Increased Asian aerosols drive a slowdown of Atlantic Meridional Overturning Circulation

Domain: earth_science
Published: 2024-01-02
Editorial outcome: minor_revision

Abstract
Observationalevidenceandclimatemodelexperimentssuggestaslowdown oftheAtlanticMeridionalOverturningCirculation(AMOC)sincethemid-1990s.Increasedgreenhousegasesandthedeclinedanthropogenicaerosols (AAs)overNorthAmericaandEuropearebelievedtocontributetotheAMOC slowdown.AsianAAscontinuetoincreasebuttheassociatedimpacthasbeen unclear.Usingensemblesofclimatesimulations,hereweshowthatthe radiativecoolingresultingfromincreasedAsianAAsdrivesanAMOCreduc- tion.TheincreasedAAsoverAsiageneratecircumglobalstationaryRossby wavesinthenorthernmidlatitudes,whichshiftthewesterlyjetstream southwardandweakenthesubpolarNorthAtlanticwesterlies.Consequently, reducedtransportofcoldairfromNorthAmericahinderswatermasstrans- formationintheLabradorSeaandthuscontributestotheAMOCslowdown. ThelinkbetweenincreasedAsianAAsandanAMOCslowdownissupportedby differentmodelswithdifferentconfigurations.Thus,reducingemissionsof AsianAAswillnotonlylowerlocalairpollution,butalsohelpstabilize theAMOC.

Introduction
The Atlantic Meridional Overturning Circulation (AMOC) transports vast amountsofenergytothenorthernhighlatitudes,playingapivotal roleinmodulatingtheglobalclimate1–3.Increasinglinesofevidence indicatethattheAMOCisdecliningoverthemodernclimaterecord4–7, although debates persist8–10. Furthermore, numerical model simula- tionsconsistentlyprojectaslowdownorevencollapseoftheAMOCas aresponsetoincreasinggreenhousegases(GHGs)inthe21stcentury, assummarizedbytheSixthAssessmentReportoftheUnitedNations IntergovernmentalPanelonClimateChange(IPCCAR6)11.

However,GHGsarenottheonlyexternalfactorinfluencingAMOC strength,andanthropogenicaerosols(AAs)havealsobeenfoundto playanimportantrole(SupplementaryFig.1a)12–18.Comparedtothe globallywell-mixedGHGs,AAsarehighlyvariableinspaceandtime duetotheirshortresidencetimeintheatmosphere.Incontrasttothe monotonicallyincreasingGHGssincetheindustrialrevolution,thereis atwo-stagetemporalevolutionoftheglobalmeanAAs(Supplemen- tary Fig. 1b): prior to the late 1980s, the global mean AAs shows a monotonicincreaseduetorampingaerosolemissionsfromboththe eastern and western hemispheres; after the early 1990s it remains stable due to a reversal of the emission trend in Europe and North America(aneffectoftheCleanAirAct).Bycontrast,theAAsemission overSouthandEastAsiacontinuestoincrease.

A growing number of literature recognizes the role of AAs in AMOC variations over the past century12–18. In particular, a multi- model of single-forcing simulations, known as the Detection and AttributionModelIntercomparisonProject(DAMIP)19,revealsatwo- stage evolution of the AAs-induced AMOC change15: a monotonic steadystrengtheningbeforethemid-1990sandarapidweakening thereafter (Supplementary Fig. 1a). Since AAs are known to be an importantradiativeforcingthatcoolstheglobalclimateanddrives anincreaseinthesurfacewaterdensityovertheNorthAtlantic18,20,21, the AAs-induced AMOC decay in recent decades is presumably attributedtothedecreasedAAsemissionovertheadjacentNorth AmericaandEurope13,14,21,22.Incontrast,theincreasedAAsoverthe easternhemisphereareanticipatedtohaveacoolingeffect,hypo- thetically strengthening the AMOC17. However, owing to their inhomogeneous spatial distributions23–25, changes in Asian AAs result in dynamic impacts on large-scale atmospheric circulations and have far-reaching effects on remote regions. A recent study, in particular, has demonstrated that increased Asian AAs can shift the westerly jet equatorward in the northern hemipshere26. However, the influence of the increased Asian AAs on the AMOCandtheunderlyingmechanismsremainunknown.Here,we find that far-field AAs over Asia weaken the AMOC despite local radiativecooling.

Results
Modelset-upandexperimentaldesign
Asdiscussed above,aerosolopticaldepth evolutionismarked bya clearspatialcontrastwithapositivetrendovertheeasternhemisphere andanegativetrendoverthewesternhemisphere(Fig.1a),knownas thezonalshiftpatternofAAsemission17,27.Toinvestigatetheclimatic effectsofchangesineasternandwesternAAschanges,weperform idealized radiative perturbation experiments17,28,29 using a widely adoptedfully-coupled climate model—the CommunityEarth System Modelversion1(CESM1)(see“CESM1model”inMethods),knownfor itsabilitytorepresentthemainphysicalprocessesinvolvedinAMOC variability3,30–32.Priortotheperturbationruns,wefirstintegratea500- yr control (CTRL) by forcing the CESM1 with GHGs, AAs, and solar insolationfixedattheyear2000levels.Tomimictheradiativeeffectof theaerosolforcing,weseparatelyreducesolarinsolationby10%over EastAsiaandSouthAsia(redboxinFig.1a)andNorthAmericaand Europe(blueboxesinFig.1a),hereafterreferredtoasEASTandWEST simulations,respectively.Assuminglinearity,thesignoftheresponse inWESTisreversedtomimictheradiativeheatingeffectoftheaerosol decrease after the 1980s. The annual-mean net radiative forcing amountsto~30Wm(cid:1)2intheperturbationexperiments,whichisthree timeslargerinmagnitudethantheactualradiativeforcingoftheorder 10Wm(cid:1)217,forthepurposeofgeneratingarobustresponsesignal.

Accordingtopaleoclimateproxyrecordsandclimatemodels,the AMOCexhibitsdecadaltomulti-decadalvariability33–36.Toensurethat thesimulatedchangesarenottheresultofinternalvariability,atotalof ten 100-yr ensemble membersare generated for eachof the CTRL, EAST,andWESTexperimentsbranchingoutfromyear350ofCTRL, whereeachmemberisrealizedbyaddingasmallrandomperturbation totheinitialsurfacetemperatureineachrealization.Sincewefocuson howincreasedAsianAAsandtheassociatedradiativecoolingaffect theAMOCthroughexcitingatmosphericteleconnectionsduringthe transientresponse stage,the main analysesshown in thisstudyare based on the difference between years 1–40 of the ensemble per- turbedrunsandthecorresponding40yearsoftheensembleCTRL.In addition,weprovideanoverviewoftheensembleresponsesduring theslow responsephaseaveragedoveryears51–100inSupplementary Fig.2,andtheconsistencybetweentheresultsaveragedoverthefirst 40yearsandthelast50yearssuggestsashortresponsetimescaleof theAMOCtochangesinAAs.

IncreasedAsianaerosolsweakentheAMOC
Itissomewhatsurprisingtoseethattheaerosol-inducedcoolingover AsialeadstoasignificantslowdownoftheAMOC(Fig.1b).Thisslow- downincludesarapiddecreaseof~2.7Sv(1Sv=106m3s(cid:1)1)withinthe first40yearsacrossallensemblemembers(Fig.1b,thincoloredlines) andagradualdeclineby~0.6Svoverthesubsequent60years.This resultdemonstratesthatthesimulateddeclineoftheAMOCisarobust climate response and not related to internal climate variability.

RegardingthetemporalevolutionoftheAMOCresponseintheWEST (Fig.1d),asexpected,theAMOCisweakenedby~1.4Svinthefirstfour decadesduetothewarmingeffectofreducedAAs,andremainsrela- tivelystablearoundthisvaluethereafter.Notethatthemagnitudesof boththetransientandlong-termresponsesoftheAMOCinWESTare smallerthanthoseinEAST,suggestingthattheremoteAsianforcing exerts an even stronger influence on the AMOC than the adjacent forcingoverNorthAmericaandEurope.

ThetransientAMOCresponsesindensityspace(see“AMOCin densityspace”inMethods)inbothEASTandWESTexhibitstrikingly similarspatialpatterns(Fig.1c,e).Thereductionoccursinthelower limbofAMOC,between36.80and37.00kgm(cid:1)3.Thisreductionsig- nifiesadecreaseinthetransportofequatorward-flowingdeepwater, indicatingnotonlyanoverallweakeningbutalsoashallowerAMOC. The wind-driven subtropical cells in the light upper limb (σ <33.5 2 kgm(cid:1)3)appeartoberarelyaffectedbytheradiativeforcing,regardless ofwhetheritoriginatesfromtheeasternorwesternhemisphere.This demonstrates that the primary impact of aerosol changes is con- centratedonthelowerbranchoftheAMOC.

AsshownintheDAMIPsingleforcingexperiments(Supplemen- taryFig.1a),therateofAAs-inducedAMOCweakeningafterthe1990s ((cid:1)0.81Sv/decade during 1990–2014) is 1.4 times greater than the AMOC strengthening before the 1990s (0.56Sv/decade during 1920–1990), despite the limited changes in global mean AAs concentration changes since the 1980s. Our model experiments suggest that the simulated acceleration in AMOC weakening after the 1990s is likely a combined result of AAs changes in both the eastern and western hemispheres. While the radiative heating caused by aerosol reduction over North America and Europe weakening the AMOC is not surprising, the mechanism behind the AMOC slowdown induced by remote radiative cooling over Asia remains puzzling. Furthermore, the remote forcing appears to have an even stronger influence on the AMOC than the adjacent forcing surrounding the North Atlantic. Next, we will delve into the mechanisms behind the AMOC slowdown in EAST.

CircumglobalteleconnectionexcitedbyincreasedAsian Aerosols
The impactofAsian AAs-induced radiative cooling on the AMOC is conductedbyexcitingstationaryRossbywavepatterns.Initially,the cooling effect leads to the suppressionof local convectionand the developmentofupper-levelconvergenceoverbroadregionsinIndia andChina(Fig.2a),inducingquasi-stationaryatmosphericplanetary wavetrainspropagatingpolewardandeastward(Fig.2b),manifested as an equatorward shift of the westerly jet in the northern mid- latitudes (Fig. 2d). The associated streamfunction anomalies pre- dominantly exhibit zonal wavenumber 4-5, characterized by a few prominent centers over the Tibetan Plateau, North Pacific, North America,andwesternEurope,whicharealignedwithinthewaveguide of the westerly jet stream. This anomalous pattern of atmospheric circulationmainlyoccurswithinthejetstreamwaveguide,resembling thewell-knowncircumglobalteleconnection37,38.Inthiscase,however, thecentersforEastAsiaandtheIndianregionsaremergedduetothe idealized configuration of the radiative forcing, covering western, central,andeasternAsia.Thecircumglobalresponsetriggeredbythe reductioninAsianAAspersiststhroughouttheyear(Supplementary Fig.3a–d),withitspeakintensityoccurringinsummer,likelytheresult ofthestrongerconvectivefeedbacktoexternalradiativeforcingand henceastrongerdiabaticwavesourceduringtheactiveAsianMon- soonseason(i.e.,summer)thanwinter(SupplementaryFig.3e).

