East Antarctic warming forced by ice loss during the Last Interglacial

Domain: earth_science
Published: 2024-02-03
Editorial outcome: major_revision

Abstract
During the Last Interglacial (LIG; 129-116 thousand years before present), the Antarctic ice sheet (AIS) was 1 to 7 m sea level equivalents smaller than at pre-industrial. Here, we assess the climatic impact of partial AIS melting at the LIG by forcing a coupled climate model with a smaller AIS and the equivalent meltwater input around the Antarctic coast. We find that changes in surface elevation induce surface warming over East Antarctica of 2 to 4 °C, and sea surface temperature (SST) increases in the Weddell and Ross Seas by up to 2 °C. Meltwater forcing causes a high latitude SST decrease and a subsurface (100–500 m) ocean temperature increase by up to 2 °C in the Ross Sea. Our results suggest that the combination of a smaller AIS and enhanced meltwater input leads to a larger sub-surface warming than meltwater alone and induces further Antarctic warming than each perturbation separately.

Introduction
During the last interglacial (LIG), summer temperatures over land areas of the northern high-latitudes were 4 to 5°C higher than the pre-industrial (PI), primarily due to differences in orbital forcing1–3. In contrast, greenhouse gas forcing was reduced, with CO2, CH4 and N2O concentrations all estimated to be slightly lower than during PI3,4. Antarctica’s contribution to the LIG sea level high-stand is estimated to have occurred between 129.5 and 124.5 ka, with a peak at 127 ka, while Greenland’s contribution slowly ramped up from 127 ka onwards5. The Greenland ice sheet is estimated to have contributed 0.4 to 4.4 m sea-level equivalent (SLE)6,7 compared with 1 to 7 m SLE for Antarctica5,8,9. A recent study suggests the AIS contributed 5.7 m SLE of sea level rise, with a likely range of 3.6 to 8.7 m (68% probability)10. However, the evolution of these ice sheets and their associated climate feedbacks are not well understood, motivating further examination with physical climate models.

Paleo-proxy records suggest 1.8±0.8°C higher Southern Ocean summer sea surface temperatures (SSTs), and annual mean SSTs up to 3°C higher at 127ka compared with PI at~40°S11,12. Recent coordinated efforts to model the LIG at 127ka, under the Paleoclimate Model Intercomparison Project 4 (PMIP4) underestimate the warm anomaly suggested by Southern Hemisphere SST proxy records13. Most of the LIG PMIP4 experiments suggest an anomaly of 1 to 2°C in austral winter south of 60°S, while simulating no significant changes in austral summer, thus leading to an annual mean SST change of −0.5 to +2°C13. The LIG 127 ka experiments also underestimate the annual mean surface air temperatures over Antarctica. Ice core records suggest an anomaly of~2.2°C compared with PI11,14, however the multi-model mean finds an increase of~0.5°C13. One of the reasons for these discrepancies might be the absence of changes in the Antarctic ice sheet in the PMIP4 experiments, which are expected to play an integral role in high latitude temperature change. Coupling a dynamic Antarctic ice sheet with the ocean-atmosphere-sea ice-land components necessary for PMIP4 simulations remains a significant technical barrier, primarily due to the problem of ice-ocean coupling necessary for a large marine-based ice sheet15. Recent progress has been made towards coupling a dynamic ice sheet with a fully coupled climate model16, however this approach is still under development, and remains challenging to adapt to a paleoclimate scenario such as the LIG. Therefore, to investigate the effects of changing ice sheets, we currently prescribe ice sheet changes through the model boundary conditions.

Ice-sheet modelling studies suggest differing mechanisms that could account for the observed LIG sea level rise, but all suggest that a sub-surface warming of the Southern Ocean is necessary for AIS retreat, with a potential collapse of the West Antarctic ice sheet (WAIS) above 2–3° sub-surface warming17. A recent model study suggests that sub-surface warming induced by weakening of the Atlantic meridional overturning circulation (AMOC) can cause an Antarctic ice mass loss of 3.42 m SLE by the early LIG18. At the higher end of ice loss estimates, an Antarctic ice-sheet modelling study was able to simulate a 7.5 m SLE increase from a disintegration of the Antarctic ice sheet (AIS) at the LIG, by forcing their model with a 3°C sub-surface warming and implementing a marine ice-cliff instability (MICI) parametrisation19. However, constraints on MICI remain uncertain, since previous sea level rise events can be explained within uncertainty bounds without invoking MICI20. Furthermore, in the recent Ice Sheet Model Intercomparison (ISMIP6) projections of 21st century Antarctic ice sheet evolution, none of the modelling groups included a MICI parameterisation21.

A limitation of current LIG studies is that either the climate is studied without including potential Antarctic ice-sheet (AIS) changes or the AIS evolution is assessed using climate model outputs as a prescribed (non-interactive) boundary condition. For example, a previous coupled climate model study assessed the processes leading to the LIG δ18O maximum and found that this could not be explained by WAIS retreat, but instead could reflect sea ice loss22. A related approach is to use iterative coupling (i.e. repeated updates of boundary conditions) between an ice sheet model and a climate model; such a method has found significant positive feedbacks between sub-surface warming and Antarctic ice melt for future warming scenarios23, but has not been implemented for the LIG. Meltwater input into the Southern Ocean at the LIG has been shown to lead to an increase in Southern Ocean stratification and thus a subsurface warming24. This positive feedback could enhance Antarctic ice-sheet disintegration25,26.

The impact of the WAIS removal on δ18O was previously studied in a coarse-resolution (2.5°×3.75° ocean and atmosphere) climate model22, assuming an idealised flattening or removal of the WAIS. In that study the oceanic response was not studied in detail. To fill this gap, here we assess the response to a more widely distributed ice sheet perturbation and meltwater forcing informed by ice sheet modelling, rather than just WAIS removal, and we study more closely the ocean circulation and climate responses.

Here we assess the impact of a partial removal of the AIS upon Antarctic and Southern Ocean climate. This partial removal consists of separate and combined contributions from changing the ice sheet topography and from enhanced meltwater forcing. Our aim is to investigate how feedbacks from partially melting the Antarctic ice sheet would impact Southern Ocean and Antarctic climate, especially in regions that affect the stability of the remaining Antarctic ice sheet.

Results
ClimateresponsetoapartialAISremoval
WeinvestigatetheimpactofapartialremovaloftheAISbyperforminganexperimentunderLIGboundaryconditionsinwhichtheAISelevationandextent,aswellasoceanbathymetryweremodifiedtorepresenta4.1mand7.1mSLEAntarcticicemassloss19,27(Fig.1andSupplementaryFig.S1,Methods;experimentsSL4.1andSL7.1).TohighlightthedistributionoficelossaroundAntarctica,wedividetheSLEperturbationinto60°sectorsoflongitude(showninFig.1a,b).WealsocalculatethecontributionfromtheWAIS,theEastAntarcticIceSheet(EAIS),andtheAntarcticPeninsula,asdefinedbydrainagebasindefinitionsofAntarctica28.TheseamountsareshowninSupplementaryDataset3.ThekeydifferencebetweenthetwoscenariosisfoundintheEAIS,wheretheicelossis1.92mSLEinSL4.1,increasingto4.63mSLEinSL7.1.

InboththeSL4.1andSL7.1cases,partialicesheetremovalleadstoa0.5–1.5°CannualmeanSSTincreaseintheWeddellSea,extendingtowardstheIndiansectoroftheAntarcticshelf(30°W–30°E),aswellasintherossSea(180–150°W;SupplementaryFig.S2a,d).BoththeWeddellandRossSeawarmingoccurpredominantlyinsummer,incombinationwitharetreatinsummerseaice(SupplementaryFigs.S3andS4),thoughanexpansionofseaiceoccursintheWeddellSeainwinter.Thoseareasofseaiceexpansionareassociatedwithaslightdecreaseinannualmeantemperature(between−0.25and0°C).EastAntarcticannualmeancoastalSSTsincreasebyaround0.5–1°Cfrom0°to60°Eincombinationwithseaiceretreat,withastrongersignalintheSL7.1casethantheSL4.1case.However,inotherregionstheSL4.1andSL7.1simulationsshowminimaldifferencesandevensomestrongeranomaliesinSL4.1.From60to180°E,SSTanomaliesareclosetozonerethecoast,withtheseaiceedgemostlyunchanged(Fig.2a,d).Thesummerwarmingisdrivenbyice-albedofeedbacksbothfromreplacingpartofthemarineicesheetwithocean,andfromaretreatinsummerseaiceinthosesectors.

SomeoftheSSTanomaliesextendtothesubsurface(SupplementaryFig.S2d),witha0.25–0.5°Csubsurfacewarmingat100–500mdepthsimulatedalongtheEastAntarcticshelfbetween0and180°EinbothSL4.1andSL7.1.A~1°CSSTcoolingissimulatedat60to65°SintheAmundsenandBellingshausenSeasinbothSL4.1andSL7.1(Fig.2a,d).Thissignalisalsomirroredina1–2°Csubsurfacecoolingintheseregions,withaslightlystrongercoolingsignalinSL4.1thanSL7.1(Fig.S2a,d).ThisregionalcoolingisassociatedwithanorthwardshiftofthesubpolarfrontoftheAntarcticCircumpolarCurrent(ACC),whichsteerscolderisothermstothenorth(FigureS5).ChangesinthepathwaysoftheACCalsomanifestinsubsurfacewarmingof>2°Caround140–180°W,45–50°S(SupplementaryFig.S5),aswellassubsurfacewarmingaround~1°Clocatedat45°SintheIndianOceansector.TheshiftsintheACCareinturndrivenbybothchangestotheAntarcticcoastalbathymetrywhentheiceisremoved,andchangestothewindfield,notablyaweakeningofthewesterlywindsinallofthePacificsectoraswellassomeoftheIndiansectoroftheSouthernOcean,withaslightlylargeranomalyinSL4.1thanSL7.1(SupplementaryFig.S6).Thisweakeningislikelyduetohigh-latitudesurfacewarmingcausingadecreaseinbaroclinicinstability,whichinturndecreasesthestrengthofthemidlatitudewesterlywinds29.Inaddition,mixedlayersdeepenintheRossSeafromlessthan50mintheLIGcontrolto400–500mintheSL4.1experiment,andtosomeextentintheSL7.1experiment(SupplementaryFig.S7).Thisdeepeningisdrivenbygreaterexposureofhigherlatitudeshelfregionstoatmosphericcooling,enablingdenserwatertoformintheRossSearegion.Sea-iceretreatintheAtlantic-Indian-Atlanticsectoralsoleadstogreaterexposureofsurfacewaterstowintercooling,thusdeepeningthemixedlayersbetween0and30°E(SupplementaryFig.S7).ThissurfaceforcingincreasesAntarcticBottomWater(AABW)formationfrom9.8SvintheLIGcontrolto13.0SvinSL4.1and11.2SvinSL7.1(Figs.3and4).

