A Simulation of the Atlantic Meridional Circulation During Heinrich Event 4 Using Reconstructed Sea Surface Temperatures and Salinities Didier Paillard, Elsa Cortijo

A simulation of the Atlantic meridional circulation during Heinrich event 4 using reconstructed sea surface temperatures and salinities Didier Paillard, Elsa Cortijo

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Didier Paillard, Elsa Cortijo. A simulation of the Atlantic meridional circulation during Heinrich event 4 using reconstructed sea surface temperatures and salinities. Paleoceanography, American Geophysical Union, 1999, 14 (6), pp.716-724. 10.1029/1999PA900045. hal-03003312

HAL Id: hal-03003312 https://hal.archives-ouvertes.fr/hal-03003312 Submitted on 18 Dec 2020

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. PALEOCEANOGRAPHY, VOL. 14, NO. 6, PAGES 716-724, DECEMBER 1999

A simulation of the Atlantic lneridional circulation during Heinrich event 4 usingreconstructed sea surface temperatures and salinities

Didier Paillardand Elsa Cortijo Laboratoiredes Sciences du Climat et de l'Environnement,Centre d'Etudes de Sachay,Gif-sur-Yvette, France

Abstract.With a simplifiedtwo-dimensionnal Atlantic Ocean model, we investigatethe responseof the deepocean circulationto reconstructedsea surface salinity and temperatureused as surfaceboundary conditions at two time slices'just beforeand during Heinrich event 4. In contrastto previousstudies, we do not makeany assumption aboutfreshwater input. Our modelresults suggest that the recordedestimations of surfacehydrological changes duringan Heinrichevent are able to inducea drasticslowdown, or even a collapse,of the overallAtlantic thermohalinecirculation. This is, to someextent, consistent with recordsof the deepsea ventilation at thesetimes.

1. Introduction sustainedoscillations [Quon and Ghil, 1995;Sakai and Peltier,1996; Winton and Sarachik, 1993 ]. Numericalmodels During the last glacial period,the climateof the Earth has of ice sheetshave also demonstratedthe possibilityof ice undergonemany abruptchanges. The first evidenceof such sheetoscillations [Oerlemans, 1983], and such a mechanism rapid changeswas found in the isotopic records of past was suggested [MacAyeal, 1993] to explain the iceberg precipitationsabove Greenland [Dansgaard et al., 1982, dischargesassociated with Heinrich events.The first tentative 1993]. These climatic excursions, with typical durations modelingexperiments coupling the ice sheetdynamics with between1000 and 3000 years,are namedDansgaard-Oeschger the ocean-atmospheresystem have since been performed events.In deepsea marine sediments from the North Atlantic [Paillard, 1995; Paillard and Labeyrie, 1994]. If the Ocean,six rapid eventshave beenalso discovered during the mechanismof the Dansgaard-Oeschgeroscillations is still a last glacialperiod [Bondet al., 1992; Heinrich, 1988]. These matter of debate, there is now a wide acceptancethat the so-called Heinrich events are characterized by an abrupt icebergdischarge during Heinrich eventsinduces a slowdown increasein the ice-rafteddebris (lithic materiallarger than 150 or even a complete disruption of the North Atlantic [tm) foundin the sedimentand correspondto massiveiceberg thermohalinecirculation, thus greatly reducingthe northward dischargesin the North Atlantic between 40øN and 55øN. oceanicheat transport and coolingthe highnorthern latitudes. These Heinrich eventsoccur every 7 to 10 kyr during glacial To addressthis question,many numericalexperiments have periods, and their duration is of the order of 1000 years been performedwith different oceanmodels, assuming some [Francois and Bacon, 1994]. The interpretation of these kind of large additional freshwater input at high northern Dansgaard-Oeschgerand Heinrich eventsas large-scaleand latitudes in the North Atlantic Ocean. If these experiments large-amplitudeclimatic changes came with the studyof high- have clearly demonstratedthe sensitivityof the thermohaline resolution marine records of sea surface conditions [Bond et circulationto freshwaterforcing, noneof them can be taken as al., 1993; Cortijo et al., 1995]. A remarkablecorrelation was even close to what really happenedduring a Heinrich event. indeed found between sea surfacetemperature proxies and Indeed,most of theseexperiments were startedfrom a present- each isotopic excursionfound in the Greenlandrecords, or day ocean circulation state. However, during glacial times, Dansgaard-Oeschgerevent. The Heinrichevents are associated when Heinrich events occurred,the oceanicdeep circulation with the coldest time periods, just preceding the largest was different from the present one. Furthermore, these increasesin temperature. experiments assumed very different freshwater forcings, Unlike the glacial-interglacialchanges, these rapid climatic rangingin intensityfrom 0.01 Sv to 1 Sv, in durationfrom 10 shifts are not associated with variations in the orbital yearsto thousandsof years, and in geographiclocations from parametersof the Earth. Broecker et al. [1985] were among Labrador Sea to some North Atlantic latitude band [Maier- the first to promote the idea that the ocean thermohaline Raimer and Mikolajewicz, 1989; Manabe and Stouffer, 1995; circulationmight have played a crucialrole, Many numerical Stockerand Wright, 1996; Mikolajewiczet al., 1997]. Besides, experimentshave since shownthat this hypothesiswas sound these experiments were all performed with oceanic mixed and that the ocean circulation can indeed switch abruptly boundaryconditions or simplified coupledocean-atmosphere betweendifferent equilibria [Bryan, 1986; Rahmstorf,1995; models.This has led to criticismand skepticismtoward these Stocker and Wright, 1991] or can even experience self- results[Zhang et al., 1993], since suchmodel configurations are far less stable than the more classicaloceanic restoring boundaryconditions. Copyright1999 by theAmerican Geophysical Union. In orderto betterrepresent what really happenedduring an Papernumber 1999PA900045. Heinrich event, and since we are not interested in further 0883-8305/99/1999PA900045 $12.00 studying the stability properties of the ocean, we used an

