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Transcript
Page 1: 4. CLIMATIC RESPONSEenvsci.rutgers.edu/~broccoli/reprints/Manabe+Broccoli_Ann_Glaciol_1984.pdfProject Members (1981) and will hereafter be identi-fied as the ice-sheet experiment.

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Annals of Glaciology 5 1984@ International Glaciological Society

ICE-AGE CLIMATE AND C()NTlr~ENTAL ICE SHEETS:

SOME EXPERIMENTS WITbl A GI:NERAL (:::IRCULATION

MODEL

by

S. Manabe and A. J. Broccoli

(Geophysical Fluid Dynamics Laboratory/NOAA, F'rinceton Univl~rsity, P.O. Box 308Princeton, New Jersey 08542, U.S.A.)

the cltmosphere, (3) higher surface albedo of morearid continents, and (4) differences in the orbitalparameters of thE! I:arth.

'~ny studies have suggested that the temporalvariation of the orbit,ll parameters of the Earth isresponsible for t.he transition between nonglacial andglacial climates. HOtiever, the values of the orbitalparameters at the peak of the last major glaciationapproximately 18 ka 8P are similar to the modemvaluE~s. Therefore!, it is not likely that they wereresponsible for maintaining the cold ice-age climate,even though they may h,lve triggered it. On the otherhand, it is probable that the reflection of a largefraction o~ insolation by continental ice sheets wasimportant in mairltaining an ice-age cl imate. Primarilyfor this reason, the climatic influence of continentalice !;heets is chosen a~; the subject for the presentstudy.

To investigate this influence, a coupled atmos-pherle/mixed 1 ayer ocean model developed by Manabe andStouffer (1980) is used. In this model, sea surfacetemperature (SST) is a predicted quantity, so theinfluence of continental ice sheets not only on theatmospheric circlJlation but also on SST can be inves-tigated. For this reason, the study is fundamentallydifferent from thclse of Gates (I976[a],[b]), Manabeand Hahn (1977), and Kutzbach and Guetter (1984),which used ice-age; SSTs reconstructed by CLIf0i4P Pro-

ATMOSPHERE.-.

ABST RACTThe climatic influence of the land ice which

existed 18 ka BP is investigated using a. climatemodel developed at the Geophysical Fluid DynamicsLaboratory of the National Dceanic and AtmosphericAanin;stration. The model cons;sts of an atmosphericgeneral circulation model coupled w;th a stat;cm;xed layer ocean model. Simulated climates are ob-tained from each of two versions of the model: onewith the land-ice distribution of the present andthe other with that of 18 ka BP.

In the northern hemisphere, the difference in thedistribution of sea surface temperature (SST) betweenthe two experiments resembles the difference betweenthe SST at 18 ka BP and at present as est;mated byCLI~P Project Members (1981). In the northern hemi-sphere a substant;al lower;ng of air temperature alsooccurs in winter, with a less pronounced coolingdur;ng summer. The mid-tropospheric flow field is;nf1uenced by the Laurentide ice sheet and features asplit jet stream straddling the ice sheet and a longwave trough along the east coast of North America.In the southern hemisphere of 18 ka BP, the ice sheethas little influence on temperature. An exam;nationof hem;spher;c heat balances ind;cates that this isbecause only a small change ;n interhem;spheric heattransport ex;sts, as the in situ radiative compensa-tion in the northern hemisphere counterbalances theeffective ref1ect;on of solar radiation by continentalice sheets.

Hydrologic changes in the model climate are alsofound, with stat;stically significant decreases insoil mo;sture occurr;ng ;n a zone located to thesouth of the ;ce sheets ;n North Amer;ca and Euras;a.These find;ngs are consistent with some geologicalev;dence of reg;onal1y drier climates from the lastglac;al maximum.

CONTI .

EQUA~~~~R IWATE~~~~.

fHERMOOYNAMICEQUATION

RAOIATION

1.. INTRODUCTIONOne of the main objectives of this study is to

investigate hC1tl the cold climate of an ice age ismaintained. In particular, this study attempts toidentify the physical factors which are responsiblefor making the climate of the last major ice age muchcolder than the modern climate. Such physical factorsmay include (1) the existence of massive continentalice sheets which reflect a large fraction of inso-lation, (2) lower concentration of carbon dioxide in

""""

._I ""... "xI HY~ROLOGY !HE~T~~ I SEA ICE--r;;EA~

CONTINENT MIXEO LAYER OCEAN

Fig.I. Box diagram illustrating the basic structureof the coupled atmo~;phere/mixed layer ocean cl imatemodel.

