Ice-sheet mass balance during the last glacial maximum

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Ice-sheet mass balance during the last glacial maximum

Gilles Ramstein, Adeline Fabre, Sophie Pinot, Catherine Ritz, Sylvie Joussaume

To cite this version:

Gilles Ramstein, Adeline Fabre, Sophie Pinot, Catherine Ritz, Sylvie Joussaume. Ice-sheet mass

balance during the last glacial maximum. Annals of Glaciology, International Glaciological Society,

1997, 25, pp.145-152. �10.3189/S026030550001394X�. �hal-03011923�

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. lllnals ofGlaciology 25 1997

(' Internati onal Glaciologica l Society

Ice-sheet tnass balance during the Last Glacial Maxitnutn

GILLES RAMSTEIN,i ADELINE FABRE,2 SOPHIE PINOT,I CATHERINE RITz/ SVLVIEjOUSSAUME¹

I

Labomloire de l \Jodilisalion du Climal el de l'Envirollnemml, CE

Sarl(~y

B allmenl 709, Dune des A Iellsiers, 91191 Cif-sur-l [elle Cede>:, Frallce 2 Laboraloire de Glaciologie el de Ceoplzysiqlle de l'Envirol1lle1l1frtL, Domaille { 'nil'ersilai re,

5-1

rile !l1olihe, HP. 96,

38-102

Sailll-,llarlill-d'Hhes Cede:c Frallce

ABSTRACT

In the framework of th e Pa leoelim ate Modelling Intercomp arison Pro- ject (PMIP ), simul a tions of the L ast Glacia l

~Iaxil1lum

(LGM ) have been performed,

~Iore

than 10 different atm ospheric ge nera l circulati on models

(AGC~Is)

have been used with th e sa me bound a r y conditions: sea-surf ace temperatures presc ribed by CLIMAP (1 981 ), ice-s hee t reconstruction provided b y Peltier (199 +), change in insola tion, a nd re- duced CO

₂

contenl. On c of the major questions is to im'estigate whether th e simulat ions of the LGM are in equ ilibrium with th e pr escr ibed ice-sheet reconstruction. To answer thi s question, wc have used two diff erent approaches. First, we a na lyze the res ults or a se t of LGM: simul ation s performed with diff e rent versions of the Labora toire de l\1etco rolo- gie D ynamiqu e (LMD ) AGCl\I a nd stud y the hydrologic a nd snow budgets o\"('r the L aurentide and F ennosea ndi an ice sheets. Second, wc use the AGCl\ [ output s to force an ice-sheet mod el in order to investigate its abilit y to maintain the ice sh eets as reconstructed by CLIMAP (1981) or Peltier (1 994-).

INTRODUCTION

Since th e fir st simul a tions of the L ast Glacial Max imum ( LGM), using atmospheric ge neral circul ation mod els (AGCl\ l s) (Williams a nd others, 1974; sce a lso Street-Perrot (1 991 ) for a review ), progress has been m ade both on the pa rameteri zation of physical processes a nd on th e setting of boundary co nditi ons. Th is last point is crucial beca use wc perform "snapshot " experiments a nd co mpute th e atmo- spheric res ponse in equilibrium with different boundary co nditio ns. Some of these co nditions a rc very accu ra tely defined, such as the insolation in the upper a tm osphere (Bcrger, 1 988), or atmospheri c composition (ma inl y CO

2

partial pressure ) (R aynaud a nd oth ers, 1 993). Oth ers, such as sea su rface temp erature (SST) or ice shects, ar e more dif - fi cult to reco nstruct. In th e 1 980s a nd a t the beginning of th e

1 990s, most simulation s of th e LGM used th e CLIMAP (1981) SSTs (Broccoli a nd Ma nabe, 1 987; Rind , 1 987; J ous- saume, 1 993), whereas in the more recent o nes

AGC~Is

arc co upled with a mi xe d-layer ocean and so the C LIl\IAP dataset is not used to prescrib e SSTs ( \ V ebb a nd others, 1997).

Of co urse, the problem is d iff erent for ice-shee t recon- stru ction.

~

[any AGCM exp eriments have st udied the sensi- ti\'it y of th e LGJ.I climate to th e ice- sheet reco nstruction (Shinn and Ban-on, 1 989; Ramstein and J o ussaum e, 1995).

Since 1 994, the Pelti cr reco nstruction has been availab le ( Peltier, 1 99+). In the framework of the Pa leoclimate Model- ing Interco mpa riso n Project (PMIP ) Uoussaume and Tay- 1 01', 1995), thi s reco n truction has been chose n as a co mmon boundary co ndition for a ll the models perform in g the LGl\I simulati on.

The a i m of this paper is to inves ti gate the ice-sheet ma ss balance through different \'C rsions of the LaboraLOire de l\feteorolog ie Dyna mique (LMD) AGCM. To achieve this, we used two different appro aches: the "direct" approach uses model o utputs m"('r the Laurenride and F e nnoscandia n ice

sheets to a nalyze ice-sheet mass balance; th e "coupl ed"

approac h uses model outputs ( mainl y temperat ure and pre- cipita tion ) to force an ice-sheet model (Ritz a nd others, 1996), th e physics and resolution of which are more refined, to investigate through a time-step simul at ion, whether ice sheets a r c mainta ined or melt (Fabre a nd others, 1 997).

