Comparing ice discharge through West Antarctic Gateways

29 Pages·2015·4.76 MB·English

TheCryosphereDiscuss.,9,1705–1733,2015 D
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doi:10.5194/tcd-9-1705-2015 s
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discharge through
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West Antarctic
Comparing ice discharge through West D
Gateways
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Antarctic Gateways: Weddell vs. s M.A.Martinetal.
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Amundsen Sea warming P
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M. A. Martin , A. Levermann , and R. Winkelmann | Abstract Introduction
1PotsdamInstituteforClimateImpactResearch,Potsdam,Germany D Conclusions References
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2UniversityofPotsdam,Potsdam,Germany c
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Received:26February2015–Accepted:3March2015–Published:17March2015 io
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Correspondenceto:M.A.Martin([email protected]) P
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PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. er (cid:74) (cid:73)
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Abstract D
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Future changes in Antarctic ice discharge will be largely controlled by the fate of the s
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floating ice shelves, which exert a back-stress onto Antarctica’s marine outlet glaciers. io 9,1705–1733,2015
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IcelossinresponsetowarmingoftheAmundsenSeahasbeenobservedandinvesti- P
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5 gatedasapotentialtriggerforthemarineice-sheetinstability.Recentobservationsand pe Comparing ice
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simulations suggest that the Amundsen Sea Sector might already be unstable which discharge through
would have strong implications for global sea-level rise. At the same time, regional | West Antarctic
ocean projections show much stronger warm-water intrusion into ice-shelf cavities in D Gateways
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the Weddell Sea compared to the observed Amundsen warming. Here we present re- c
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10 sults of numerical ice sheet modelling with the Parallel Ice Sheet Model (PISM) which ssio M.A.Martinetal.
show that idealized, step-function type ocean warming in the Weddell Sea leads to n
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moreimmediateicedischargewithahighersensitivitytosmallwarminglevelsthanthe a
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same warming in the Amundsen Sea. This is consistent with the specific combination e TitlePage
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of bedrock and ice topography in the Weddell Sea Sector which results in an ice sheet
| Abstract Introduction
closetofloatation.Inresponsetoevenslightoceanwarming,icelossincreasesrapidly,
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peaks and declines within one century. While the cumulative ice loss in the Amundsen Dis Conclusions References
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Sea Sector is of similar magnitude after five centuries of continued warming, ice loss us Tables Figures
increases at a slower pace and only for significantly higher warming levels. Although s
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there is more marine ice stored above sea level in close vicinity of the grounding line n (cid:74) (cid:73)
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compared to the Weddell Sea Sector, the ice sheet is farther from floatation and the a
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grounding line initially retreats more slowly. r
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1 Introduction Dis FullScreen/Esc
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Projecting sea-level rise for the next centuries requires a deep understanding of the s
physical processes involved and remains a major challenge for climate-change re- sion Printer-friendlyVersion
search. One of the biggest uncertainties lies within the dynamic contribution from the P InteractiveDiscussion
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Antarctic Ice Sheet, especially the marine sectors (Bamber et al., 2009; Joughin and e
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Alley, 2011; Rignot et al., 2011) and the ocean-ice interaction beneath the buttressing D
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iceshelves(DupontandAlley,2005;PritchardandLigtenberg,2012).Lossofbuttress- c TCD
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ingasaresultofincreasedsub-shelfmeltingcanleadtogroundinglineretreat,butthe s
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bed topography cannot serve as the sole factor for the determination of glacial re- n
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5 sponse in a real three-dimensional system (e.g., Gudmundsson, 2013; Gudmundsson a
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et al., 2012). e Comparing ice
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Significant warming of the Amundsen Sea (Jacobs et al., 2011) and the ongoing discharge through
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changes(Favieretal.,2014;Jenkinsetal.,2010;Joughinetal.,2014;Mouginotetal., West Antarctic
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2014; Rignot et al., 2014; Shepherd et al., 2002) of Pine Island and Thwaites Glaciers Gateways
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10 have put the Amundsen Sea Sector into special focus (Docquier et al., 2014; Parizek u
s M.A.Martinetal.