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Ml~be and B7'OccoU: GCM tO1' ice-age ctinnte and ice sheets

ject Members (1981) as a prescribed lower boundarycondition for their models.

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2. /o{)OEL STRUCTUREAs the box diagram of Figure 1 indicates, the

mathematical model used for this study consists ofthree basic units: (1) a general circulation model ofthe atmosphere, (2) a heat and water balance modelover the continents, and (3) a simple model of theoceanic mixed layer. A brief description of thesethree units follows. A more detailed description ofthis atmosphere/mixed layer ocean model can be foundin Manabe and Stouffer (1980)..

The atmospheric general circulation model computesthe rates of change with time of the vertical com-ponent of vorticity, horizontal divergence, tempera-ture, moisture, and surface pressure using the so-called spectral method. The dynamical component ofthis model is developed by Gordon and Stern: (1982).Manabe and others (1979) and Manabe and Hahn (1981)discuss the structure and performance of thisatmospheric model in detail.

Over the continents, the assumption of zero sur-face heat storage is used to determine surfacetemperatures from energy flu~es at the surface. Snowis allowed to accumulate on the surface, with thechange in snow depth predicted as the net contri-bution from snowfall, sublimation, and snow-me't.A higher surface albedo is used when snow is present.Also used is a water balance model which co,mputeschanges in soil moisture from the rates of rainfall,

COMPUTED

e'iaporat i on, snOllI-me 1 t and run-off. Further deta i1 sof the hydro1 ogic: computat~ ions can be found inM!nabe (1969).

The oceanic mixed-layer model consists of avlertical1y isothE!rmal layer of static water of un i-f'~rm depth. This model includes the effects ofolceanic evaporation and the' heat capacity of theo.ceanic mixed layer, but neglects the effects ofh'~rizonta1 heat transport b,y ocean currents and ofthe heat exchange betlleen the mixed layer and thedeeper parts of the OCI!an. Sea ice is predicted whentlr,e temperature of the mi):e'd 1a:ter falls below thefreezing point of sea-~/ater' (-2 C), and a highersurface albedo is used wher'e sea ice is present. Thedistribution and thickness of continental ice isprescribed at the star1: of an experiment and does notclhange during its cour~.e, t,ut ablation and accretionrates are computed.

r"onabe and Stouffer (1980) compared the geo-graphical distribution of climate from this modelwith observed climatic data. They found that, despitesome exceptions, the model succeeds in reproducingthe general characteri~.tics of the observed distri-bution of surface air 'temperature and precipitation.The success of the modl!l inl simulating the geograph-ical distribution of c1 imat~e and its seasonalvariation encouraged the authors to conduct thepresent study by use 01: th is model.

3. EXPERII-EIffAL DESIGNIn order to investigatE! the influence of contin-

ental ice sheets on the climate of an ice age, two

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Fig.2. February monthly mean 55T difference (degrees Kelvin, stippling indicatespositive difference). Top: diff~rence betoleen ice-sheet and stilndard experi-ments. Bottom: difference betolej!n 18 ka BP and present (as reconstructed byCLIMAP Project Members 1981). Areas covered by sea ice in the ice-sheet experi-ment and at 18 ka BP are indicated by black shading.

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MInabe and Broccoli.. GCM fo1' ice-age cZil7rIte and ice sheets

long-term integrations of the atmosphere/mixed layerocean model described in the preceding section areconducted. The first time integration, hereafteridentified as the standard experiment, assumes as aboundary condition the modern distribution of contin-ental ice. The second time integration assumes thedistribution of continental ice at the time of thelast glacial maximum as reconstructed by CLItoIAPProject Members (1981) and will hereafter be identi-fied as the ice-sheet experiment.

A difference in sea-level between the two experi-ments of 150 m is prescribed, consistent with theglacial lowering of sea-level as estimated by theClimate: Long-range Interpretation, Mapping and Pre-diction (CLItoIAP) project. Orbital parameters in bothexperiments are set at modern values, making the dis-tribution of insolation at the top of the atmospherewith latitude and season the same in both experiments.This simplification is reasonable, since the orbitalparameters at 18 ka BP are not very different fromthe modern values. The distribution of the surfacealbedos of snow- and ice-free areas is prescribed tobe the same in both experiments.

The initial condition for both time integrationsis a dry, isothermal atmosphere at rest coupled withan isothermal mixed layer ocean. In both cases, themodel is time-integrated for 20 seasonal cycles. Aquasi-equilibrium model climate is achieved after15 model years, and the subsequent five-year periodis used for analysis in each experiment.