METHODOLOGY Direct approach

Cs ing the Pelti er (19 9+) reconstruction, wc performed the LGM simulati o ns with three different "ersion s of th e

L~1D

AGC?\I: Ll\ID +,

L~ID+.3

a nd

L~ID5.3.

?\Ioreover for the Ll\fD+ \'Crsion, we constrained the LGl\f si mul ati on with tv,o different ice-shee t reconstructions:

CLI~IAP

(1 981 ) a nd Pelti er (1994). The impac t of the ice-shee t reconstruc- tion on its mass balance is weaker than the changes in model parameterization (sce belo\\·). Thus wc decided to use only the 1110re recent Peltier

(199-~)

reconstruction for th e experi- ments with Ll\ID+.3 and Ll\ID5.3. :\lo reove r, a ll these runs were performed using presc ribed SSTs deduced from CLIMAP (19 81 ). In Table I, wc li st a synth es is of the four different

LG~I

runs.

Table 1.

~Ylllhesis

qf Ihe LI ID fl eel/ simula lions qf Ihe Lasl Glacial JIj{lIlmllm

.1 loddI'mioll

L:\IO+

UdD+

UdD+.3 1,\1053

Hr.lO/lIlioll

f8 x 36 +8 x 36 -18 x 36 6-t x 50

/(£>-,1/'('('1 r(,(Ol1f/rllrtioll

eLl:\!.\!' 1981 Pcliicr 199+

Pcliier (199+) Pdtirr 199-1)

Downloaded from https://www.cambridge.org/core. 18 Dec 2020 at 11:40:54, subject to the Cambridge Core terms of use.

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Ramstein and others: fee-sheet mass balance during the LGlI f

Commonfeatll res and differences between the three Li\JD versions All th e num eri cal ex perim ents describ ed below havc been perf orm ed using the UvfD AGC M. This is a gridpoint model using a n Ara kawa C-grid (Sadourny and L aval, 1 984) . Th e radi ation scheme is th e sa me as tha t used in the Europea n Ce nter for l\Iedium-Ra nge ''\leather F orecas ts (ECMWF ) model: th e sola r part is a refined ve rsion of the scheme developed by Fouqua rt a nd Bonn el (1 980) a nd th e terr es trial co mponelll is fr om j \10rcrette (1991 ). The bound- ary layer is para meteri zed using a diffusive equ ati on in which th e mi xing coefficients depend on a presc ribe d length scale and a diagnos tic determin a tion of th e turbul ent ki- neti c energy. Co ndensation is parameteri zed sepa rately for convective a nd non- co nvec tive elo uds. Convecti on is pa ra- meteri zed using, in scq uenee, a moist adi abatic acUust ment and a modifi ed version ofth c Kuo (1965) algo rithm. A prog- nos tic equa tion for cloud water is included in th e model:

sources and sinks of cloud- condensed wa ter a re pa ra meter- ized and the la rge-sca le transport is ta ken into account.

U I/D4 and LMD4.3 imjnovemenls

T he main sink of clo ud wa ter is the prec ipitati on process, which uses different pa rameteriza tions in th c LMD 4 a nd LMD 4.3 versio ns (L e Treut a nd oth ers, 1 994).

(i) T he fir st a pproach, used in the bas ic LMD4 ve rsion of th e GCM , simply co nsists of presc ribing a thrcshold for the prccipita tion of cloud wa ter: the water in cxcess of thc threshold is precipitated to th e g round. It is necessa ry to di - tingui sh between warm a nd co ld clouds, a nd a lower thres h- old is used f or cloud s the top layer of which is below - 1 5°C.

This is a ve ry simple way to acco unt for th e Bergeron pro- cess, by which ice cl ouds precipit ate more efficiently. A vali- dati on of this fir st scheme in term s of cloud cover, using th e Intern ationa l Satellite Cloud C lim atology Project (ISCCP ) data over the Me teosat area, gave good res ults (L e Treut and Li , 1 988).

(ii ) Dcfi ciencics of the simpl e pa ra meterization des - crib ed above became obvious wh en compari sons with Sca n- nin g l\Iul tichannel Microwave R adiometer (SMMR ) estim ates showed tha t the integrated cloud water CO lllent was too low. In additi on, compa risons with Ea rth R adi ation Budget Experiment (ERBE ) data showed th at th e long- wave c loud forcing was to o la rge a t high lati tud es. The dis- co ntinuities illlrodu ccd by th e threshold approach also presc nted a se riou s probl em for further comp a riso ns with observati ons (Yu and others, 1 996) , as well as elimate sensi- ti vity studies (Li and L eTreut , 1 992). The LMD 4.3 version of the model was therefore develo ped. C loud precipit ation in the liquid ph ase is desc ribed using a f o rmula first proposed by Sundqvist (1 981 ). It a llows a tra nsiti on from a thin , non- precipit ating to a thi ck, strongly precipitating cloud regime th at is moot her th an with th e threshold me thod. Prec ipita- tion fr om ice clouds is treated through a simple ice-sedimen- ta tion mecha nism, in which th e prec ipitati on rate depends on th e termin a l velocit y of the fallin g crys ta ls. Fin ally, a lin- ear tra nsitio n betwee n ice a nd water clouds is also defined between O ° C and - 1 5°C. The fr ac ti on of a gi\·e n clo ud in the liquid phase decreases linea rl y fr om 1 to 0 betwee n these t wo temp era tures.

