Assimilation of Antarctic velocity observations provides evidence for uncharted pinning points

Abstract. In ice flow modelling, the use of control methods to assimilate the dynamic and geometric state of an ice body has become common practice. These methods have primarily focussed on inverting for one of the two least known properties in glaciology, namely the basal friction coefficient or the ice viscosity parameter. Here, we present an approach to infer both properties simultaneously for the whole of the Antarctic ice sheet. After the assimilation, the root-mean-square deviation between modelled and observed surface velocities attains 8.7 m a−1 for the entire domain, with a slightly higher value of 14.0 m a−1 for the ice shelves. An exception in terms of the velocity mismatch is the Thwaites Glacier Ice Shelf, where the RMS value is almost 70 m a−1. The reason is that the underlying Bedmap2 geometry ignores the presence of an ice rise, which exerts major control on the dynamics of the eastern part of the ice shelf. On these grounds, we suggest an approach to account for pinning points not included in Bedmap2 by locally allowing an optimisation of basal friction during the inversion. In this way, the velocity mismatch on the ice shelf of Thwaites Glacier is more than halved. A characteristic velocity mismatch pattern emerges for unaccounted pinning points close to the marine shelf front. This pattern is exploited to manually identify seven uncharted features around Antarctica that exert significant resistance to the shelf flow. Potential pinning points are detected on Fimbul, West, Shackleton, Nickerson and Venable ice shelves. As pinning points can provide substantial resistance to shelf flow, with considerable consequences if they became ungrounded in the future, the model community is in need of detailed bathymetry there. Our data assimilation points to some of these dynamically important features not present in Bedmap2 and implicitly quantifies their relevance.

[1]  Eric Rignot,et al.  Roles of marine ice, rheology, and fracture in the flow and stability of the Brunt/Stancomb-Wills Ice Shelf , 2009 .

[2]  J. Bamber,et al.  Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice Sheet , 2009, Science.

[3]  E. Rignot,et al.  Larsen B Ice Shelf rheology preceding its disintegration inferred by a control method , 2007, Geophysical Research Letters.

[4]  Bernd Kulessa,et al.  Marine ice regulates the future stability of a large Antarctic ice shelf , 2014, Nature Communications.

[5]  Eric Rignot,et al.  Inversion of basal friction in Antarctica using exact and incomplete adjoints of a higher‐order model , 2013 .

[6]  R. Arthern,et al.  Flow speed within the Antarctic ice sheet and its controls inferred from satellite observations , 2015 .

[7]  Antony J. Payne,et al.  An improved Antarctic dataset for high resolution numerical ice sheet models (ALBMAP v1) , 2010 .

[8]  E. King,et al.  Seabed topography beneath Larsen C Ice Shelf from seismic soundings , 2013 .

[9]  Duncan J. Wingham,et al.  Sustained retreat of the Pine Island Glacier , 2013 .

[10]  C. Schoof Ice sheet grounding line dynamics: Steady states, stability, and hysteresis , 2007 .

[11]  Eric Rignot,et al.  A Reconciled Estimate of Ice-Sheet Mass Balance , 2012, Science.

[12]  Douglas R. Macayeal,et al.  A tutorial on the use of control methods in ice-sheet modeling , 1993, Journal of Glaciology.

[13]  Bo Sun,et al.  Bedmap2: improved ice bed, surface and thickness datasets for Antarctica , 2012 .

[14]  M. E. Peters,et al.  New boundary conditions for the West Antarctic Ice Sheet: Subglacial topography of the Thwaites and Smith glacier catchments , 2006 .

[15]  B. Scheuchl,et al.  Ice-Shelf Melting Around Antarctica , 2013, Science.

[16]  David G. Vaughan,et al.  BEDMAP: a new ice thickness and subglacial topographic model of Antarctica , 2001 .

[17]  B. Smith,et al.  Marine Ice Sheet Collapse Potentially Under Way for the Thwaites Glacier Basin, West Antarctica , 2014, Science.

[18]  Frank Pattyn,et al.  Antarctic subglacial conditions inferred from a hybrid ice sheet/ice stream model , 2010 .

[19]  F. Gillet-Chaulet,et al.  Interactive comment on “ Greenland Ice Sheet contribution to sea-level rise from a new-generation icesheet model ” , 2012 .

[20]  Robin E. Bell,et al.  Progressive unpinning of Thwaites Glacier from newly identified offshore ridge: Constraints from aerogravity , 2011 .

[21]  E. Rignot,et al.  Creep deformation and buttressing capacity of damaged ice shelves: theory and application to Larsen C ice shelf , 2013, The Cryosphere.

[22]  Ian Joughin,et al.  Ice-sheet velocity mapping: a combined interferometric and speckle-tracking approach , 2002, Annals of Glaciology.

[23]  R. Armstrong,et al.  The Physics of Glaciers , 1981 .

[24]  N. Glasser,et al.  Present stability of the Larsen C ice shelf, Antarctic Peninsula , 2010, Journal of Glaciology.

[25]  B. Scheuchl,et al.  Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011 , 2014 .

[26]  Angelika Humbert,et al.  A comparative modeling study of the Brunt Ice Shelf/ Stancomb-Wills Ice Tongue system, East Antarctica , 2009, Journal of Glaciology.

[27]  R. Arthern,et al.  Initialization of ice-sheet forecasts viewed as an inverse Robin problem , 2010, Journal of Glaciology.

