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A New Tide Model for the Antarctic Ice Shelves and Seas

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A New Tide Model for the Antarctic Ice Shelves and Seas Annals of Glaciology 34 2002 # InternationalGlaciological Society Anew tide model forthe Antarctic ice shelves and seas Laurie Padman,1 Helen A. Fricker,2 Richard Coleman,3, 4 Susan Howard,1 Lana Erofeeva5 1Earth &Space Research,1910 FairviewAve.E.,Suite102 ,Seattle,WA98102-3620,U.S.A. 2Institute of Geophysics and Planetary Physics,Scripps Institution of Oceanography,University of California SanDiego , LaJolla,CA92093-0225 ,U.S.A. 3Antarctic CRCand School of Geography and Environmental Studies,University ofTasmania,Box 252-80,Hobart,Tasmania 7001,Australia 4CSIROMarine Research, Box 1538,Hobart,Tasmania 7001,Australia 5College of Oceanic &Atmospheric Sciences,Oregon StateUniversity ,Ocean Admin. Bldg.104,Corvallis,OR97331-5504,U.S.A. ABSTRACT.Wedescribe anewtide modelfor the seas surroundingAntarctica, includ- ingthe oceancavities under the floatingice shelves. Themodel uses dataassimilation to improveits fit toavailable data. T ypicalpeak-to-peak tide rangeson ice shelves are1^2 m but canexceed 3 mforthe Filchner^Ronneand Larsen Ice Shelvesin the WeddellSea. Springtidal ranges are about twice these values.M odelperformance is judgedrelative to the 5^10cmaccuracythat isneededto fully utilize ice-shelf heightdata that willbe collected¹ with the Geoscience Laser Altimeter System,scheduledto be launched on the Ice, Cloudand land Elevation Satellite in late 2002. Themodel does not yet achieve this levelof accuracyexcept very near the fewhigh-quality tidal records that havebeen assimilated into the model.Some improvement in predictive skill is expectedfrom increased sophistication of modelphysics, but wealso require better definitionof ice-shelf groundinglines andmore accuratewater-column thickness datain shelf seas andunder the ice shelves. Long-duration tide measurements (bottom pressure gaugeor globalpositioning system) incritical data- sparse areas,particularly near and on the Filchner^Ronneand R oss Ice Shelvesand Pine IslandBay ,arerequired to improve the performanceof the data-assimilationmodel. INTRODUCTION (ICESat).GLASwill providemeasurements ofice-shelf sur- faceheight at 5cm accuracyduring its expected3 year ¹ Most ofthe mass loss from the Antarcticcontinent takes mission lifetime. On this time-scale, however,the height placefrom the floatingice shelves, viaiceberg calving from changesmay be small comparedwith the high-frequency their outer marginsand basal melting beneath them (Ja- variabilityof tide height:perhaps tens ofcentimeters per year cobsand others, 1992).Thesurface heightof an ice shelf var- comparedwith the order1 mstandarddeviation of the tides. ies intime withocean tides, atmospheric pressure, ocean Tides,with periods of 0.5 and 1day,areundersampled by ¹ ¹ andice density,snowloading, firn compaction, ablation or satellite repeat periods.I CESat,for example, will have a accretionof ice atthe ocean/ice interface,and ice dynamics. 183day repeat intervalfor most ofits planned3 yearmission, Theseprocesses actover a widevariety of time-scales: from after 3months ofan 8 dayrepeat cycle.Orbits canbe ¹ hoursto decades and longer .Themain cause of short-time- designedto allow removal of tides froma longrecord of satel- scales: (less thanseasonal) height variability will generally lite altimeter data(Parke and others, 1987).Atthe time of beocean tides, withpredicted peak-to-peaktide-induced writing,however,TOPEX/Poseidon(T /P)is the onlysatellite displacements being 43mundersome shelves. Thelargest that is designedfor this purpose(Smith, 1999;Smith and tides occurin the WeddellSea: a gravimeterrecord from others, 2000),andit onlyprovides coverage to latitude nearthe groundingzone of the RutfordIce Stream inthe 66.2³S. Therefore,the most effectivemethod for predicting ¹ southern RonneIce Shelfshows peak-to-peak tidal changes tides forremoval from satellite dataover Antarcticice shelves ofabout6 m(Doake,1992).Toaccuratelymonitor long-term is throughnumerical modeling . trends inice-shelf surface height,the tide componentmust Inthis paperwe describe recent progress inmodeling beremovedfrom the heightmeasurement, such that allice- tides inthe SouthernOcean. Wetreat tides asnoise that must shelf heightsare referred toa ``tide-free’’datum. beremoved from satellite datacollected over ice shelves,so Thevertical displacement ofanice shelf canbe meas- wefocus here onpredictionof tidal height rather thantidal uredto high accuracy using global positioning system currents.Wenote,however ,that tides contributedirectly to (GPS) receivers placedon the ice (e.g.Kingand others, the dynamicsand thermodynamics of ice shelves (see, e.g. 2000),andinterferometry from time-separated satellite syn- MacAyeal,1 984;Makinson and Nicholls, 1 999)andplay