716 PAILLARD AND CORTIJO: ATLANTIC MERIDIONAL CIRCULATION 717 oceanmodel with restoringboundary conditions. In addition maximum [Fichefet et al., 1994; Ganopolskiet at., 1998]. to beingmuch more stable, this also allows us to avoidmaking Though they differ in many respects, these previous any assumptionabout the freshwater input intensity or experimentshave proven the usefulnessof two-dimensional location.For this purpose,we needreconstructions of the sea models to representthe large-scale features of the oceanic surfacetemperature (SST) and salinity (SSS) fields during thermohalinecirculation. The model used here is very similar some particularHeinrich event. Such a reconstructionwas to the Winton [1997] model, where the meridionalvelocity is made for Heinrich event 4 (HE 4) [Corto'oet al., 1997]. This related to the meridional pressuregradient using a Rayleigh event occurred at about 35 ka. It was chosen for such a dampingas a parameterizationfor the friction on meridional reconstruction because it is well marked in all North Atlantic velocity. This is, to first order, equivalent to the more marine sediment cores, which is not the case for HE 3. In sophisticatedWright and Stocker approach[Stocker et al., contrast to HE 1 and HE 2, it occurs at a time of nearly 1992; Wright and Stocker, 1992]. The SouthernOcean needs constantice volume and nearly constantinsolation forcing: specialattention. Since there are no eastor west boundariesin The climatic variations associated with HE 4 are therefore not this part of the ocean,at least in the top 2000 or 3000 m, there linked to the Milankovitch theory and glaciationcycles but can be no zonal pressure gradient, and therefore the result solely from internal (ice sheetor oceanic)variability. geostrophicmeridional velocity must vanish. In the Wright The reconstructedtemperatures during the event are 0øC to and Stocker approach,this is taken into accountby setting 4øC cooler than they were just before the event, while the their "closure"parameter to zero in the top 3000 m between salinitiesare similarly loweredby 0%oto 2 or 3%o.The HE 4 55øS and 65øS. Equivalently, in our case,we simply set the sea surface reconstruction extends from 40øN to 60øN in the Rayleighcoefficient r to infinity in this samepart of the ocean. North Atlantic, which encompassesthe areawhere the iceberg For convection, we use an enhanced vertical diffusion instead dischargeoccurs. The largestvariations are observedin the of a more classical convective adjustment scheme. This iceberg melting latitudinal band, between 40øN and 50øN, improves notably the numerical behavior of the model while sites above 50øN did not experience significant [Schmidtand Mysak, 1996]. temperature and salinity changes. Most importantly, this We impose restoring surface boundary conditions for reconstructionreveals a coherentspatial pattern for SST and temperatureand salinity usingthe relaxationtime constantsZr SSS changesand gives us confidenceto use them as an input and Zs and the observedsurface temperatures and salinities. of an oceanic model. The numerical implementation uses standard integration Thougha more global data set would be neededto perform techniques.The modelhas 12 vertical layersdown to 5000 m, precisethree-dimensional oceanic modeling experiments,the the top layer being 100 m deep. There are 21 latitude bands. availabledata can still be usedwith a simplified oceanmodel The model equations are given in the appendix, and the in order to obtain the main characteristics of the ocean numericalvalues for the parametersare listedin Table 1. circulationat this time. The simplifiedtwo-dimensional ocean In order to assessthe ability of the model to reproducethe model that we are using will also permit a larger numberof main features of the thermohaline circulation, we first experimentsin orderto assessthe robustnessof our results. performed an experimentwith present-daysurface boundary conditions. We used the Levitus [1982] temperature and 2. Model Description and Control Experiment salinity fields, averagedzonally over the Atlantic Ocean, as the prescribedsurface forcing for our control experiment.A The Atlantic and SouthernOceans have a prevalent role. crucial choice for such a two-dimensional model is the The relative amountof deep water formed in the southand in definition of the averagingarea in the deep water formation the north determinesthe characteristicsof the deep waters in regions. Indeed, if we choose to average zonally the Arctic the rest of the world ocean. More importantly, the heat Oceansurface conditions, the resultingprescribed salinities in transportednorthward in the Atlantic is directly linked to the the high northernlatitudes of the model will be much too low, intensityof North Atlantic Deep Water (NADW). Though the becauseof the low salinities in the Bering Strait area of the global oceaniccirculation may have been entirely different in Arctic Basin. In sucha situation,the model cannot form deep the remote past, there is no evidence in the late Quaternary waters in the North Atlantic. It makes much more sense, from that deepwater formationoccurred anywhere but in the North an oceanographicpoint of view, to use only the Norwegian Atlantic and around Antarctica, just as nowadays. Sea sector of the Arctic Basin as our prescribedboundary Furthermore, the reconstructionsof past temperaturesand salinities,during the Last Glacial Maximum as well as during Table 1. Numerical Values of the Model HE 4, are concentratedin the Atlantic basin. It seemstherefore naturalto concentrateour modelingonly on the Atlantic sector Parameters of the global ocean. Our simplified two-dimensionalocean Parameter Value Unit model is a sectorextending from 80øS to 90øN and 70ø wide -2 inlongitude. Thebottom isflat at 5000 m depth. Nmln2 10'7 s days -I investigateTwo-dim.•n•icm•!the stabilityoco.•n modelsof the havethermohaline been usedcirculation recently to •rc 1300. s [HovineandFichefet, 1994; Marotzke etal., 1988; Sakai and Kv 10'4 m2.s-1 Peltier,1997; Stocker et al., 1992;Thual and McWilliams, K, 100 m2.s-1 1992;Winton, 1997; Wright and Stocker, 1992] and also to •T 30 days reconstructthethermohaline circulation during the last glacial zs 100 days 718 PAILLARD AND CORTIJO' ATLANTIC MERIDIONAL CIRCULATION