The temporalevolution of streamfunction anomalies along the mean path of the stationary wave shows that the teleconnection responseemergeswithinafewmonthsandstabilizesinthefirstfew years(SupplementaryFig.4).Thissuggestsarelativelyshorttimescale for the northernhemisphere atmospheric circulation to respond to changesinAsianAAs.Onthistimescale,theoceanicfeedbacktothe atmosphericcirculationisexpectedtohaveasecondaryinfluence.The responseoftheatmosphericcirculationislargelybarotropic.Overthe subpolar North Atlantic, the induced sea level pressure response is characterizedbyayear-roundmeridionaldipole(Fig.2c,colorsand Supplementary Fig. 5), resembling the negative phase of the North Atlantic Oscillation (NAO). Notably, there are positive pressure anomalies over Iceland and negative anomalies in lower latitudes. Thesesea level pressureanomalies correspond to a weakening and equatorwardshiftoftheprevailingwesterlieslocatedtothesouthof Greenland(Fig.2c,vectors).Theequatorwardshiftofthewesterlyjet over the North Atlantic since the 1980s has been observed and simulated23,39,andarecentstudyhaspinpointedtheprimaryroleof increased Asian AAs in driving this shift in purposefully designed experiments26, thus corroborating our findings. The next natural questionis:Howdothesechangesinatmosphericcirculationweaken theAMOC?

Suppressedwesterlywindshinderthedeepwaterformation
Atmospheric circulation changes over the North Atlantic (especially NorthAtlanticOscillation-relatedchanges)havebeenprovenimpor- tantintransformingupperoceanwatersintothedensestclassofthe North Atlantic Deep Water40–43. To investigate the regional surface ocean drivers responsible for the reduced transport of lower limb waters(Fig.1c),weconductawinter-timesurfacewatermasstrans- formation(WMT)analysis(see“Watermasstransformation”inMeth- ods).Climatologically,thepartitioningofthetotalWMTduringwinter into different regions varies significantly across different density classes (Fig. 3a). In particular, in the higher density range (>36.80 kgm(cid:1)3),theWMTprimarilyoccursintheLabradorSea,indicatingits importantroleinmaintainingthetransportofthelowerlimbwaters. The Greenland-Iceland-Norwegian Seas contribute to the WMT in a densityrangegreaterthan37.00kgm(cid:1)3,whichdeviatesalotfromthe maximumAMOCdensities(σ =36.73kgm(cid:1)3;Fig.1c,contours)andis 2 thusunlikelytocontributetotheAMOC.Theseclimatologicalresults agree well with previous modeling studies44,45. Although the con- tributionoftheLabradorSeatothemeandeepconvectionmaybe overestimatedinclimatemodels46,recentstudiessuggestedthatlong- term changes in the AMOC are still dominated by changes in deep convectionintheLabradorSea45,47.

In response to the radiative cooling over Asia, a pronounced reduction ofthewinter WMToccursinthe subpolarNorthAtlantic (Fig.3b).Thisreductioniscenteredatσ =36.88kgm(cid:1)3andreachesa 2 maximum value of 15.1Sv, and the annual-mean reduction (5.6Sv) matchestheweakenedAMOCinasimilardensityrange.Amongthe fourmajordeepconvectionsites,theLabradorSeastandsoutasthe dominantcontributortotheWMTreduction,whiletheotherbasins havelittlecontribution.Inaddition,theLabradorSeaexperiencesan enhancementoftheWMTatlowerdensity,withapeakvalueof6.1Sv atσ ~36.78kgm(cid:1)3.Therefore,theWMTanomalyintheLabradorSea 2 manifestsasanoverallshifttowardsalowerdensityrange,andthe simulatedAMOCslowdowncanbeentirelyattributedtothereduction oftheWMTintheLabradorSea.

ToinvestigatethedrivingmechanismbehindtheWMTreduction, we analyze the response of surface density flux in different basins, which measures the changes in surface buoyancy due to heat and freshwater exchanges between the atmosphere and the ocean (see “Watermasstransformation”inMethods)48.ThereducedWMTalmost entirelyresultedfromthesuppressedheatlossinthesurfaceLabrador Sea (Fig. 3c, also see Supplementary Fig. 6 for the negligiblehaline contribution),resultinginamorebuoyantsurfaceoceanthathinders the transformation of lighter upper waters into denser waters. By quantifying the thermally-driven WMT anomaly associated with dif- ferentsurfaceheatfluxcomponents,wefindthattheWMTreduction ismainlydrivenbyreducedturbulentheatloss(alsoseeSupplemen- taryFig.7a-bfortheirspatialpatterns).Bothlatentandsensibleheat fluxescontributetothereductionintheLabradorSea,withtheformer contributing(cid:1)3.5Svandthelatter(cid:1)4.1Sv.

ThereducedturbulentheatfluxovertheLabradorSeaisprimarily drivenbythesuppressedwesterliesoverthesubpolarNorthAtlantic. The totalturbulent heatflux changes (11.5Wm(cid:1)2) can be linked to changesinSST,surfacewindspeed,surfacerelativehumidity,andair- seatemperaturedifference(see“Turbulentfluxdecomposition”in Methods).Thelargestcontributionarisesfromthereductionintheair- seatemperaturedifference(7.8Wm(cid:1)2;Fig.3d,secondblueandyellow bars),whichinturncanbeattributedtotheweakenedsurfacewes- terliessouthofGreenland(Fig.2c).Specifically,thesuppressedsurface wind reduces the transport of cold air from the upstream North American continent to the deep convection site, generating an anomalouslywarmerboundaryairthanthesurfaceocean,andthus inducingananomalouslyzonalwarmtemperature advectionovera substantial region of the Labrador Sea (Supplementary Fig. 7c–f). Moreover,thereductioninwindspeedalsoactstowarmthesurface oceanbysuppressingheatlossfromlatentandsensibleheatfluxes (2.9Wm(cid:1)2;Fig.3d,thirdblueandyellowbars).Therefore,thesup- pressedwesterliesleadtoadecreaseinbothair-seatemperaturedif- ferenceandsurfacewindspeed,collectivelyhinderingturbulentheat loss and impeding the deep water formation in the Labrador Sea(Fig.4).

Therate ofAMOCweakeningslowsdown substantiallyastime progressesintotheslowresponsestage(Fig.1b).GiventhattheAMOC isresponsibleformostoftheheattransportintheAtlanticbasin,its weakeningresultsinreducedpolewardoceanicheattransport(Sup- plementaryFig.8a).Asaresult,anomalouslycoldwater(Supplemen- taryFig.8b)propagatesdownstreamalongthecyclonicsubpolargyre, eventuallyreachingtheLabradorSea.Thisprocessactsasanegative feedback, balancing the surface heat gain in the Labrador Sea and preventingfurtherreductionofdeepwaterformation.Consequently, thisself-limitedprocesshelpsslowdowntheweakeningoftheAMOC.

Additionalmulti-modelevidenceconfirmingtheAsianAAs- AMOClink
The above responses in atmospheric teleconnection and surface- forced WMT identified for weakening the AMOC are behaviors expected from fundamental physical principles, but the results are basedsolelyontheCESM1model.Itremainstobeseenwhetherthe main conclusion holds in different model settings. Therefore, we performacomprehensiveanalysisincludingmulti-modelresultsand large-ensemblesimulationstotesttherobustnessofourfindings.

Weusesimulationsfrom10modelsparticipatingintheAerosol ChemistryModelIntercomparisonProject(AerChemMIP)49,whichisa subset of Coupled Model Intercomparison Project Phase 6 (CMIP6) dedicatedtounderstandingthefastresponsesoftheatmosphereto aerosolchanges(see“CMIP6andCESM2-SF-LEsimulations”inMeth- ods).WemakeuseoftwosetsofAerChemMIPsimulations,piClim- controlandpiClim-SO2,eachspanning30yearswithfixedclimatolo- gical mean SST and sea ice conditions corresponding to the pre- industriallevels.Theonlydifferencebetweenthesetwosetsofsimu- lationsistheemissionsofanthropogenicaerosolprecursorsofSO : 2 while piClim-control employs preindustrial emissions fixed at 1850 levels,piClim-SO2adoptsemissionsfixedat2014levels.Notably,due to the implementation of the Clean Air Act in Europe and North America, SO emissions in the western hemisphere have already 2 decreased significantly by 2014, making Asian emissions the pre- dominant forcing in the piClim-SO2 simulations (Supplementary Fig.9a,contours).Therefore,this2014sulfateforcingoperatesasa comparableagenttothatusedinourEASTsimulations(Fig.1a),albeit beingmoreconcentratedinEastAsia.

Toassessthetransientresponsesofatmosphericcirculationto Asian AAs, we calculate the difference between the piClim-SO2 ensemble and the piClim-control ensemble for years 1–30. Con- sistently, the AerChemMIP simulations show a similar atmospheric responsepatterncomparedtoourEASTsimulations(Supplementary Fig.9a,colors),especiallythecircumglobalteleconnectionstructurein thenorthernmidlatitudes.Theanomalouslow-pressurecentersofthe teleconnection patterns inthe AerChemMIP simulations are slightly shifted compared to those in our EAST simulations (Fig. 2b), due possibly to the differences in the distribution of aerosol forcing. Nevertheless,theequatorwardshiftofthewesterlyjetandthenega- tiveNAO-likeresponseareevident(SupplementaryFig.9b),accom- panied by suppressed westerlies over the Labrador Sea. These agreements between the results of AerChemMIP and our CESM1 simulations provide support for the findings from our CESM1 simulations, adding confidence in the robustness of the atmospheric teleconnections excited by increased Asian AAs. How- ever,theresponseoftheAMOCiselusiveintheAerChemMIPsetting duetotheabsenceofanactiveoceanmodel.

