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A four decade record of elevation change of the Amery Ice Shelf, East Antarctica

A four decade record of elevation change of the Amery Ice Shelf, East Antarctica . 1 Matt King , 2,3,4 Richard Coleman, 5 Helen Amanda Fricker, 2,3,4 Ben Galton-Fenzi, 6 Benoit Legresy, 7 Laurie Padman, 2,4,8 Roland Warner

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A four decade record of elevation change of the Amery Ice Shelf, East Antarctica

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  1. A four decade record of elevation change of the Amery Ice Shelf, East Antarctica 1Matt King, 2,3,4Richard Coleman, 5Helen Amanda Fricker, 2,3,4Ben Galton-Fenzi, 6Benoit Legresy,7Laurie Padman, 2,4,8Roland Warner 1Newcastle University, UK; 2University of Tasmania, Australia; 3CSIRO, Australia; 4ACE CRC, Australia;5Scripps Institution of Oceanography, USA; 6CNRS-CNES, France; 7Earth and Space Research, USA; 8Australian Antarctic Division, Australia Acknowledgements:Max Corry, Jo Jacka, Bill Budd, Andrew Ruddell

  2. Early work • Seminal work of Budd et al. [1982] based on 1968-69 survey & glaciological data • Angles, distances, astronomical observations • Levelling • Accumulation • Ice core • One of the first large-scale surveys of an Antarctic Ice Shelf

  3. Early work • Seminal work of Budd et al. [1982] based on 1968-69 survey & glaciological data • Angles, distances, astronomical observations • Levelling • Accumulation • Ice core • One of the first large-scale surveys of an Antarctic Ice Shelf

  4. Since then… • Major calving event in ~1963 • Progressive growth since • Multi-decadal forcing • Potential climate effects • No data on ocean temperature change • Atmospheric temperature change small-to-negligible • Upstream sub-glacial lakes (Lambert Glacier) • Is there any observable ice shelf response? Monaghan et al. [2006]

  5. AIS elevation change • Zwally et al. [2005] ERS-1/2 results show net elevation increase of AIS 1992-2001 • Infers net thickening 1992-2001 • Does this same trend continue over longer timescales? • Longer period reduces impact of intra-decadal spatio-temporal accumulation variations • Also, what change, if any, is seen in the velocity field?

  6. Velocity data • Terrestrial data • Traversing on a moving conveyor belt over 500km • Angles (Wild T3), distances (Tellurometer MRA3) and star observations • Two closed traverses measured between April 1968 and Feb. 1970 • Analysis using “reduction to epoch” method • Large (>2.5km, ~43’) and unidentified misclose in previous traverse analysis

  7. Velocity data • Reanalysis • Identify and correct errors causing misclose in Budd et al. [1982] analysis using raw field notes/computer output • Difficult due to the weakly constrained traverse • Multiple angle errors were identified • Addition of data previously not used (star observations for azimuth, extra ties to fixed locations) • Re-analysed the observations using least squares

  8. New velocity results • Miscloseless than 5m or 11” (500km of traversing) • May still be some undetectable gross errors • Average velocity uncertainties (1 sigma) 1.4m/yr and 0.3 • Variations from previous analysis • Up to 4km in position (bore hole position varies by ~3km) • Up to 110m/yr and 38, but median differences ~5m/yr and 0.8 • Conclusions of Budd et al. [1982] still supported King, M. A., R. Coleman, P. J. Morgan, and R. S. Hurd (2007), Velocity change of the Amery Ice Shelf, East Antarctica, during the period 1968-1999, J. Geophys. Res., 112, F01013, doi:01010.01029/02006JF000609.

  9. Comparison to ~1997 “InSAR” velocities Centreline differences (terrestrial minus RADARSAT) • Systematic biases evident between terrestrial and RADARSAT-derived velocities of Young and Hyland [2002] • Similar magnitude but different pattern in Joughin [2002] • Differences on grounded ice 1-3m/yr, suggesting tides + “inverse barometer” source • Also geo-location issues? • Real change possible? Magnitude (m/yr) Direction (°)

  10. GPS velocities • GPS velocities from data 1991-1999 • In good agreement with terrestrial velocities • Suggests RADARSAT results are biased in this region by up to ±20-30m/yr • Suggests possible slow-down of 0.6%, 1968-1999