et al., 2013). And while observations in the Weddell Sea Sector show that the Filchner s
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RonneIceShelfiscurrentlythickening(Shepherdetal.,2010),regionalocean-sea-ice n
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projections (Hellmer et al., 2012) indicate the possibility of an abrupt warming of the a
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surrounding waters by more than 2K within the 21st century, due to the elimination of r
15 aprotectiveoceanicfrontthatcurrentlyshieldstheice-shelfcavityfromwarmerAntarc- | Abstract Introduction
ticcircumpolarwaters.TheInstituteandMöllerGlaciersfeedingtheFilchnerRonneIce
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Shelf are located in a basin with a steep reverse bed slope showing few small-scale is
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topographic features which could halt retreat of the grounding line between grounded us Tables Figures
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ice sheet and floating ice shelf (Ross et al., 2012). Within the group of ice streams io
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feeding the Filchner Ronne Ice Shelf, these glaciers have been identified as the ones (cid:74) (cid:73)
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mostquicklyrespondingtomeltingattheirgroundinglinesbyarecentmodellingstudy p
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(Wright et al., 2014). Reconstructions based on radio-echo sounding suggest that this r
area might have been ocean floor during the last interglacial (Bingham and Siegert, | Back Close
2009;Joughinetal.,2006),indicatingthatpotentialsignificantgrounding-lineretreatis
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not unprecedented. is FullScreen/Esc
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Here,wecomparetheresponseoftheAntarcticIceSheettoidealized,step-function s
typeoceanwarmingintheWeddellandAmundsenSeaSectors.Theice-andbedrock sio Printer-friendlyVersion
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topographiesforthetwoassociatedWestAntarcticgatewaysareveryspecific(Fig.1). P InteractiveDiscussion
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Both regions exhibit a reverse bed slope and are thereby potentially prone to a marine p
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ice-sheet instability. One major difference lies in the specific combination of bed deep- D
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ness and ice thickness: while a large area of the ice sheet in the Weddell Sea Sector c TCD
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(namely the Institute and Möller Glaciers flowing around the Bungenstock Ice Rise) is s
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currently close to floatation such that comparatively small changes in ice thickness at n
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5 the grounding line lead to its retreat, this is not the case in the Amundsen Sea Sector. a
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Althoughthesteepicesurfaceslopethereresultsinahugeamountoficestoredabove e Comparing ice
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sea level in close vicinity to the grounding line, that very ice needs to get into motion discharge through
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first in order to reach the floatation criterion before contributing to sea-level rise. West Antarctic
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2 Experiments s M.A.Martinetal.
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In order to identify which aspect of this differing ice- and bedrock topography domi- P
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nates, we investigate the ice-dynamic response of these two basins to an idealized p
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step-function type ocean warming using the Parallel Ice Sheet Model (PISM, Bueler
andBrown,2009;Martinetal.,2011;Winkelmannetal.,2011).Inordertocaptureice- | Abstract Introduction
model uncertainty, an ensemble of 22 parameter combinations and resolutions from D Conclusions References
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15 2fe4r tino t2h.e5kmmodaerel puasreadm.eSteimrsuglaotivoenrsnianrgebsatasartlerdesfrisotmanaceseatnodficsetesaodfytnsetsastebsuwt hgiicvhe tdhife- cus Tables Figures
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best match to observed grounding-line positions in both Weddell and Amundsen Sea ion
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Sectors within a wider range of parameters (see Appendix A3). The qualitative fea- P
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tures of the ice surface gradients of Institute and Möller Glaciers (comparatively flat) e (cid:74) (cid:73)
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andThwaitesGlacier(steepestnearthegroundingline)arewell-captured(Fig.1cand
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d), although the initial ice thickness in the former ice streams is overestimated; possi-
ble consequences are discussed below. Both gateways are investigated for the entire Dis FullScreen/Esc
ice-parameter ensemble within the same model set-up allowing for direct comparison c
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at 12km resolution, while a limitation to representative ensemble members and a re- sio Printer-friendlyVersion
gional setup was necessary due to computational constraints for the simulations with n
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5 and 2.5km resolution. The model uncertainty resulting from the range of tested ice a
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model parameters, however, is bigger than that from the range of resolutions, which e
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is spanning almost one order of magnitude (see Appendix A1 and A3). So while the D
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question of resolution is certainly crucial for model-based projections, the uncertainty c TCD
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of model results is also heavily impacted by the choice of ice model parameters. We s
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aim to show the full uncertainty within PISM by varying both, ice model parameters n
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At resolutions of 2.5 to 5km, the dynamic representation of the transition zone discharge through
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and the sub-grid treatment of the grounding line allow for a qualitative (i.e. “shape West Antarctic
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ofthecurve”regardingtimeandforcingdependence)representationofthegrounding- Gateways