The water condensed in co n\"ec ti\"( cloud s is reta ined as cloud water, and precipita tes according to th e sam e parame- teri zati ons as th ose used in the str ati form case. This feature, which is not sha red by ma ny models, pa rtly expl a ins th e strong associatio n of convec tive o r strati form cloud s with

the intertropical cO Il\·erge nce zone. This is a notabl e fealUre of th e LMD GCM.

LiHD 4.3 and LN/D5.3 imjJrollemenls

The ma in new features o f the LYlD5.3 version a re the treat- menLs o f the surface and the surface- a tmos phere inter- ac tion. Thi s vers ion inc ludes a sub-grid representati on of fr acti ona l sea-ice cover. A distributi on between eight biomes represe nts, f or each ccll, the type of vegeta tion. The surface sc heme represenLin g the hydrologic exchanges betwee n a tmosphere a nd biosphere is SEC HIBA (Schcmatisa tion des E cha nges H ydriques a ^l' ^Interface ^Bi osphere Atm o- sphere; Duco udrc a nd Lava l, 1 993) . Albedo is comp uted fr om th e \·alu es of snow de pth, cover a nd age, and vegeta- tion cover (C halit a and Le Treu t, 1 994).

Resolution

We use two diff erent resolutions. F or LMD4 a nd LMD4 ·.3, we use the low resolution: 48 po ints regul a rly spaced in longitude, 36 points reg ul arl y spaced in sine of the la titude a nd 11 vertical leve ls. For the Ll\1D5.3 ve rsion, we use th e sta nda rd reso lutio n, which has a better hori zo nta l resolu- tion: 64 points in longitud e and 50 points in latitude. E ach horizont a l grid cell has a consta nt a rea. The \"C rti cal coordi- nate is the norma li zed pressure.

lee-slzeet

recoflSl rucl iOIl

A major difference be twee n prese nt a nd LGM clim ate is the large ice shee ts covering northwestern Europ e (Fenno- sca ndi a ) a nd the northern part of North America (L a uren- tide ) . The shape and size of th ese ice shee ts h ave a majo r impact o n the atm ospheric dynamics ove r the Northe rn He misph ere during the LGM, especia ll y co nce rning the split of th e j et th at may have occurred as a res ult ol"L a uren- tide ice shee t influence on pl aneta ry waves (Kutzbach a nd Gucuer, 1 986; Rind, 1 987) . De nton and Hugh es (1 981 ) pro- vid ed two reconstructions ref erenced as MIN a nd l\I IAX, co rres pond ing to eustatic sea -levcl rise \·alu es of 1 27 m an d 1 63 m. Peltier (1 994-) showed th at even the MIN reco nstruc- tion was excess ive a nd proposed a new reconstructi on base d on a gravi tationa ll y self-co nsistent theo ry of relative sea- level cha nges. It co rresponds

to

a n eustatic sea-l evel rise of 105 m. In Fi gure I, we show the c ha nge in elevation b etween the LG?vf a nd th e prese nt climate f or th e reconstructions de-

\·ised by Peltier (1 994) (for both resolutions) a nd C LIJ\IAP (onl y the low resolution is used ). The ex tent of the ice sheets is not very diff erent, but the elevation is 1 500 m hi gher over the L aurentide, a nd about 1000 m higher over Fennosca ndia in th e CLIMAP recon stru ction. Moreover, the small er sea- Ic\·cl rise in Peltier's reconstruction leads to a reducti on of emerged land s. \Ve take this into accoun t and use diff erent sea- la nd mas ks for the [WO simula tions.

Coupled approach: AGCM outputs driving an ice- sheet model

The ice-shee t model is a three-dimensional ( 3-D ) therm o- mechanicall y coupl ed model. It takes into acco unt the co u- pling be tween temperature a nd velocity fields, a nd belongs to the same catego ry of ice-sh eet model s as those developed by Huybrec hts a nd T'siobbel (1995) and Greve and Hutter (1 995) . The evolution of ice-shee t geometr y is a fun cti on of mass ba la nce, velocit y, temperature field s a nd bedrock posi- tion. The ice-shee t model (see Ritz and others, 1 996) for a comprehensive descripti on) was used successfull y on th e

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90

~~

60

""

³⁰

Cl

~

....

~ q,

«

CJ

-l ·30

-&l

·90

·180 -120 -&l 60 120 180

a

LONGITUDE

90

60

""

³⁰

Cl

~

....

~ q,

«

CJ

-l -30

...,

·90

·180 -120 -&l 60 120 180

b

^LONGITUDE

90

60

""

³⁰

Cl

~

....