80øS 60oS 30oS 0ON 30ON 60ON 90ON 80ø'S'6•øS''' :•0•'S ' ' ' 0ø'N.... •3•ø'N''' 66ø'N''' •6øN Latitude Latitude

80øS 60os 30os 0ON 30ON 60ON 90ON 80'oS' 6(•o s --30:S . . 0o,N.... 36O.N-..66O'N-'' 90ON Latitude Latitude Figure1. (top) temperature and(bottom) salinity annual mean fields in the model for the control run (on the right) compared tothe Levitus [1982]climatology (onthe left). Contours areevery IøC for temperature andevery 0.29'oo forsalinity.

conditions.In this area (north of 60øN),we therefore used only the 60øW-60øE sector of the ocean. 3. PastSST and SSSReconstruction Accuracy Similarly,Antarctic Bottom Waters (AABW) are formedin Sincewe will investigatein the followingthe response of extremelynarrow regions,on the east side of the Antarctic thismodel to paleoclimaticforcings, it is importantto examine peninsulain the westpart of the WeddellSea, and also in the the accuracyof the availabledata. SST are obtained usually west part of the Ross Sea. Using a global averageof from faunalassemblages, and the standardstatistical estimates temperaturesand salinitiesall aroundAntarctica completely givean errorof about1 o to 2øC[Prell 1985].The sea surface overrepresentsregions were no deep water formation is salinities(SSS) are estimatedusing the isotopiccomposition possible.We thereforemade our zonal averaging in onlya 10ø of plankticforaminifera. A carefulerror analysis shows that longitudeband around the deepwater formation areas. It must themain sources of uncertaintyfor theseSSS estimates are the be emphasizedthat thesesomewhat arbitrary choices are SSTvalues and, to a smallerextent, the slopeof the $]80- crucialin determiningthe exactbalance between NADW and salinityregression [Cortijo et al., 1997;Schmidt, 1999]. In our AABW. Thismisrepresentation of the deepwater formation study,we use two extreme cases for the $]80-salinity slope, regionsis one of the major limitation of two-dimensional andthe errorsarise mainly from the SST errors.Indeed, a 2øC oceanic models. error on SST may eventually lead to a 196oerror on SSS, Using the wind stressfrom the Hellerman and Rosenstein which is very important. However, it is then crucial to [1983] climatology,we haveintegrated the modelfrom rest understandhow this may affect density, since the for at least10,000 years, up to the equilibrium.The seasonal thermohalinecirculation is drivenby densitygradients, not cycle is explicitlyresolved, which is crucialfor a good directlyby temperature or salinity gradients. The error analysis representationof the high-latitude convection areas. The musttherefore go beyonda simpleexamination of SST and resultingannual mean temperature and salinityfields are SSSerrors. In particular,these errors are not independentand comparedto the climatologyin Figure1. We notethe good cannotbe addedlike randomerrors. On the contrary,they agreement between the modeled SST and SSS fields and the mostlycancel out when computingdensity. Indeed, the two Levitus [1982] climatology. The annual mean overall statisticallyindependant estimates are the $]80 of calciteand meridionalstreamfunction is shown in Figure2a: TheNADW thetemperature. If we usea $]80-Sslope of 2, weget the andAABW strengthsare alsoin reasonableagreement with followingdifferential: currentestimates [Macdonald and Wunsch,1996], as aretheir d•18Ocaco3= d•518Omo - 0.25 dT= 0.5dS - 0.25dT. respective depths and extensions.The low-latitude wind- Thedifferential of densityis givenby dp= [3dS - (xdT, with drivencells, in the upperthermocline, are superimposedon (xand [3 being the thermal and haline expansion coefficients. this general thermohaline circulation. The "no east-west This gives boundaryregion" in theSouthern Ocean is alsodriven by the dp=2 [3d•18Ocaco 3+ (0.5 [• - (g)dT. upperwind stress,and the inducedoverturning contributes With[3 = 0.8 (%o-density/%o-salinity),a = 0.3 (%o-density/øC) significantlyto the interhemisphericwater massexchange for warmwaters, and ct = 0.1 (%o/øC)for coldwaters, we get [Toggweilerand Samuels, 1993]. Op/0T = 0.5 [3 - (x= 0.1%o/øCfor warmwaters and 0p/OT PAILLARD AND CORTIJO: ATLANTIC MERIDIONAL CIRCULATION 719