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grounding-linemotion(Feldmannetal.,2014;Pattynetal.,2013)(formoredetailssee s
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Appendix A1). In a set of simple forcing experiments, a step change in ocean tem- n
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peratures between 0.1 and 2K is instantaneously switched on in each sector and the a
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additional discharge in terms of sea-level-relevant ice loss is compared. r
| Abstract Introduction
3 Results D Conclusions References
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We find that the same ocean warming leads to fundamentally different responses of us Tables Figures
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the two sectors. The strongest response in terms of the rate of sea-level rise (Fig. 2) ion
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occurs within the first century in the Weddell Sea Sector and is associated with rapid P
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grounding-line retreat, mostly along the Institute and Möller Glaciers (Fig. 1a). The e (cid:74) (cid:73)
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samewarmingoftheAmundsenSeainitiatescomparativelyslowgrounding-lineretreat
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of Thwaites Glacier at first, followed by increasing rates of ice loss as the grounding
line eventually moves deeper into Central West Antarctica. Dis FullScreen/Esc
Thesensitivityoficelosstothestrengthofoceanwarmingalsofundamentallydiffers c
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for the two gateways (Fig. 3). We observe strong incremental ice loss per degree of sio Printer-friendlyVersion
warming in the Weddell Sea Sector for warming levels up to 0.8K (Fig. 3a). Especially n
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for very low warming levels of up to 0.3K, the grounding lines retreat significantly, in a
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particular for the Institute and Möller Glaciers (Fig. 3b). Beyond 0.8K, additional ice e
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lossislesspronouncedanditoccursthroughthetopographicallystrongerconstrained D
Evans and Rutford ice streams. The situation is different for Amundsen Sea warming. isc TCD
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While ice loss after five centuries is negligible for warming levels up to 0.9K (Fig. 3c) ss
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because the grounding line does not significantly retreat (Fig. 3d), strong incremental n
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5 ice loss per degree of ocean warming is triggered beyond 0.9K. a
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4 Discussion |
West Antarctic
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The vulnerability of an ice sheet in terms of susceptibility to positive feedbacks asso- is
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ciated with grounding line retreat is – for our experimental setup – not a question of s M.A.Martinetal.
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self-sustained ice loss, which occurs over much longer time scales (Mengel and Lev- io
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ermann, 2014). We rather test the vulnerability to continued forcing, the “gear” that P
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translates warmer ocean waters into loss of buttressing, increased ice discharge and p
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groundinglineretreat(Drouetetal.,2013).Weproposethatthisgearisinteraliachar-
acterized by the specific combination of ice- and bedrock topography of the gateways | Abstract Introduction
where the major ice streams flow into the ocean. Figure 1 visualizes the grounding- D Conclusions References
15 loinceeamnigwraartimoningfo.rInrepthreesWenetadtdiveell eSnesaemSebcletomr,etmhebeorvserwdiethep5eknmingrebseollouwtionInsatnitdute2Kanodf iscus Tables Figures
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Möller Glaciers is associated with an ice sheet just thin enough to be close to floata- io
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(cid:74) (cid:73)
tion (Fig. 4). This means that even slight changes in ice thickness at the grounding P
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line lead to its retreat, exposing the underside of the newly formed ice shelf to the e (cid:74) (cid:73)
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continuing forcing by warm ocean waters, leading to further thinning and retreat. This
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mechanism ultimately results in high rates of sea-level rise: although the amount of
ice above floatation is limited near the grounding line, the speed of grounding line re- Dis FullScreen/Esc
treat over-compensates this limitation. In the Amundsen Sea Sector, by contrast, the c
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ice sheet upstream of the grounding line is characterized by a steep surface gradient. sio Printer-friendlyVersion
Therefore, although there is a comparably pronounced overdeepening of the bedrock, n
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the ice sheet has to thin considerably more in order to reach floatation such that the a
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groundinglinecanretreat.Thisargumentationsupportstheassertionthattheoveresti- e
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mationoftheinitialicethicknessinInstituteandMöllerGlaciersreducesthesimulated D
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speed of grounding line retreat and therefore counteracts the reported high rate of ice c TCD
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loss, although the effect is difficult to quantify. ss
Other possible reasons for the different consequences of Weddell and Amundsen ion 9,1705–1733,2015
5 Sea warming include basal resistance, ice hardness and differences in ice shelf but- Pa
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tressing. While it has been argued that basal resistance is slightly larger in Thwaites e Comparing ice
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GlacierthanintheInstituteMöllerBasin(Joughinetal.,2006,2009;Rossetal.,2012) discharge through
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this is not the case in our simulations, which compute basal resistance according to West Antarctic
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a pseudo-plastic sliding law involving yield stress of saturated till (Fig. 5). Therefore, it Gateways
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theeffectdiscussedhere;itis,however,unlikelytocounterit.Thesamelineofreason- ss M.A.Martinetal.