E= «

-l -30

·60

·90

·180 -120 ·60 60 120 180

C LONGITUDE

Fig. 1. lee-sheet reconstructions and AGe \l grid JOT (a) CLfIlLiP (1 98/ ) ice shee/s illter/ Jola ted on th e 48

x

36 grid userlfor LAID 4, (b) Peltier

(/99-/-)

ice sheets inlelpolated on the -/-8

x

36 grid llsedJar LAID-/- and LAID-/-. 3, (c) Peltier (1 99-/-) ice sheets interpolated

077

the 6-/- x 50 grid llsedJor LlfD53

G ree nla nd ice shee t (F a bre a nd others, 1 995) a nd has since bee n extended

to

th e whole o f the :"Jorthern H emisphere for prese nt and LGf-.r climates (F abr e a nd oth ers, 1 997). Wc now describ e briefl y the m ass-bal a nce calcula tion.

The mass ba lance is the sum offour sepa r ately comput ed terms: acc umula tion, ablati on, ca king and melting a t th e bO llom of th e ice sheet. Acc umula tion is a key fac tor co m- puted from interp ola ted AGC f-.I temp eratures a nd precipi- ta tion. Abl ati on is co mputed usin g th e "positive deg ree day meth od" (Ree h, 1 991 ). there is no treatment of ice shelves in the modeL Calving is computed by se tting th e ice thi ckncss to zero on a "coasta l I i ne" dete rm i ned by th e sea level. A sea- level drop is taken into acco ulll for th e LG.M co rres ponding

to

1 05 m for th e Pclti er (1994) reco nstruction, a nd of 163 m for the C LlMAP (1 981 ) maximum reconstru cti on. The iso- stati c \'ariation of the bedrock, re. ulting from a n evolving icc load, is governed by two comp onents : a n as thenos pheri c flow with a cha r ac teristic r espo nsc time o f 3000 yea rs a nd a lithos ph eric rigidity th at is ta ken into acco unt by the spati a l sha pe of th e defl ccti on.

R amsteill and others: la -sheet mass balance during the LG "!

ANALYSIS OF THE RESULTS

Direct approach

The clima te of th e LG:'.I is globally colde r than present. In all ofth cse simulations, th e a nnua l mea n diff ere nce of temp- erature be tween the LGM and a prese nt co ntrol run is - 3.3 C (R a mstein a nd j oussa ume, 1 995; R a mstein a nd other s, in press ). Temper atu re cha nge is a strong f uncti on of latitude, with a coolin g of more tha n 1 0 C a t high latitudes, a nd only I C for th e tropics. f-.Ioreo\Tr, th e temper ature dec rease is la rgc over the icc sheets, especially in the summ er when orogr aphy and a lbedo impac ts a rc sup erim- posed, reac hin g \'alu es between - 1 5 a nd 25 C, dependin g o n the versions used. An other region of drast ic coo lin g is the north ern Atl a mic Occa n. This is due

to

sea-i ce extent a nd cha nge in a tmospheric circul ation (ma inl y spliLling o f the jet that brings co ld p ola r air m asses toward s th e North Atl antic (Kut zbac h a nd Guetler, 1 986; Rind, 1 987)). In this a rea, th e annu a l mea n tempe ratu re cooling is abo ut 25

C.

The hyd rologic cycle is also strongly mod ified. A co lde r climate lea ds

to

a global decrease in th e hyd rologic cyc le, whereas th e wa ter tra nsport from E qu ator-ta-Pole is incr cased due to th e enha ncement of th e H a dlc y cells (R a m- stein a nd o thers, in press ). The decrcase is wea ker a nd more uniform in wint er (-80/0 ofwatcr \'a por in th e atm os phere), a nd mor e pronounced a nd loca lized O\'er the Northern Hemi sphe re mid a nd hig h latitudes (- 1 6% ) during summer (R amstein a nd other s, in press).

In thi s pape r, we will foc us our di sc ussion on th e La ur- entide a nd F enn oscandi an ice-sheet m ass balance through different \Trsions of th e L?\ID AGC M . To qua ntify the impac t of th e ice-shee t reco nstru ctions o n mass bala nce, wc fir st p erf orm ed t wo simula tions using

L~1D4

and two diff erent ice-shee t rcco nstructions ( CLIf-. IAP (1 981 ) and Peltier (1 99+)). In Fig ure 1 , wc show the OJ'og r aphic diff er- ence betwee n the LG?\I a nd th e present for the

CLI~IAP

(1981 ) ice-shee t reco nstructi on interpola ted on th e Iow-reso- luti on hori zont a l grid , a nd th e Pelticr (1 994) reco nstructi on interpola ted on the low- a nd sta nda rd-resoluti on ho rizonta l grid. From th ese ma ps, we de fine two spati al a reas cO\'ering the L aurcntide a nd F e nnosca ndia n ice sheets. TnT able 2, we prescnt for eac h ice sheet all the simul ati ons of the a nnua l s ummer a nd winter a\'er ages of th e wa ter ba lance (diff er- ences betwee n precipit atio n a nd e\'apo rati on, plus l ' unofD, eac h term of thi s ba la nce equati on, and surface temper- a ture. \\'e a lso present th e mean annu al net snow acc umula- tion (snowfa ll minus snowmelt ). For all LGM runs, we obseITe a n a nnua l positive hydr ologic bala nce, but the acc umul atio n has different amplitudes within th e different

\'CrS lOns.