RecognizingthelimitationsofAerChemMIPandtheincapability ofdistinguishingtheeffectsofaerosolchangesbetweentheeastern and western hemispheres in DAMIP experiments, we turn to the CESM2SingleForcingLargeEnsemble(CESM2-SF-LE).Thisensemble extendsits15-memberAAs-onlysimulationstotheyear2050under the shared socioeconomic pathways 3-7.0 scenario, referred to as SSP370-AAs simulations. In this scenario, Asian AAs emissions are projectedtocontinuetoincreaseinthefuture,whileAAsinthewes- ternhemisphereshowlittlechangeastheyhavealreadydecreasedto low levels (Supplementary Fig. 10b, contours). Consequently, the increasedAsianAAsandtheassociatedradiativecoolingemergeasa dominantforcingduringtheperiod2015to2050.Thisprovidesan analogtoourEASTcase(Fig.1a)andaddssomemodeldiversityto ouranalysis.

ConsistentwithourEASTcase,theSSP370-AAssimulationsalso showacircumglobalteleconnectionstructureandanegativeNAO-like trendintheatmosphericcirculation(SupplementaryFig.10b–d).In addition, the weakened westerlies over the Labrador Sea lead to a reductioninheatlossthroughchangesinturbulentfluxes,ultimately resultinginaweakeningoftheAMOC.Inparticular,theAMOCweak- eningalsoexhibitstwodistincttimescales(SupplementaryFig.10a):an initialrapiddeclinefollowedbyarelativelyslowerrateofchange.Such analignmentoftheseresultssuggeststhattherelationshipbetween AsianAAsandtheAMOCisnotuniquetoCESM1,lendingmoresup- port to our conclusion that the increased Asian AAs can weaken theAMOC.

Methods
CMIP6andCESM2-SF-LEsimulations
To estimate the impact of different anthropogenic forcing on the AMOC during the historical period, we use outputs of AMOC and aerosolopticaldepthat550nmfromtheDetectionandAttribution ModelIntercomparisonProject(DAMIP)19basedondataavailability. TheDAMIPsimulationsincludescenarioswithGHGsonly(Hist-GHGs), AAsonly(Hist-AAs)simulationsandcovertheperiodof1850-2014.

To validate the contributions of increased Asian AAs on atmo- spheric circulation changes,we use Aerosol Chemistry Model Inter- comparisonProject(AerChemMIP)49outputsofaerosolopticaldepth at550nm,zonalandmeridionalwindvelocity,sealevelpressuredata from the piClim-control and piClim-SO2 simulations. These simula- tionsareintegratedfor30yearsandareidenticalinallways,except that piClim-control employs preindustrial SO emissions, whereas 2 piClim-SO2adoptspresent-daySO emissions.Onlythefirstmember 2 of each model is used to ensure equal weight in the multi-model ensemble mean analysis. A detailed summary of these CMIP6simulationscanbefoundinSupplementaryTable1.

In addition, to further validate the contributions of increased AsianAAsonAMOCweakening,weuseCESM2SingleForcingLarge Ensemble(CESM2-SF-LE)50outputsfrom15ensemblemembersofthe AAs-onlysimulations.AllsimulationsusethesameAAsforcingbased ontheSSP370scenariofrom2015to2050.Ensemblemembersare generated by round-off level differences in theirinitialair tempera- turefields.

CESM1model
The modelweuseis the CommunityEarth System Modelversion 1 (CESM1)51,withagx1v6globalconfigurationthatcomprisesanatmo- sphere(CommunityAtmosphereModelversion5),aland(Community LandModelversion4),anocean(ParallelOceanProgramversion2), and a sea ice (Community Ice CodE) component model. The atmo- sphereandlandmodelsrunonanominal2°horizontalgrid,andthe seaiceandoceanmodelsuseanirregularhorizontalgridofanominal 1° resolution, but telescoped meridionally to ~35–50km around Greenlandand theLabradorSea(“f19_gx1v6”horizontalresolution). Vertically, the atmosphere model discretized on 30 uneven vertical levels,andtheoceanmodelhas60unevenlevelswiththethickness varyingfrom10mnearthesurfaceto250mnearthebottom.

AMOCindensityspace
WeanalyzetheAMOCindensityspace,asitfacilitatesrelatingAMOC changestovariationsinwatermasstransformationinthesurfaceocean. TheAMOCindensityspaceisdefinedas45:

AMOC(y,σ,t)=max(∫xW xE v(x,y,σ,t)dx)

whereσ isthepotentialdensityreferencedto2000m,yisthelati- tude, t is time, x and x are the westernand eastern longitudinal W E limitsoftheAtlanticbasin,visthemeridionalvelocity,andhisthe bottomdepth.

Watermasstransformation
Becauseofthedominanceofsurfacebuoyancyexchangeindrivingthe density-space overturning in the deep convection sites47, surface- forcedWMTanalysisservesasapowerfultoolforstudyingdeepwater formation and AMOC variation in both observations and models. Following the methodology in previous studies47,48,52, the surface densityflux(SDF)canbecalculatedas

SDF= (cid:1)α Q (cid:1)β F S, 
Cp

where the first term represents the thermal contribution related to surfaceheatfluxes(Q)andthesecondtermisthehalinecontribution associatedwithfreshwaterfluxes(F).Thethermalcomponentincludes contributions from latent and sensible heat fluxes, shortwave, and longwave radiations. The haline component mainly comprises pre- cipitation,evaporation,runoff,meltingflux,andsaltfluxduetoseaice formation/melt. α and β are the thermal expansion and haline con- tractioncoefficientsasfunctionsoftimeandlocation,C isthespecific p heatcapacityofseawater,andSistheseasurfacesalinity.Tocompute thesurface-forcedWMTasafunctionofdensity,theSDFisintegrated overallsurfacedensityoutcropregionsineachdensitybin:

WMT(σ)=1/Δσ ∫σ σ+Δσ SDFdA

Where σ is the potentialdensity referenced to 2000 m. The above analysisisbasedonmonthlymeanmodeloutputs.

Turbulentfluxdecomposition
The latent heat flux response can be decomposed to contributions from the air-sea surface temperature difference (ΔT), surface wind speed(W),relativehumidity(RH),andSST(T )53:

QLHΔT0 =QLH(1-βRH)ΔT0/(1-βRH-βΔT)

QLHW0 =QLH(W0/W)

QRH0 = -QLH(eβΔT/(1-βRH-βΔT))RH0

Qo0 =βQLH T0s

whereβ=Lv/(RvTs2)withLv thelatentheatofcondensationandRv the idealgasconstantforwatervapor.Theoverbarrepresentstheclima- tologicalstateinCTRL,andtheprimedenotesthedeviationfromthe climatologicalstate.Similarly,thesensibleheatfluxresponsecanbe decomposed to contributions from the air-sea surface temperature differenceandsurfacewindspeed:

QSHΔT0 =QSH(ΔT0/ΔT)

QWSH0 =QSH(W0/W)

Dataavailability
TheCMIP6dataispubliclyavailableat:https://esgf-node.llnl.gov/.The CESM2-SF-LEdataispubliclyavailableat:https://www.cesm.ucar.edu/ community-projects/lens. The RAPID data is publicly available at: https://rapid.ac.uk/rapidmoc/.TheCESM1simulationdatatosupport theanalysisisavailableathttps://doi.org/10.5281/zenodo.10374334.