0o/•.... 30•N 60•øN 90•ON

II I1• , '"'"",1/ ' 2 Illl•lllC•l/ __.•/•K. I

5 80o:•' ' b0o'S.... 30o•.... 0oi•1.... 3o•,N' ' ' 6ogN ' ' ' 90'ON80o• ' ' 60'oS.... 30o•.... 0Ol•l.... 30•oN' ' ' bO'•N' ' ' 90'oN

Latitude Latitude Figure2. (a) Streamfunction obtained in thecontrol run (contours every 2 Sv).The maximum of theNorth Atlantic Deep Water (NADW) overturningis 16.9 Sv at about 60øN, while the maximum of theAntarctic Bottom Water (AABW) is 13.2Sv at 70øS.(b) The Last Glacial Maximum(LGM) experiment. The maximum of theNADW (or Glacial North Atlantic Intermediate Water (GNAIW)) overturning is20.2 Svat about40øN, while the maximum of theAABW is 20.8Sv always at 70øS.(c) Temperatureand (d) salinityannual mean fields in the modelfor the LGM run (contoursevery IøC and0.2%0, respectively).

--0.3%o/øC for cold waters. This means that for a 2øC error on estimatedby CLIMAP [1981] for the temperaturesand by the SST estimates,the correspondingdensity error will be Duplessyet al. [ 1991] and Wanget al. [ 1995] for the salinities. between -- 0.2%0 and -- 0.6%0. This is considerablydifferent In the SouthernOcean, where no salinity reconstructionis yet from the independentaccumulation of a 1%oerror on salinity available,we useda SSS anomalyequal to the global shift of with a 2øC error on temperature. In particular, if CLIMAP +1.1%o, resulting from the about 120 m overall sea level [1981] is wrong at low latitudes,this has only a minor effect lowering. Since the overall surfacewind patternsremain the on the estimationof the densityof seawater,since it depends same during the LGM and since the influence of the wind almostentirely on the estimation 8•80 of calcitealone, as also stressis mostly limited to the upper thermocline ocean, we noticedby Lynch-Stieglitzet al. [1999].If we usea 8•80- think that, in contrastto SST and SSS changes,wind stress salinity slope of 1, the situation is even more favorable. changeswill not affect significantlythe generalthermohaline Indeed, we get 3o/3T = 0.25 [3- (• ---0.1%o/øC for warm structure.We are thereforeusing the present-daywind stress watersand 3o/3T -- +0.1%o/øCfor coldwaters. For a 2øC error climatology as a surfaceboundary condition for the glacial on the SST estimates,the correspondingdensity error will ocean.The model is then integratedfor severalthousand years, alwaysremain below 0.2%o. up to the equilibrium.The resultingoceanic deep circulation is If the errors on temperatureand salinity reconstructions presentedin Figure 2b, while the correspondingtemperatures may be quite large, up to 2øC and up to 1%o,respectively, in and salinitiesin the oceaninterior are shownin Figures2c and some cases, this translates into much smaller errors for the 2d. The circulationchanges in this experimentare essentially density of seawater,between 0.2%0and 0.6%0 in the worst located in the northern Atlantic. The deep water formation case. Since the thermohaline circulation results from large- occurs at lower latitudes, around 40ø-50øN instead of 60 ø- scale density gradients, and since the spatial coverage 70øN in the control run. The resultingNADW flow is also provided by the paleodata shows coherent large-scale much shallower and remains above 3000 m, and therefore anomalies in SST and SSS, we believe it does make sense to owes the new name of Glacial North Atlantic Intermediate use paleotemperatureand paleosalinity reconstructionsas Waters (GNAIW) [Duplessyet al., 1988]. The Atlantic deep input of an oceanmodel to estimatethe impact of sea surface waters are therefore coming mainly from the south, even changeson the deepocean circulation. thoughthe AABW flow remainsalmost unchanged at about2 to3 Sv. The ocean bottom waterscool by about 1øC as a result of the greaterextent of the cold AABW contribution.For the 4. The Last Glacial Maximum Experiment same reason, the bottom waters are also fresher by about In order to investigatethe role of temperatureand salinity 0.1%o,in additionto the global +1.1%oshift. Conversely,since changeson the oceanic circulation during the Last Glacial the North Atlantic waters are sinking at lower latitudes, the Maximum (LGM), we performed an experiment where the temperaturesaround 2000 m are slightly warmer in the LGM SST and SSS fields are relaxed to their LGM values, as experimentthan duringthe controlrun. 720 PAILLARD AND CORTIJO: ATLANTIC MERIDIONAL CIRCULATION