Tabl e 2 shows a n importa nt feature: th e seaso na l be ha- yi or is different for L?\ID5.3 comp a red to the Ll\ID + models. For th e pre\·ious versions, ther e is a strong acc umu- la tion durin g winter, a nd a wea ker "melting" during s ummer beca use temper atures reach p ositi\'e \·a lues. to r Ll\ID5.3, accumul ati on occ urs fo r both seasons with the same or der of m agnitude a nd, over Fenn oscandi a, more acc umul ati on occ urs during M ar ch, April a nd May a nd during September, O ctober a nd Novemb er (not shown ).

Over both ice sheets, Lf-.lD 5.3 has acc umula tion rates 3- 5 tim es higher than th e oth er versions (LMD4- a nd LMD+.3, r es pectively). This in crease r esults pa nly from cooler temp eratur es th at a ll ow more snowfa ll

^III

summ er,

1+7

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Ramstein and others: Ice-sheet mass balance durin g the LGN!

Table 2. Annual, summer and winter averages over Laurentide and Fennoswndian ice sheets qf the surfoce temperature (0 ° e),

water mass balance in mm d ',( jJrecij)italiorl, evaporation, runqfJ) and annually averaged net snow aCCllmu/atiorl in cmJor all LGJII simlllations

Ices",,1 Period ?frills L.\1 D·J L.\If)-J L.\/D4.3 L.\/DS.3

eLl,\I,IP Pe/I;('/" Pell;er Pellier

Fennoscanclia Al\"N ~Cl snow acculllulation (CI11) +6.6 +6.86 30.99 10+.59

(annual) [sol ( C) -15.21 -16.01 16.77 -21.25

precipitation (mm cl ^I) 1.18 1.22 1.33 1.16

evaporation (mm cl ^I) 0.50 0.58 0.73 0.22

runolr (mm d ^I) 0.36 0.42 OM 0.00

prccipiullioll~c\·aporalion runoCT (mill cl 0.32 0.22 0.16 0.9+

DJF tsol (

Cl

32.1+ 32.25 38.70 ·U.19

(December precipitation (I11m cl ¹ 0.73 0.79 0.72 0.73

February) c,·aporation (m m cl ^I) 0.03 0.06 0.05 0.0+

runofT(mmcl ^I) 0.00 0.00 0.00 0.00

prcci piWl ion-eva ponuioll rUllolT(lllm cl ^I) 0.70 0.73 0.67 0.77

J.JA

^L^sol⁽^C) 0.86 0.32 1.66 2.20

Uune prccipitaLion (I11m cl ¹ 1.39 1.65 1.81 1.10

August) evaporation (mm cl ^I) 0.99 1.22 1.+4- 0.62

runofT (I11m cl ^I) 0.92 0.95 0.63 0.00

precipitation-evaporation runofT'(mm cl ^I) 0.52 -0.52 0.26 0.78

LaurCl1ticle AN01 l\Ct snow acculllulation (rm) 66.9~ 7:>.08 34.92 119.58

(annual) 1501 ( C 10.1·2 9.36 9.55 l(i.OO

prccipilaLion (mill cl ¹ 1.90 1.85 2.12 1.73

n·aporation (mill cl ^I) 0.68 0.70 0.89 0.21

runofT (Illm cl ^I) 0.68 0.71 0.98 0.01

precipitation-evaporation runoff (Illm cl ^I) 05·, 0.44 0.25 1.51

QJF tsol CC) -21.26 -20.70 22.70 27.6+

(DccclllbcI- precipitation (1ll1TI cl ¹ Ui2 1.56 1.62 1..19

Fcbrual·Y) evaporation (nllll cl ^I) 0.35 0.35 0.24- 0.08

runofr (111111 cl ^I) 0.08 0.13 0.28 0.00

precipitation-evaporation runofT (Illm cl ^I) 1.19 1.08 1.10 IAI

JJA

^{tsol (}^C) ^0.61 ^1.17 ^1.43

³⁺⁵

(june precipitation (mm cl ^I) 2.23 2.0 .. 2.62 2.06

Augusl) cTaporation (n1111 cl ¹ 1.01 0.9+ 1.+1 0.39

runofr (mm cl ^I) 1.59 1.+3 1.82 0.0'>

prccipitation c\"aporation rUlloff (111111 cl ^I) 0.37 0.33 0.61 1.62

but it is mainly due to the drastic dec rease in evapora tion and runoff (Table 2). In contras t to th e oth er vers ions, LMD5.3 always gi\"es negative summer temp eratur es over th e ice shee ts, which lead to a seve re d ecrease o f evaporat ion a nd inhibit the runoff.