AnimportantfeatureofoursimulationsistheabilitytomodeltheAntarcticcontinentalclimateresponsetoapartiallossoftheicesheet.Therearefourproxyrecordsfrom~127kathatrecordAntarcticsurfaceairtemperature(SAT)anomaliesof0.9to3.3°CwithrespecttoPI11,30.TheseAntarctictemperatureanomaliesarenotcapturedbyrecentPMIP4experimentsthatsimulatetheLIGclimatewithoutexplicitlyincludingAntarcticiceloss.OurLIGcontrolexperimentproducesasimilarresulttothePMIP4ensemble;AntarcticSATdoesnotchangesignificantlycomparedwithPI(SupplementaryFig.S9e).Incontrast,theicelossexperimentSL7.1capturesanannualmeanSATincreaseof2to4°CintheAntarcticcontinentalinterior,ingoodagreementwithproxydata(Fig.5b).TheSL4.1experimentalsoshowsEastAntarcticwarminginthecontinentalinterior,althoughtoalesserextentthanSL7.1.ThesechangesarecloselylinkedtoreductionsiniceelevationinEastAntarctica(Fig.1a,b),indicatinglapserateeffectshaveamajorimpactinexplainingthiswarmingsignal.However,someofthewarmingsignalisalsoduetoice-albedoeffectswarmingthenewlyformedoceansurface(i.e.lossofmarineicesheet),allowingadditionalheattobetransportedintothecontinentviatheatmosphericcirculation.

Surfacewindanomaliesindicateadecreaseinthepredominant(southerly)offshorewindsoffEastAntaricticainboththeSL4.1andSL7.1experiments(SupplementaryFig.S6b,e).ThisdecreaseinsoutherlywindsenableswarmeroceanicairmassestoreachfurtherinlandoverEastAntarctica.SurfacewindanomaliesalsoindicateaminorenhancementofeasterlywindsalongtheAntarcticcoast(SupplementaryFig.S6).Wesuggestthatthesewindpatternsmaybedrivenbytwofactors.First,theweakeningoftheequator-to-polegradient(duetoAntarcticsurfacewarming)drivesaweakeningofthemidlatitudewesterlies,sincepolarwarmingatthesurfacehasbeenfoundtoreducebaroclinicinstabilityandthereforeweakenthemidlatitudewesterlies29.Second,theloweringoftopographicbarriersespeciallyinWestAntarcticaallowsagreaterexchangeofsurfacewindsintonear-polarlatitudes.Inreality,thesetopographicchangeswouldalsoimpactthekatabaticwinds,buttheseremainpoorlyresolvedwithanatmosphereresolutionof~2°.ThesouthwardshiftoftheeasterliesalsostrengthenstheAntarcticSlopeCurrent(ASC;NatureCommunications|( 2024)1 5:1026 3).Thenewoceangatewayat73°S,86–80°Wopenedbytheremovaloficesheet,allowsasouthwardtransportof0.7Svand1.1SvrespectivelyinSL4.1andSL7.1throughthegatewayfromthePacifictotheAtlanticsector.

ClimateresponsetomeltwaterinputaroundAntarctica
WefurtherinvestigatetheimpactofmeltwaterforcingaroundAntarctica,equivalentto4.1mand7.1mSLEofAISdisintegration(Methods,experimentsFW4.1andFW7.1).Thismeltwaterinputleadstoapproximately1–2°CSSTdecreasefrom150to30°WoffthecoastofAntarcticaintheFW4.1case(Fig.2b).Asimilarpattern,butstrongermagnitudeofSSTdecrease(2–3°C)occursintheFW7.1case.Thiscoolingisassociatedwithalargeexpansionofseaice(Fig.2).Thesechangesaredrivenbythefreshwaterforcingcreatingastrongerhaloclinenearthesurface,whichensuresthatsurfacewatersremainstablystratifiedundercoolingconditions.Amorestablehaloclineenablesbothgreaterseaiceformation,andareductionofdeepoceanconvectionintheWeddellSeaasindicatedbychangestomixedlayerdepths(SupplementaryFig.S7).TherateofSouthernOceandeepwaterreducesfrom9.8SvintheLIGcontrol,to7.8SvintheFW4.1caseand5.8SvintheFW7.1case(Figs.3and4).

Duetotheincreasedsurfacestratificationandreduceddeepoceanconvection,subsurface(100–500mdepth)temperaturesintheFW4.1experimentincreasebyupto2°CintheRossSea(SupplementaryFig.S2b),withaslightlystrongerincreaseinFW7.1(SupplementaryFig.S2e).Themeltwater-inducedstratificationisolatesthesurfacefromthewarmercircumpolardeepwater(CDW),whichisthentransportedfurthersouthandwarmsAntarcticsubsurfacewaters.TheincreasedsouthwardadvectionofCDWisindicatedbyaflatteningofneutraldensitysurfacesinthehighsouthernlatitudes,enablingsouthwardisoneutraltransportofwarmerdeepwaters(Figs.3and4).ThiseffectissimilartothatfoundinpreviouscoupledclimatemodelsimulationsoftheLIG24andfuturewarmingscenarios31,whichfoundthatmeltwaterperturbationstendedtolowerAntarcticSSTwhilewarmingthesubsurface.

MeltwaterforcingalsoreducesAntarcticSAT,similartotheSSTresponse(Fig.5b,e).Thisperturbationinitselfdoesnotproduceagoodagreementwithpaleo-proxyrecordsfromEastAntarctica,whichshowawarmanomalycomparedwithPI11,30.ThestrengthofmidlatitudewesterlywindsincreasesintheFW4.1andFW7.1experiments(SupplementaryFig.S6c,f),likelyduetoanenhancedequator-to-polegradient.ThisistheoppositeresponseofthesamemechanismseenintheSL4.1andSL7.1windchanges,inwhichsurfacepolarwarmingreducesbaroclinicinstability29.

ClimaticresponsetoacombinedpartialAISremovalandenhancedmeltwaterinputaroundAntarctica
Finally,weassessTheclimaticimpactofthecombinedicesheetremovalandmeltwaterinputsof4.1mand7.1mSLE(Methods,experimentsCOMB4.1andCOMB7.1).ThesecombinedexperimentsleadtomostlywarmingoverAntarcticaandcoolingovertheSouthernOcean,resemblingacombinationofSATanomaliesfromboththeicelossandmeltwaterexperiments(Fig.5c,f).WhiletheCOMB7.1experimentshowssomemodestwarminginthefourproxylocations11,12,30,theinfluenceoflowerSSTsduetothemeltwaterforcingproducesapooreragreementwiththeproxydatainthecombinedcasesthanintheiceloss-onlycases.Aquantitativecomparisonbetweentheproxydata,andthemodelledSATchangesateachproxylocationisshowninSupplementaryDataset1.

Themeltwaterforcingdominatestheocean’ssurfaceresponsewithanincreaseinsurfaceoceanstratificationallaroundAntarctica,andadecreaseindeep-waterformationintheWeddellSea(SupplementaryFig.S7).Theannualmeansea-iceextentis4.7%and12.8%largerthantheLIGcontrolinCOMB4.1andCOMB7.1respectively(herewecountgridcellsonlyiftheyexistasoceaninbothsimulations).Theseaiceexpansionisassociatedwith1–1.5°Cand1.5–2°CSSTdecreasesintheWeddellSeainCOMB4.1andCOMB7.1respectively(Fig.2c,f).Similarly,thereisa~1°Cand~2–3°CdecreaseintheAmundsenandBellingshausenSeasinCOMB4.1andCOMB7.1respectively.AlongtheEastAntarcticcoast,SSTchangeisslightlynegative(~-0.5–0°C),thoughitiswarmerthanthemeltwatersimulationsinregionswhereicesheetisconvertedtoocean(Fig.1).AcomparisonofannualmeanSSTproxydata11,12,32fromtheSouthernOceanwithmodelanomaliesisgiveninSupplementaryDataset1,andplottedinFig.2,whilesummer(DJF)averageSSTproxydata11andmodelanomaliesarecomparedinSupplementaryDataset2.ThesecomparisonsindicatethatthesimulatedSSTsaregenerallytoolowcomparedtotheSSTproxydata.MostoftheSouthernOceanSSTproxiesindicatewarmingofatleastseveraldegees,althoughafewsitesalsoindicatestrongcooling(Fig.2),withtheproxyanomalydatarangingfrom−6.8°Cto11.5°C(SupplementaryDataset1).Ingeneral,thebestagreementisfoundintheicelossonlyexperiments(SL4.1andSL7.1),whileapooreragreementisfoundwhenintroducingmeltwaterbothseparately(FW4.1andFW7.1)andcombinedwiththeiceloss(COMB4.1andCOMB7.1).Thesedatasuggestthat,inthecontextofuncertaintimingofAntarcticmeltingattheLIG5,10,thatthepeakmeltwaterpulseintheSouthernOceandidnotoccursimultaneouslywiththewarmestSSTanomalies,sincemeltwatertendstostronglycoolthesurfaceoftheSouthernOcean.

ThesubsurfaceresponseinCOMB4.1alsostronglyresemblesthepatternoftheFW4.1experiment.A2°CsubsurfacewarmingoccursintheRossSea.ElsewherearoundtheAntarcticcoast,thereismodestwarming(<0.5°C),buttheCOMB4.1experimentshowssomeslightadditionalwarmingcomparedtothemeltwater-onlycase,similartoSL4.1.ComparedtoSL7.1,inCOMB7.1theRossSeaalsoexhibitstrongsubsurfacewarming(>2°C),butthereisfarmorewidespreadsubsurfacewarmingaroundallofEastAntarctica(0.5–1°C).Withthelarger(7.1mSLE)perturbations,theiceremovalandmeltwatereffectscompoundeachothermorestrongly.