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4. CLIMATIC RESPONSE4(a). Sea surface temperature---To illustrate the effect of continental ice onthe distribution of SST, Figure 2 is constructed. This

SEAICE

300

290

280

270

30 EQ90 60LATITUDE

Fig.3. Winter (DJF) zonal mean SST (degrees Kelvin)and sea-ice thickness (scale represents 5 m) forthe North Atlantic Ocean from the standard andice-sheet experiments.

figure shOlols, a:. an example, the geographical distri-butions of the ~;ST difference of the model mixed layerocean between thE! ice-sheet and standard experiments forFebruary. This I:an be compared to the distribution ofSST difference bE!tween 18 ka BP and the present asdetermined by CI_Iro\A.P Project Members (1981), which isadded to the lowE,r half of the figure.

In the northErn hemisphere, SSTs from the ice-sheet experimen't are sign ificantly lower than thecorresponding tj~lTIperatures in the standard experiment.The SST differences are most pronounced in the mid-latitudes of thl~ North Atlantic and the North Pacific,with the cool in!} over the Atlantic generally largerthan that over 'the Pacific. This is in good qual it-ative agreement Ijith the difference between thedistributions of SST now and in 18 ka BP obtainedby I:LIro\A.P Projec:t Members (1981).

In contrast, the :;ST differences in the southernhemisphere of the mod,~l are very small whereas theS~ .differenc~s as estimated by CLIl-'Ap'are of sig-n1f1cant magnltude. ,~~; most changes in the distri-bution of contirlental ice between the two experimentsare located in l;he northern hemisphere, this resultsuggests that t~le pre~;ence of an ice sheet in onehemisphere has relatil/ely 1 ittle ,influence on thedistribution of SST in the other hemisphere.

It is significant that, in winter, the SSTdifference is largest in the mid-latitudes of theNort.h Atlantic and North Pacific oceans. This is nearthe southern mar'gin 01' the ex tens ive sea ice wh ich ispre!;ent in the ice-shE!et experiment. One can appreciatethe reason for this location by consulting Figure 3whic:h shows the latitudinal distributions of zonalmean SST over the Norl;h Atlantic Ocean for winterobtained from bOtil experiments. As this figure indi-catE~s, SST always remclins at the freezing point(i.E!. -2.C) beneath SE!a ice and increases equatorwardof l;he sea-ice margin in both experiments. It is,therefore, reasonilbl e that the SST difference islar£lest in the neighborhood of the sea-ice marginin the ice-sheet I~xperiment.4(b). Atmospheric circ:ulation

[n-the preced~;ubsection, it is shown that theextensive sea ice whic:h forms in the North AtlanticOcea,n exerts a strong influence upon the distributionof S,ST in the ice'.sheE~t experiment. Some of the keyprocesses responsible for the formation of this seaice may be identilFied based upon the analysis of theatmospheric circulation obtained from the ice-sheetexpEriment.

Figure 4 contilins the December-January-February(DJF) map of the I}eopotential height at 500 mbarwhich is obtained from the ice-sheet experiment.From this map, onl~ carl infer the flCM field at500 mbar level of the model atmosphere with the aid

Fig.4. Winter (~F) 500 mbar geopotential height I:m) from the ice-sheet experiment.

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MImbe and B7'Occoli GCM j'O7' i~j~-age ~Zinnte and i~e sheets

Laurentide and the I;reenland ice sheets, eventuallyreaching the surface of the North Atlantic Ocean.

This is clearly ,illustrated in Figure 5 whichcontains the map of !;urface streaml ines for the DJFseason from the ice-!;heet experiment. By the timethis air mass reache!; the North Atlantic Ocean, itis extremely cold and is,responsible for theformation of the thi<:k sea ice over the model ocean.This is impl fed in tile upper portion of Figure 6,which illustrates tJhE! distribution of the difference

DJf 9O'N

of the geostrophic relationship. According to thisfigure, the mid-tropospheric flow field in the ice-sheet experiment is characterized by (1) the doublejet streams straddling the laurentide ice sheet and(2) a planetary wave trough along the east coast ofthe North A~rican continent. Along the southernbranch of the jet located near the southern boundaryof the ice sheet, cyclone waves propagate eastwardinducing intense snowfall. Under the northern jet,cold air flows around the northern periphery of the

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Fig.6. Difference in surface air temperature between the two expE!riments (degreesKelvin, stippling indicates positive difference). Top: DJF. Bo1:tom: JJA.