The patternof a shallowerand southward shifted NADW 0 (or GNAIW)flow is consistentwith •5•3Cand A•4C measurementson the shells of benthic foraminifera living at [ this time. Indeed, deep waters in the Atlantic Ocean are systematicallydepleted in •4C and enriched in •3C during the • g-3 LGM, suggestinga reduceddeep ventilation. However, the E -4 intermediate waters, above 2500 m, exhibit the opposite -5 meltingarea changes[Duplessy et al., 1988],which indicates a possibly • 0 betterthan presentintermediate water ventilation. Previous E •-1 experimentswith a two-dimensionnal(2-D) oceanmodel • '•-2 .. forcedby reconstructedSST and SSS for theLGM [Fichefetet •[ "' ""-• : slopeS/1518 O= l al., 1994]have shown the samegeneral southward shift of the '• -4 I ....'"::.-•...-•-..-'i-- '[ - -,-- slope S/•5180 2 -5 deepwater formation area and the same shallower sinking of 35 40 45 50 55 60 65 thesewaters. However, in contrastto our results,the NADW Latitude (øN)

(or GNAIW) strengthwas significantlyweaker in the LGM Figure 3. Summaryof the Heinrich event4 (HE 4) surfacehydrological run than in the controlrun. This probablyresults from slight perturbation[from Cortijo et al., 1997]. The differencesin summer sea differences in model configurationnear the deep water surfacetemperature and surfacesalinity during HE 4 and beforethe event formationareas. In particular,their top level is only 50m deep are plotted here to emphasizethe cooling (between 3ø and 4øC) and the instead of 100m here. We believe 100 m is more freshening(between 1 and3960, depending on the choice of b•80-Sslope) in the icebergmelting area. representativeof what the foraminiferalassemblages are able to record in terms of anomalous SST and SSS. A coupled ocean-atmospheresimulation has also been performed for this time period [Ganopolskiet al., 1998], for theHE 4 experiments,the major limitation comes from the where a 2-D ocean was coupledto a simplified atmospheric very restricted area where such surface hydrological circulation model. In this simulation where LGM SST and reconstructionsare available. Indeed, the model needs a full SSS fields are not used as input, the correspondingNADW SST and SSS coverageof the Atlantic Ocean as boundary strengthis very similar to the controlrun. This result is quite conditions,whereas these fields are available only in the comparable to our results. The correspondingdepth and 40øN-60øN latitude band for time slicesaround HE 4. A major locationof the deep water formation are also very similar to difficultyin extendingthis datacoverage southward concerns our findings.In contrast,another LGM simulation,using a 3-D the precisedating and correlationof paleoclimaticproxies oceanmodel coupled to an energyand moisture balance model accross the whole Atlantic Ocean. If such a correlation is [Weaver et al., 1998], exhibits a much reduced and much possiblein the vicinityof the icebergmelting area, with the deeperNADW formation, without significantchanges in the clear identificationof the ice-rafted debrisdefining Heinrich location of the deep water formation area, togetherwith an layers,the problemis muchmore difficult for moresouthern almost complete wanishing of AABW, in contradictionto sediment cores. paleoceanographic data. However, in contrast to the We decidedto usethe availabledata together with the LGM Ganopolskiet al. model,water vaporis diffused,not advected, boundaryconditions in orderto run the model.The following in the atmosphere. This may lead to quite unrealistic HE 4 experimentsare thereforeperturbation experiments, precipitationpatterns and probablyhas somestrong influence around the LGM state, to changes in the hydrological on the resultingthermohaline circulation. propertiesof the surfaceocean near the icebergmelting area. Both the comparisonof the controlrun with the climatology The data are indeedavailable precisely in this area,where the and the quite reasonable results obtained for the LGM icebergsassociated with the Heinrich eventsare melting and experimentmake us confidentthat our simplifiedocean model depositingtheir debristo the sediment,but this latitudeband is is suitablefor simulatingthe stateof the Atlantic Oceanbefore also the area where the LGM North Atlantic Deep Water (or and duringa Heinrich event, usingthe availableSST and SSS GNAIW) forms. We can therefore experiment, with our data. simplified model, how the SST and SSS changesin this criticalarea affect the overallthermohaline circulation pattern. We have performedthree experiments.The first one uses 5. The Heinrich Event 4 Experiments the reconstructedSST and SSSjust beforethe Heinrichevent. The reconstructedSST and SSS anomalies[Cortijo et al., The situation during the event correspondsto the second 1997] are shown in Figure 3. The methodologyused to obtain experiment,assuming that the •5•80-salinity relationship is 1:1, these fields is strictly similar to the one used for the LGM. In whichassumes that the surfacesalinity decrease is due only to particular,it is possibleto reconstructboth winter and summer the meltwater from the ice sheets,in the same fashion as the SSTs and thus to have an idea of the seasonal cycle for global glacial-interglacialtransitions [Fairbanks, 1989]. The temperature.However, the reconstructedSSS anomaly fields third experimentis also for the HE 4 event,assuming that the are assumed to be annual mean values, and the salinity •5•80-salinityrelationship is 1:2. Thisis the present-day seasonalcycle remainstherefore identical to the present-day relationshipobserved for surfaceocean waters, which results one. This may representa significantlimitation of the method, from the evaporationand precipitationprocesses [Craig and since the high-latitude convection is mainly seasonal and Gordon, 1965]. For each of these experiments,the model is dependsstrongly on the surfacesalinity fields. Nevertheless, integratedfor 3000 years, up to the equilibrium. It is worth PAILLARD AND CORTIJO: ATLANTIC MERIDIONAL CIRCULATION 721