Below1000m,theicelossandmeltwatereffectscompoundnon-linearly.InthedeepSouthernOcean,zonalmeantemperaturesshowaclearcoolinginSL4.1,andamodestwarminginFW4.1(Fig.3d,f).However,thecombinedexperimentCOMB4.1showsastrongerandlargerwarmingsignalthanFW4.1,i.e.thetwoperturbationsdonotcombinelinearly(Fig.3h).Thereisalargeregionofwarming(0.2–0.4°C)thatextendsfromAntarcticato50–60°Satintermediatedepths,andupto30°Sbelow4000m(Fig.3).Likewise,inCOMB7.1,thedeepSouthernOceanshowsstrongerwarming(0.5–1.0°C)andgreaterspatialextentthanFW7.1,despitethefactthattheiceloss-experiment(SL7.1)coolsthedeepocean;i.e.anon-linearwarmingeffect.

AABWformationisreducedto6.0and4.7SvintheCOMB4.1andCOMB7.1respectively,representingevenstrongerreductionsthaninthemeltwater-onlycases(7.8and5.8SvforFW4.1andFW7.1).DeepwaterformationremainsintheAtlanticsector,butshiftsnorth-eastoutoftheWeddellSea,andisabsentfromtheRossSea.Thereisaclearflatteningofisoneutralslopesinthecombinedperturbationexperiments,asinthemeltwatercase,suchthatrelativelywarmerdeepwaterisbroughtintotheSouthernOcean.Wefindthatthecombinationofflatterisoneutralslopes,adecreaseinventilationandSSTwarmingfromtheicelossperturbationproducesthiscompoundwarmingofthedeepocean.Thisisincontrasttotheicelossonlycases(SL4.1andSL7.1),wherethesteepeningofisoneutralslopesmeansthatthedeepoceanismorestronglyconnectedwith(colder)highlatitudeatmosphereforcing.

Thenewoceangatewayat73°S,86–80°Wopenedbytheremovaloficesheetallowsanorthwardtransportof0.1Svand1.0SvinCOMB4.1andCOMB7.1respectivelythroughthegateway(i.e.AtlantictoPacificsector;SupplementaryFig.S8d,g),whichisintheoppositedirectionasthetransportinSL4.1andSL7.1.Thisthroughflowdoesnotgreatlyaffectthesurfacetemperatureandsalinityinthedeepwaterformationregions.

Methods
Modeldescription
TheexperimentsarecarriedoutusingthecoupledclimatemodelGFDLCM2.141. The ocean component of the model uses a resolution of 1° latitude×1° longitude×50 levels, with a latitudinal grid refinement in the tropics down to 0.33° at the equator. The model also uses a tripolar grid, with a regular latitude-longitude grid south of 65°N, transitioning to a bipolar grid in the Arctic, with poles centred over Siberia and Canada42. The ocean model is modified from the original GFDL CM2.1 configuration to use MOM version 5.1.0, and uses a bottom roughness scheme for vertical mixing43. The sea ice model is the Sea Ice Simulator (SIS), which is a dynamical model with three vertical layers, one snow and two ice, with five ice thickness categories. It uses an elastic-viscous-plastic method to calculate ice stresses44, and the thermodynamics use a modified Semtner three-layer scheme45. The sea ice component uses the same horizontal resolution as the ocean. The atmosphere component is the GFDL AM2, and uses a resolution of 2° latitude×2.5° longitude×24 levels41. The land surface model is LM2, and is configured for PI conditions as in the original CM2.1 experiments. The land cover type is based on a historical land use distribution dataset, based on ref. 46. This classifies the land surface into 10 vegetation or land surface types46, with corresponding properties of albedo, surface roughness, stomatal resistance, root depth and snow masking depth.

Pre-IndustrialandLastInterglacialexperiments
Themodelisrunintwoconfigurations:aPIcontrolrun,andaLIGrun.Bothofthese runsfollowtheexperimentalprotocolofref.3,whichspecifiesthegreenhousegasconcentrations,orbitalparametersandsolarconstantforboththePIandLIGsimulations.TheseparametersaregiveninTable1ofref.3.Allvegetationtypes,soiltypesandrunoffschemeareleftunchangedfortheLIGsimulationsandperturbationexperiments(below).Wekeeptheriverrunoffrelocationmapfixedregardlessofchangestotheicesheets,since(i)thedrainage-basinscalerunoffchangestoriverrunoffarerelativelysmall,and(ii)riverrunoffisasmallsignalcomparedtothemeltwaterperturbations,allowingustoattributechangesmoredirectlytomeltwatereffects.ThePIandLIGsimulationsareinitialisedfromamodernobservationaldatasetoftemperatureandsalinity47,andrunfor1500years.Bothofthesesimulationsreachaquasi-equilibriumwithglobalmeanSSTincreasingby0.02–0.03°Cpercenturyoverthelast500years.Forthelast100yearsofsimulation,thetemperaturetrendat4000misbetween0and0.03°Cpercenturyinallsimulationsincludingtheperturbationexperiments.WeaccountforthissmalldriftinouranalysisbytreatingtheLIGsimulationasa‘control’experiment,andsubtractingitfromtheperturbationexperimentsatthesamenumberofmodelyears.ThesurfacetemperaturechangesfoundintheLIGcomparedwiththePIsimulationshowsimilarseasonalchanges(SupplementaryFig.S9)tothePMIP4results13 (theirFig.5),andsomedifferencesintheannualmeanresponse.AsinthePMIP4ensemble,ourmodelsimulatesstrongnorthernhemispherelandwarminginborealsummer(JJA)andlandcoolinginborealwinter(DJF)(SupplementaryFig.S9a,c).ThereisacontrastingsignalofborealsummercoolingovertropicalAfricaandtoalesserextentIndia;theseresultsarealsopresentinthePMIP4ensemble.OurresultsdiffertoPMIP4inthatwehaveanoverallcoolingintheannualmean,andarelativelackofArcticwarming,wherethePMIP4ensemblerecordsanannualmeananomalyofaround2°C.WhilePMIP4findsanensemblemeanglobalanomalyof0.5°C,wefindaglobalanomalyof−0.59°C,similartothelowerendofthePMIP4range(−0.5to+2°C).Below,wedescribetheperturbationexperiments.

TheMOCcirculationinthePIandLIGrunsispresentedinSupplementaryFig.S10.Thenortherncell(AMOC)transportsasimilarvolumeinbothsimulations;26.3Svand26.1SvinthePIandLIGsimulationsrespectively.ThedepthoftheAMOCincreasesslightly,accountingfortheminorchangesinthestreamfunctiondifferenceplot(SupplementaryFig.S10c).AABWformationdecreasesfrom11.1SvinthePIto9.8SvintheLIG.Thisdecreaseisevidentinthepositivedifferencearound70°SinSupplementaryFig.S10c.ThisdecreaseinAABWformationisalsoevidentfromadecreaseinmixedlayerdepthsinboththeRossandWeddellSeas(SupplementaryFig.S11).IntheWeddellSea,whereAABWispredominantlyformed,mixedlayersaregenerallyseveralhundredmetresshallower,whileintheRossSea,deepconvectionceasescompletely(SupplementaryFig.S11).

PartialAntarcticicesheetremovalexperiments
Sea-levelduringtheLastInterglacialwasestimatedtobebetween1and9mhigherthanpresentday7.Herewesimulatetheeffectsofthisicemelt,usingaperturbationderivedfromGolledgeetal.27,usingtheirscenarioofRCP4.5forcingatyear5000,includingthesub-gridbasalmeltinterpolation(seeFig.1f; ‘high’ scenario). We chose this scenario, since it represents a 4.3 m SLE rise with respect to PI, i.e. in line with mid-range estimates of ice loss at the LIG. We use their modelled ice elevation as an indicative estimate of an Antarctic ice sheet reduced by 4.3m, given sea level constraints from proxy information5,8. However, when comparing their ice sheet elevation for the historical period (1900 CE) with the control ice sheet in GFDL CM2.1 derived from observations but on a far coarser grid, there are some differences in baseline elevation (both higher and lower in different regions). To account for these discrepancies in baseline ice sheet elevation, we applied a perturbation to the GFDL CM2.1 model in two ways: 1. using the anomaly of ice sheet elevation, taken as the difference between the RCP4.5 and 1900 CE simulations of Golledge et al.27, and 2. using the absolute value of elevation in the RCP4.5 simulation of Golledge et al.27. In both cases, we made further adjustments so that the perturbed ice sheet was always less than or equal to the control (here the GFDL CM2.1 control) ice sheet elevation, and the anomalous elevation does not go below sea level.

This procedure yielded two estimates of ice loss: 1. Using the anomaly-derived perturbation, the resulting ice loss was 4.1 m SLE, and 2. using the absolute value of elevation, the resulting ice loss was 7.1 m SLE. For each of these values of total ice loss, we also perform a separate perturbation using the equivalent volume of freshwater forcing, and a combined ice loss and freshwater forcing experiment, as follows:
1. SL4.1: Removal of 4.1 m sea level-equivalent ice sheet using the RCP4.5-1900 CE anomaly from Golledge et al.27 (Fig. 1a, c);
2. FW4.1: Freshwater forcing over 500 years adjacent to the Antarctic coast, with total volume equivalent to the ice loss derived in SL4.1;
3. COMB4.1: Combined perturbations of ice sheet removal (SL4.1), and freshwater forcing (FW4.1).
4. SL7.1: Removal of 7.1 m sea level-equivalent ice sheet using the RCP4.5 absolute ice elevation from Golledge et al.27 (Fig. 1b, d);
5. FW7.1: Freshwater forcing over 500 years adjacent to the Antarctic coast, with total volume equivalent to the ice loss derived in SL7.1;
6. COMB7.1: Combined perturbations of ice sheet removal (SL7.1), and freshwater forcing (FW7.1).

All perturbations are started from year 1000 of the LIG experiment and using LIG orbital parameters and greenhouse gases. A summary table of these experiments is given in Supplementary Dataset 3.

We chose to derive perturbations from this model27 because it is one of few publicly available Antarctic ice sheet reconstructions that matches the total sea-level change for the LIG. Several other ice modelling studies have also examined ice loss changes at the LIG17,19; these generally have a lower or similar total sea level change (3–4 m) than the scenario we have chosen. Part of our motivation for the choice of ref. 27 is to obtain a large signal of change (within the bounds of LIG sea-level change), in order to more clearly examine the mechanisms of climate responses, rather than finding the best possible LIG ice sheet reconstruction. In addition, the spatial pattern of AIS loss in these experiments is similar to previous simulations of the LIG Antarctic ice sheet17–19. Therefore, the AIS ice-mass loss forcing used here is consistent with modelling and paleo-proxy estimates.