Page 5: 4. CLIMATIC RESPONSEenvsci.rutgers.edu/~broccoli/reprints/Manabe+Broccoli_Ann_Glaciol_1984.pdfProject Members (1981) and will hereafter be identi-fied as the ice-sheet experiment.

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l-hnabe and Broccoli: GCM fo1' ice-age cLimte and ice ,;heets

in surface air temperature between the ice sheet andthe standard experiments. As can be noted by com-paring this figure and Figurt! 2, the difference insurface air temperature betwt!en the two experimentsover the North Atlantic Ocean is much larger than thecorresponding difference in !;ea surface temperature<JtIing to the thermal insulation by sea ice. Theintense outf1<Jt1 of cold air described above partlyexplains why the difference in surface air tempera-ture over the North Atlantic Ocean between the twoexperiments is significantly larger than the corres-ponding difference over the North Pacific in winter.

Over the continents in the northern hemispherethe February difference in surface air temperaturebetween the two experiments is large, particularlynear southern continental ice margins where coldkatabatic winds blow southward from the massive icesheets. In summer (June-Ju1y-August (JJA», thedifference in surface air temperature over the con-tinents of the northern hemisphere is much smaller,as indicated in the l<Jt1er half of Figure 6. One caneven note isolated areas of positive differencewhere the ice-sheet experiment is simulated to bewarmer than the standard experiment. These positivedifferences usually occur in the areas where thearidity of the soil increase!; from the standard to theice-sheet experiments. This topic is the subject ofthe discussion in a subsequent subsection.

The smallness of the difference in summer tempera-ture described above is not consistent with the recentcompilation of paleoclimatic data from 18 ka BP byPeterson and others (1979), which indicates largerdifferences in temperature between 18 ka BP and thepresent during all seasons. It may be necessary toincl ude other factors such a!; the differences insurface albedo and atmospher1ic concentrations ofcarbon dioxide in climate models in order to repro-duce cold surface air temperatures in summer overthe continents in middle and high latitudes of thenorthern hemisphere.

Partly because of the war-m continental air des-cribed above, very rapid me11:ing is indicated atthe southern periphery of both the Scandinavian andLaurentide ice sheets. It is found, h<Jtlever, thatthe rate of ablation of an ice sheet dependscritically upon the specific values of some keyparameters such as the surface albedo of melting iceand the fraction of meltwater which becomes run-off.Further studies are required before one can deter-mine these parameters, and the factors which in-fluence them, reliably.4(c). Hemispheric heat balance-To evaTuate-tne-,ack-or-response in the southern

hemisphere to the presence of widespread continental

ice in the northE'rll hernisphere, the hemispheric heatbudgl?ts of the mcldel a1:mosphere are obtained from thestandard and ice-s'heet experiments. For the northernhemi!;phere, the net d~lnward flux of solar radiationat tile top of the' modell atmosphere in the ice-sheetexperiment is 5.6 ill m-'! 1 ess than the correspondingflux in the stanclard e)(periment. This is primarilydue 1;0 the reflection of insolation by the large areaof continental ice. ThE! difference in incoming solarradiation is essentially counterbalanced by a diff~r-ence of 5.8 W m-2 in outgoing terrestrial radiationat tile top of the lnodel atmosphere in the northernhemi!.phere. Althou'~ t/'le rate of interhemisphericheat exchange is also different in the two experi-ment!., the magnitu,je of the difference is only0.4 ~, m-2, and fs Inuch sma 11 er than the d iff erencesin ttle net incomin'~ solar radiation and net outgOi.'!,gterrE!S trial radiat Ion in the northern hemisphere.In ttle southern he'nisp~lere, there is 1 ittl e changein both the incomillg solar radiation and the out-goin~, terrestrial radiaition.

These results Indicate that, in the ice-sheetexperiment, the ef1'ectfve reflection of incomingsolar radiation reduces the surface and atmospherictempE'ratures fn thl? nor'thern hemisphere of themodel and, accordingly, the outgoing terrestrialradiation at the top of the atmosphere. Therelat.ively low surl'ace temperature in the northernhemisphere induces a sff~ll increase in the heatsuppl ied from the ~'armer atmosphere in the southernhemisphere. However, the radiative compensation inthe northern hemisphere is much more effectivethan the thermal adjustment through the inter-hemispheric heat e)tchange fn the model atmosphere.4(d). Hydrologic rE~se-rhe geographiC111-dlStribution of the percentagechange in annual mE!an soil moisture between the twoexperiments is sh~ln fn Figure 7. As discussed in thefigure caption, no contours are drawn in regions ofincrease in soil moisture. Large areas with areduction of soil nloisture in the ice-sheet experimentare evident south of the Laurentfde and Scandinavianice sheets. Tests for statistical significance of thedifference in soil moisture at each gridpoint, usingthe method of Chervin and Schneider (1976), indicatethat the differen':E!S fn soil moisture are signffi-cant in a broad b'el t south of the ice sheet in thewestern Soviet Union and a smaller area in the GreatPlains of North AmE~rica.