80• '•' ' -j0• --- a• - - - j&ff--•" •' •!•'1•- •- ' I

Lati•de Latitude Latitude Figure4. (top) Before HE 4. (middle) During HE 4, with •80-S slope = 1. (bottom) During HE 4, with fi•80-S slope = 2. (right) Stream functions(contours eveff 2 Sv).Maximum of clockwiseflow: (top) 21.5 Sv, (middle) 11.2 Sv, (bosom) 9.0 Sv. Maximum of •BW: (top)19.4 Sv, (middle) 31.6 Sv, (bosom) 33.3 Sv. (middle) Temperatures (contours eveff loC). (le•) Salinities(contours eveff 0.2%0).

noticingthat an Heinrichevent lasts probably much less than 6. Discussion 3000years, and these experiments cannot pretend to reproduce exactlywhat happens during HE 4. Nevertheless,after a few The collapseof the thermohalinecirculation in the North hundredyears of model integration,the North Atlantic Atlanticduring Heinrich event 4 obtainedin our numerical circulationis very closeto its equilibriumstate. The results experimentsis entirelyconsistent with the resultsof Vidalet shownhereafter are thereforerepresentative of a Heinrich al. [1997],which have demonstrated that the 8•3C of the eventwhich would last at leastseveral hundred years, which is benthicforaminifera, living in the deep sea at this time, was a reasonableassumption [Francois and Bacon, 1994]. The considerablydepleted during each Heinrich event, thus data usedfor the SST and SSS are actuallyaverages on a time suggestinga muchreduced deep water ventilation.Such a periodof theorder of 1000years. It is thereforequite sensible reduction is also expected in order to account for the to look at the equilibriumresponse of the oceanto SST and considerablecooling observed in ice core recordsat these SSSchanges. These results are shown in Figure4. times[Dansgaard et al., 1993].Indeed, as alreadymentioned Since the reconstructedtemperatures and salinitiesjust in section 1, the Atlantic Ocean transportsnorthward a before the event.are only sligthly different from their LGM considerableamount of heat. A reductionor a collapseof the values,this first experimentis a small perturbationof the thermohaline circulation is therefore probably the best LGM state obtained before. The results are therefore not explanationfor therecorded coolings in Greenland.We have significantlydifferent from the LGM ones.On the contrary, plotted,in Figure5a, the Atlanticnorthward heat fluxes the SST and SSS are considerablylowered during the event, obtainedfrom our experiments.As expected,the heattransport and the thermohalinecirculation obtained by the model is during HE 4 is much reduced.It is also interestingto note notablyaffected. The reductionof North AtlanticDeep Water from Figure5 that,though the LGM experimentoverturning is formation is suchthat there is, in our model, no more outflow strongerthan the controlone, the LGM oceanicheat transport of NADW from the North Atlantic to the south and, is much smaller. This is due mainly to the more southerly consequently,to other basins. Since the near-surface locationof deepwater formation during LGM, whichtherefore overturningis controlledin a largepart by the wind stress,the considerably reduces the efficiency of meridional heat resultspresented here suggesta nearlycomplete collapse of transport.This heat transportis somewhatlarger before HE 4 the circulationduring Heinrich event 4. This collapseis even than during LGM, which is also consistentwith the higher more evident when the 1:2 slope is used for salinity temperaturesrecorded during marine isotopic stage 3. The reconstructions.Since the deepwaters are now fueledonly by interhemisphericheat transportis northward for the control, the southernflow, the deepNorth Atlanticis evencooler and the LGM, and the "before HE 4" experiments, while it fresherthan it was in the LGM experiment. becomes southward for both HE 4 ones. This is consistent 722 PAILLARD AND CORTIJO: ATLANTIC MERIDIONAL CIRCULATION