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Weddell Sea Sector and can thereby not explain the effect. a
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The thick and confined Filchner Ronne Ice Shelf has the potential to exert more r
15 buttressing than the ice shelves in the Amundsen Sea Embayment, such that its disin- | Abstract Introduction
tegration could imply a stronger dynamical disruption. However, we chose not to com-
pare the ice response of the different gateways to the same hypothetical reduction in Dis Conclusions References
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buttressing,butrathercomparetheiceresponsetothesamechangeinoceantemper- us Tables Figures
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ature,andlettheresponseoftheiceshelvesandthereforetheicesheetbedetermined io
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by – if not elaborated but at least plausible – basic physical principles (compare Ap- (cid:74) (cid:73)
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relative increase in actual sub-shelf melting instead of ocean temperature, one can r
estimate that the qualitative and quantitative difference of the ice response would re- | Back Close
main.Theratioofactualicethicknessandfloatationicethicknessisapparentlyagood
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measure for the grounding line retreat that can be expected for continued warm-water is FullScreen/Esc
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forcing (Fig. 4): ice that is thinner than about 160% of the floatation ice thickness is s
likely to get afloat within one century of 2K warm-water forcing. sio Printer-friendlyVersion
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5 Conclusions D
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While it cannot be claimed that PISM represents the temporal evolution of the ice re- s
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sponse to ocean warming in a quantitatively accurate way, the qualitative difference io 9,1705–1733,2015
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between the Weddell and Amundsen Sea Sectors can be explained by the different P
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the ice-dynamics in the model. Our results are not to be understood as a projection, discharge through
mainlybecauseoftheexperimentalsetupofstep-functiontypeforcing,butalsoforvar- | West Antarctic
iousotherreasons:thoseincludethebasicparameterizationofsub-shelfmeltratesas D Gateways
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afunctionofoceantemperaturesandshelfbaseelevation,thelimitedspatialresolution c
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10 of bed andice topography and the fixedsurface accumulation. Also relevant isthe fact ssio M.A.Martinetal.
that we use steady states of the ice sheet as starting points for our simulations, which n
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are presented as either an un-weighted mean of the ice physics ensemble or focus a
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on a representative ensemble member. Nevertheless our results can provide insight e TitlePage
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about potential future differences between major West Antarctic gateways. In spite of
| Abstract Introduction
its current stability compared to the Amundsen Sector, the ice sheet draining into the
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Weddell Sea might be more susceptible to relatively small climatic changes than pre- Dis Conclusions References
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viously thought or even – providing a significant amount of warm water continuously us Tables Figures
reaches the grounding line – become a major contributor to sea-level rise from West s
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Antarctica in the future. n (cid:74) (cid:73)
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Appendix A: Methods D
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A1 Ice sheet model s
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SimulationswerecarriedoutwiththeParallelIceSheetModel(PISM,basedonversion P
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stable0.5) at a horizontal resolution of 24, 15, 12, 5km and for the Amundsen Sea p
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Sector also 2.5km (compare Figs. 7 and 8). Although even this resolution is arguably r
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too low for capturing the details of the stress balance in features such as narrow ice |
West Antarctic
streams (compare Favier et al., 2014), 10 to 20 grid cells for a 50km wide ice stream D
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aresufficientforthestrictlycomparativeanalysispresentedhere.Sub-gridscalebasal isc
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drag and sub-shelf melting at the grounding line ensure a more realistic grounding- s M.A.Martinetal.