We use the Antarctic bedrock map contained in ref. 27 to determine where the ice sheet overlies either marine or terrestrial points. Where the ice sheet is removed from marine locations, we replace the ice with ocean grid cells (formerly land grid cells in the model). Ocean grid cells are set to the depth of the marine bedrock, with an additional constraint that new ocean grid cells must have a minimum depth of 100m. This minimum depth ensures numerical stability of the newly created shelf region. Where ice is removed from terrestrial locations, the land grid cells are lowered to the bedrock altitude27. In this case, the albedo and vegetation parameters are kept as the ‘ice’ type, to reflect the fact that the landscape is still predominantly ice- or snow-covered. Note that in the control run, the ocean grid is limited to 82°S, while in the SL4.1 configuration, the ocean grid is extended to 85°S (Fig. 1 and Supplementary Fig. S1).

Freshwaterforcing
Wederiveafreshwaterforcingdistributionbasedonthetotalicelostbetweenthepre-industrialandSL4.1andSL7.1icesheets,convertedtoanequivalentfreshwatervolume.WespreadthisaroundAntarcticainbinned60°longitudinalsectors;wherethetotalicechangeineachsectorisusedtodeterminethetotalfreshwaterforcingforthatsector(seeFig.1a,b).Thetotalfreshwatervolumeisthenspreadaroundthecoastline,inthe2surfaceoceangridcellsadjacenttothenearestAntarcticlandpoint;thisdistributionoffreshwaterforcingisshowninSupplementaryFig.S12.Therateofforcingissetaccordingtoaconstantfreshwaterforcingperiodof500years,whichamountsto0.09Svand0.16SvfortheFW4.1andFW7.1experimentsrespectively.Theseratesarecomparabletoamaximalmeltingratearound127ka19,butlowerthanapreviousclimatemodellingstudy,whichforceda3.5mSLEfreshwaterperturbationemulatingaWAIScollapsein100years22.FortheCOMB4.1andCOMB7.1experiments,weapplyboththeicesheetremovalandfreshwaterperturbationssimultaneously.Inthesecases,therearemoreoceangridcellspresent,especiallysouthof80°SalongtheAntarcticcoast.Thereforethefreshwaterforcingregionissomewhatlargerinlatitudinalextent(SupplementaryFig.S12),howeverthetotalfluxremainsthesameasintheFW4.1andFW7.1experiments.