Codeavailability
The NCAR Command Language (NCL v6.5.0) is used for plotting. Codestoreproducethefiguresareavailableathttps://doi.org/10.5281/ zenodo.10374334.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of “Increased Asian aerosols drive an unexpected slowdown of Atlanfic Meridional Overturning\nCirculafion” by Liu et.al.\nThis study invesfigates the role of the increasing AA emissions over Asia on the AMOC changes,\nespecially for a decline in AMOC through designed experiments using the CESM climate model. The main\nargument of the study is that the increasing AA over Asia contributes to a decline in AMOC after 1990s.\nThe results from the experiments are clear and supporfive of the argument drawn from the analysis.\nHowever, in my view, the authors need to show more rigor in their analysis with some further\nclarificafions to establish the novelty of their results according to the standard of this journal.\nMain Comments:\n1. I am not sure why the authors claim this response “unexpected”. I find this phrasing unnecessary.\nBecause I don’t find any clear evidence of expecfing anything else to be happening from AA\naerosol emissions from Asia. There could be other hypothesis which their results do not support. But a\nhypothesis described in one study is not enough to set an expectafion. This is the first study looking at\nthis aspect systemafically and finding a clear connecfion from the AA emissions in Asia to AMOC\nchanges. Moreover, I can’t also see any basis to consider the response from Asian AA emissions to be in\nsimilar nature to the European or North American AA emissions. Because the AA emission over Asia is\nremote in nature and hence its connecfion to AMOC can’t be considered in similar lines. I would\nrecommend to omit this “unexpected” term or else to provide clear evidence of previous study looking\ninto the very same response and finding a totally opposite outcome.\n2. The other glaring major issue is that the study is based on sensifivity experiments conducted on a\nsingle model CESM1. There is no analysis conducted on finding the relafion of the AA changes in the Asia\nand AMOC strength change in observafions. Because the observed behaviors of AMOC most probably be\nvery different from the models such as CESM1 (see Lozier et.al. 2019). The main influence region for\nAMOC from Asia AA emissions in the model is over Labrador Sea. Recent observafional studies are\nshowing that the observed AMOC sensifivity to Labrador Sea deep convecfion changes maybe minor\ncompared to what some standard coupled climate models depicts. This means that though the analysis\nshows a valid way of Asian AA emission influence to AMOC through Labrador Sea water mass\ntransformafion changes, such change may not be happening in reality due to differences in sensifivifies\nin the observed world from a model world. A more rigorous analysis on observed changes and its\nconnecfion to observed AMOC changes is necessary to give this study a stronger foundafion. The authors\ncan look at observed year to year AA changes over Asia and invesfigate if there is any connecfion to the\nobserved AMOC changes. Further, the same could be invesfigated for the teleconnecfion pathways. Do\nwe see a similar teleconnecfion pathway from the Asia in the observed upper-level winds? There must\nbe some signal present in the observafional geopotenfial height, winds and stream funcfion field in\nassociafion with observed AA changes over Asia, if a similar mechanism is at play. Do we also find a\nnegafive NAO like response connecfion to Asian AA changed over the North Atlanfic region. Previous\nstudies could have provided some hint on this, though I think it is essenfial to provide a clear picture of\nsuch observafional associafion in this manuscript.\n3. It would be also useful and highly confirmatory to show if CMIP6 models altogether show a same\nconnecfion of AA emissions changes over Asia and AMOC. Among the CMIP6 models, when there are\ndifferent future AA emission levels or aerosol radiafive effect (under the same scenario) over East Asia,\ndo they show any correlafion with their respecfive AMOC changes? The goal is if the authors could show\nsomehow the model independency of the main findings and a similar connecfion from AA emissions\nover Asia to AMOC across mulfiple models.\nSpecific comments:\nLine 76: what do you mean by “present day level”?\nLine 77: why 10% ?\nLine 80: here the response is reversed, which means the authors assume a linear nature of the response.\nThis has to be stated and if the authors could provide some evidence behind such an assumpfion that\nwould be even befter.\nLine 85: why only 10 years of simulafions are performed? This is quite small and unusual. I understand\nthat the response fime for aerosol would be small but a fime of emergence analysis on the AMOC\nstrength should be done considering its variability in the model to determine the minimum durafion of\nthe run to find a discernible forced signal from internal variability. I am okay with the claims made\nqualitafively from this limited length data, though the quanfificafion could be different and could show a\nfurther robust esfimate if the runs are performed for longer fimes. I would urge the authors to do this if\nthere is no major computafional constraint or put some strong reasoning to perform such short\nsimulafions. It is not ideal to set such experimental precedence without any strong reasoning.\nLine 98: Why is there a rapid decrease in the first two decades? Could the authors give a\nreason/hypothesis for this?\nLine 100: Similarly, why is there a gradual decline afterwards?\nLine 108-111: Are these separate experiments performed with only South Asian AA and East Asian AA?\nThe descripfion of such experiments should be clearly stated then and the supplementary figure 3 also\ndo not provide the images of two separate experiments. Hence it remains unclear what the authors have\ndone.\nLine 145: please remove “greatly”. And there are key differences in the CGT paftern and the CGT-like\npaftern that the authors find in this experiment. That has been pointed out too. I am curious if the\nauthors could show such resembling paftern in observafions with respect to observed Asian AA\nvariafions.\nLine 150: is there an associated seasonal dependency in the AMOC response to Asian AA?\nFigure 2c : in this figure the locafion of the mean westerlies in contours must be shown.\nLine 165-167: here the authors claim that previous studies from CMIP5 and CMIP6 models already\ncorroborate the link of the equatorward shift of the jet and AA emission over Asia. In a similar line, could\nthe authors also link the changes in AA over Asia and AMOC changes in those CMIP5 and CMIP6 models?\nLine 180: these are model specific behaviour of the AMOC and authors must be careful to generalise\nsuch things. Could the author confirm the same in observafions? Lozier et.al. suggest that observed\nAMOC behaves in a different fashion than what the models show and AMOC is more sensifive to the\nchanges in the eastern side of subpolar North Atlanfic than the western side over Labrador Sea. Hence\nthis potenfial difference in observed and model behaviour and its effect on the results needs to be\nclarified.\nLine 214: could the authors show the changes in horizontal (zonal and meridional) temperature\nadvecfion to confirm that the less cold air is gefting advected over the Labrador Sea from North\nAmerica? This analysis would complete the picture by clearly showing anomalous horizontal warm air\nadvecfion.\nLine 470: cures --> curves\nM. S. Lozier et al. A sea change in our view of overturning in the subpolar North Atlanfic. Science\n363,516-521(2019). DOI:10.1126/science.aau6592", "reviewer_2": "Key Results\nRecent work has highlighted how aerosols over the North Atlanfic region can strengthen AMOC by\ncooling the deep water formafion region, which strengthens meridional density gradients driving AMOC.\nThis study finds a new and potenfially important contrary effect: aerosols over South Asia and East Asia\ncan weaken AMOC by shifting the Atlanfic Westerly winds southward, decreasing heat loss and warming\nthe deep water formafion regions. Heat budget analysis makes it clear that the main mechanism is the\nreducfion in transport of cold confinental air over the Labrador Sea, which reduces ocean heat loss,\ncooling, and density increase there.\nValidity, Data and Methodology\nThe authors themselves point out what I see as the main quesfion mark about the validity of the result,\nwhich is that climate models such as theirs do not do a good job capturing the overflow component\n(from the Nordic Seas) of deep water formafion and hence may over-emphasize the role of the Labrador\nSea. Since this is a very challenging feature to model correctly, it is a shortcoming of virtually all studies\nof AMOC, and should not hinder publicafion.\nSignificance\nIn recent decades, the largest aerosol increases have been in Asia, and this study is the first to show a\nclear link between these increases and possible weakening of AMOC. Therefore it is a highly important\nstudy. The fact that there has been increasing discussion of weakening of AMOC, presumably due to the\neffects of anthropogenic global heafing, will heighten interest in this paper because it suggests an\nalternafive explanafion for dropping AMOC strength.\nClarity & Context/ Suggested Improvements\nThe paper is very well wriften. I have some comments on what is wriften about the context which also\nhas some bearing on how the paper presents the significance of these findings.\n1) Previous work on Asian Aerosols and teleconnecfions to N Atl\nThe last paragraph before the Discussion secfion discusses prior work, and it (or at least the first part of\nit, before the discussion of discrepancy with Kang et al and new results) should be placed at the end of\nthe Introducfion secfion. I assume the mofivafion for pufting it later was that the paragraph is mofivated\nby this paper’s results that the main influence on AMOC of the Asian AA is through a shift of the\nwesterlies. However, the paragraph (if placed in Intro) would make sense to a reader since it talks about\neffects of climate perturbafions in Asia on the North Atlanfic region, which is plausibly relevant to the\ntopic at hand.\nOne sentence in that paragraph makes me think that this paper should summarize its results a liftle\ndifferently. The sentence says “experiments forced with … aerosol emissions over the eastern\nhemisphere… has shown a weakening of the AMOC… providing direct support for our finding.” If that\nwere true, doesn’t the existence an earlier paper with similar results call into quesfion the novelty of the\nfinding? I have not thoroughly read the paper in quesfion (Diao et al, 2021), but as far as I can tell, Diao\net al find the Asian aerosol effect on moving the Westerlies southward over the Atlanfic, but not the\nfurther effect on the AMOC.\nTherefore, the abstract and Intro should be rephrased a bit to make it clear that this study finds that a\npreviously-discovered shift in Westerlies makes the Asian AA have the effect of decreasing AMOC.\n2) Are the new results surprising?\nThis is more subjecfive, but I don’t think the result is quite as surprising as they portray. The strength of\nthe AMOC depends less on density than on density differences between the northern North Atlanfic and\nother parts of the Ocean (see Klinger and Haine, Ocean Circulafion in Three Dimensions, Chapter 9).\nThus, the fact that cooling the North Atlanfic with aerosols strengthens AMOC does not imply that\ncooling a different lafitude and longitude with aerosols should have the same affect. The Kang et al\n(2021) study cited here does suggest opposing AMOC increase rather than decrease, so maybe the real\nquesfion this study is raising is not “what does Asian AA forcing do to AMOC?” but “what does Asian AA\nforcing at lower lafitudes do to AMOC?”\n3) Some minor suggesfions\nI recommend changing name of last secfion to \"Discussion and Conclusions\" and moving the\ncomparison to earlier studies (end of last paragraph before discussion) to this secfion."