To simplify th e desc ription of the results, the LMD 5.3 silTlUlati on, v,·hich is the mos t recent ve rsion and has the highes t reso lution run, is presented in more deta il. Figure 2 gives the an nual mean total precipit ation a nd snowfall for The ice-sheet m ass bala nce produced by LMD4.3 is the

lowest (a fac tor of2 lower than LMD4-). The mai n reason for thi s decrease is that thi s version has the highes t summer temperatures, a nd therefore the highest evaporation a nd run off rates during summ er. This co mpensates for the sli ght precipita tion increase, a nd leads

to

a nea rly ba lanced water budget. The colder surface temperatures of LMD5.3, espe- cially in summer, a rc the consequence of th e hi gher snow albedo specified over the ice shee t (Pin ot, 1 995). If we now compa re the simulations pe rfo rmed with UVfD4 using th e two diffe rent ice-sheet reconstructions, the accumu la tion rates for both ice sheets a rc higher than for LMD4.3. This is mainl y due to lower, positive, summer surf ace temper- atures. There a rc ve r y sma ll differences in the water balance betwee n th e two Ll\IID4· simulations us ing different ice sheets. These res ults indicate th a t the impac t of the different parameterizations used in the models on ice-sh eet mass balance is g reater than the cha nges in the ice-sheet recon- st ructions.

·180 -150 -120 -90 .so -30 30

LONGITUDE

Fi.g. 2. Annual mean Jar LMD5.3 LGM simulation. Top:

snowfall (mm d ') and bottom: jmcijJitation (mm d )

60

(6)

this simulation. If mo st of the precipita tion over the ice sheet is snowfall , then th e hydro logic ba la nce disc usse d abmT is indeed consistent with the ne t snow accumu lation.

Precipita tion is very low over th e ce ntral Laurentide, but higher on th e eastern and wes tern sid es (where yalues as high as 1 0 mm d

I

are reached ). Over the northeas tern Pac i- fic there a rc hi gh precipitation rat es due to th e warm LGl\i[

SST given by th e C LIMAP (1 981 ) r eco nstruction. This r egion is an important source of th e water vapour that pro- duces the hi gh precipitation rates over the ex treme western pa rt of the Laur entide ice she et. On the eastern side, there are also hi gh precipitation rates co rrespondin g to winter storm s occurring a t the location of th e sea-i ce extent, and the Icel a ndic winter low. Over the Fenn oscandi a n ice shee t, precipit a tion a nd snowfall arc also sim il ar, ha\·ing low va lues a t abo ut I mm d

1

To understa nd be tter the snow m ass balance over the ice sheets in LMD5.3, wc ha\T plotted the annual mea n snow- fa ll, snowmelt, and snow acc umul ation durin g th e 1 6 simu lated year s of the run on the same diagram ( Fig. 3).

Once mor e, wc obsenT th at there is a n im portal1l snow ac- cumul a tion oyer th e extr eme western a nd eastern part s of the L aure ntide ice sheet, but th er e is littl e or n o acc umula- tion O \Tr the ce ntral region. O\·er Fennoscan dia, the re is a more homoge neo us acc umulation. The data prese nted in Figure 3 sugges t, therefore, that snow acc umula tion reOec ts the spatia l pattern of snowf a ll. M elting a lso occ urs m·er the same a reas, but is weaker due to very cold temperatur es and is five times lower than the snowfall rate. 10 compare this resu lt to the net snow acc umulation of th e other model ver- sions, Fi gure

4·

shows the net snow acc umul ation for a ll th e L Gl\ I runs. Snow acc umul ation is co mputed m·er the lengt h of the run, which is 1 6 yea rs for L l\ID 5.3 and Ll\fD+.3, but on ly 6 years for Ll\lD 4.

To compa re more easily the different plots shown in Fig- ure +, we di\·ided by three the sca le used for th ese two last runs. (F o r a nnu al net accumu lation , sec a lso th e values in Table 2.) Again, we observe that the la rges t net acc umula- tion occurs in LMD5.3. The geog raphi ca l pattern of the net snow acc umulation is simil ar for a ll the model Yersions, with a n area of accumu lation locali zed on th e wcstcrn a nd eas tern m arg in s of the L aurel1lide ice sheet, a nd a more homoge neous distribution o\'er the F e nnosea ndia n ice shee t. This spatia l similarity, despit e d iff erences in th e a mplitude, shows th at prescrib ed SSTs that a re a co mmon

90

""

Q

:::>

E--

e::

..,;

....:I

30

·180 -150 -120 -90 -60 -30 30 60

a

_LONGITUDE

""

Q :::>

E--

e::

..,;

....:I

-180 -150 -120 -90 -60 -30 30 60

C LONGITUDE

Ramstein al/d others: lce-slzeetmass halance during the LC1 11

3O+---,---~--~,-~--,_--_,--~_,-===~--_T

-180 -150 -120 -90 -60 -30 30 60

LONGITUDE

9O+---L----L----~ __ -m~n

~+---_.----~----,_L--r----~-i_,~==~--_T

·180 -150 -90 -30 30

LONGITUDE

Fig. 3 . . , lllflllal mean fin LMD5.3 LC,\ I simulation. To/x sl1ouial/ (mm d ), middle: snow melt ( mm d ) and net snow aCCllmlllatiolljor the 16j1ears oflhe rUII ( nun d ).