Neutraldensitysurfaces
Forthecalculationofω-surfacesofneutraldensityusedinFigs.3and4,weusethemethodofStanleyetal.48.Theω-surfacesaregeneratedfroma50-yearaverageoftemperature,salinityandpressureoutputfromthemodel,whicharethenzonallyaveraged.Theω-surfacesaregeneratedatthereferencelocation(49.5°S,20.5°W)atdepthsof(100,200,500,1000,1500,2000,2500,3000)m.ThesecoordinatesareintheWeddellSeasector,butnorthofthezoneofdeepmixedlayerdepths,inordertoberepresentativeoftheSouthernOceanstratificationbutoutsideofthemarginallystratifiedconvectionzone.Notethatneutralsurfacesaretangentialtolocalisopycnalsurfacesattheirreferencedepth48,representingpathwaysofalong-isopycnalmixingandtransportinthemodel.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of : \"East Antarctic warming triggered by West Antarctic ice loss during the Last Interglacial\"\nHutchinson et al. investigate potential feedbacks of Last Interglacial (LIG) Antarctic Ice Sheet (AIS) retreat acting on the atmospheric and oceanic circulation. Their motivation is the apparent discrepancy between modelled LIG climate conditions (e.g. in PMIP4) and the actual observed state revealed by ice core proxy data (surface air temperatures, SAT) and marine sediment cores (sea surface temperatures, SST). This discrepancy is a long-standing issue in the paleo-modelling community and very plausibly caused by missing ice-atmosphere-ocean feedbacks in the relatively coarse resolution PMIP4 models. Fully coupled climate models including interactive and evolving ice sheets are still difficult to realize due to prohibitive computational constraints and technical challenges such as evolving ice shelf cavities. To overcome these limitations ice sheet boundary conditions can be adjusted to the respective climate epoch and freshwater hosing can mimic increased ice discharge e.g. in Interglacials. This is the approach Hutchinson et al. take in this work. Their main finding is, that a 500 year freshwater hosing during the LIG, sourced from the West Antarctic Ice Sheet (WAIS) and combined with a modified ice sheet topography (partial WAIS collapse, including bathymetric changes), has considerable effects on the Southern Ocean thermal regime as well as on SATs over the AIS. The LIG SAT anomalies found in Hutchinson’s model simulations are closer to ice core reconstructions compared to the results of the PMIP4 ensemble. The subsurface ocean temperature anomalies the authors model here are of sufficient magnitude to affect grounding line dynamics during the LIG. I congratulate the authors on an interesting piece of work. Overall, this is an interesting and well written study focused on a highly relevant research question in earth system science. It could be a valuable contribution; however, I have several comments on how the author present their findings. Additionally, I find parts of the methodological approach to be inconsistent. To make this a viable contribution to Nature Comm. in my opinion major changes to the manuscript and potentially additional experiments are necessary. While I certainly hope so, I leave it to the editor and the authors to decide whether this is possible. In the following I will outline my main comments followed by some smaller comments.\nMajor points:\n1. Hutchinson et al. write in their conclusions, that their’s is the first study “demonstrating … the contrasting climatic impacts of Antarctic ice removal and Southern Ocean meltwater input”. This sentence needs moderation as to my knowledge several studies (e.g. Holloway et al. 2016 in Nature Communications) follow similar experimental procedures albeit with different foci in their analysis. When reading the manuscript by Hutchinson et al., I was missing a discussion how their findings differ from previous studies and what novel methodologies they apply. This should be expanded on, certainly in the methods section, but also in the main manuscript.\n2. The title claims that the climate response to changes of WAIS is investigated here. However, the authors write that they consider a scenario akin to 4.5 m AIS SLE volume change during the LIG which is inconsistent. Should the whole WAIS collapse, global sea levels would rise by “only” 3-3.5 m (see e.g. Bamber et al. 2009). In light of the “4.5 m scenario” this would imply at least some East Antarctic Ice Sheet (EAIS) contributions. Furthermore, the boundary conditions used here clearly show only partial WAIS collapse (e.g. Figure 1) thus the EAIS contribution must be considerable.\n3. This brings me to the last main point. The experimental setup is designed to focus on freshwater hosing from the WAIS, specifically from the Amundsen/Bellinghausen Sea embayment as well as the Ross and Weddell Sea. This is inconsistent with the topographic boundary conditions applied: Figure 1 shows that e.g. the Recovery Basin glaciers are clearly retreated. The Recovery Basin is technically East Antarctica but drains but the freshwater from ice sheet retreat would end up in the Weddell Sea, so that’s consistent with the hosing approach. However, large swathes of the coastline of Dronning Maud Land are gone and Wilkes and Aurora Basin Glaciers have retreated. For the sake of consistency, freshwater should be channeled through these regions as well. Additional experiments including freshwater hosing from the EAIS, and analysis of the impact of regional hosing on the different ocean sectors would greatly strengthen this study (see below).\nSpecific Comments:\nI think there are several places in the manuscript which would benefit from references\ne.g.\nL36, L48, L50 here e.g. Sutter et al. 2016, Pollard et al. 2016. L51 e.g. Golledge et al. 2019\nL59: Figure 1 shows the ice sheet representation on the GCM-grid resolution. You mention in the methods that you take the ice sheet model output from the RCP4.5 simulation at the year 5000 in Golledge et al. 2015 and then regrid it to the native GCM-resolution? This means (as one can see in Figure 1) that you not only remove parts of the WAIS but also the EAIS (e.g. in the Recovery Basin, Wilkes subglacial basin and Aurora basin, coastline of Dronning Maud Land exhibits drastic changes as well). This means you can't strictly assign the climatic changes to WAIS topographic LIG-responses exclusively. I suggest you discuss this in the manuscript and potentially adjust the framing accordingly. Furthermore, looking at Figure 1 in Golledge et al. 2015 from which you take the boundary conditions, it seems that the WAIS configuration in your GCM boundary conditions is the “low scenario” i.e. 2.77 m SLE, is that correct? In that case, most of WAIS seems to be still intact in your representation, which means that the majority of sea level equivalent ice volume change has to come from the East Antarctic Ice Sheet. A complete loss of WAIS amounts to ca. 3 m sea level rise (e.g. Bamber et al. 2009). You state that yours is a configuration with 4.5m sea level rise but only partial WAIS collapse. Please elaborate and expand the discussion/caveats of your approach accordingly.\nFigure 2: I suggest you include place names in figure 1 as an orientation for the reader unfamiliar with Southern Ocean sector names. Also, consider including an elevation map from the original ice sheet model showing the ice sheet thk changes compared to the ctrl (e.g. with a diverging RdBu cmap).\nL115 surface cooling alone\nL118 I would argue that this is a very striking finding as subsurface ocean temperature ultimately drive ice sheet retreat/advance (the grounding line does not \"see\" SST). In general, the effect of AIS changes on the subsurface ocean conditions should be highlighted and discussed more in the manuscript as it reveals a positive feedback if including freshwater forcing (as e.g. shown and discussed in Golledge et al. 2019, Bronselaer et al. 2018) and I think goes beyond what e.g. Holloway et al. 2016 discussed. However, I think that additional experiments would be necessary as the boundary conditions you apply clearly exhibit substantial EAIS changes as well. How would these affect ocean circulation? Would your results change (e.g. the large Ross Sea ocean warming) if you would add a freshwater feedback in East Antarctica as well? What mechanisms would be in place then?\nL161 again, this statement is incomplete, as you also include EAIS topographic changes (but only WAIS freshwater forcing, which is inconsistent). Furthermore, the largest feedback of WAIS retreat seems to be a substantial subsurface warming of the Ross Sea.\nL166 e.g. Holloway et al. 2016 conduct similar experiments also focused on the LIG.\nL190 Ice core constrained ice sheet modelling show's that this (Wilkes Subglacial Basin) sector very likely remained stable during the LIG (Sutter et al. 2020). Uranium isotopes in subglacial precipitates also hint at LIG ice sheet stability in the Wilkes Subglacial Basin (Blackburn et al. 2020). Furthermore, Sutter et al. 2016 show sea level rise of ca. 3-4 m (mid range) for the LIG without invoking MICI.\nL197 this sentence appears to be incomplete?\nL198 see e.g. Holloway et al. 2016 and other studies investigating the climate response to ice sheet topographic changes and freshwater hosing. In general, I suggest to significantly expand the method section by detailing how your experimental setup goes beyond/differs from previous studies.\nL253 there are at least two studies explicitly simulating the AIS during the LIG (Sutter et al. 2016, DeConto et al. 2016). Furthermore, Golledge et al. 2019 discusses the effect of changes in ocean stratification due to freshwater release from the AIS.\nL255- No, see below.\nL266- not consistent with the physical constraints. 1. WAIS only amounts to ca. 3 m SLE ice volume (e.g. Bamber et al. 2009), 2. only partial collapse of WAIS in boundary conditions, -> substantial contr. from EAIS.\nBern, 05.07.2022\nJohannes Sutter", "reviewer_2": "This paper presents modelling attempt to illustrate the impacts of Antarctic-ice patial removal and the associated Southern Ocean meltwater input on the Antarctic climate change in the LIG with respect to the PI. New to existing studies, their ESM modelling results have suggested the contrasting climatic impacts of Antarctic ice removal and Southern Ocean meltwater input on the spatial pattern of the LIG Antarctic surface air temperature during the LIG. This work is of significance to the field of paleoclimatology and has implications for present day global warming. However, in the manuscript, both the modelling set-ups and climatic mechanism remain to be further clarified. I would encourage a re-submission of a revised manuscript.\nComments:\n1. In 6.3, the SL4.5 experiment set-up is discribed that: Removal of 4.3 m (here it should be 4.5?) sea level-equivalent ice sheet from Antarctic topography and adjustment of ocean bathymetry. So, I would expect that the entire topography of the Antarctic ice and the coatal line in the LIG are changed. However, in the manuscript, the mechanism is only attributed to the West Antarctic ice loss, e.g. as mentioned in the title. How to undersrtand this?\n2. In Fig.1, a figure panel for LIG-to-PI topography anomolies will be useful to understand the ice change\n3. l.234-235, ‘The surface temperature changes found in the LIG compared with the PI simulation are broadly in agreement with PMIP4 results5 (Figure S7).’. Acturally, this is not true. The Figure S7 is quite different from Fig. 5 in ref.5.\n4. l.227, How to understand ‘the runoff scheme are left unchanged’? E.g. river freshwater discharge amount, river mouth location.\n5. l.44-45, ‘implies a larger sea level rise by 2100 than many process-based studies’, how to understand by 2100?\n6. l.54-55, ‘the stability’ is not discussed in this work, but a shift of mean climate state.\n7. l.62-63, The Weddell-Sea sea ice edge shows W-S opposite pattern in sea ice edge change, rather than a common retreat.\n8. l.64, sea ice edge around most of East Antarctica is acturally reduced, not steady, see Fig. 2a.\n9. l.67-68, 100-500 m depth in the the Ross Sea becomes colder rather than warmer, see Fig. 2d.\n10. l.69-72, according to Fig. S2a, an anti-clockwise atmosphere circulation over the Amundsen, Bellingshausen and Ross Seas is shown. It seems as a key pattern to explain the regional climate. Is it related to the Land-sea-mask change, e.g. enlarged Ross-Sea bay due to coastal line retreat?\n11. l.101-102, ‘First, the weakening of the equator-to-pole gradient (due to Antarctic surface warming) drives a weakening of the midlatitude westerlies’, by what mechanism?\n12. l.105-106, Fig. S5 is not able to present ‘The southward shift of the easterlies also strengthens the Antarctic Slope Current (ASC; Figure S5).’, please add a figure for anomalies.\n13. l.119-120, ‘and by 0.5-1 C along most of the West Antarctic coast (180E - 60W, Figure 2e).’, Fig.2e is not able to present this change.\n14. l.129-131, ‘The strength of midlatitude westerly winds increases in the FW4.5 experiment, likely due to an enhanced equator-to-pole gradient, contrary to the circulation changes in the ice loss experiment.’ Similarly, by what mechanism?\n15. l.139-140, ‘since the ice elevation does not change at these proxy locations.’ Same as comment #2, a figure panel for LIG-to-PI topography anomolies is requested.\n16. l.147-149, ‘On the other hand, a small SST increase is simulated along the East Antarctic coast, which may be due to the minor retreat in the land ice edge in this region (Figure 1).’ Does this mean land-ice grid becomes ocean grid cells in the model? Please clarify.\n17. l.158-159, ‘is driven by a southward expansion of CDW below the freshwater layer’ Why to exclude the impact of reduced AABW formation. Any modelling signal to indicate this?