}, "author_response": "We thank the reviewers for their insightful comments and detailed suggestions, which are helpful\nin improving the manuscript. We have conducted further analysis with observations, CMIP6, and\nCESM2 large ensemble results to confirm that the link between increased Asian Anthropogenic\nAerosols (AAs) and the AMOC is not unique to CESM1, the model used in this study. In addition,\nwe have extended all 30 simulations (EAST, WEST, and CTRL) for an additional 80 years to\nvalidate the results of our study. We also thank both reviewers for their constructive comments that\nmotivated us to write more meticulously. In the following, we provide a point-by-point response\nto both reviewers.\nNote that major changes are highlighted in red in the revised text.\nReviewer #1 (Remarks to the Author):\nReview of “Increased Asian aerosols drive an unexpected slowdown of Atlantic Meridional\nOverturning Circulation” by Liu et.al.\nThis study investigates the role of the increasing AA emissions over Asia on the AMOC changes,\nespecially for a decline in AMOC through designed experiments using the CESM climate model.\nThe main argument of the study is that the increasing AA over Asia contributes to a decline in\nAMOC after 1990s. The results from the experiments are clear and supportive of the argument\ndrawn from the analysis. However, in my view, the authors need to show more rigor in their\nanalysis with some further clarifications to establish the novelty of their results according to the\nstandard of this journal.\nResponse: We appreciate your perceptive comments, which have helped us improve the\nmanuscript substantially. As you can see from the revised version of our manuscript, considerable\nefforts have been made to address your comments, particularly in the following areas:\n(1) Extension of Simulations: We have extended all 30 simulations by an additional 80 years to\nfurther validate our results.\n(2) Multi-Model Validation: To demonstrate that our results are not model-dependent, we have\nanalyzed a large number of simulations from two sets of AerChemMIP and one set of CESM2-\nLE, providing a multi-model validation of the findings from our CESM1 results.\n(3) Regression Analyses: We have conducted regression analyses using atmospheric reanalysis\nto investigate the relationship between changes in Asian AAs and atmospheric circulations.\n(4) References: We have incorporated references that examine long-term AMOC changes in\nhigh-resolution models, particularly emphasizing the driving role of the Labrador Sea. This\ncontextualizes our work within the existing literature.\n(5) Language Refinement: We have refined the language, especially in the Introduction section,\nto highlight the novelty of this study without implying prior expectations.\nBelow are our point-by-point responses to your comments.\n1. I am not sure why the authors claim this response “unexpected”. I find this phrasing unnecessary.\nBecause I don’t find any clear evidence of expecting anything else to be happening from AA\naerosol emissions from Asia. There could be other hypothesis which their results do not support.\nBut a hypothesis described in one study is not enough to set an expectation. This is the first study\nlooking at this aspect systematically and finding a clear connection from the AA emissions in Asia\nto AMOC changes. Moreover, I can’t also see any basis to consider the response from Asian AA\nemissions to be in similar nature to the European or North American AA emissions. Because the\nAA emission over Asia is remote in nature and hence its connection to AMOC can’t be considered\nin similar lines. I would recommend to omit this “unexpected” term or else to provide clear\nevidence of previous study looking into the very same response and finding a totally opposite\noutcome.\nResponse: Thank you for your comment. Here we systematically investigate the connection\nbetween Asian AA emissions and AMOC changes. We called our result \"unexpected\" based on a\nsingle previous study (Kang et al. 2021) that seems to imply a strengthened AMOC in response to\nincreased Asian AAs.\nFollowing your suggestion, we have removed the word \"unexpected\". We have also rephrased the\nrelevant sentences to better emphasize the novelty and uniqueness of our findings, without\nimplying prior expectations.\nComment 2. The other glaring major issue is that the study is based on sensitivity experiments\nconducted on a single model CESM1. There is no analysis conducted on finding the relation of the\nAA changes in the Asia and AMOC strength change in observations. Because the observed\nbehaviors of AMOC most probably be very different from the models such as CESM1 (see Lozier\net.al. 2019). The main influence region for AMOC from Asia AA emissions in the model is over\nLabrador Sea. Recent observational studies are showing that the observed AMOC sensitivity to\nLabrador Sea deep convection changes maybe minor compared to what some standard coupled\nclimate models depicts. This means that though the analysis shows a valid way of Asian AA\nemission influence to AMOC through Labrador Sea water mass transformation changes, such\nchange may not be happening in reality due to differences in sensitivities in the observed world\nfrom a model world.\nResponse: Thank you for raising the issue. As you pointed out, the observation-based studies have\nindicated a limited role of the Labrador Sea in the mean deep convection and AMOC. However,\nrecent studies have demonstrated that the Labrador Sea still plays a dominant role in long-term\nAMOC changes despite its weak mean deep convection and overturning in the mean state. For\nexample, Oldenburg et al. (2021) compared the locations of deep convection in different low-\nresolution models and found that mean deep convection sites vary considerably between models,\n2\nyet the Labrador Sea drives the low-frequency AMOC variability, even in models where the time-\nmean deep convection occurs in the Irminger Sea or the Norwegian Sea. This result holds in high-\nresolution models (Yeager et al. 2021; Oldenburg et al. 2022), which reproduce the weak mean\noverturning along the OSNAP west line and can be seen as an analog to observations. These above\nfindings reconcile the observation- and model-based perspectives on the subpolar origins of the\nAMOC: although the Labrador Sea plays a secondary role in driving the time-mean AMOC, it is\nstill the most important driver of the long-term changes in the deep convection and the AMOC.\nWhile we acknowledge that model simulations may not perfectly represent the observed AMOC,\nour study provides a starting point for investigating the influence of Asian AAs on the AMOC.\nWe hope that our findings will stimulate research in model-based and observational studies.\nA related discussion regarding the role of the Labrador Sea has been added to the revised text\n(Lines 186-188), and relevant papers (Lozier et al. 2019; Yeager et al. 2021; Oldenburg et al. 2022)\nhave been cited in the revision.\nReferences\nLozier, M. S., F. Li, S. Bacon, F. Bahr, A. S. Bower, S. A. Cunningham, M. F. De Jong, et al. 2019.\n“A Sea Change in Our View of Overturning in the Subpolar North Atlantic.” Science 363\n(6426): 516–21. https://doi.org/10.1126/science.aau6592.\nOldenburg, Dylan, Robert C.J. Wills, Kyle C. Armour, Luanne Thompson, and Laura C. Jackson.\n2021. “Mechanisms of Low-Frequency Variability in North Atlantic Ocean Heat Transport\nand AMOC.” Journal of Climate 34 (12): 4733–55. https://doi.org/10.1175/JCLI-D-20-\n0614.1.\nOldenburg, Dylan, Robert C.J. Wills, Kyle C. Armour, and Lu Anne Thompson. 2022. “Resolution\nDependence of Atmosphere–Ocean Interactions and Water Mass Transformation in the North\nAtlantic.” Journal of Geophysical Research: Oceans 127 (4): 1–18.\nhttps://doi.org/10.1029/2021JC018102.\nYeager, Stephen, Fred Castruccio, Ping Chang, Gokhan Danabasoglu, Elizabeth Maroon, Justin\nSmall, Hong Wang, Lixin Wu, and Shaoqing Zhang. 2021. “An Outsized Role for the\nLabrador Sea in the Multidecadal Variability of the Atlantic Overturning Circulation.”\nScience Advances 7 (41). https://doi.org/10.1126/sciadv.abh3592.\nA more rigorous analysis on observed changes and its connection to observed AMOC changes is\nnecessary to give this study a stronger foundation. The authors can look at observed year to year\nAA changes over Asia and investigate if there is any connection to the observed AMOC changes.\nFurther, the same could be investigated for the teleconnection pathways. Do we see a similar\nteleconnection pathway from the Asia in the observed upper-level winds? There must be some\nsignal present in the observational geopotential height, winds and stream function field in\nassociation with observed AA changes over Asia, if a similar mechanism is at play. Do we also\nfind a negative NAO like response connection to Asian AA changed over the North Atlantic region.\nPrevious studies could have provided some hint on this, though I think it is essential to provide a\nclear picture of such observational association in this manuscript.\nResponse: Thank you for your insightful comments and we agree on the importance of\nobservational evidence to strengthen the foundation of our findings. Following your suggestion,\nwe have performed several analyses to explore the observational connections between changes in\nAsian AAs and atmospheric circulations.\nFirst, we conducted a regression analysis using an atmospheric reanalysis dataset Modern-Era\nRetrospective analysis for Research and Applications, Version 2 (MERRA2), to examine the\nconnection between changes in Asian AAs and atmospheric circulation in the real world.\nMERRA2 was chosen because it provides aerosol optical depth data. However, we encountered\nchallenges in distinguishing the individual impacts of AAs and greenhouse gases (GHGs) due to\nthe relatively smooth, long-term increase in both AAs and GHGs over the years, as shown in Fig.\nR1a,d. These commonalities resulted in the generation of very similar spatial patterns in the\nregression analyses for both GHGs and AAs (Fig. R1b,c,e,f), complicating the task of separating\ntheir distinct effects on atmospheric circulations.\nWe have performed an EOF analysis to identify the fingerprints in the atmospheric circulation\nassociated with the Asian AAs changes. Figure R2 shows the first two empirical orthogonal\nfunctions (EOFs) modes of 200-hPa geopotential height in the northern midlatitudes, of which the\nvariance fractions are 41.5% and 14.3%, respectively. The first EOF mode (Fig. R2a) and the\ncorresponding principal component (PC) time series (Fig. R2b) indicate a monotonic poleward\nshift of the subtropical jet over the past few decades, primarily attributed to increased GHGs (Lu\net al. 2010; Fu et al. 2011). The second EOF mode (Fig. R2c) is characterized by a circumglobal\nwave pattern and an equatorward shift of the westerly jet in the midlatitudes, exhibiting some\nresemblance to the teleconnection pattern induced by increased Asian AAs (Fig. 2b in the revision).\nThe corresponding PC2 time series (Fig. R2d) also exhibits a similar temporal evolution as Asian\nAAs (Fig. R1a): an increase from 1990 to 2010 and then a decrease afterward. Regarding\ncirculation changes over the subpolar North Atlantic, the leading EOF of SLP (Fig. R3a),\nresembling the negative phase of NAO, explains 50.9% of the total variance. The corresponding\nPC1 (Fig. R3b) also mirrors the evolution seen in Asian AAs since the 1990s (Fig. R1a), indicating\nweakened (strengthened) westerly winds over the Labrador Sea from 1990-2010 (2010-present).\nDespite the above consistency in teleconnection patterns between reanalysis data and the responses\ninduced by Asian AAs, caution is warranted in establishing causal relationships for several reasons:\n4\ni) Statistical analysis alone cannot determine the causality, as the relationship can be\ninfluenced by common drivers or indirect links via a third process, which is common in\nclimate science (e.g., Runge et al., 2019).\nii) The sparse observational record of AAs (since the 1980s) and the AMOC (since the 2000s\nfor RAPID, and 2014-2020 for OSNAP) makes it difficult to discern changes and\nattributions over a relatively short period.