  11. Levelling Data(1968-69) • 500km of geodetic levelling in 1968-69 • From 1968-69 ice front south for ~250km, 2 lateral lines • Raw field notes re-analysed • Propagated errors <0.2m • Datum • Connection to instantaneous sea level NE Amery • Levelling lat/lonbased on traverse locations

  12. Levelling Data(1968-69) • 500km of geodetic levelling in 1968-69 • From 1968-69 ice front south for ~250km, 2 lateral lines • Raw field notes re-analysed • Propagated errors <0.2m • Datum • Connection to instantaneous sea level NE Amery • Levelling lat/lon based on traverse locations Sea level connection

  13. GPS data(1995-2006) • Data • Profiles collected in 1995, 1998-99, 1999-2000, 2003 and 2005-6 • Some profiles attempted to follow the levelling lines • Processing stream • Profiles relative to local ice shelf base station in Track software • Base station motion (tidal etc) determined using kinematic positioning in GIPSY software (kinematic precise point positioning mode) • Apply model for tides and Inverse Barometer Effect (IBE)

  14. ICESat data(2003-7) • October 2003 to April 2007 from ICESat campaigns: Laser 2a, 2b, 3b, 3c, 3d, 3e, 3f, 3g, and 3h • Release 428 • Low gain saturation correction applied • Converted to WGS84 ellipsoid • Re-tided, then more accurate tide models and IBE corrections applied

  15. ERS-1&2 data(1992-2003) • Data • from http://icesat4.gsfc.nasa.gov/index.html, as Level 2 data records (version 5) • NASA/GSFC V5 range-retrackingalgorithm, etc. • 35-day repeat data in ice and ocean mode • Re-tided, then more accurate tide models and IBE corrections applied

  16. Datum connections • Elevation from levelling (H) and GPS/ICESat (h) is not the same • Corrections • h= H+ N – T – IB - MDT • Geoid (N) ~15m (GRACE geoid: EIGEN-GL04C) • Tides (T) ~1-2m (TPXO6.2) • Inverse Barometer Effect (IB) <=0.4m • Mean Dynamic Ocean Topography (MDT) ~-1.5m (altimetry minus geoid) • Further correction for MSL change (~0.002m/yr) not applied Ice shelf Instantaneous T+IB Mean MDT N (Geoid-Ellipsoid) Geoid Ellipsoid

  17. Crossovers • 84 GPS-levelingcrossovers (27-38 years) • 119 ICESat-levelingcrossovers (35-39 years)

  18. Crossover dh/dt • ICESat-levelling crossovers normally distributed, close to 0 m/yr • GPS-levelling crossovers not normally distributed • smaller-scale features playing a more dominant role? ICESat-levelling GPS-levelling

  19. Multi-decadal picture is close to zero change Mean dh/dt values -0.003 m/yr for GPS-levelling +0.013 m/yr for ICESat-levelling Timeline of change

  20. Elevation fluctuations over shorter periods Agreements between techniques Timeline of change Net thickening Infer small net thinning ~1968-1996.5

  21. Elevation fluctuations (ERS)

  22. Elevation fluctuations (ICESat) Ice Shelf Grounded Ice

  23. Elevation fluctuations (ERS) Grounded Ice Grounded Ice Ice Shelf Ice Shelf Grounded Ice Ice Shelf

  24. Origins of post-1996.6 change • What is the role of surface densification variations? ? Surface lowering Mean Jan Temp Surface raising ? Amery Ice Shelf proxy and observed January temperature data(Courtesy Ian Allison)

  25. Liu et al [2006] Passive Microwave Melt Extents Tedesco [2009]

  26. Origins of post-1996.6 change • Ocean/ice shelf modelling suggests • A 0.5 degCthermal forcing in the surface ocean gives a ~0.3 m/year increase in the melt rate at AM03 • This corresponds to a temperature change in the layer adjacent to the ice base of 0.08 degC • Surface ocean temperatures changes of 0.5 degCpossible • Could the oceans play a part? Location in northern Amery

  27. Conclusions (Amery) • Elevation of northern Amery Ice Shelf is unchanged over period 1968-present • Variations of up to ~±0.2m/yr over periods up to ~1 decade; high spatial variability • Net thickening ~1992-2003 (agreement with altimetry) • Net thinning ~1968-1992; 2003-present • Signals may originate in densification variation but oceans could contribute • Velocity over 1968-1999 has slowed by 0.6%, in agreement with little elevation change over this time • Places new constraints on multi-decadal changes of a large Antarctic ice shelf • Further info: King et al. [2007, 2009], JGR Earth Surface

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