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Mass Balance of East Antarctic Glaciers and Ice Shelves from Satellite Data

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Mass Balance of East Antarctic Glaciers and Ice Shelves from Satellite Data Annals of Glaciology 34 2002 # InternationalGlaciological Society Massbalance ofEast Antarctic glaciers and ice shelves fromsatellite data Eric Rignot JetPropulsion Laboratory,California Institute ofTechnology,4800OakGrove Drive,Pasadena, CA9 1109-8099,U.S.A. ABSTRACT.Thevelocity and mass dischargeof nine ma jorEast Antarcticglaciers not draininginto the Ross orFilchner^R onneI ce Shelvesis investigatedusing interferometric synthetic aperture radar(InSAR) datafrom the EuropeanR emote-sensing Satellite1and2 (ERS-1/2)andRAD ARSAT-1.Theglaciers are: David, Ninnis, Mertz,Totten,Scott, Denman, Lambert, Shiraseand Stancom b-Wills. InSARis used tolocate their groundingline with pre- cision.Ice velocityis measured witheither InSARor aspeckle-trackingtechnique. Ice thickness is deducedfrom prior -determined ice-shelf elevationassuming hydrostatic equi- librium.Mass fluxesare calculated both at the groundingline and at a fluxgate located downstream.The grounding-line flux is comparedto a mass inputcalculated from snow accumulationto deduce the glaciermass balance.The calculation is repeatedat the flux gatedownstream of the groundingline to estimate the averagebottom melt rate ofthe ice shelf understeady-state conditions.The main results are:( 1)Groundinglines arefound severaltens ofkm upstream ofprior-identified positions, not because of a recent ice-sheet retreat butbecause of the inadequacyof prior-determined grounding-linepositions. ( 2)No grossim balancebetween outflow and inflow is detected onthe nineglaciers being investi- gated,with an uncertainty of 10^20%.Prior-determined, largelypositive mass imbalances weredue to an incorrect localizationof the groundingline. ( 3)High rates ofbottommelting (24 7 m ice a^1)areinferred neargrounding zones, where ice reaches the deepest draft.A few§glaciers exhibit lower bottom melt rates (4 7 m ice a^1).Bottommelting, however , appearsto be a majorsource ofmass loss onAntarctic § ice shelves. 1.INTR ODUCTION lation,but largecoastal sectors arethinning rapidly ,with thinningconcentrated in channels occupied by fast-moving Thestate ofmass balanceof the vastmajority of Antarctic outlet glaciers(Krabill and others, 2000).Similarstudies glaciersis notknown at present. Thisincomplete knowledge conductedalong the coastalsectors ofAntarcticaare timely . casts doubton anyestimate ofthe contributionof the Ant- Severalapproaches have been used todetermine the mass arctic ice sheet toglobal sea-level rise. Nochangein ice- balanceof Antarcticglaciers, all with their ownadvantages sheet elevationis detected inthe vastinterior ofthe ice sheet andlimitations. The approach considered here iscommonly byEuropean Remote-sensing Satellite (ERS) radaraltim- referred toas the mass-budget method.I tcomparesmass etry,exceptperhaps in the AmundsenSea sector ofW est accumulationin the interior withmass dischargeacross a Antarctica(Wingham and others, 1998).Theseresults, how- fluxgate. Thedifference between these twolarge values deter- ever,whichare limited inspatial resolution to about 250 km, mines whetherthe glacieras awholeis losingor gaining mass. donot provide any indication of the state ofmass balanceof Recent advancesin satellite radarinterferometry (InSAR) the glaciersat low elevation, along the coast.In Greenland, mappingof grounding lines andice velocity(Rignot and recent results fromairborne laser altimetry showthat the others, 1997),topographicmapping of the ice sheet (Bamber ice-sheet interior is more orless inbalance with accumu- andBindschadler ,1997)anddigital mapping of snowaccu- Table1.Mapping of East Antarctic glaciers using ERSand RSAT Glacier ERS velocity RSATvelocity ERS groundingline ERS topography (ERS-1/2orbit pairs) (orbitpair) (ERS-1/2orbit pairs) (ERS-1/2orbit pairs) David 24676-5003,24765-5092 ^ 23763-4090,24765-5092 23806/4133,24808-5 135 Ninnis 24838-5165 ^ 24838-5165,25339-5666 ^ Mertz 24967-5294 ^ 24967-5294,25468-5 795 23965-4292,24967-5294 Totten 24868-5195 ^ 24868-5195,24367-4694 ^ Denman 24840-5167 ^ 24339-4666,23838-41 65 ^ Scott 24840-5167 ^ 24339-4666,23838-41 65 ^ Lambert 23853-4180,24436-4763 10196-9853 23810-4137,24311-4638 ^ Shirase 24766-5715 10183-9840 24766-5715,25267-5 714 43575-23902,44076-24403 Stancomb-Wills22482-2809, 24497-4824 10208-9865 22482-2809,24486-48 13 ^ Notes:ERS velocityis determinedcom bining across-trackInSAR with along-trackspeckle tracking if onlyone orbit pair is listedin column2, and a combination ofInSAR ascending and descending if twopairs arelisted. RSATvelocityis determinedusing speckle tracking both across and along track. 