60

feature of a ll these run s (see the me thodology section) play a major role in th e snowfall patter n. The warm Pacific pro-

\·ides hi gh snowfa ll on the western ma rgi n of th e LaLII·en- tide, wh il e winter srorms, loca li zed m-cr th e northwestern Atl a lllie, contribute ro snowfa ll on th e eastern margin. The prescribed SSTappears to be more importa nt than the dif- ferences bet\\Tell model \·ersions or ice-shee t reconstru c- ti ons.

Ice-sheet Inodel driven by AGCM outputs

In th e ice-shee t model, Pellier (1 99+) or CLIMAP (1 981 ) re- co nstructio ns Ill ay be used as an initi a l co nditi on for the LGl\f runs. To account

for

the isos ta tic rebound, a cr ude approximation is used in which the bedroc k lowering is a third of the ice thickness. The isosta tie rebo und is th en co m-

""

Q

:::>

E--

e::

..,;

....:I

b

·180 -150 ^-120 -90 -60 -30

LONGITUDE

30 60

""

Q :::>

E--

e::

..,;

....:I

cl

-180 ^-150 ^-120 ^-90 -60 -30

LONGITUDE

60 30

F ig. 4. Snow aCCllmlllationJor LCJIl sim lllaliolls. ( a) LMD4 C L11 IJAP (1 981 ) 5}ear ru n, ( h) LJIlD4 P eltier (1 99-1) 5)lear rlll1, (c) LJJD4.3 1 5}earTlIIZ, ( d) L IID5.3 1 5j1earrun.

1 +9

(7)

Ramstein and others: l ee- sheet mass balance during Ihe LCI \1

puted by the model. In the ice-sheet m odel , snow acc umul a- tion a nd abl ation are deri" ed from AGC M outputs. Snow acc umula tion is computcd from a nnu al mean surface temp- erature a nd precipitati on, and abl ati on is computed fr om summ cr Uune, Jul y a nd Aug ust) and annu al surf ace temp- eratures. Note that these AGCM fi eld s a re not directl y used in the ice-shcet model (see F a bre a nd others (1997) for a detail ed di scussion of the couplin g method ).

In the following secti on, wc desc ribe th e treatment of the AG C M output before it is used in the ice-sheet mod el. A ma jor problem is the difference in resolution of both models.

The initi a l d atabase provided by Pelti er (1994) has a I x I ' reso lution, while CLIl'vIAP (1 98 1 ) has a 2 x 2° resolution, which is interpola ted on th e Cartesia n grid of the ice-shee t model (50 x 50 km ). The int erpolati on on th e AGCM g rid smooth es the el evation b ecause of th e coarser grid boxes. F or temp erature field s, we used a "reconstructed" temperature defined as th e diff erence of temp era ture between th e LGM and the control (from AGCM outputs), which is a dded to presc nt climatol ogy:

T.-ec = l1

g¹¹¹^- Tetrl

+

^Telim .

(1) The reaso n for such a methodology is th a t the C LIl\ fA P datase t onl y g ives Aug ust a nd l:;e brua ry SST, a nd it is better to keep a realistic seasonal cycl e by using the "recon- structed" surfa ce temperatures. These values a re th en co r- rected for th e el evation difference (Altne\\' a nd Alt

olcl)

to acco unt for th e better resolution of th e ice-shee t model.

The tempera ture gradient chosen is SO C km

-I

as observed in pola r regions. Thus:

T.,cw

= Told -

0.008

X

(Altnt'\\' - Alt

olcl ) .

(2) Us ing this meth od leads to negati ve values for some gridpoints in precipitati on fields, so wc used a ra tio rather th a n a difference:

Precrec

=

^(P^{TeCIgm/ P}^TeCetr^l)^x^Precclim .

(3) As previously noted, a ll LGM simul ations have been perfo rmed using prescrib ed

SS1~

which re sults, for the ice- sheet model, in a di sco ntinuity in coastal reg ions' sur[a ee temperatur es because these a re computed over land by the model. This disc ontinui ty has co nsequences [or the hydrolo- gic cycle, a nd to minimi ze th cm we use the AGCM sur[aee a ir temperature a t 2 m h cight th at i comput ed for all grid- points.

Acco unting [or all th ese probl ems before coupling, we performed three 20000 run s using AGCM LMD 5.3 and LMD 4 ·.3 outputs a nd th e Pelti er (1 994) reco nstruction as bound a ry conditions, a nd the LMD4 outputs with th e CLIMAP ice-sheet recon struction. Th e results are shown in Fig ure 5. Using LMD5.3 outputs, th e ice-sheet model mainta in s th e ice sheet, wh ereas with other run s, the L aur- entide a nd Fennosca ndi an ice shee ts coll apse. For the run co upl ed with Ll\ID 5.3 outputs, the best results a re the ma in- tena nce of th e L aurentide and Fenn oscandia n ice sh eets in a stead y state near the Pelti er recon stru ction a nd the absence of an ice cap over Siberia and Alaska, which is co nsistent with data [or the LGM. A weaker p oint is the large acc umu- la tion dec rease over Gree nla nd, which leads to a lowering of the ice sheet. However, thi s may be due to the

AGCl\II

coa r- ser grid at high latitudes.