\niii) Year-to-year changes in the mid-latitude atmospheric circulation are strongly affected by\ninternal variabilities such as ENSO, North Atlantic Oscillation (NAO), and North Pacific\nOscillation (NPO). The atmospheric conditions in the subpolar North Atlantic, the focus of\nthis study, are strongly modulated by the NAO (e.g., Visbeck et al. 2001). Therefore, on\ninterannual timescales, the contribution of forcing by AAs or even GHGs is not comparable\nto these internal variabilities. Even on decadal to interdecadal timescales, both the PC2 of\nthe geopotential height (Fig. R2d) and the PC1 of the SLP (Fig. R3b) exhibit notable\ninterdecadal variabilities, particularly in correlation with the Atlantic Multidecadal\nOscillation (positive/negative phase after/before the 1990s; r=0.60 and 0.76, respectively).\nThis correlation suggests the possibility that the observed circumglobal wave pattern and\nthe NAO-like response could be manifestations of internal variability.\nTo reduce the impact of internal variability and to establish causality, an ideal approach is to use\nlarge ensembles of simulations, each initialized with different conditions but subjected to the same\nAAs forcing over Asia. This methodology allows for a more accurate assessment of the individual\nimpacts of Asian AAs and GHGs, and has been adopted in DAMIP and numerous other studies to\nsupport climate change detection and attribution assessments (e.g., Dong et al. 2021, 2022; Menary\net al. 2020). Following this methodology, we have collected multiple lines of supporting evidence\nfrom CMIP6 and CESM2-LE simulations to demonstrate that the proposed link is not unique to\nCESM1, as detailed in our response to your subsequent comment.\n5\na d\nAerosol GHGs\nb e\nc f\nFigure R1 | a, Temporal evolution of AOD at 550 nm (unit: 1) averaged over Asia based on\nMERRA2 dataset. Note that the values for the years 1982, 1983, 1991, and 1992 have been\nreplaced with the means of adjacent years in order to eliminate the global impact of volcanic\neruptions of El Chichon and Mount Pinatubo. b, Regression of 200 hPa geopotential height onto\nthe Asian AOD time series (unit: m/1). c, Regression of sea level pressure (SLP) onto the Asian\nAOD time series (unit: Pa/1). d-f, Same as a-c, but associated with global-mean GHGs\nconcentrantion based on EDGAR v6.0 Greenhouse Gas Emissions dataset. Regression analyses\napplied to time series of Asian AAs and GHGs show similar results.\na c\nb d\nFigure R2 | a,c The first two EOFs of 200-hPa geopotential height based on MERRA2 dataset.\nb,d, the corresponding PC time series. The 2nd EOF mode is characterized by a circumglobal\nwave pattern, and its corresponding PC2 shows similarities with the evolution of Asian AAs\nsince the 1990s.\n6\na c\nb d\nFigure R3 | a,c The first two EOFs of SLP over the subpolar North Atlantic based on MERRA2\ndataset. b,d, the corresponding PC time series. The 1st EOF mode is characterized by a negative\nNAO-like pattern, and its corresponding PC1 shows similarities with the evolution of Asian\nAAs since the 1990s.\nReferences\nDong, Buwen, and Rowan T. Sutton. 2021. “Recent Trends in Summer Atmospheric Circulation\nin the North Atlantic/European Region: Is There a Role for Anthropogenic Aerosols?”\nJournal of Climate 34 (16): 6777–95. https://doi.org/10.1175/JCLI-D-20-0665.1.\nDong, Buwen, Rowan T. Sutton, Len Shaffrey, and Ben Harvey. 2022. “Recent Decadal\nWeakening of the Summer Eurasian Westerly Jet Attributable to Anthropogenic Aerosol\nEmissions.” Nature Communications 13 (1): 1–10. https://doi.org/10.1038/s41467-022-\n28816-5.\nFu, Qiang, and Pu Lin. 2011. “Poleward Shift of Subtropical Jets Inferred from Satellite-Observed\nLower-Stratospheric Temperatures.” Journal of Climate 24 (21): 5597–5603.\nhttps://doi.org/10.1175/JCLI-D-11-00027.1.\nLu, Jian, Gang Chen, and Dargan M. W. Frierson. 2010. “The Position of the Midlatitude Storm\nTrack and Eddy-Driven Westerlies in Aquaplanet AGCMs.” Journal of the Atmospheric\nSciences 67 (12): 3984–4000. https://doi.org/10.1175/2010JAS3477.1.\nMenary, Matthew B., Jon Robson, Richard P. Allan, Ben B.B. Booth, Christophe Cassou,\nGuillaume Gastineau, Jonathan Gregory, et al. 2020. “Aerosol-Forced AMOC Changes in\nCMIP6 Historical Simulations.” Geophysical Research Letters 47 (14).\nhttps://doi.org/10.1029/2020GL088166.\nRunge, J., Bathiany, S., Bollt, E., Camps-Valls, G., Coumou, D., Deyle, E., et al. (2019). Inferring\ncausation from time series in Earth system sciences. Nature Communications, 10, 2553.\nhttps://doi.org/10.1038/s41467-019-10105-3\nAtlantic Oscillation: Past, Present, and Future.” Proceedings of the National Academy of\nSciences of the United States of America 98 (23): 12876–77.\nhttps://doi.org/10.1073/pnas.231391598.\n3. It would be also useful and highly confirmatory to show if CMIP6 models altogether show a\nsame connection of AA emissions changes over Asia and AMOC. Among the CMIP6 models,\nwhen there are different future AA emission levels or aerosol radiative effect (under the same\nscenario) over East Asia, do they show any correlation with their respective AMOC changes?\nThe goal is if the authors could show somehow the model independency of the main findings and a\nsimilar connection from AA emissions over Asia to AMOC across multiple models.\nResponse: Thank you for your valuable suggestion. In the revision, we have incorporated new\nanalyses and discussions to provide additional multi-model validation of the Asian AAs-AMOC\nlink:\ni) Validation of teleconnection patterns through AerChemMIP: To assess the robustness of\nthe atmospheric teleconnections induced by increased Asian AAs, we have analyzed simulations\nfrom 10 models participating in the Aerosol Chemistry Model Intercomparison Project\n(AerChemMIP) (Collins et al. 2017), a subset of CMIP6. Two sets of AerChemMIP simulations,\npiClim-control and piClim-SO2, were used. Both simulations span 30 years with fixed\nclimatological mean SST and sea ice conditions corresponding to the pre-industrial levels. The\nonly difference is in the emissions of anthropogenic aerosol precursors of SO2: piClim-control\nuses preindustrial emissions, while piClim-SO2 employs present-day (2014) SO2 emissions.\nNotably, due to the implementation of the Clean Air Act in Europe and North America, SO2\nemissions in the western hemisphere had already significantly decreased by 2014, making Asian\nemissions the dominant forcing in the piClim-SO2 simulations. Therefore, this 2014 SO2 forcing\n(Fig. R4a, contours), albeit being more concentrated in East Asia, operates as a comparable agent\nto that used in our EAST simulations.\nOur analysis reveals consistent patterns of atmospheric response in the AerChemMIP simulations\n(Fig. R4) compared to our EAST simulations (Fig. 2). These patterns include a circumglobal\nteleconnection pattern (Fig. R4a) and an equatorward migration of the westerly jet in the northern\nmidlatitudes (Fig. R4c). Additionally, a negative NAO-like response is evident in the subpolar\nNorth Atlantic, accompanied by suppressed westerlies over the Labrador Sea (Fig. R4b). This\nrobust agreement between the results of AerChemMIP and our CESM1 simulations provides\nconfidence in the robustness of the atmospheric teleconnections excited by increased Asian AAs.\nHowever, due to the absence of ocean models in the AerChemMIP simulations, further\nexamination of AMOC responses is not feasible with these simulations.\n8\nii) Validation of AMOC responses through CESM2-LE: Recognizing the limitations of the\nAerChemMIP simulations in examining AMOC responses and the challenges of disentangling\naerosol changes in the eastern and western hemispheres in DAMIP experiments, we turn to the\nCESM2 Large Ensemble (CESM2-LE) (Simpson et al. 2023). This ensemble extends its AAs-\nalone ensemble simulations with 15 members into the year 2050 under the shared socioeconomic\npathways 3-7.0 (SSP3-7.0) scenario.\nIn this high-emission scenario, Asian AA emissions continue to rise in the future, while AAs in\nthe western hemisphere show limited changes as they have already diminished to low levels.\nConsequently, the increased Asian AAs and the associated radiative cooling emerge as the\ndominant forcing during the period 2015 to 2050 (Fig. R5b, contours). This provides analogs to\nour EAST results and adds some model diversity to the analysis. The circumglobal teleconnection\npattern (Fig. R5b, color) and the negative NAO-like response (Fig. R5c) are also found in the\nCESM2-LE simulations. In addition, the CESM2-LE simulations show that the weakened westerly\nover the Labrador Sea suppresses the heat loss, resulting in a rapid decline of AMOC during the\nperiod 2015 to 2035 (Fig. R5a). Such agreement across different models suggests that the\nrelationship between Asian AAs and AMOC is not unique to CESM1, and supports our argument\nthat the increased Asian AAs can weaken the AMOC by exciting atmospheric teleconnections and\nhindering deep water formation at the deep convection site.\nIn the revision, the above analysis and related discussion have been added as a new section entitled\n\"Additional multi-model results confirming the Asian AAs-AMOC link\" before the conclusion\n(Lines 237-292). We are delighted at the consistency between different models that provides a\ncompelling line of evidence for our Asian AAs-AMOC link. Thank you for the suggestions.\n9\na\nStreamfunction and Rotational Wind at 200 hPa\nb c\nSLP and Surface Wind Jet Stream response\nFigure R4 | Responses in atmospheric circulation to 2014 sulfate emission forcing in\nAerChemMIP simulations. a, Responses of aerosol optical depth (contours; unit: 1), 200-hPa\nstreamfunction (colors; unit: 10! 𝑚\" 𝑠#$) and rotational wind (vectors; unit: 𝑚 𝑠#$) calculated as\nthe difference between the piClim-SO2 ensemble and the piClim-control ensemble for years 1-30.\nb, Responses of sea level pressure (colors; unit: Pa) and surface wind (vector; unit: 𝑚 𝑠#$) over\nthe North Atlantic of the multi-model mean. Blue and black vectors denote anomalous winds that\nweaken and intensify the climatological winds. c, Response of the zonal-mean zonal winds (colors;\nunit: 𝑚 𝑠#$) of the multi-model mean, and superimposed is the corresponding climatology in\npiClim-control. The Asian AAs-induced atmospheric responses are validated by the\nAerChemMIP simulations.\n10\na\nb\nAMOC Response AOD & Streamfunction and Rotational Wind at 200 hPa\nc d\nSLP and Surface Wind Jet Stream response\nFigure R5 | Responses in atmospheric circulation and AMOC to increasing Asian AAs in\nCESM2-SF-LE simulations. a, AMOC time series (unit: Sv) in CESM2-SF-LE SSP370-AAs\nsimulations. The AMOC index is obtained as the maximum of streamfunction between latitudes\n25° N–75° N and between depths of 500 and 2000 m. b-d, Same as Supplementary Fig. 9a-c, but\nfor the difference between years 2040-2050 and years 2015-2025 of the ensemble mean from\nSSP370-AAs simulations. The Asian AAs-AMOC link is validated by the CESM2-SF-LE\nsimulations.\nReferences\nCollins, J. William, Jean François Lamarque, Michael Schulz, Olivier Boucher, Veronika Eyring,\nI. Michaela Hegglin, Amanda Maycock, et al. 2017. “AerChemMIP: Quantifying the Effects\nof Chemistry and Aerosols in CMIP6.” Geoscientific Model Development 10 (2): 585–607.\nhttps://doi.org/10.5194/gmd-10-585-2017.\nSimpson, Isla R., Nan Rosenbloom, Gokhan Danabasoglu, Clara Deser, Stephen G. Yeager,\nChristina S. McCluskey, Ryohei Yamaguchi, et al. 2023. “The CESM2 Single-Forcing Large\nEnsemble and Comparison to CESM1: Implications for Experimental Design.” Journal of\nClimate 36 (17): 5687–5711. https://doi.org/10.1175/JCLI-D-22-0666.1.\nLine 76: what do you mean by “present day level”?\nResponse: We apologize for not explaining this more clearly in the manuscript. “present-day level”\nrefers to the levels of GHGs, aerosols, and solar insolations observed in the year 2000. For example,\nthe global mean GHG mixing ratio is 367.0 ppmv. This clarification has been included in the\nrevised text (Line 83).\nLine 77: why 10% ?\nResponse: The choice of a 10% perturbation was made to generate a robust response signal in our\nsimulations. This level of perturbation results in a radiative forcing amplitude of approximately 30\nW/m², which is confined to specific regions over Asia, North America, and Western Europe. The\nrationale for using a stronger forcing is to ensure that the effects of Asian aerosols on the AMOC\ncan be clearly distinguished from the background noise introduced by natural variability, such as\nthe Atlantic Multidecadal Variability (AMV). We have incorporated this explanation into the\nrevised manuscript (Lines 88-90).