The reduction of soil moisture in these regionsis consistent with geological evidence of loess inboth areas. One of the models for loess deposition(Smalley 1972) sug!lests that the particles arecarried by the wind from the outwash of glacial sedi-

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Fig.7. Percentage change in annual mean soil moisture from tJ'Ie s1:andard to theice-sheet experiment. Areas covered by continental ice are blacked out;stippl ing indicates areas with an increase of soil moistLlre. (~iince thestatistical significance of the change in soil moisture is smalll in regionsof increase of soil moistuY1e, no contours arE! drawn in tl1ese rE!gions to,-~~ -,,- -,..,- .£ -'-- ", ,--,

Page 6: 4. CLIMATIC RESPONSEenvsci.rutgers.edu/~broccoli/reprints/Manabe+Broccoli_Ann_Glaciol_1984.pdfProject Members (1981) and will hereafter be identi-fied as the ice-sheet experiment.

MInabe and Broccoli GCM fo1' ice-age clil7rIte a1ld ice sheets

.

,II

J

less Ulan the CUrrE!nt concentration of 340 ppmv(Berner and others 1980, Delmas and others 1980,StauffE!r and others, 1984). If such a reduction didoccur, it could account 1'or much of the cooling ofthe southern hemis~lhere indicated in the CLI/o'APresults.

ACKNOWl.EDGEr-ENTSD Hahn played an important role in the design of

this experiment and )uperv1sed its execution.D Daniel managed the timE! integration of the numericalmodel carefully and efficiently. K B(:1IIman and N-C Laumade constructive colllnenl:s on th is work. J Kennedytyped the manuscript CheE!rful1y and swiftly, andJ Conner, P Tunison, W Ellis and M Zadl'iorny preparedthe final versions 01' thE! illustrations.

ments. Some have suggested that the existence ofloess is indicative of a dry (or at least seasonallydry) climate during the time of its deposition.

An examination of the CaUSE!S of the model re-duction in soil moisture indicates that a reductionof precipitation is a primary i'actor. In NorthAmerica, this reduction occurs primarily in summerand results from the small amount of moisturesuppl fed to the atmosphere by !;ubl imation from thecold Laurentide ice sheet. In Eurasia, similar pro-cesses also operate. In addition, the reduction inEurasia during the warm season is related to theshift of the middle-latitude precipitation beltfrom.a position several hundred kilometers south ofthe ice-sheet margin to a position along the ice-sheet margin. This shift is probably due to enhancedbaroclinicity in this region resulting from thelarge contrast in surface temperature across the ice-sheet margin. REFERENCES

Berner W, Oeschger H" Stauffer B 1980 Informationon the CO2 cycle from ice core studies. Radio-capbon 22( 2): 227 -;~35

Chervin R M, SchneidE~r S H 1976 On determining thestatistical significance of climate experiments withgeneral circulation models. JoUPnaL of the Atmos-phePic Sciences 33(3): 405-412

CLlKA.P Project Members 1981 Seasonal reconstructionsof the Earth's surf'ace at the last glacial maximum.GeoLogicaL Society of AmePica M1p and Chari SePiesMC-36

Delmas I~ J, Ascencio J-M, Legrand M 1980 Polar iceevidence that atmospheric CO2 20,000 yr BP was 50%of pr'~sent. MztU~1 284(5752): 155-157

Gates W L 1976[a) ~tJde1ing the ice age climate.Scien(}e 191: 1138..1144

Gates W L 1976[b) The numerical simulation of ice-age climate with il global general circulation model.JoUPnaL of the AtmosphePic Sciences 33: 1844-1873

Gordon C T, Stern W F 1982 A description of theGFDL global spectral model. MonthLy Weathep Revie~110: 625-644

Kutzbach J, Guetter P J 1984 Sensitivity of 1ate-glacial and Holocene c1 ilnates to the combined effectsof orbital parameter changes and lower boundary con-dition changes: "!inapsh'ot" simulations with a gen-eral circulation model for 18, 9, and 6 ka BP.AnnaLs of GLacioLo9Y 5: 85-87