For LMD4.3, the Fennoscandi a n a nd Greenland ice shee ts survive, despite an importa nt lowering, but the L a ur- entide ice sheet is reduced

to

a longitudinal belt on th e western part of North Am erica. Thi s may be due to th e pre- 1 50

Fig. 5. Resulls rif th e ice- sheet model. The steady state is obtained a f ter 20 000 years integration qf the model. (a) In- IJlIls are de du ced from Li\lD5.3 o u/jJuls and Pe ltie r (1994 ) reconstructio n is th e initial condition, ( b) inputs aTe deduced Jro m LMD 4.3 outp 1lts and Pe/tier (1 994) reconst11l ction is the initial condition, (c) in /JUts are deducedJrom LMD" O llt- I luls and CLlilIAP (1981 ) reconstruction is the initial condi-

lion. Th e thin line relnesenLs th e land over sea level.

3.0

2.0

1.0

0.5

n,

3.0

2.0

1.0

0.5

0.1

3.0

2.0

1.0

0.5

0.1

(8)

scribed CLII\IAP (1981 ) SSTs, which gi\'e a warm pool over the mid latitudes of the north Pacific that are able to pro- duce moi sture that precipitates over the cont inent. Finall y,

L~ID+

outputs show that. despite the raet that wc start with the

CLI~lAP

ice sheets that arc 1000 m higher, a weaker extent of the Laurentide ice sheet, but a sli ght ly higher . Fennoseandian ice sheet, are obtained.

The ice-sheet model is therefore very sens itive to

AGC~I

outputs. The best result is obtained with Ll\IDS.3 in this approach, whereas in the previous approach, the weaker ice-sheet mass balance was obtained for LI\ lD4.3

CONCLUSIONS

\Ve ill\Tstigated, thr ough two different methods, the mass balanc e of the ice sheets for the LG11. Wc expect little or no net snow accumulation. Arc these tools able lo tell us whether the presc ribed ice sheets (Peltier or CLIMAP) arc in equi librium or nea r the equil ibrium with the computed

LG~l

climat e? The a nswer to thi s question is !lot clear cut beca use our results s uggest that the conclusions \'ary de- pending on the methodology used.

~Ioreo\Tr,

these results are \Try different from those of Hall and others (1996) who found that for the modrl de\'eloped by the UK Uni\ 'ersities' Global Atmospheri c l\lodcling Program, the ma intenance of ice shcets was duc to a net accumu lat ion in the central part and a melting at the edges. In the present rcsearch, we [ound that there is still accumu lati on O\'e r the ice sheets at the LGM. Ho\\'e\'er, these results arc quite different for the

L~ID5

a nd LJ\fD4 \Trsions.

L~'fD5

surface temperatures arc co lder a nd always below freez ing, \\'hich leads to a decrease or c\'aporation and

to

a hi gh accumu lat ion. Ill. the other model \'Crsions, the summer temperatures arc abo\'e freezing point and ice-sheet mass balance is reduced to lower \'alue s.

In th e method using LGJ\I AGCi\I inputs and an ice- sheet model, the bes t result is obtained with Ll\rDS.3 inputs, which is the on ly onc that maintains the ice sheet. The fact that thi s version is the best candidate to maintain the ice sheet is not surprising because it has the highest accumu la- tion and also the highest resolution. What is puzzling is why th e LMD4.3 \'ersion, which from the direct approach seems the close st to a zero mass balance, gi\'('s such poor results when usi ng a n ice-shee t model. There arc two mai n reasons.

First, in the direct approach the high Ll\ID+.3 summer sur- face temperatures enhance summer melting, which partly compensate for winter snowfall, but in the coupled approach wc use 2 m height air surface temperatures that strongly dampen the summer temperature differences between the Ll\IDS.3 and earlier \'e rsions. Secondly, the coarser grid of the LM.D+.3 \'e rsion is more difficult to cou- ple with the smaller ice-sheet Cartesia n grid , especiall y when using precipitation.

A way to a\'oid the first problem is to use a LGi\I simul- ation with computed SSls that will a ll ow us

to

use directly the surrace temperature in the ice-sheet model. This simul- ation has a lread y been performed , and will be coupled with the ice-sheet model. A major advance in this topic should be ach ieved through the Ice Sheet Mass Balance Sub-Project led by D. Pollard in the framework of the PI\IIP program Uoussaume a nd Taylor, 1995). !\'fore th an 15 model results for LGl\[ simulat ion using the same boundar y cond itions, will then be analyzed in term s of ice-sheet mass balance.

Ralll sleill al/d others: Ice-s heell1lass balance during th e LC,!!

ACKNOWLEDGEMENTS

\\'e grateful l y acknowledge the Labora toi re de l\Ieteorolo- gie Dynamiqu e (C:-\RS, Paris, France) far prO\'iding us with their GCl\I,J.Jouzel for fruitful disc ussion, D. Lewden for participating in the simulations a ndJ\'. Peterschmitl

(L~ICE)

for the graphi c outputs .

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