a thetic aperture radar(SAR) images(e.g .Rignotand majorrole in setting the oceanicand sea-ice conditionsnorth others, 2000).Satellitealtimetry is the best approachto ofthe ice shelves (Robertsonand others, 1998;P admanand monitoringchanges in ice-shelf heightover time-scales of Kottmeier,2000).Thedistribution of tidal currents rather months toseveral years. A significantnew dataset on ice- thanheight variability is the most significantfactor affecting shelf surface heightwill be obtained from the Geoscience these processes; therefore accurateprediction of currents is Laser Altimeter System (GLAS),scheduledfor launch in alsoan important goal of our tidal studies, but is not late2002 on the Ice, Cloudand land Elevation Satellite discussed here. 247 Padman and others:Tide modelforAntarctic ice shelves and seas TIDE-MODEL DESCRIPTION aremodeled on a gridwith node spacing of 1/4³61/12³(about 10kmspacingnear the Antarcticcoast at 70³S) ,usingthe ¹ Severalmodels oftides inthe SouthernOcean already exist, depthgrid described inthe Appendix.The ` `prior’’solution includingthe globalFinite-Element Simulation,version 95 .2 (i.e. the dynamicallybased ` `first estimate’’)is alinearized (FES95.2:le Provostand others, 1997)and regional models, formof the CATSmodel, version 00 .10(CATS00.10).Four such asthose forthe Ross Sea(MacA yeal,1 984)andthe diurnal (O1, K1, P1, Q1),foursemi-diurnal (M 2, S2, K2, N2) WeddellSea (Smithson andothers, 1996;R obertsonand andtwo long-period (Mm, Mf) constituents aremodeled. others,1998;Makinson and Nicholls, 1999).TheCircum-Ant- arctic TidalSimulation (CA TS)(Padman and Kottmeier , 2000)coversthe entire SouthernOcean south of 56³ S. Table 1.Tidal records used in model for assimilation or ¹ While the predictiveskill of recent versionsof CA TSis better validation thanthat ofprevious models, comparisons of predictionswith avarietyof datasets indicatethat significantfurther improve- ment isstill required. No. Name Location Type Record Comments length Errors inpresent-day tide models arise from three sources: errors inforcing, primarily due to errors inopen- days boundaryspecifications; simplifications in model physics; anderrors inthe water-columnthickness grid.Simplifica- 1 Faraday 65.25³S, 64.27³W CTG 365 2 Forster 70.77³S, 11.87³W BPR 307.25NU 1 tions ofthe physicsare necessary toreduce computation 3 Rothera 67.57³S, 68.12³W CTG 365 time fora modelrun. Some simplifications (e.g .parameter- 4 Signy 60.70³S, 45.60³W BPR 384.96 izationsof bottomfriction and lateral mixing) apply to all 5 PTC 4.2.1 77.12³S, 49.05³W BPR 4.2NU :shortrecord 2 tide models.When ice shelves areincluded, additional terms 6 PTC 4.2.2 74.43³S, 39.40³W BPR 30NU 7 PTC 4.2.3 74.38³S, 37.65³W BPR 180 arerequired toaccount for some ofthe inelasticbehavior of 8 PTC 4.2.5 70.43³S, 8.30³W BPR 324 the ice. Inour model, the shelves aretreated aspassiveele- 9 PTC 4.2.6 71.05³S, 11.75³W BPR 367 ments freelyfloating on a perturbed ocean` `free’’surface, 10 PTC 4.2.7 72.88³S, 19.62³W BPR 316 andtheir onlyeffects areto reduce the waterdepth and to 11PTC 4.2.1960.05³S, 47.08³W BPR 408 12PTC 4.2.2073. 13³S, 72.53³W BPR 357RonneEntrance providean additional frictional surface (the ice/oceaninter- 13PTC 4.2.246 1.47³S, 61.28³W BPR 320 face)for dissipation of tidalenergy (MacA yeal,1984).Water- 14PTC 4.2.2760.85³ S, 54.72³W BPR 378 columnthickness ( d)isthe same aswaterdepth for the open 15PTC 4.2.3059. 73³S, 55.50³W BPR 377 ocean,and is the verticaldistance between the ice baseand 16PTC 4.2.3362. 13³S, 60.68³W BPR 358 17PTC 4.2.3470. 13³S, 2.65³W BPR 653 the seabedunder the ice shelves (see fig.1inSmithson and 18 PTC 4.1.3 60.02³S, 132.12³E BPR 21NU:shortrecord others (1996)fora diagramof tide-model geometry).Errors 19 Mawson 67.60³S, 62.87³E BPR 365 in d x; y arelargest under the ice shelves. Indeed,even the 20 Davis 68.45³S, 77.97³E BPR 365 … † precise locationof the groundingline for sections ofsome ice 21 Casey 66.28³S, 110.53³E BPR 365 22 RIS Base 82.53³S, 166.00³W Grav.44NU :validation shelves is still notknown, although progress is nowbeing 23 RIS C13 79.25³S,1 70.37³E Grav.34NU :validation madethrough satellite mapping(see, e.g.,Grayand others 24 RIS C16 81.19³S, 170.50³E Grav.45NU :validation 2002;F ricker andothers, inpress) .Thewater-depth grid is 25 RIS C36 79.75³S, 169.05³W Grav.34NU :validation slowlybeing improved (see Appendix)using additional 26 RIS F9 84.25³S,1 71.33³W Grav.58NU :validation 27 RISJ9 82.37³S, 168.63³W Grav.30NU :validation datafrom cruises andre-analysesof existingice-penetrating 28 RIS LAS 78.20³S, 162.27³W Grav.32 radardata, but more depthdata are needed. 29RIS McMurdo 77.85³S, 166.66³E BPR 365 Inthe longterm, improvements intide modelingwill 30 RIS O19 79.53³S, 163.36³E Grav.39NU :validation result froman increase inmodel sophistication combined 31 RIS RI 80.19³S, 161.56³W Grav.36NU
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