[28]  C. Swithinbank,et al.  Proposed new terms and definitions for snow and ice features , 1977, Polar Record.

[29]  Mika Malinen,et al.  Capabilities and performance of Elmer/Ice, a new-generation ice sheet model , 2013 .

[30]  D. Macayeal,et al.  Catastrophic ice-shelf break-up by an ice-shelf-fragment-capsize mechanism , 2003, Journal of Glaciology.

[31]  A. Vieli,et al.  Causes of pre-collapse changes of the Larsen B ice shelf: Numerical modelling and assimilation of satellite observations , 2007 .

[32]  T. Scambos,et al.  Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica , 2004 .

[33]  M. Broeke Depth and Density of the Antarctic Firn Layer , 2008 .

[34]  R. Hindmarsh The role of membrane-like stresses in determining the stability and sensitivity of the Antarctic ice sheets: back pressure and grounding line motion , 2006, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[35]  R. Bindschadler,et al.  The Landsat Image Mosaic of Antarctica , 2008 .

[36]  H. Rott,et al.  Rapid Collapse of Northern Larsen Ice Shelf, Antarctica , 1996, Science.

[37]  M. R. van den Broeke,et al.  Calving fluxes and basal melt rates of Antarctic ice shelves , 2013, Nature.

[38]  Eric Rignot,et al.  Spatial patterns of basal drag inferred using control methods from a full‐Stokes and simpler models for Pine Island Glacier, West Antarctica , 2010 .

[39]  F. Nitsche,et al.  The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0 , 2013 .

[40]  T. Nagler,et al.  The imbalance of glaciers after disintegration of Larsen-B ice shelf, Antarctic Peninsula , 2010 .

[41]  J. Bamber,et al.  Antarctic ice-shelf thickness from satellite radar altimetry , 2011, Journal of Glaciology.

[42]  F. Nitsche,et al.  The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0—A new bathymetric compilation covering circum‐Antarctic waters , 2013 .

[43]  Gaël Durand,et al.  Greenland ice sheet contribution to sea-level rise from a new-generation ice-sheet model , 2012 .

[44]  M. Morlighem,et al.  Representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics: Application to Brunt/Stancomb‐Wills Ice Shelf, Antarctica , 2014 .

[45]  Douglas R. Macayeal,et al.  Large‐scale ice flow over a viscous basal sediment: Theory and application to ice stream B, Antarctica , 1989 .

[46]  Eric Rignot,et al.  Antarctic grounding line mapping from differential satellite radar interferometry , 2011 .

[47]  E. Rignot Evidence for rapid retreat and mass loss of Thwaites Glacier, West Antarctica , 2001, Journal of Glaciology.

[48]  Thomas Zwinger,et al.  A three-dimensional full Stokes model of the grounding line dynamics: effect of a pinning point beneath the ice shelf , 2011 .

[49]  B. Scheuchl,et al.  Ice Flow of the Antarctic Ice Sheet , 2011, Science.

[50]  E. Rignot,et al.  Acceleration and spatial rheology of Larsen C Ice Shelf, Antarctic Peninsula , 2011 .

[51]  G. Gudmundsson Ice-shelf buttressing and the stability of marine ice sheets , 2012 .

[52]  Per Christian Hansen,et al.  Analysis of Discrete Ill-Posed Problems by Means of the L-Curve , 1992, SIAM Rev..

[53]  Eric Rignot,et al.  A Reconciled Estimate of Ice-Sheet Mass Balance , 2012, Science.

[54]  David M. Holland,et al.  Sensitivity of 21st century sea level to ocean‐induced thinning of Pine Island Glacier, Antarctica , 2010 .

[55]  C. J. van der Veen,et al.  Fundamentals of glacier dynamics , 1999 .

[56]  I. Joughin,et al.  Rheology of the Ronne Ice Shelf, Antarctica, inferred from satellite radar interferometry data using an inverse control method , 2005 .

[57]  W. Paterson Hydraulics of Glaciers , 1994 .

[58]  F. Pattyn,et al.  Using ice-flow models to evaluate potential sites of million year-old ice in Antarctica , 2013 .

[59]  H. Fricker,et al.  Properties of a marine ice layer under the Amery Ice Shelf, East Antarctica , 2009, Journal of Glaciology.

[60]  D. Vaughan,et al.  Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets , 2009, Nature.

[61]  R. Bell,et al.  Inversion of IceBridge gravity data for continental shelf bathymetry beneath the Larsen Ice Shelf, Antarctica , 2010, Journal of Glaciology.

[62]  Eric Rignot,et al.  Recent Antarctic ice mass loss from radar interferometry and regional climate modelling , 2008 .

[63]  M. Nodet,et al.  Investigating changes in basal conditions of Variegated Glacier prior to and during its 1982–1983 surge , 2011 .

[64]  D. Vaughan,et al.  Antarctic ice-sheet loss driven by basal melting of ice shelves , 2012, Nature.

[65]  John W. Holt,et al.  New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island Glacier , 2006 .

[66]  T. Zwinger,et al.  The ISMIP-HOM benchmark experiments performed using the Finite-Element code Elmer , 2008 .

[67]  Ice Shelves: A Review , 1979 .

[68]  Theodore A. Scambos,et al.  Mass loss of Larsen B tributary glaciers (Antarctic Peninsula) unabated since 2002 , 2012 .

[69]  A. Vieli,et al.  Numerical modelling and data assimilation of the Larsen B ice shelf, Antarctic Peninsula , 2006, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.