\nLine 80: here the response is reversed, which means the authors assume a linear nature of the\nresponse. This has to be stated and if the authors could provide some evidence behind such an\nassumption that would be even better.\nResponse: Thank you for your comment, and we have clarified that a linear assumption is adopted\nin the revision (Line 86). Your point has inspired us to consider the linearity of the AMOC response\nto changes in AAs, which we plan to explore in future research.\nLine 85: why only 10 years of simulations are performed? This is quite small and unusual. I\nunderstand that the response time for aerosol would be small but a time of emergence analysis on\nthe AMOC strength should be done considering its variability in the model to determine the\nminimum duration of the run to find a discernible forced signal from internal variability. I am okay\nwith the claims made qualitatively from this limited length data, though the quantification could\nbe different and could show a further robust estimate if the runs are performed for longer times. I\nwould urge the authors to do this if there is no major computational constraint or put some strong\nreasoning to perform such short simulations. It is not ideal to set such experimental precedence\nwithout any strong reasoning.\nResponse: We agree with your point about the length of the simulation. In the original manuscript,\nwe presented the ensemble simulations for the first 20 years because our primary focus was on\nhow Asian radiative cooling affects atmospheric circulation and leads to a rapid reduction of the\nAMOC during the first two decades. Beyond this period, the AMOC tends to weaken at a\nconsiderably slower rate. This deceleration in AMOC weakening is attributed to the anomalous\ncold advection associated with a weakened AMOC, which acts to balance the surface heat gain\noccurring in the Labrador Sea and suppress further decrease of the deep water formation (please\nsee details in our response to your next comment).\nmembers. In Fig. R6, we present the updated temporal evolution of the AMOC index in the EAST\nand WEST experiments, and the main findings remain consistent. In the EAST experiments, the\nAMOC experiences a rapid decline of ~2.7 Sv in the first four decades, followed by a slower\ndecline of ~0.6 Sv over the subsequent 60 years. In the WEST experiments, the AMOC is\nweakened by ~1.4 Sv in the first four decades and remains relatively stable at this level for the\nnext 60 years. Importantly, the eastern hemisphere forcing still exhibits a greater efficacy in\nweakening the AMOC compared to the adjacent western hemisphere forcing. Furthermore, the\nspatial patterns of responses of the AMOC and atmospheric circulations averaged over the last 50\nyears (Fig. R7) are consistent with those averaged over years 1-40 (Figs. 2b-c,3b), again\ndemonstrating a short response timescale of the AMOC to aerosol changes.\nIn the revision, we still focus on the transient responses to emphasize how increased Asian AAs\nweaken the AMOC through the excitation of atmospheric teleconnections. However, the results\nare updated based on the average over the first 40 years. In addition, we provide an overview of\nthe ensemble responses during the slower response stage (years 51-100) in the supplementary,\ndemonstrating the limited changes in response patterns over time. We have revised the figures\nand the corresponding text in the revision (Lines 107-113).\na b\nAMOC Index [EAST] AMOC Index [WEST]\nFig. R6 a,b Ensemble-mean evolution of annual-mean AMOC index (unit: Sv) in EAST and\nWEST smoothed with a 5-yr running-mean filter, with the thin curves denoting each member’s\nanomalies. The radiative forcing due to changes in aerosols over both Asia and North America\n& Europe causes the AMOC to slowdown.\n13\na\nStreamfunction and Rotational Wind at 200 hPa\nb c\nSLP and Surface Wind WMT Response\nFig. R7 Responses of atmospheric and deep water formation to increased Asian AAs during\nthe slow response stage. a, Responses of 200-hPa streamfunction (colors; unit: 10! 𝑚\" 𝑠#$) and\nrotational wind (vectors; unit: 𝑚 𝑠#$) in the northern hemisphere averaged over year 51-100. b,\nResponses of sea level pressure (colors; unit: Pa) and surface wind (vector; unit: 𝑚 𝑠#$) over\nthe North Atlantic in EAST simulations averaged over year 51-100. c, WMT responses (unit: Sv)\nover the subpolar North Atlantic (SPNA) and contributions from Irminger Sea, Labrador Sea, and\nGreenland-Iceland-Norwegian Seas (GIN). The long-term responses of atmospheric circulation\nand water mass transformation show consistent patterns compared to the transient\nresponses.\nLine 98: Why is there a rapid decrease in the first two decades? Could the authors give a\nreason/hypothesis for this?\nResponse: The AMOC response, as depicted in Fig. R6a, exhibits two distinct timescales: a\ntransient phase during years 1-40 and a slow phase from years 41-100. During the transient phase\nof the response, the atmosphere acts primarily as a forcing on the ocean, and the feedback from\nocean dynamical processes is relatively minor, and the weakened westerlies lead to reduced heat\nloss from the surface of the Labrador Sea, effectively weakening the AMOC strength.\nHowever, the importance of the ocean’s feedback is increasing as time progresses into the slow\nresponse phase. Since the AMOC is responsible for the majority of the mean heat transport in the\nConsequently, anomalous colder water (Fig. R8b) and positive density anomalies propagate\ndownstream along the cyclonic subpolar gyre, reaching the Labrador Sea. This process balances\nthe surface heat gain in the Labrador Sea, preventing further reduction of deep water formation\nand thus alleviating the weakening of the AMOC. This above explanation has been included in the\nrevised manuscript to provide a clearer understanding of the dynamics involved (Lines 228-235).\na b\nNorthward Heat Transport at 40°N SST Anomaly\nFig. R8 a. Ensemble-mean evolution of oceanic northward heat transport at 40N (unit: PW) in\nEAST smoothed with a 5-yr running-mean filter, with the thin curves denoting each member’s\nanomalies. b, SST response (unit: °C) in EAST averaged over year 51-100. The weakened\nAMOC reduces the northward oceanic heat transport and advects anomalously cold water\ninto the deep convection site, balancing the surface heat gain there.\nLine 100: Similarly, why is there a gradual decline afterwards?\nResponse: As mentioned in our previous response, the gradual decline of the AMOC during the\nslow response phase is a result of the atmospheric forcing and oceanic feedback processes, with\nthe latter contributing to a decreased northward heat transport.\nLine 108-111: Are these separate experiments performed with only South Asian AA and East\nAsian AA? The description of such experiments should be clearly stated then and the\nsupplementary figure 3 also do not provide the images of two separate experiments. Hence it\nremains unclear what the authors have done.\nResponse: We apologize for not explaining this clearly in the manuscript. In addition to the EAST\nexperiment that represents the total impact of Asian AAs, in this study, we also performed the East\nAsia experiment and inferred the influence of South Asian AAs by comparing it to the EAST\nexperiment. The relative contribution of these two regions, East Asia and South Asia, to AMOC\nchanges is an interesting and complex issue, as East Asia has experienced a decrease in AAs\nemissions since the 2010s, while South Asia has experienced a continuous increase in AAs\nemissions.\nHowever, we are aware that a single member of the simulation is insufficient to accurately estimate\nthe contributions of either East Asia or South Asia. Given the scope of this study, we have decided\nto focus on the impact of increased Asian AAs as a whole on AMOC strength and leave for the\n15\nfuture a more in-depth investigation of the relative role of East Asia and South Asia on AMOC\nstrength changes.\nLine 145: please remove “greatly”. And there are key differences in the CGT pattern and the CGT-\nlike pattern that the authors find in this experiment. That has been pointed out too. I am curious if\nthe authors could show such resembling pattern in observations with respect to observed Asian\nAA variations.\nResponse: Thank you for pointing this out. We have removed \"greatly\" from the sentence as\nsuggested and simplified the terminology to \"circumglobal response\" instead of “CGT-like\nresponse” to describe the wave pattern identified in the experiments.\nIn addition, as shown in Fig. R2c, the second EOF of observed geopotential height is characterized\nby a circumglobal wave pattern, exhibiting a resemblance to the teleconnection pattern induced by\nincreased Asian AAs. The corresponding PC2 time series (Fig. R2d) also exhibits some similarity\nto the temporal evolution of Asian AAs since the 1990s (Fig. R1a). However, as mentioned in our\nresponse to your major comment #2, the regression analysis cannot establish causality. Besides,\nthe observed circumglobal wave pattern could be a manifestation of internal variability.\nLine 150: is there an associated seasonal dependency in the AMOC response to Asian AA?\nResponse: Not much, and the AMOC weakening is evident in all seasons (Fig. R9), likely due to\nthe significantly longer response timescales associated with the deep western boundary currents\nand horizontal recirculations that collectively constitute the AMOC (Zhang 2010).\na b e\nMAM JJA Seasonal variability\nc d\nSON DJF\nFig. R9 a-d, Seasonal mean responses of AMOC to increased Asian AAs. e, Seasonal mean\nstreamfunction anomaly averaged over 50-65N, 36.75-37.0 𝑘𝑔 𝑚#%. The AMOC weakening is\nevident in all four seasons.\nReference\n(AMOC) Variations.” Geophysical Research Letters 37 (16): 1–6.\nhttps://doi.org/10.1029/2010GL044474.\nFigure 2c : in this figure the location of the mean westerlies in contours must be shown.\nResponse: Thank you for your comment, and the mean zonal wind at 200 hpa has been\nsuperimposed in the revised Fig. 3b to highlight the location of the waveguide.\nLine 165-167: here the authors claim that previous studies from CMIP5 and CMIP6 models\nalready corroborate the link of the equatorward shift of the jet and AA emission over Asia. In a\nsimilar line, could the authors also link the changes in AA over Asia and AMOC changes in those\nCMIP5 and CMIP6 models?\nResponse: We apologize for the incorrect description in our original manuscript. To clarify,\nprevious studies based on observations and CMIP models have indeed reported the equatorward\nshift of Atlantic jets from the 1970s to the 2010s (Dong et al. 2021). However, the link between\nthis shift and AAs changes has only been established through DAMIP single forcing experiments\n(Undorf et al. 2018; Dong et al. 2022) or purposefully designed atmospheric experiments (Dong\net al. 2021). In other words, these studies did not explicitly attribute these changes to AAs over\nAsia, as AA emissions from North America and Europe exhibit significant variations during the\nsame time period. On the other hand, using the CESM1, Diao et al. (2021) attributed a central role\nto Asian AAs in driving the equatorward shift of the jet, but the atmospheric changes in the\nsubpolar North Atlantic didn’t receive attention in their study, let alone the impact on the AMOC.\nWe have revised the text accordingly in the manuscript (Lines 168-170) to provide a more accurate\nrepresentation of the previous studies and their findings. In addition, as in our response to your\nmajor comment #3, we have made an extensive effort to explore the atmospheric circulation and\nAMOC response to AAs changes in AerChemMIP simulations and CESM2-SF-LE simulations.\nAll of these modeling results provide robust corroborating evidence for our findings. Please see\nour new section in the revision “Additional multi-model evidence confirming the Asian AAs-AMOC\nlink” for details.\nReferences\nDiao, Chenrui, Yangyang Xu, and Shang Ping Xie. 2021. “Anthropogenic Aerosol Effects on\nTropospheric Circulation and Sea Surface Temperature (1980-2020): Separating the Role of\nZonally Asymmetric Forcings.” Atmospheric Chemistry and Physics 21 (24): 18499–518.\nhttps://doi.org/10.5194/acp-21-18499-2021.\nDong, Buwen, and Rowan T. Sutton. 