Transcript Patagonia Ice Fields

Patagonia Ice Field Melting Observed by GRACE
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
Joint International GSTM and DFG SPP Symposium, October 15-17, 2007 at GFZ Potsdam
Image of the Patagonia Ice Fields
Motivations:
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Patagonia Ice Fields (PIF) are second largest SH ice mass
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Northern Patagonia Ice Field (NPIF ), ~ 4200 km2 - Chile;
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Southern Patagonia Ice Field (SPIF), ~ 13000 km2 -Chile and Argentina
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Observed historical / contemporary melting / retreat.
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GRACE RL04 solutions show improved spatial resolution, though
coarse relative to ~hundred km scale of PIF.
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PIF mass change may be estimated from RL04 and:
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Filtering to suppress noise;
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Forward modeling, using known location of the PIF;
GRACE Data and Processing
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CSR GRACE RL04 solutions
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April 2002 through December 2006;
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53 monthly solutions;
Filters
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De-correlation filtering [Swenson and Wahr, 2006]
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300 km Gaussian smoothing
GRACE Data Processing
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Global Mass Change Rate Map
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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
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Least squares fit every time series with annual, semiannual, 161-day
S2 ocean tide alias, and linear mass rate.
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Examine maps of mass rate for globe and smaller areas of interest
such as the PIF
Global Mass Rates (cm/year): Apr 2002 - Dec 2006
GRACE Apparent Mass Rates in the PIF Region
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
Forward Modeling Scheme
Forward Modeling
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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.
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Expand in SH to degree, order 60; apply same noise
decorrelation and 300 km Gaussian smoothing filters.
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Adjust model rates to agree with GRACE rate maps,
and sum of values over the red area to agree with
GRACE rate maps.
GRACE and Modeled PIF Mass Rates
Model: - 24.3 km3/year mass
rate uniform over PIF
To Obtain an Ice Mass Rate, Other
Signals Must Be Estimated
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Land Water Storage Change
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LadWorld land surface model;
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April 2002 through November 2006;
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Estimate is ~ - 5.4 km3/year land water mass change.
Postglacial Rebound (PGR)
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Regional PGR rate from Erik Ivins [Ivins and James, 2004).
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PGR effect is sensitive to asthenosphere viscosity.
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Assume 65 km lithosphere and mantle viscosity of 1.0 x 10 19 Pa s.
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Estimated PGR contribution ~ + 6.1 km3/year.
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Terrestrial Water Storage + PGR estimated to approximately cancel.
Conclusions:
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GRACE RL04 data are of sufficient spatial resolution to be useful in
studying features as small as the PIF.
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Forward modeling with known geography is a useful interpretive
tool and provides quantitative estimates.
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Predicted water storage and PGR effects are of opposite sign and
nearly cancel.
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PIF ice melting rate is estimated to be 25.0 ± 9.2 km3/year.
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Uncertainty estimate combines least squares fit errors and 100% of
predicted hydrological and PGR effects. (Note: errors could be
larger than 100%!)
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25.0 km3/year agrees reasonably well with others based on
topography and remote sensing data
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.