1.2 also smaller during the Heinrich event (Figure 5b), which ..-- Control seemsto contradictthe considerablefreshening recording by • LGM 0.s _ . Before H4 the SSS data and causedby the iceberg discharge.First, there During H4 (slopel) -. - During H4 (slope 2) is no reason to think that, during an Heinrich event, the 0.4 meltwaterflux at high latitudeis compensatedby evaporation at lower latitude. In other words, during a real Heinrich event, 0 . oO the water budgetof the oceanis probablynot equilibrated.The equilibrium freshwater fluxes shown in Figure 5b have ß A therefore a quite limited relevance to the Heinrich event situation.Nevertheless, it is interestingto note that, when the -0.8 • I • I I I '. ', -80 -60 -40 -20 0 20 40 60 80 Atlantic thermohalinecirculation is greatly reduced,or even Latitude (øN) collapsed,the freshwaterflux at high latitude required to maintainthis stateis quitelow. This probablyresults from the 0.8 Precipitation or hysteresis phenomena associated with the thermohaline eberg melting ß Control multiple equilibria [Rahmstorf,1995; Stommel, 1961]: Once 0.6 ["] LGM [] Before H4 the advectivecirculation is shutdown by a freshwaterinput, it 0.4 •lDuring H4 (slope1) can stay down even when the freshwaterinput vanishes.Of [] During H4 (slope2) 0.2 course, the stability of the equilibrium states found in our studyneeds to be investigatedwith a morecomplex, coupled 0ø-40øN 0 ß ! ocean-atmospheremodel, and, ultimately, the understanding : . 40o.90ON 40 ø.50øN -0.2 of the physicsof Heinrichevents requires a detailedmodelling of the ocean,atmosphere, and ice sheetfreshwater budgets. -0.4