\n18. In general, how deep is the ocean channel between the WAIS Island and Antarctic main land, when the ice is reduced? Then, how is the water exchange between the Weddell Sea and Amundsen Sea? This is also key to undertand the full process.\n19. Please add the information about the MOC anomalies between LIG and PI. Also important to understand the PI-to-LIG ocean change.", "reviewer_3": "Review of NCOMMS-22-16471-T: “ East Antarctic warming triggered by West Antarctic ice loss during the Last Interglacial”.\nIn this paper the authors discuss the impact of both WAIS removal and freshwater input on the climate and ocean state in the southern Hemisphere during the LIG. They use a climate model with a prescribed ice sheet and freshwater forcing. One issue they investigate is if the mismatch between the LIG127ka PMIP4 simulations and southern hemisphere proxy records (SAT and SST) was due to the choice of an PI AIS in the model setup. The authors focus on indefinity changes in southern hemisphere climate state specially from ice-sheet driven feedbacks (changes in bathymetry/elevation) from those related to freshwater by running three climate simulations.\nThe paper is well written, and the results are clearly presented. However, I would the authors to expand on previous publications which have investigate the combination of WAIS and FWF for the LIG, although they may be from older generation of climate models. For example, Holloway et al., 2016 also performed climate model simulations with a reduced AIS and FWF.( I was not associated with this study). I would recommend for publication with the minor changes below.\nSection 2: Question about lapse rate correction: I would like more details on the reasoning of this correction for the final COMB4.5 results (shown in the paper). The results in Fig3 are the SAT with the lapse-rate correction, so removing any warming signal associated with a change in altitude due to the retreat of the WAIS. However, as the aim of the SL4.5 is to investigate the impact of a reduced WAIS, this correction removes/reduces this impact. Why are the corrected results not shown in the SOM? I am not sure of the rational? Is this to make the results more consistent with FW4.5?\nMinor text corrections:\nLine 35: “Coupling a dynamic ice sheet with ocean-atmosphere-sea-ice-land ... remains a significant technical barrier”. There have been simulations with dynamic ice sheet (Sommers et al, 2021). To be clearer, I would state a dynamic Antarctic ice sheet. This is a challenge due to the ice-ocean coupling required for a large marine based ice sheet.\nResults section2: SL4.5 experiment compared to COMB4.5\nFrom the SL4.5 (Fig2d) there is a pronounced warming signal ( > 2C) at 100-500m between 140W and 180W. What is the origin of this? It does not occur when running the COMB4.5 experiment, unlike most other signals (Wind speed, SAT)? Is there a 2nd feedback so the FWF dominates the response in the ocean?\nDiscussion:\nLine 160 (related to FigS6)” Moreover, the East Antarctic subsurface anomaly is more positive than in FW4.5, which partly reflects the warming caused by the continental ice sheet reduction” It could be the colour scale but comparing Fig.2e and f most of EAIS appear to surrounding by ‘white’ region, so a very similar temperature as in FW4.5 (so colder), apart from close to the Wilkes Basin. Also, in FigS6, with deeper SST anomalies the ice sheet only simulation (FigS6d), there is a strong cooling along the coast.\nLine 182:”there are also areas for East Antarctica’ where reverse-sloping bedrock...’ Can you mention which of these three regions coincide with simulated warming/cooling anomalies. It is interesting the large warming anomaly near the Wilkes SG Basin, but the Amery is associated with a cooling.\nFigures:\nFigure 2-3. Could you increase the latitude extent to be consistent with SOM plots. The lateral extent of the warmer anomalies (i.e Fig.2d) is chopped off.\nFigure 3: what are the circles? Are there the ice core sites? Are the anomalies larger than 5C? if so can you edit the scale bar to indicate this?\nMethods:\nTable: Model setup: could the authors add a table in the SOM of model set up (forcings), names; ice sheets(small/large); size of FWF etc. Just as summary for the model setup and boundary conditions.\nTable: Proxy data: Throughout the text the authors compare the SAT/SST from the simulations to proxy data and other values from PMIP4 simulations. Can these be summarised in a table. For example, the 4 proxy ice core records (line 80); SST temperature records (line 25-26). Can the authors compare to other proxy records, for example those shown in the Otto-Bliesner et al., 2021, summary PMIP4 LIG127ka paper.\nReferences:\nHolloway, M., Sime, L., Singarayer, J. et al. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse. Nat Commun 7, 12293 (2016). https://www.nature.com/articles/ncomms12293\nOtto-Bliesner et al., 2021 Large-scale features of Last Interglacial climate: results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate Modeling Intercomparison Project (PMIP4)\nhttps://cp.copernicus.org/articles/17/63/2021/"}, "author_response": "We thank the reviewers for their constructive and thoughtful comments, and positive encouragement for resubmission. We have made substantial revisions to address the paper concerns, including revising and re-running all but one of the perturbation experiments to achieve better consistency between the ice sheet and meltwater forcing, and more thoroughly addressing previous literature. We have also expanded our experiment scope to include a smaller and larger perturbation of ice (4.5 m SLE and 7.5 m SLE), with equivalent amounts of freshwater forcing in each case. Importantly, we have taken a more careful approach exactly describe the ice perturbations and match the location of freshwater forcing around the Antarctic coast to align with where the ice has been reduced. We believe that the new experimental design and updated manuscript thoroughly address Dr Sutter’s concerns.\n\nREVIEWER 1\n\n1. Hutchinson et al. write in their conclusions, that their’s is the first study “demonstrating … the contrasting climatic impacts of Antarctic ice removal and Southern Ocean meltwater input”. This sentence needs moderation as to my knowledge several studies (e.g. Holloway et al. 2016 in Nature Communications) follow similar experimental procedures albeit with different foci in their analysis. When reading the manuscript by Hutchinson et al., I was missing a discussion how their findings differ from previous studies and what novel methodologies they apply. This should be expanded on, certainly in the methods section, but also in the main manuscript.\n\nWe have now included at several points in the Introduction and Discussion a comparison with Holloway et al. (2016), and added further references covering LIG ice sheet modelling and observations. We acknowledge that there are some similarities to Holloway et al.’s work, but also several differences in our methods, and importantly, we address our best fit to the LIG proxies in East Antarctica for different reasons than their study (which argued sea ice reduction was the most important factor).\n\n2. The title claims that the climate response to changes of WAIS is investigated here. However, the authors write that they consider a scenario akin to 4.5 m AIS SLE volume change during the LIG which is inconsistent. Should the whole WAIS collapse, global sea levels would rise by “only” 3-3.5 m (see e.g. Bamber et al. 2009). In light of the “4.5 m scenario” this would imply at least some East Antarctic Ice Sheet (EAIS) contributions. Furthermore, the boundary conditions used here clearly show only partial WAIS collapse (e.g. Figure 1) thus the EAIS contribution must be considerable.\n\nFirst, we agree that our original description of the WAIS component of the perturbation was incorrect and we thank Dr. Sutter for catching this. There was indeed a considerable component of EAIS change in our experiments, and we have revised the manuscript to address this. Second, in the process of revising the manuscript, we realized that there were some unintended perturbations to East Antarctica that we had not scrutinized properly.\n\nOur control simulation is forced with an imposed ice sheet based on present day observations while the sensitivity experiments in our first submission were forced with ice elevations directly from the Golledge et al. (2015) LIG ice sheet model scenario. This was an inconsistent approach, because the Golledge et al. simulations started with a simulated, and therefore slightly different, control ice sheet. This resulted in differences between the ice sheets used for our sensitivity runs and the control run that were not just due to climate forcing, but also to the inherent difference between the two control ice sheets. As a result, some regions of the East Antarctic ice sheet were lower because of the mismatch of the control ice sheets. In the revised manuscript, we have corrected this inconsistency by calculating the difference between Golledge et al.’s LIG simulation and their year 5000 control run and then adding this “anomaly” in ice sheet elevation to our control run ice sheet. This enabled us to create a perturbation that was more in line with their simulated warming effects: such that the ice removal was more focused on marine ice sheets and glacial valleys where melting is likely to occur. This perturbation is labelled 4.5 m SLE, representing a 4.5 m SLE change in ice.\n\nIn addition, we decided to retain our original ice sheet perturbation (previously called 4.5 m SLE), since it produced interesting results. And, after properly accounting for the anomalies in ice elevation, we have now renamed this experiment 7.5 m SLE, since in our grid configuration, the total ice volume change is in fact 7.5 m SLE. The new perturbations are presented more clearly in our new Figure 1, with ice elevation anomalies shown as requested. The new 4.5 m and 7.5 m ice elevation anomalies are shown in Figure 1a,b, while the change in bathymetry is shown in Figure 1c,d. The absolute values of elevation and bathymetry are shown in Supplementary Figure 1. We now also quantify the sea level equivalent (SLE) ice sheet change for each 30 deg sector of Antarctica. Furthermore, we quantify the contributions of the WAIS, EAIS and Antarctic Peninsula sectors according to the drainage basin definition of each sector, presented in Supplementary Table 1.\n\n3. This brings me to the last main point. The experimental setup is designed to focus on freshwater hosing from the WAIS, specifically from the Amundsen/Bellingshausen Sea embayment as well as the Ross and Weddell Sea. This is inconsistent with the topographic boundary conditions applied: Figure 1 shows that e.g. the Recovery Basin glaciers are clearly retreated. The Recovery Basin is technically East Antarctica but drains but the freshwater from ice sheet retreat would end up in the Weddell Sea, so that’s consistent with the hosing approach. However, large swathes of the coastline of Dronning Maud Land are gone and Wilkes and Aurora Basin Glaciers have retreated. For the sake of consistency, freshwater should be channeled through these regions as well. Additional experiments including freshwater hosing from the EAIS, and analysis of the impact of regional hosing on the different ocean sectors would greatly strengthen this study (see below).\n\nWe agree that the runoff in the original experiments was not consistent with the distribution of ice sheet changes. We therefore redesigned the runoff to be more widely distributed around Antarctica. We used the 30 deg sector-by-sector values noted above (Figure 1a,b) to define the magnitude of freshwater forcing for each sector in each ice loss scenario, in order to achieve consistency between the ice sheet changes and the freshwater forcing.\n\nADDITIONAL COMMENTS\n\nI think there are several places in the manuscript which would benefit from references e.g. C-0, C.2, C/* here e.g. Sutter et al. 2016, Pollard et al. 2016. C/+ e.g. Golledge et al. 2019\n\nThese references have been added accordingly.\n\nC/34 Figure 1 shows the ice sheet representation on the GCM-grid resolution. You mention in the methods that you take the ice sheet model output from the RCP4.5 simulation at the year 5000 in Golledge et al. 2015 and then regrid it to the native GCM-resolution? This means (as one can see in Figure 1) that you not only remove parts of the WAIS but also the EAIS (e.g. in the Recovery Basin, Wilkes subglacial basin and Aurora basin, coastline of Dronning Maud Land exhibits drastic changes as well). This means you can't strictly assign the climatic changes to WAIS topographic LIG-responses exclusively. I suggest you discuss this in the manuscript and potentially adjust the framing accordingly. Furthermore, looking at Figure 1 in Golledge et al. 2015 from which you take the boundary conditions, it seems that the WAIS configuration in your GCM boundary conditions is the “low scenario” i.e. 2.77 m SLE, is that correct? In that case, most of WAIS seems to be still intact in your representation, which means that the majority of sea level equivalent ice volume change has to come from the East Antarctic Ice Sheet. A complete loss of WAIS amounts to ca. 3 m sea level rise (e.g. Bamber et al. 2009). Please elaborate and expand the discussion/caveats of your approach accordingly.\n\nFor clarification, we took our boundary conditions from the scenario shown in Golledge et al. (2015) Figure 1f, using the “high” scenario, which includes a sub-grid melting parameterization. This entails a SLE change of 4.5 m, rather than 2.77 m for the “low” scenario. In the issue of WAIS contribution, our revised perturbation is indeed less than the 3 m estimated by Bamber et al. (2009), and we have quantified this amount more clearly in the supplementary material. The WAIS component of ice loss is 1.5 m and 2.2 m SLE in 4.5 m and 7.5 m SLE respectively.