26 parameters
withobservedtransientwarming. Wenotethatthetim- simultaneously) does not facilitate a direct compari-
ing,magnitudeandspatialdistributionofthemeltwater 34 son with the qualitative differences which arise under
forcingislargelyindependentoftheDOMEFchronol- different hosing assumptions due to increased “warm-
ogy (derived by tuning the O /N ratio of trapped air climate”AMOCstabilityintheirmodel(c.f.Ganopol-
2 2
bubbles to 21 December insolation at 77◦S); the melt- ski and Rahmstorf, 2001). The ensemble distribution
waterforcingisderivedfromaninterpolationofthespa- of simulated warming does not appear sufficient to re-
tial distribution of Ice-4G onto the benthic δ18O stack produceobservationsalone,althoughthetailofthedis-
(with an age model derived from a simple ice model tribution does encompass observations. We again note
drivenby21Juneinsolationat65◦N). that absolute temperatures in Antarctica at this time
(in all three GENIE-1 ensembles) may be understated
ii) The TII ensembles provide a quantification of the un- due to diffusive heat transport that is driven by cool-
certainty that arises from structural, parametric and ingduetoNorthernHemisphereicesheetsinGENIE-1.
boundary condition sources. The meltwater-induced TheremovalofWAISincreasesensemble-averageEast
collapseofAtlanticoverturningleadstosimulatedEast Antarctic temperature to 1.6±1.3◦C above preindus-
Antarctic warming at 128500BP (of 1.6±1.0◦C) to trial. Thus,theinclusionofthisfeedback(evenassum-
0.5±1.0◦C above preindustrial. The ensemble av- ingtheextremeofcompleteWAISretreat)initiallyap-
eraged warming is consistent with Ganopolski and pearsbarelysufficienttoexplainEastAntarcticwarmth
Roche (2009), who simulated ∼1◦C and 2◦C warm-
Clim. Past,6,431–443,2010 www.clim-past.net/6/431/2010/