1. Patagonia Ice Field Melting Observed by GRACE Joint International GSTM and DFG SPP Symposium, October 15-17, 2007 at GFZ Potsdam J.L. Chen 1, C.R. Wilson 1,2,4, B.D. Tapley 1, D.D. Blankenship 3,4 Center for Space Research, University of Texas at Austin, USA 1 Department of Geological Sciences, University of Texas at Austin, USA 2 Institute for Geophysics, University of Texas at Austin, USA 3 Jackson School of Geosciences, University of Texas at Austin, USA 4

2. Image of the Patagonia Ice Fields Image of the Patagonia Ice Fields

3. Motivations:  Patagonia Ice Fields (PIF) are second largest SH ice mass  Northern Patagonia Ice Field (NPIF ), ~ 4200 km 2 - Chile;  Southern Patagonia Ice Field (SPIF), ~ 13000 km 2 -Chile and Argentina.  Observed historical / contemporary melting / retreat.  GRACE RL04 solutions show improved spatial resolution, though coarse relative to ~hundred km scale of PIF.  PIF mass change may be estimated from RL04 and:  Filtering to suppress noise;  Forward modeling, using known location of the PIF;

4. GRACE Data and Processing  CSR GRACE RL04 solutions  April 2002 through December 2006;  53 monthly solutions;  Filters  De-correlation filtering [Swenson and Wahr, 2006]  300 km Gaussian smoothing

5. GRACE Data Processing  Global Mass Change Rate Map  53 point monthly time series of surface mass change relative to mean field (equivalent water layer thickness change relative to mean) at 1 x 1 degree grid points  Least squares fit every time series with annual, semiannual, 161-day S2 ocean tide alias, and linear mass rate.  Examine maps of mass rate for globe and smaller areas of interest such as the PIF

6. Global Mass Rates (cm/year): Apr 2002 - Dec 2006

7. GRACE Apparent Mass Rates in the PIF Region

8. GRACE Mass Change Time Series at Point A red curve: seasonal + 161-day tidal alias + linear trend; vertical scale is mass layer equivalent relative to mean GRACE field (not mass rate) blue curve: time series at A after removing seasonal & 161-day tidal alias

9. Forward Modeling Scheme

10. Forward Modeling  Assume: Mass change concentrated in PIF; but limited spatial resolution causes spatial leakage to surrounding areas.  Assign mass rates to NPIF and SPIF regions, uniform over rectangular regions. Retain GRACE mass rates outside red area.  Expand in SH to degree, order 60; apply same noise decorrelation and 300 km Gaussian smoothing filters.  Adjust model rates to agree with GRACE rate maps, and sum of values over the red area to agree with GRACE rate maps.

11. GRACE and Modeled PIF Mass Rates Model: - 24.3 km 3 /year mass rate uniform over PIF

12. To Obtain an Ice Mass Rate, Other Signals Must Be Estimated  Land Water Storage Change  LadWorld land surface model;  April 2002 through November 2006;  Estimate is ~ - 5.4 km 3 /year land water mass change.  Postglacial Rebound (PGR)  Regional PGR rate from Erik Ivins [Ivins and James, 2004).  PGR effect is  PGR effect is sensitive to asthenosphere viscosity.  Assume 65 km lithosphere and mantle viscosity of 1.0 x 10 19 Pa s.  Estimated PGR contribution ~ + 6.1 km 3 /year.  Terrestrial Water Storage + PGR estimated to approximately cancel.

13. Conclusions:  GRACE RL04 data are of sufficient spatial resolution to be useful in studying features as small as the PIF.  Forward modeling with known geography is a useful interpretive tool and provides quantitative estimates.  Predicted water storage and PGR effects are of opposite sign and nearly cancel.  PIF ice melting rate is estimated to be 25.0 ± 9.2 km 3 /year.  Uncertainty estimate combines least squares fit errors and 100% of predicted hydrological and PGR effects. (Note: errors could be larger than 100%!)  25.0 km 3 /year agrees reasonably well with others based on topography and remote sensing data

14. The authors would like to thank Erik Ivins for providing the PGR model data, and Chris Milly for providing the LadWorld land water storage data. The above analysis is being published in - Chen, J.L., C.R. Wilson, B.D. Tapley, D.D. Blankenship, Patagonia Ice Field Melting Observed by GRACE, Geophys. Res. Lett., 2007.