\n\nFigure 2: I suggest you include place names in figure 1 as an orientation for the reader unfamiliar with Southern Ocean sector names. Also, consider including an elevation map from the original ice sheet model showing the ice sheet thk changes compared to the ctrl (e.g. with a diverging RdBu cmap).\n\nWe have included some place names in Figure 1 as suggested, and included a red-blue diverging color reflection of the ice sheet perturbation compared to the control. We have also noted on the difference plot the 30 deg sector-by-sector ice perturbations as discussed above.\n\nC++0 surface cooling alone\n\nThis has been added.\n\nC++2 I would argue that this is a very striking finding as subsurface ocean temperature ultimately drive ice sheet retreat/advance (the grounding line does not \"see\" SST). In general, the effect of AIS changes on the subsurface ocean conditions should be highlighted and discussed more in the manuscript as it reveals a positive feedback if including freshwater forcing (as e.g. shown and discussed in Golledge et al. 2019, Bronselaer et al. 2018) and I think goes beyond what e.g. Holloway et al. 2016 discussed. However, I think that additional experiments would be necessary as the boundary conditions you apply clearly exhibit substantial EAIS changes as well. How would these affect ocean circulation? Would your results change (e.g. the large Ross Sea ocean warming) if you would add a freshwater feedback in East Antarctica as well? What mechanisms would be in place then?\n\nWe have revised and re-run the perturbation experiments so that there is now a component of freshwater forcing along the coast of East Antarctica, consistent with the ice sheet changes in that region. The results are modified, but still show many of the same features as the original experiments: namely a significant surface warming in the Ross Sea, and deep sea changes that align with changes to the rate of AABW formation and flattening of isopycnal slopes.\n\nC+0+ OK, this statement is incomplete, as you also include EAIS topographic changes (but only WAIS freshwater forcing, which is inconsistent). Furthermore, the largest feedback of WAIS retreat seems to be a substantial subsurface warming of the Ross Sea.\n\nWe have revised the WAIS/EAIS discussion accordingly and highlight the surface warming of the Ross Sea.\n\nC+00 e.g. Golledge et al. 2019 conduct similar experiments also focused on the LIG. As discussed above, we now include comparisons to Golledge et al. (2019).\n\nC+3* Ice core constrained ice sheet modelling show's that this (Wilkes Subglacial Basin) sector very likely remained stable during the LIG (Sutter et al. 2020). Uranium isotopes in subglacial precipitates also hint at LIG ice sheet stability in the Wilkes Subglacial Basin (Blackburn et al. 2020). Furthermore, Sutter et al. 2016 show sea level rise of ca. 3-4 m (mid range) for the LIG without invoking MICI.\n\nWe now acknowledge constraints provided in Sutter et al. (2020) and Blackburn et al. (2020) which suggest ice sheet melting in the Wilkes Subglacial Basin was less than 0.5 m SLE between 100 and 200 ka, whereas in 7.5 m SLE experiment there is 1.5 m between 100 and 200 ka, i.e. covering a larger region than the Wilkes Subglacial Basin.\n\nC+31 this sentence appears to be incomplete?\n\nThe sentence has been revised.\n\nC+32 see e.g. Holloway et al. 2016 and other studies investigating the climate response to ice sheet topographic changes and freshwater hosing. In general, I suggest to significantly expand the method section by detailing how your experimental setup goes beyond/differs from previous studies.\n\nAgreed, we have expanded our discussion of previous work including Holloway et al. (2016).\n\nC,-/ there are at least two studies explicitly simulating the AIS during the LIG (Sutter et al. 2016, DeConto et al. 2016). Furthermore, Golledge et al. 2019 discusses the effect of changes in ocean stratification due to freshwater release from the AIS.\n\nWe have added further references to Sutter et al. (2016), DeConto and Pollard (2016) and Golledge et al. (2019) on this subject.\n\nC,-// E], see below.\n\nThis section has been revised.\n\nC,00' not consistent with the physical constraints. 1. WAIS only amounts to ca. 3 m SLE ice volume (e.g. Bamber et al. 2009), 2. only partial collapse of WAIS in boundary conditions, -> substantial contr. from EAIS.\n\nWe now clearly delineate contributions from the WAIS and EAIS, as discussed previously.\n\nREVIEWER 2\n\nThis paper presents modelling attempt to illustrate the impacts of Antarctic-ice patial removal and the associated Southern Ocean meltwater input on the Antarctic climate change in the LIG with respect to the PI. New to existing studies, their ESM modelling results have suggested the contrasting climatic impacts of Antarctic ice removal and Southern Ocean meltwater input on the spatial pattern of the LIG Antarctic surface air temperature during the LIG. This work is of significance to the field of paleoclimatology and has implications for present day global warming. However, in the manuscript, both the modelling set-ups and climatic mechanism remain to be further clarified. I would encourage a re-submission of a revised manuscript.\n\nWe thank the reviewer for their constructive comments, and we have revised the manuscript accordingly.\n\n1. In 6.3, the SL4.5 experiment set-up is discribed that: Removal of 4.3 m (here it should be 4.5?) sea level-equivalent ice sheet from Antarctic topography and adjustment of ocean bathymetry. So, I would expect that the entire topography of the Antarctic ice and the coatal line in the LIG are changed. However, in the manuscript, the mechanism is only attributed to the West Antarctic ice loss, e.g. as mentioned in the title. How to undersrtand this?\n\nAs mentioned in response to Reviewer 1, we have now redesigned the perturbation experiments and conducted new simulations to correct some inconsistencies and apply freshwater forcing around the East Antarctic coast in line with the ice sheet changes in that region. We now clearly quantify how much ice is removed in each sector of Antarctica (Figure 1a,b), and cite a breakdown of the WAIS, EAIS and Antarctic Peninsula components, as discussed previously.\n\n2. In Fig.1, a figure panel for LIG-to-PI topography anomolies will be useful to understand the ice change\n\nThese anomalies are now presented in Figure 1.\n\n3. l.234-235, ‘The surface temperature changes found in the LIG compared with the PI simulation are broadly in agreement with PMIP4 results5 (Figure S7).’. Acturally, this is not true. The Figure S7 is quite different from Fig. 5 in ref.5.\n\nWe have revised this statement to emphasize similarities in the seasonal patterns with as in PMIP4, while noting differences in the annual mean response and in particular regions such as the Arctic.\n\n4. l.227, How to understand ‘the runoff scheme are left unchanged’? E.g. river freshwater discharge amount, river mouth location.\n\nWe choose to leave the river runoff location the same in all experiments, since (i) the basin scale changes to river to runoff are relatively small and (ii) river runoff is a small signal compared to the freshwater perturbations, allowing us to attribute changes more directly to meltwater effects.\n\n5. l.44-45, ‘implies a larger sea level rise by 2100 than many process-based studies’, how to understand by 2100?\n\nThe sentence has been revised: we mainly highlight alternative approaches to ice sheet modelling still predict significant reductions in the WAIS by the 21st century without invoking MICI.\n\n6. l.54-55, ‘the stability’ is not discussed in this work, but a shift of mean climate state.\n\nHere we have clarified that our study investigates impacts on climate in regions that affect the stability of the remaining ice sheet, a shift of mean climate state.\n\n7. l.62-63, The Weddell-Sea sea ice edge shows W-S opposite pattern in sea ice edge change, rather than a common retreat.\n\nWe now acknowledge that an expansion of sea ice occurs in the Weddell Sea, and since the experiment design has changed, the results have been rewritten accordingly.\n\n8. l.64, sea ice edge around most of East Antarctica is acturally reduced, not steady, see Fig. 2a.\n\nWe have now rewritten this sentence to distinguish between regions with sea ice retreat (0-200 deg E) and regions with relatively unchanged (200-360 deg E).\n\n9. l.67-68, 100-500 m depth in the the Ross Sea becomes colder rather than warmer, see Fig. 2d.\n\nThe cooling identified by the reviewer was a neutral signal in the new perturbation experiment. We have revised our description of the new results. We have also re-organized our figures to focus more on the zonal mean ocean temperature sections, which better illustrate the flattening of isopycnal slopes that ultimately drive the surface warming (Figure 3, 4), while the 100-500 m surface temperature anomalies are now in the supplementary material (Figure J, 2).\n\n10. l.69-72, according to Fig. S2a, an anti-clockwise atmosphere circulation over the Amundsen, Bellingshausen and Ross Seas is shown. It seems as a key pattern to explain the regional climate. Is it related to the Land-sea-mask change, e.g. enlarged Ross-Sea bay due to coastal line retreat?\n\nSince we have updated the perturbation experiments, we have updated our characterization of this result. We now characterize this change as a weakening of the midlatitude westerlies than an anti-clockwise circulation, and we have added the control values of wind to clarify this point (Figure J, 0a).\n\n11. l.101-102, ‘First, the weakening of the equator-to-pole gradient (due to Antarctic surface warming) drives a weakening of the midlatitude westerlies’, by what mechanism?\n\nWe have added the following reasoning: “polar warming at the surface has been found to reduce baroclinic instability and therefore weaken the midlatitude westerlies (Sutter et al., 2020).”\n\n12. l.105-106, Fig. S5 is not able to present ‘The southward shift of the easterlies also strengthens the Antarctic Slope Current (ASC; Figure S5).’, please add a figure for anomalies.\n\nWe have added an anomaly plot for the surface ocean velocities.\n\n13. l.119-120, ‘and by 0.5-1 C along most of the West Antarctic coast (180E - 60W, Figure 2e).’, Fig.2e is not able to present this change.\n\nThis statement has been revised, and the new results are presented.\n\n14. l.129-131, ‘The strength of midlatitude westerly winds increases in the FW4.5 experiment, likely due to an enhanced equator-to-pole gradient, contrary to the circulation changes in the ice loss experiment.’ Similarly, by what mechanism?\n\nAs noted above, we have revised our discussion of westerly wind changes to include the effect of polar surface cooling causing an increase in baroclinic instability (Sutter et al., 2020).\n\n15. l.139-140, ‘since the ice elevation does not change at these proxy locations.’ Same as comment #2, a figure panel for LIG-to-PI topography anomolies is requested.\n\nThis statement has been revised, and the new results are presented.\n\n16. l.147-149, ‘On the other hand, a small SST increase is simulated along the East Antarctic coast, which may be due to the minor retreat in the land ice edge in this region (Figure 1).’ Does this mean land-ice grid becomes ocean grid cells in the model? Please clarify.\n\nYes, land-ice grid cells become ocean grid cells in the model. We have clarified this in the text.\n\n17. l.158-159, ‘is driven by a southward expansion of CDW below the freshwater layer’ Why to exclude the impact of reduced AABW formation. Any modelling signal to indicate this?\n\nThe AABW formation is indeed reduced, and we have added a sentence to clarify this.\n\n18. In general, how deep is the ocean channel between the WAIS Island and Antarctic main land, when the ice is reduced? Then, how is the water exchange between the Weddell Sea and Amundsen Sea? This is also key to undertand the full process.\n\nThis channel is relatively shallow, and the flow through the gateway is only a few hundred meters (see Figure 1). The total transport through the gateway is 0.1 Sv and 0.2 Sv in 4.5 m and 7.5 m SLE respectively (specific to Atlantic to Pacific sector), and 0.1 Sv and 0.2 Sv in 4.5 m and 7.5 m SLE respectively (Pacific to Atlantic sector) through the gateway at 65 deg S, 280 deg E. These numbers have now been added to the manuscript.\n\n19. Please add the information about the MOC anomalies between LIG and PI. Also important to understand the PI-to-LIG ocean change.\n\nWe have added an anomaly figure for the MOC showing the LIG minus PI change in the supplementary material (Figure J, 1), and briefly discuss the MOC changes in the manuscript.\n\nREVIEWER 3\n\nIn this paper the authors discuss the impact of both WAIS removal and freshwater input on the climate and ocean state in the southern Hemisphere during the LIG. They use a climate model with a prescribed ice sheet and freshwater forcing. One issue they investigate is if the mismatch between the LIG127ka PMIP4 simulations and southern hemisphere proxy records (SAT and SST) was due to the choice of an PI AIS in the model setup. The authors focus on indefinity changes in southern hemisphere climate state specially from ice-sheet driven feedbacks (changes in bathymetry/elevation) from those related to freshwater by running three climate simulations.\n\nThe paper is well written, and the results are clearly presented. However, I would the authors to expand on previous publications which have investigate the combination of WAIS and FWF for the LIG, although they may be from older generation of climate models. For example, Holloway et al., 2016 also performed climate model simulations with a reduced AIS and FWF.