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line motion. While without the former PISM fails the reversibility test in the original io
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MISMIPintercomparisonat16kmresolutionandsucceedsat1km(Pattynetal.,2013), P
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intermediateresolutionsaretestedbyFeldmannetal.(2014),alongwithnewsub-grid e TitlePage
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features. They enhance the model performance considerably, even at resolutions as
| Abstract Introduction
coarse as 5 or 12km.
PISM captures lateral resistance as well stress transmission across the grounding D Conclusions References
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line by using the hybrid shallow approximation (Bueler and Brown, 2009; Winkelmann c
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et al., 2011). The model hence simulates the loss of buttressing caused by ice-shelf s
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thinning(Gagliardinietal.,2010)andthepossibleresultinggrounding-lineretreat.The n
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kinematic calving law allows for a gradual retreat of the calving front in response to a
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changes in the velocity field. Therefore, buttressing can also reduce as a result of e (cid:74) (cid:73)
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calving front retreat behind pinning points like, e.g., Berker Island.
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The simulated ice topography is especially sensitive to basal drag (Gladstone et al.,
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2012).Inordertocoverthefullsliding-specificuncertaintyinPISM,athree-dimensional is FullScreen/Esc
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parameter space for basal sliding is therefore sampled in the experiments presented u
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For steady state ice surface velocities compare Fig. 10. n
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A2 Boundary conditions D
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PISM is run using the ALBMAP bedrock topography (Le Brocq et al., 2010) like in s
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the SeaRISE effort (Bindschadler et al., 2013; Nowicki et al., 2013). Compare also io 9,1705–1733,2015
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Fig. 11 for an Antarctica-wide display of this measure from ALBMAP data. The influ- P
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5 enceofbedrockroughnessongroundinglinemigrationfordifferentresolutionsishard pe Comparing ice
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to quantify. At least for the 5km simulations the original bedrock data did not need to discharge through
besmoothed.SteadysurfacetemperatureandmassbalancefieldsfromRACMO(Van | West Antarctic
de Berg et al., 2006) serve as climatic input data. Constant-in-time present day (av- D Gateways
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eraged 1950–1980) temperature- and salinity fields from ocean modeling with BRIOS c
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10 (Timmermann et al., 2002) are used for preparation of the initial states of the Antarc- ssio M.A.Martinetal.
tic Ice Sheet, translated into melt rates by a basic three equation system (Scheduikat n
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andOlbers,1990).Themeltratesthereforeroughlyadapttochangingice-shelfdepth, a
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while effects of sub-shelf circulation changes are ignored. Although this procedure is e TitlePage
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not particularly realistic, it suffices for the comparative type of analysis of idealized
| Abstract Introduction
experiments that we are performing. In the experiments, a uniform and time-constant
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anomaly in ocean temperatures between 0.1 and 2K is applied. Gaps in the original Dis Conclusions References
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ocean-temperatureandsalinityfieldsintheice-shelfcavity,whichbecomelargerasthe us Tables Figures
grounding line retreats and the ice-shelf cavities extend to formerly grounded marine s
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areas, are filled using a simple 2d-lateral diffusion equation. n (cid:74) (cid:73)
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A3 Experimental design e (cid:74) (cid:73)
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In this study we are using PISM exclusively. Since alternative approximations to the
stress balance, among other things, can strongly influence a model’s response to loss Dis FullScreen/Esc
in buttressing (Favier et al., 2014), we aim at exploring a broad range of free parame- c
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ters within PISM. In order to cover the full sliding-specific uncertainty, which has been sio Printer-friendlyVersion
identified to have the strongest impact on the models response to ice shelf thinning n
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in PISM, we prepare an ensemble of initial steady states. To that end, we sample a
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