Manabe S 1969 Climate and the ocean circulation.1. The atmospheric: circIJ1ation and hydrology of theEarth's surface. ~bnthLy Weathep Revie~ 97: 739-774

Manabe ~;, Hahn D G 1977 Simulation of the tropicalclimate of an ice age. JoUPnal of Geophysical Re-Seareh 82(27): 3889-3911

Manabe S, Hahn D G 1981 Simulation of atmosphericvariabi1 ity. Monthly W,gathep Revie~ 109: 2260-2286

Manabe 5" Stouffer R.J 1980 Sensitivity of a globalclimate model to an increase of CO2 concentrationin the atmosphere. JouPnal of Geophysical Researeh85(10): 5529-5554

Manabe S, Hahn D G, Ho11o~lay J L Jr 1979 Climatesimulations with GFDL spectral models of the atmos-phere: effect of spectrcl1 truncation. GARP Publica-tions SePies 22: 41-94

Peterson G M, Webb TIll, Kutzbach J E, van derHammen T, Wijmstra T A, Street F A 1979 Thecontinental record of environmental conditionsat 18,000 yr BP: an ini1tial evaluation. Quate1'-napY Researeh 12(1): 47..82

Smalley I J 1972 The in1:eraction of great riversand large deposits of primary loess. Tronsactionsof the Ne~ Yopk Acaiemy of S~iences Ser 2 34:

534-542Stauffer B, Hofer H, Oeschger H, Schwander J,

Siegenthaler U 1984 A1:mospheric CO2 concentrationduring the last gla,:iat~ion. AnnaZs of Glaciology5: 159-163

Suarez M J, Held I M 1982 The sensitivity of anannual mean diffusive energy balance model with anice sheet. JouPOOl of (;eophysical Researoh 87(C12):9667-9674

5. CONCLUSIONSIn this study, it was found that the distribution

of continental ice, as modeled for 18 ka BP, exertsa major influence on the climate of a coupledatmosphere/mixed layer ocean model. Large reductionsof SST in the northern hemisphere occur which arequalitatively similar to the SSTs for 18 ka BP whichwere reconstructed by CLIMAP Project Members (1981).A major reduction of surface air temperature in thenorthern hemisphere is also produced for the wiAterseason, but is not present in summer.

In addition, the Laurentide ice sheet stronglyinfluences the general circulation of the model atmos-phere, particularly in winter, when a double jetstream forms which straddles the ice sheet. Under thenorthern jet stream, an extremely cold air mass flowsalong the northern periphery of the ice sheet,eventually reaching the North Atlantic Ocean andforming thick sea ice. Along the southern jet stream,cyclone waves propagate eastward and are responsiblefor intense snowfall prevailing along the southernboundary of the ice sheet. The role of this intensesnowfall in the growth and maintenance of the icesheet is the subject of future investigation.

It is of interest that both the Laurentide andScandinavian ice sheets induce soil aridity in zonalbelts located just to the south of the ice sheets.In particular, the simulated dry belt located to thesouth of the Scandinavian ice sheet is pronounced dueto the poleward shift of the precipitation belt tothe southern boundary of the ice sheet. It is desir-able to perform further assessment of this result inthe light of a wide variety of geological evidence.

The results of this study also suggest that theeffects of the extent of the increased continentalice alone are insufficient to explain the glacialclimate of the southern hemisphere, although vari-ations in the Earth's orbital parameters may be!responsible for inducinq the large fluctuationS ! in the extent of ice sheets in the northern hemisp ere

during the Quaternary. Thus it is necessary to lookfor mechanisms other than the interhemispheric x-change of heat in the atmosphere in order to ex lainthe low temperature in the southern hemisphere. JThis is consistent with the result$ of Suarez a'1dHeld (1982) using a simple energy balance modelltostudy the astronomical theory of the ice ages. I

Among the potential mechanisms for the coolingof the southern hemisphere during glacial times isa change in the cross-equatorial heat transport bythe ocean circulation. Other processes which cancause an almost simultaneous change of temperatJrein both hemispheres are fluctuations in the con-centration of carbon dioxide or the loading ofaerosols in the atmosphere. Indeed, the results fromthe recent analysis of ice cores from the Antarcticand Greenland ice sheets suggest that the atmos-

1pheric concentration of CO2 during the last glacialmaximum was about 200 ppmv and is significantly I