-0.6 7. Conclusion Evaporation -0.8 We have performed numerical experiments, with a Figure 5. (a) The northwardoceanic heat transportfor the five numerical simplified two-dimensionnalocean model, in order to better experiments.The maximum Northern Hemisphere values are, for the understandthe physicalconsequences, on the deep ocean control run, 1.04 PW; for the LGM, 0.65 PW; before HE 4, 0.87 PW; circulation,of the recordedNorth Atlanticsurface cooling and duringHE 4 (slope= 1), 0.43 PW; and during HE 4 (slope= 2), 0.20 PW. fresheningduring a Heinrichevent. In contrastto previous (b) The implied surface freshwater fluxes for different latitude bands: between 0 ø and 40øN, where evaporationdominates; between 40 ø and studies,we madeno assumptionabout freshwater input. The 90øN, where precipitation dominates;and the iceberg meltwater band main limitation of our experimentscomes from the limited between40 ø and 50øN. In this last area, the freshwaterfluxes requiredto availability of detailedquantitative sea surfacetemperature maintainthe equilibriumare not largerduring HE 4 than they were before and salinity reconstructionsfor such critical time periods. HE 4. Assuming that the ocean state outside the North Atlantic is similar to the LGM situation, we have shown that the reconstructedSST and SSS for the HE 4 time period are with the warming observedin Antarctica during the cold stronglysuggesting a nearly completecollapse of the North phasesof the northernNorth Atlantic associated with Heinrich Atlantic thermohaline circulation. This result is consistent with events[Blunier et al., 1998]. recordsof the deepsea ventilation during Heinrich events, like We must nevertheless keep in mind that our HE 4 the$•3C of benthicforaminifera. Such a collapsehas been experimentsare actuallyperturbation experiments around the predicted by many authors [Manabe and Stouffer, 1993; LGM state of the Atlantic Ocean, since the coverageof SST Rahmstorfand Ganopolski, 1998; Stocker and Schmitther, and SSS reconstructionsis limited to the high latitudesof the 1997] in the contextof future, greenhouse-induced,global Atlantic.It may be possiblethat SST and SSS in the equatorial warming. It would therefore be of great value to better and south Atlantic are, before and during a Heinrich event, constrainthe thresholdson thermaland haline forcing, beyond different from their LGM values. Nevertheless, if the deep which such a collapse will inevitably occurs. A detailed water formation in the North Atlantic before a Heinrich event reconstruction of the climatic conditions around a Heinrich is in the 40ø-50øN latitude band as is the case at LGM, then event like HE 4 couldbe extremelyhelpful in assessingthe the decreasein salinityreconstructed from the 5•SO oddsof suchan eventuality. measurementsduring HE 4 is such that the collapseof the thermohalinecirculation is probablyinevitable, as suggested Appendix: Model Equations by our model. With our oceanmodel, it is also quite straightforwardto In the realocean, the maindynamical balance is essentially computethe surfacefreshwater fluxes required to maintainthe the geostrophicone: The Coriolisforce balances the pressure equilibrium.These fluxes are plottedin Figure5b for the low gradientone: and high latitudesof the North Atlantic. We note that the 1 3p evaporationat low latitudesand precipitation at high latitudes f0sin0 v-- • • + (higher- orderterms), (1) is reducedduring the LGM comparedto the controlrun, which P0Rcos032 is expectedfor a climatemuch cooler and therefore drier than where f0 is-the Coriolis parameter,0 is the latitude, v is the meridionalvelocity, P0 is the mean seawaterdensity, R is the _ today.More surprisingly,these required freshwater fluxes are PAILLARDAND CORTIJO'ATLANTIC MERIDIONAL CIRCULATION 723 radiusof theEarth,/T is thelongitude, and p is he pressure.In --+V--• + w-- = tcH cos0 a two-dimensionalmodel, it is impossible to represent 3t R 30 o•z R2cos 0 c) 0 explicitlythe zonal gradients. We need a newequation that relatesthe meridionalvelocity v to the merionalpressure c)2S gradient/)p/a0.Two main different approaches have been used so far. The firstis neglectingthe rotation of the Earthand +(c + tc V)--+F o•z2 S(S) ' (4b) replacingthe geostrophicbalance with a dissipativeone where FT(T) and Fs(S) are the surfacetemperature and salinity [Marotzkeet al., 1988;Sakai and Peltier, 1997; Thual and forcings,null in the oceaninterior and equal to the following McWilliams,1992; Winton, 1997]. The alternative, introduced at the surface: by Wrightand Stocker[1991], is to assumethat the mean zonalpressure gradient/)p/aX isproportional to the meridional FT(T)= (T-TR), Fs(S)= -- SR , one,/)p/a0.We areusing the Winton [1997] parameterization, r T r s whichis, to first order,equivalent to the moresophisticated in which ZT and Zs are the relaxation time constantsand TR and Wrightand Stocker approach. The model equations are: SRare the observedsurface temperatures and salinities. In equations(4a) and (4b), •CHand •cv are the horizontaland -1 cgp Fw vertical eddy diffusivity. The variable c is the contributionof v - + , (2) vertical convective mixing to vertical diffusivity. Following rP0Rc•0 f0sin0 [Schmidtand Mysak, 1996], we usethe formulabelow for c: wherer is the ad hoc Rayleigh damping coefficient (ins '•) and I 1-tanh( whereFw is a forceapplied only in thetop level of themodel, c + tcv = tcv , (6) to accountfor the wind forcing: whereN is the Brunt-V•iis•il•ifrequency: r W (3) N2= __•_g 0/9 PO•E ß (7) P0 3z in whichZw is thezonal wind stress and 8 E is thethickness of N min is the marginaly stable value of N, and •c is the thetop layer of themodel. enhancedvertical diffusivity in the fhlly convectiveregime. Sincethere are no eastor westboundaries in the Southern This lastparameter can alsobe written 2 Ocean,at leastin the top2000 or 3000m, therecan be no Az zonalpressure gradient, and thereforethe geostrophicK C = max , (8) meridionalvelocity must vanish. This is taken into account by r C settingthe Rayleigh coefficient r to infinity.The vertical where Azmax is the largest ve•ical grid spacingand Zc is a velocityw is obtainedfrom the mass balance equation, the characteristic time for ve•ical convection. pressurep is given by the hydrostatic equation, and the density The numerical implementationuses standardintegration p iscomputed asa functionofT (inCelsius) and S (inpermil) techniques.The modelhas 12 ve•ical layers.The baseof each usinga third-orderpolynomial approximation [Cox et al., layer lies at the following depths: 100, 300, 600, 1000, 1500, 1970].We use the rigid lid approximation atthe surface of the 2000, 2500, 3000, 3500, 4000, 4500, and 5000 m. There are model. 21 latitudebands positioned between the following latitudes: The evolutionequations for thetemperature and salinity 80øS, 70øS, 65øS, 60øS, 55øS, 50øS, 40øS, 30øS, 20øS, 10øS, fields are 5øS, 0øN, 5øN, 10øN, 20øN, 30øN, 40øN, 50øN, 60øN, 70øN, 80øN, and 90øN. The numerical values for the different parametersare givenin Table 1.

Acknowledgments.Basic support from CEA and CNRS to the LSCE and from PNEDC and the EU Environmentprogram ENV4- CT97-0643 is acknowledged.This paper greatly benefitedfrom the commentsof J.C. Duplessyand three anonymousreviewers. Part of • + F ,(T) (4a) this work was performedwhen D. Paillard was visiting NCAR with +(c+tCV) ø•2Toqz2 •I ' supportfrom NATO. This is LSCE contribution305A.

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