( I was not associated with this study). I would recommend for publication with the minor changes below.\n\nWe thank the reviewer for the positive recommendation and constructive comments, which we have addressed below.\n\nSection 2: Question about lapse rate correction: I would like more details on the reasoning of this correction for the final COMB4.5 results (shown in the paper). The results in Fig3 are the SAT with the lapse-rate correction, so removing any warming signal associated with a change in altitude due to the retreat of the WAIS. However, as the aim of the SL4.5 is to investigate the impact of a reduced WAIS, this correction removes/reduces this impact. Why are the corrected results not shown in the SOM? I am not sure of the rational? Is this to make the results more consistent with FW4.5?\n\nWe reconsidered our approach, and we agree with the reviewer that it is better to present the uncorrected result in the manuscript. The lapse-rate correction is potentially interesting for analyzing feedbacks, but in light of our new experimental design and revised conclusions, the lapse-rate correction is no longer of central interest to this study and we have removed it.\n\nMinor text corrections:\n\nLine 35: “Coupling a dynamic ice sheet with ocean-atmosphere-sea-ice-land ... remains a significant technical barrier”. There have been simulations with dynamic ice sheet (Sommers et al, 2021). To be clearer, I would state a dynamic Antarctic ice sheet. This is a challenge due to the ice-ocean coupling required for a large marine based ice sheet.\n\nWe have adjusted the text to “dynamic Antarctic ice sheet”, and agree that ice-ocean coupling is a major challenge, and we have referenced a recent modelling effort to achieve this (Bader et al., 2021), and progress towards fully coupled models (Bader et al., 2021).\n\nResults section2: SL4.5 experiment compared to COMB4.5\n\nFrom the SL4.5 (Fig2d) there is a pronounced warming signal ( > 2C) at 100-500m between 140W and 180W. What is the origin of this? It does not occur when running the COMB4.5 experiment, unlike most other signals (Wind speed, SAT)? Is there a 2nd feedback so the FWF dominates the response in the ocean?\n\nIn general, the freshwater perturbation response tends to dominate the ocean response in the combined case (both surface and subsurface), while the ice sheet loss perturbation tends to dominate the atmospheric response over Antarctica. However, there is also a non-linear effect whereby the deep ocean warms more strongly in the combined case than the freshwater-only case (despite the surface cooling), which we now discuss more clearly in the manuscript.\n\nDiscussion:\n\nLine 160 (related to FigS6)” Moreover, the East Antarctic subsurface anomaly is more positive than in FW4.5, which partly reflects the warming caused by the continental ice sheet reduction” It could be the colour scale but comparing Fig.2e and f most of EAIS appear to surrounding by ‘white’ region, so a very similar temperature as in FW4.5 (so colder), apart from close to the Wilkes Basin. Also, in FigS6, with deeper SST anomalies the ice sheet only simulation (FigS6d), there is a strong cooling along the coast.\n\nFirst, we accept that the differences were small in the original version. In the updated perturbation experiments, there is a discernible increase in temperature around East Antarctica when comparing the combined experiment to the freshwater-only experiment.\n\nLine 182:”there are also areas for East Antarctica’ where reverse-sloping bedrock...’ Can you mention which of these three regions coincide with simulated warming/cooling anomalies. It is interesting the large warming anomaly near the Wilkes SG Basin, but the Amery is associated with a cooling.\n\nThere is significant localized warming (up to 2 deg C) and associated ice sheet loss at the Wilkes Ice Sheet, and the Amery is associated with a cooling. We have now noted this in the text.\n\nFigures:\n\nFigure 2-3. Could you increase the latitude extent to be consistent with SOM plots. The lateral extent of the warmer anomalies (i.e Fig.2d) is chopped off.\n\nWe have adjusted the latitude extent so that all polar plots extend to a cutoff of 45 deg S.\n\nFigure 3: what are the circles? Are there the ice core sites? Are the anomalies larger than 5C? if so can you edit the scale bar to indicate this?\n\nYes, the circles are the ice core sites. None of the proxies exceed 5 deg C, so we have kept the scale bar as is.\n\nMethods:\n\nTable: Model setup: could the authors add a table in the SOM of model set up (forcings), names; ice sheets(small/large); size of FWF etc. Just as summary for the model setup and boundary conditions.\n\nWe have added a table summarizing the boundary conditions to the supplementary material.\n\nTable: Proxy data: Throughout the text the authors compare the SAT/SST from the simulations to proxy data and other values from PMIP4 simulations. Can these be summarised in a table. For example, the 4 proxy ice core records (line 80); SST temperature records (line 25-26). Can the authors compare to other proxy records, for example those shown in the Otto-Bliesner et al., 2021, summary PMIP4 LIG127ka paper.\n\nWe have added a table showing comparisons to ice core and SST proxy records, as in Otto-Bliesner et al. (2021).\n\nReferences:\n\nHolloway, M., Sime, L., Singarayer, J. et al. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse. Nat Commun 7, 12293 (2016). https://www.nature.com/articles/ncomms12293\n\nOtto-Bliesner et al., 2021 Large-scale features of Last Interglacial climate: results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate Modeling Intercomparison Project (PMIP4)\nhttps://cp.copernicus.org/articles/17/63/2021/\n\nWe have added these references.\n\nREVIEWER 1 (SECOND ROUND)\n\nThe authors have addressed all my previous major comments and I congratulate them on an interesting study providing new insights into the missing pieces in understanding LIG Antarctic climatic changes. I still have some remaining minor comments which should be addressed before publication (see below) but these should be resolved easily.\n\nWe thank Dr Sutter for his positive feedback.\n\nGeneral comment:\n\nThe fact that including a meltwater-feedback worsens the match with SST-proxies in the Southern Ocean is somewhat glossed over and merits a more extensive discussion (see specific comments below). As of now the abstract highlights the improved match with SAT records while ignoring the increased mismatch with SST-proxies. Both should be mentioned.\n\nDue to word limits, we were not able to add this statement to the abstract, but we instead removed the “improved match” with SAT proxies for balance. We now discuss the comparisons to both SAT and SST data in the Results and Discussion section.\n\nI missed this in the last round of reviews, but I think there are a couple of SST reconstructions for the LIG (e.g. in Figure 5 in Capron et al. 2014 I count at least 5 SST reconstructions which fall in your domain shown in fig 2 and 3, I’m sure there must be more recent studies as well). In Figure 3 you show ice core reconstruction but no SST proxies (neither in Fig. 2). If it would be possible to include the SST-comparison (at least in the figure) as well that would be fantastic. Having now read further I see that you provide a comparison in supp tables 1+2. I think adding the points in the figures as well would be nice if possible.\n\nWe have overlaid on Figure 2 the SST proxy anomalies from the compilations of Capron et al (2017) and Hoffman et al (2017), as used in the PMIP intercomparison of Otto-Bliesner et al (2021), and additional new proxy data from Chandler and Langebroek (2021). Since Figure 2 presents SST model data, we feel this is the appropriate place to show SST proxies. In Figure 3 (now re-ordered to Figure 5) we are only showing surface air temperature (SAT), therefore we only use the SAT proxies from Antarctica here.\n\nWe have also updated our Supplementary tables to include the new Chandler and Langebroek (2021) data, with model-based estimates provided for each location.\n\nBelow I list some minor edits and comments (please note that line numbering refers to the track-changes document)\n\nL1 : was ca. 1 to 9 …\n\nWe have inserted “approximately”.\n\nL5 : both from topographic changes and\n\nDone\n\nL6 : we find that changes in surface elevation and gradients …\n\nWe have changed this to “changes in surface elevation”\n\nL10: the combination of a diminished ice sheet and enhanced …\n\nDone\n\nL11 : and induces further warming … both of which could increase ice sheet retreat\n\nDone\n\nL25 : suggested by Southern Hemisphere …\n\nDone\n\nL59 : … climate model, assuming an idealised flattening or removal of WAIS. In this study the oceanic response was not assessed in detail.\n\nDone\n\nL63: Here, we assess …\n\nDone\n\nL75: … is found in the EAIS …\n\nDone\n\nL79 : both the Weddell and Ross Sea warming …\n\nDone\n\nL116/117: Here you could speculate as to how the katabatic winds would change in response to the topographic changes. Maybe there are studies which have looked at that?\n\nWe considered this suggestion, but since we don’t have a strong argument for katabatic wind changes we prefer not to speculate here.\n\nL189-194: I think this paragraph is a bit positivistic. From supp table 1 and 2 I gather that including the meltwater feedback actually leads to a negative SST anomaly while the proxies suggest SST-warming during the LIG. The elevation feedback alleviates this somewhat but the net effect is still a LIG cooling. So, while improving the match with ice core proxies the combined feedbacks actually worsens the match with respect to SST? This merits a little more discussion of the involved effects (i.e. what’s missing in the model) and caveats.\n\nWe have added the following discussion on this subject:\n\n“These comparisons indicate that the simulated SST are generally too low compared to the SST proxy data. Most of the Southern Ocean SST proxies indicate warming of at least several degrees, although a few sites also indicate strong cooling (Figure 2), with the proxy anomaly data ranging from -6.8 deg C to 11.5 deg C (Supplementary Table 1). In general, the best agreement is found in the ice loss only experiments (SL4.1 and SL7.1), while a poorer agreement is found when introducing meltwater both separately (FW4.1 and FW7.1) and combined with the ice loss (COMB4.1 and COMB7.1). These data suggest that, in the context of uncertain timing of Antarctic melting at the LIG [Rohling et al. 2019; Barnett et al. 2023], that the peak meltwater pulse in the Southern Ocean did not occur simultaneously with the warmest SST anomalies, since meltwater tends to strongly cool the surface of the Southern Ocean.”\n\nL156: 1-2 degree decrease in what?\n\nL156 did not include the words “1-2 degree decrease”, but L256-257 stated that “…anomalies are found … (1-2 deg C)” without explicitly saying of which field. Therefore, I updated that statement to “SAT anomalies are found… (1-2 deg C)”.\n\nL243 There should be one or two sentences pointing towards the fact that this increases the mismatch to SST-proxies in the region (see comment above) and suggestions as to why that may be and what could be done to remedy this (e.g. resolution, missing feedbacks, fully (incl. ice sheets) coupled models).\n\nWe have added the following:\n\n“This cooling generally does not agree with Southern Ocean SST proxies from the LIG, which may indicate that the peak in meltwater from Antarctic ice melt at the LIG possibly succeeded the peak positive SST anomalies in the Southern Ocean. We note, however, that Southern Ocean SST proxies are almost all located between 40 and 50 deg S, while most of the SST anomalies simulated in our experiments are located poleward of 50 deg S, where no proxy records are available.”\n\nFigure 1. colorbar label please indicate change of what?? I assume it is surface elevation from the figure caption? Also the colorbar is very large compared to the plots.\n\nWe have changed the colorbar titles in Figure 1 to “Change in elevation (m)” and “Change in depth (m)”. We have also slightly reduced the size of the colorbars.\n\nFigure 2 and 3: add a proper label to the colorbars (e.g. SST anomaly and SAT anomaly). I suggest to cut the polar plots a little more to the south as you don’t discuss temp changes in Patagonia. Cutting at 60 South would make enlarge the Antarctic portion of the model output considerably, makes it easier to read. If you decide to include the SST proxies as well ignore the comment with regard to cutting at 60 south.\n\nIn Figure 2 and 3 we updated the colorbar labels to “SST anomaly” and “SAT anomaly” respectively. In Figure 2, we decided to include the SST proxies as suggested by the reviewer, and therefore we retain the larger domain (up to 45 deg S), rather than cutting to 60 deg S.\n\nNOTE: Figure 3 (SAT anomaly) has now been re-ordered to Figure 5, to match the order of reference in the manuscript.\n\nFigures 4 & 5. Colorbars don’t have labels.\n\nWe have added colorbar titles to Figure 4 and 5.\n\nNOTE: Figure 4 and 5 (MOC and zonal mean temperatures) have been re-ordered to Figure 3 and 4, to match the order of reference in the manuscript.\n\nREVIEWER 3\n\nThank you for the extensive replies to my comments. I would recommend this paper for publication\n\nWe thank the reviewers for their positive response, and Reviewer 1 (Dr Sutter) for his additional comments which have helped to improve the manuscript. As before, we outline our response to the comments in blue.\n\nRegards,\nDavid Hutchinson"}]

Decision letter
We thank the reviewers for their positive response, and Reviewer 1 (Dr Sutter) for his additional comments which have helped to improve the manuscript. As before, we outline our response to the comments in blue.
Regards,
David Hutchinson