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1 Andrew Shepherd 1 Zhijun Du 2 Andreas Vieli 1 University of Edinburgh 2 University of Durham

Melting and freezing beneath LIS. 1 Andrew Shepherd 1 Zhijun Du 2 Andreas Vieli 1 University of Edinburgh 2 University of Durham. Melting and freezing beneath LIS. NSIDC 2006. D. Vaughan, BAS. Surface forcing. Warming…. D. Vaughan, BAS. Surface forcing. Melting…. T. Scambos, NSIDC.

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1 Andrew Shepherd 1 Zhijun Du 2 Andreas Vieli 1 University of Edinburgh 2 University of Durham

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  1. Melting and freezing beneath LIS 1Andrew Shepherd 1Zhijun Du 2Andreas Vieli 1University of Edinburgh 2University of Durham

  2. Melting and freezing beneath LIS NSIDC 2006

  3. D. Vaughan, BAS Surface forcing Warming…

  4. D. Vaughan, BAS Surface forcing Melting… T. Scambos, NSIDC

  5. D. Vaughan, BAS W. Rack, AWI Surface forcing Collapsing… T. Scambos, NSIDC

  6. Other forcing? Lowering…

  7. Other forcing? Compacting…

  8. Other forcing? Thinning…

  9. Other forcing? Sea level change Sea density change Surface accumulation Ice flow Elevation change Firn densification Basal melting

  10. Other forcing? <10% <5% <10% Sea level change Sea density change Surface accumulation Ice flow <20% Elevation change Firn densification Basal melting

  11. Basal melting • 25 m thinning / decade

  12. Basal melting • 25 m thinning / decade • Deep ocean transport toward Larsen Muench and Gordon, JGR, 1995

  13. Basal melting • 25 m thinning / decade • Deep ocean transport toward Larsen • Weddell deep water has warmed 0.4C since 1970 Roberston and Visbeck, DSR, 2002 NSIDC 2006

  14. Basal melting • 25 m thinning / decade • Deep ocean transport toward Larsen • Weddell deep water has warmed 0.4C since 1970 • 1C warming melts 10 m Rignot and Jacobs, Science, 2002 NSIDC 2006

  15. Basal melting • 25 m thinning / decade • Deep ocean transport toward Larsen • Weddell deep water has warmed 0.4C since 1970 • 1C warming melts 10 m • 0.25C warming sufficient to melt Larsen Rignot and Jacobs, Science, 2002 NSIDC 2006

  16. Basal melting • 25 m thinning / decade • Deep ocean transport toward Larsen • Weddell deep water has warmed 0.4C since 1970 • 1C warming melts 10 m • 0.25C warming sufficient to melt Larsen • Ocean 0.3C above freezing off shelf X NSIDC 2006

  17. Basal melting • 25 m thinning / decade • Deep ocean transport toward Larsen • Weddell deep water has warmed 0.4C since 1970 • 1C warming melts 10 m • 0.25C warming sufficient to melt Larsen • Ocean 0.3C above freezing off shelf • MWDW mixed with ISW, no evidence of infiltration Nicholls et al., GRL, 2002 NSIDC 2006

  18. Ice acceleration ERS InSAR data

  19. Ice acceleration ERS InSAR velocity

  20. Ice acceleration ERS InSAR velocity error

  21. Melting and freezing beneath LIS Ice acceleration • ~20 % acceleration • in 4 years VLOOK 1995 VLOOK 1999-1995 NSIDC 2006

  22. 1999 Ice acceleration Modelled flow • 2D depth-averaged ice flow • model with uniform rheology • shows poor fit •  Try data assimilation T= -10oC Larsen B 1995/96 Observed flow Modelled flow

  23. Ice acceleration • Simultaneous optimization for rheology & boundary velocity • Opt. flow agrees, spatial pattern & magnitude • Non-opt. Flow, too slow & no shear margins Optimized velocity Observed velocity Non-Optimized velocity Velocity fit -10oC Vobs (m/y) Vmod (m/y)

  24. Ice acceleration Ice rheology B (Pa y1/3∙106) • Rheology shows weak, soft bands along margins • Coincide with fracture zones

  25. Ice acceleration VLOOK 1995 • Potential causes of • acceleration? • Ice thinning • Tributary flow • Frontal retreat • Softening VLOOK 1999

  26. Ice acceleration VLOOK 1995 • Potential causes of • acceleration? • Ice thinning • Tributary flow • Frontal retreat • Softening • Artefacts in VLOOK 1999 VLOOK 1999

  27. Ice acceleration • Reference: • model adjusted to 1995 • Experiment: • instant response to • perturbation Observed acceleration

  28. 1999 1995/96 Ice acceleration • Reference: • model adjusted to 1995 • Experiment: • instant response to • perturbation • Experiment #1: • Frontal retreat • Results: • Acceleration too low • Wrong pattern Observed acceleration

  29. Ice acceleration • Reference: • model adjusted to 1995 • Experiment: • instant response to • perturbation • Experiment #2: • Uniform thinning • Results: • Slow down • Stabilising feedback Observed acceleration

  30. Ice acceleration • Reference: • model adjusted to 1995 • Experiment: • instant response to • perturbation • Experiment #3: • 50% tributary acceleration • Results: • Required speedup too high • Effect rather than cause Observed acceleration

  31. Ice acceleration • Reference: • model adjusted to 1995 • Experiment: • instant response to • perturbation • Experiment #4: • Softening of weak zones • Results: • Reasonable acceleration • Uniform softening not enough Observed acceleration

  32. Ice acceleration • Reference: • model adjusted to 1995 • Experiment: • instant response to • perturbation • Experiment #5: • Retreat + softening • Results: • Similar magnitude • Similar pattern Observed acceleration

  33. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  34. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  35. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  36. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  37. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  38. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  39. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  40. Conclusions • Ice shelf thinning before, during, after collapse • Uniform, high basal melting unlikely • Ice shelf accelerated by 20% prior to collapse • Thinning at odds with acceleration • Frontal retreat + margin softening explains acceleration • Flow & collapse mainly controlled by weak zones • Surface & basal melting weaken margins • Stability of other ice shelves remains uncertain

  41. Implications Scambos et al., GRL, 2004 Rignot et al., GRL, 2004 Glacier drawdown after collapse

  42. Implications • Ice shelves fringe Antarctica

  43. Implications • Ice shelves fringe Antarctica • CDW nearby several

  44. Implications • Ice shelves fringe Antarctica • CDW nearby several • Submarine sectors are all in retreat

  45. Implications • Ice shelves fringe Antarctica • CDW nearby several • Submarine sectors are all in retreat • Ocean temperatures set to rise • > 10 m esl exposed Kim et al., GRL, 2005

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