217 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 10:33:59, subject to the Cambridge Core terms of use. Rignot: Mass balance of East Antarctic glaciers and ice shelves Fig.1.Balance velocity (personal communication fromJ.L.Bamber ,2000)on alogarithmic scale showing the drainagebasins of nine East Antarctic glaciers,on apolar stereographic grid.The drainage basins of fiveWest Antarctic glaciers,studied with the same methodology but not discussed here,are shown for reference. mulation(V aughanand others, 1999;Giovinetto and Zwally , tobe useful for this study.Althoughit offers improvedmap- 2000)areprompting a revisionof earlier mass-budget assess- pingof the coastalrange, its accuracyis pooron floating ice ments ofAntarctic glaciers. In this study,wepresent afirst whereold cartography was used. Thisstudy uses the DEM of step towardthis modernrevision. InSAR is used tomap the Antarcticaderived from ERS radaraltimetry alone(Bamber locationof grounding lines, ice velocityin vector form, and andBindschadler ,1997),whichis accurateat the meter level surface topographyof ninemajor outlet glaciersof East Ant- onthe nearlyflat surface oflarge ice shelves. Performance arcticathat donot drain into the Ross orFilchner^R onneI ce decreases rapidlyover sloping terrain, nearmountain ranges Shelves.The results areused toestimate the mass fluxesand orat high latitude where the densityof crossoverpoints is low. mass balanceof the glaciers,and the rates ofbasalmelting of Insome cases, ERS radaraltimetry hadto be supplemented the ice shelves infront of them understeady-state conditions. byan InSAR -derivedDEM ofthe glacier(T able1 ). Table 2.Mass discharge of East Antarctic glaciersat the METHODS grounding line, ¿GL,and mass input from snow accumulation, ¿A,over adrainage basin of area A. H and V are,respectively, Thisstudycombines InSAR data (T able1)withprior-deter- the glacier-center thickness and velocity at the grounding line mined digitalelevation maps (DEMs) andaccumulation maps ofAntarctica. Glacier H V ¿GL A ¿A Topographyand accumulation m m a^1 km3 ice a^1 km2 km3 ice a^1 Topographyhas four main usages: ( 1)toremove the topo- David 3000500 1 5.4 22120251 6.4 4 graphicsignal from InSAR measurements ofice velocity § § Ninnis 1600900 2 1.9 3170550 24.8 1 andgrounding-line position; ( 2)to rectify andgeoreference § § Mertz 1350850 1 9.8 2 83 080 21.3 0 § § the InSARproducts onto an Earth-fixed grid; ( 3)toestimate Totten 2100900 69.9 753473068.8 10 § § ice thickness fromice-shelf elevationassuming hydrostatic Scott 1200 500 9.2 2 27340 10.9 3 § § Denman 20001 60035. 1 5176240 37 .4 9 equilibrium;and ( 4)todelineate the glacierdrainage basins. § § SeveralDEMs ofAntarctica have been produced in Lambert 3000800 5 7.5 5953670 55.2 1 Shirase 9002300 1 5.1 §3195670 1 6.9 §0 § § recent years.The RAD ARSAT-1(RSAT)AntarcticMap- Stancomb-Wills1 200700 1 6.6 2 98710 15.7 2 § § pingProject (RAMP) DEM (Liu andothers, 1999),used to Total 260.5 112452020 267 .4 15 § § controlthe mosaickingof RAD ARSATdata,was not found 218 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 10:33:59, subject to the Cambridge Core terms of use. Rignot:Mass balance of East Antarctic glaciers and ice shelves Fig.2.David Glacier,Victoria Land:(a)vector velocity map (ma ^1)derived from ERS;(b)grounding-line position and tidal motion derived fromERSInSAR (each color cycle represents a30mm increment in vertical displacement of the ice surface due to changes in oceanic tide);surface topography (200mcontour) derived from (c)ERS InSAR and (d)ERSradar altimetry.The grounding-line and basal-melt flux gates are,respectively,shown in blue anddotted blue in (a),and black and dotted black in (c, d).Latitude is plotted every 0.25³. Longitude is plotted 1³. Ice thickness isdeducedfrom the ERS DEM (referenced ingsections ofT otten, Denman,Shirase, Stancomb- Wills tosea level using the OSU 91geoidmodel) by multiplying andNinnis Glaciers. Threemeasurement pointson Mertz the ice-shelf elevationby 8. 05.Thisheuristic isbasedon a GlacierTongueindicate an underestimationof ice thickness comparisonof ice-sounding radar measurements with by3% or30 m. On Lambert Glacier,17datarecords indicate DEM elevationson Pine Island and Thwaites Glaciers anoverestimation of ice thickness by3% or5 7m. Ice-thick- (Rignot,200 1).Themultiplicative coefficient reflects the ness datarecords exist inthis areaboth above and below the densityprofile of ice andsea water ,aswellas geoidalheight groundingline, but not at the groundingline, which is the offsets. Errors ingeoidheight could be 10m(e.g.J enkins zoneof confluence of Mellor,Fisher andLambert Glaciers, § andDoake, 1 991),thus inducinguncertainties inice thick- andhence where ice isthe thickest. Basedon this limited set ness of 100m from hydrostaticequilibrium. Errors inthe ofcomparisons, and potential uncertainties inthe geoid,ice § densityprofile of ice andsea water are likely less significant. thickness employedin this studyhas
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