2021. “Recent Trends in Summer Atmospheric Circulation\nin the North Atlantic/European Region: Is There a Role for Anthropogenic Aerosols?”\nJournal of Climate 34 (16): 6777–95. https://doi.org/10.1175/JCLI-D-20-0665.1.\nWeakening of the Summer Eurasian Westerly Jet Attributable to Anthropogenic Aerosol\nEmissions.” Nature Communications 13 (1): 1–10. https://doi.org/10.1038/s41467-022-\n28816-5.\nUndorf, Sabine, M. A. Bollasina, and G. C. Hegerl. 2018. “Impacts of the 1900-74 Increase in\nAnthropogenic Aerosol Emissions from North America and Europe on Eurasian Summer\nClimate.” Journal of Climate 31 (20): 8381–99. https://doi.org/10.1175/JCLI-D-17-0850.1.\nLine 180: these are model specific behaviour of the AMOC and authors must be careful to\ngeneralise such things. Could the author confirm the same in observations? Lozier et.al. suggest\nthat observed AMOC behaves in a different fashion than what the models show and AMOC is\nmore sensitive to the changes in the eastern side of subpolar North Atlantic than the western side\nover Labrador Sea. Hence this potential difference in observed and model behaviour and its effect\non the results needs to be clarified.\nResponse: Thank you for your comment on this. As addressed in our response to your major\ncomment #2, the deep convection in the LS is overestimated in climate models, but it has been\nfound in recent studies (Yeager et al. 2021; Oldenburg et al. 2021, 2022) to drive AMOC low-\nfrequency variability, even in high-resolution models that exhibit reasonable consistency with the\nweak overturning in the OSNAP observations.\nWe have added a discussion in the revised manuscript (Lines 186-188) that highlights the debate\nover the role of the Labrador Sea in the AMOC and the potential differences between modeled and\nobserved behavior. We also cite relevant studies, including Lozier et al. (2019), to acknowledge\nthe variability in AMOC sensitivity in the revision.\nLine 214: could the authors show the changes in horizontal (zonal and meridional) temperature\nadvection to confirm that the less cold air is getting advected over the Labrador Sea from North\nAmerica? This analysis would complete the picture by clearly showing anomalous horizontal\nwarm air advection.\nResponse: Thanks for the comments. Following your suggestion, we have decomposed the\nhorizontal advection terms into contributions from zonal and meridional processes in\nSupplementary Fig. 7e-f, and it does indeed illustrate the changes in zonal temperature advection\ndominates, highlighting the less cold air getting advected over the Labrador Sea from North\nAmerica. We have discussed this in the revised manuscript (Line 220).\nLine 470: cures --> curves\nResponse: Thank you, and it has been corrected.\nKey Results\nRecent work has highlighted how aerosols over the North Atlantic region can strengthen AMOC\nby cooling the deep water formation region, which strengthens meridional density gradients\ndriving AMOC. This study finds a new and potentially important contrary effect: aerosols over\nSouth Asia and East Asia can weaken AMOC by shifting the Atlantic Westerly winds southward,\ndecreasing heat loss and warming the deep water formation regions. Heat budget analysis makes\nit clear that the main mechanism is the reduction in transport of cold continental air over the\nLabrador Sea, which reduces ocean heat loss, cooling, and density increase there.\nValidity, Data and Methodology\nThe authors themselves point out what I see as the main question mark about the validity of the\nresult, which is that climate models such as theirs do not do a good job capturing the overflow\ncomponent (from the Nordic Seas) of deep water formation and hence may over-emphasize the\nrole of the Labrador Sea. Since this is a very challenging feature to model correctly, it is a\nshortcoming of virtually all studies of AMOC, and should not hinder publication.\nResponse: We appreciate your understanding of this challenge, which is indeed a common\nlimitation in AMOC studies due to the complex nature of modeling deep water formation processes\nin the North Atlantic.\nIt's worth noting that while the climate model may not simulate the mean overturning well, recent\nstudies (Yeager et al. 2021; Oldenburg et al. 2021, 2022) have highlighted the role of the Labrador\nSea in driving long-term changes in deep convection and AMOC, even in high-resolution models\nthat exhibit reasonable consistency with the weak mean overturning in the OSNAP observations.\nIn light of this, we believe that our study provides valuable insights into how increased Asian AAs\nmay contribute to the weakening of the AMOC. Thank you for recognizing the importance of our\nstudy despite this acknowledged limitation.\nReferences\nOldenburg, Dylan, Robert C.J. Wills, Kyle C. Armour, Luanne Thompson, and Laura C. Jackson.\n2021. “Mechanisms of Low-Frequency Variability in North Atlantic Ocean Heat Transport\nand AMOC.” Journal of Climate 34 (12): 4733–55. https://doi.org/10.1175/JCLI-D-20-\n0614.1.\nOldenburg, Dylan, Robert C.J. Wills, Kyle C. Armour, and Lu Anne Thompson. 2022. “Resolution\nDependence of Atmosphere–Ocean Interactions and Water Mass Transformation in the North\nAtlantic.” Journal of Geophysical Research: Oceans 127 (4): 1–18.\nhttps://doi.org/10.1029/2021JC018102.\nYeager, Stephen, Fred Castruccio, Ping Chang, Gokhan Danabasoglu, Elizabeth Maroon, Justin\nSmall, Hong Wang, Lixin Wu, and Shaoqing Zhang. 2021. “An Outsized Role for the\nScience Advances 7 (41). https://doi.org/10.1126/sciadv.abh3592.\nSignificance\nIn recent decades, the largest aerosol increases have been in Asia, and this study is the first to show\na clear link between these increases and possible weakening of AMOC. Therefore it is a highly\nimportant study. The fact that there has been increasing discussion of weakening of AMOC,\npresumably due to the effects of anthropogenic global heating, will heighten interest in this paper\nbecause it suggests an alternative explanation for dropping AMOC strength.\nResponse: We sincerely appreciate your recognition.\nClarity & Context/ Suggested Improvements\nThe paper is very well written. I have some comments on what is written about the context which\nalso has some bearing on how the paper presents the significance of these findings.\n1) Previous work on Asian Aerosols and teleconnections to N Atl\nThe last paragraph before the Discussion section discusses prior work, and it (or at least the first\npart of it, before the discussion of discrepancy with Kang et al and new results) should be placed\nat the end of the Introduction section. I assume the motivation for putting it later was that the\nparagraph is motivated by this paper’s results that the main influence on AMOC of the Asian AA\nis through a shift of the westerlies. However, the paragraph (if placed in Intro) would make sense\nto a reader since it talks about effects of climate perturbations in Asia on the North Atlantic region,\nwhich is plausibly relevant to the topic at hand.\nOne sentence in that paragraph makes me think that this paper should summarize its results a little\ndifferently. The sentence says “experiments forced with … aerosol emissions over the eastern\nhemisphere… has shown a weakening of the AMOC… providing direct support for our finding.”\nIf that were true, doesn’t the existence an earlier paper with similar results call into question the\nnovelty of the finding? I have not thoroughly read the paper in question (Diao et al, 2021), but as\nfar as I can tell, Diao et al find the Asian aerosol effect on moving the Westerlies southward over\nthe Atlantic, but not the further effect on the AMOC.\nTherefore, the abstract and Intro should be rephrased a bit to make it clear that this study finds that\na previously-discovered shift in Westerlies makes the Asian AA have the effect of decreasing\nAMOC.\nResponse: Thank you for your suggestions. We agree with you to move the discussion of Diao et\nal. (2021) to the Introduction. We have made the necessary revisions to both the Abstract and\nIntroduction to better introduce the topic and provide context for the readers (Lines 66-72).\nDiao et al. (2021) indeed identified the role of increased Asian aerosols in shifting the westerlies,\nthey did not explore the further impact on the AMOC. To validate the results of our experiments\nwith TOA radiative forcings, we requested the AMOC output from the authors of Diao et al. (2021)\nand examined the AMOC response, as shown in the original supplementary Fig. 8. To avoid any\npotential confusion, in the revised manuscript we have refrained from presenting the AMOC\nresponse derived from the data of Diao et al. (2021).\nGiven that both Diao et al. (2021) and our study employ the same model CESM1, a better way to\nvalidate our findings is through multi-model or large-ensemble simulations. In the revision, we\ncomprehensively examined the atmospheric and AMOC responses to increased Asian AAs using\nthe AerChemMIP and CESM2 single-forcing large ensembles. These independent analyses\nyielded results consistent with our study, including a circumglobal teleconnection pattern, a\nnegative NAO-like response in the North Atlantic, reduced heat loss in the Labrador Sea, and a\nweakening of the AMOC. In the revised manuscript, we have added a new section entitled\n\"Additional multi-model results confirming the Asian AAs-AMOC link\" before the conclusion\nsection (Lines 237-292) to emphasize the consistency between different models in supporting the\nproposed link between Asian aerosols and AMOC changes.\n2) Are the new results surprising?\nThis is more subjective, but I don’t think the result is quite as surprising as they portray. The\nstrength of the AMOC depends less on density than on density differences between the northern\nNorth Atlantic and other parts of the Ocean (see Klinger and Haine, Ocean Circulation in Three\nDimensions, Chapter 9). Thus, the fact that cooling the North Atlantic with aerosols strengthens\nAMOC does not imply that cooling a different latitude and longitude with aerosols should have\nthe same affect. The Kang et al (2021) study cited here does suggest opposing AMOC increase\nrather than decrease, so maybe the real question this study is raising is not “what does Asian AA\nforcing do to AMOC?” but “what does Asian AA forcing at lower latitudes do to AMOC?”\nResponse: We agree with you that radiative cooling at different latitudes can lead to different\nclimatic impacts. The mechanism through which the Asian AAs forcing impacts the AMOC in\nKang et al. (2021) relies on downstream advective processes, in contrast to our study where we\nemphasize the importance of exciting Rossby exciting Rossby waves in facilitating this influence. The specific\nlocation of the increased Asian AAs along the path of the mean subtropical jet is a crucial factor\nthat enhances their influence on altering atmospheric circulation patterns and, by extension, the\nAMOC. For radiative forcings positioned at the same latitude but outside of the waveguide, their\nimpact on the AMOC could be very different.\nIn the revision, we have moved the comparison with Kang et al. (2021) to the last section (Lines\n302-308) and highlighted the sensitivity of the global climate system to the location of the forcing.\n21\n3) Some minor suggestions\nI recommend changing name of last section to \"Discussion and Conclusions\" and moving the\ncomparison to earlier studies (end of last paragraph before discussion) to this section.\nResponse: Thank you. Following your suggestions, we have updated the name of the last section\nto \"Discussion and Conclusions\" and moved the comparison with previous studies to this section\n(Lines 302-308).\n22"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have throughly revised the arficle based on my previous comments. I am more than safisfied\nby the arguments provided by the authors and totally agree with the limitafions menfioned. The results\nare promising and unique and would surely make an impact on the ongoing research in this field. I\nrecommend acceptance of this manuscript and congratulate for this fine work.", "reviewer_2": "For original submission, I believe I recommended publicafion subject to response to my comments. I am\nsafisfied with the response and now recommend publicafion."}, "author_response": ""}]

Decision letter