Climate-Driven Deformation of the Solid Earth from GRACE and GPS
Davis, J. L., P. Elósegui, J. X. Mitrovica, and M. E. Tamisiea (2004), Climate-driven deformation of the solid Earth from GRACE and GPS, Geophys. Res. Lett., 31, L24605, doi: 10.1029/2004GL021435.
GRACE data indicate large seasonal variations in gravity that are assumed to be related to climate-driven fluxes of surface water. Seasonal redistribution of surface mass should deform the Earth, and our calculations using GRACE data suggest vertical deformations of 13 mm in the region of greatest flux, the Amazon River Basin. To test the GRACE gravity-hydrology connection, we analyzed GPS data acquired from sites in this region. After accounting for degree 1 variations not observable with GRACE, we find that annual deformation measured with GPS correlates highly with predictions calculated from GRACE measurements. These results confirm the variations in surface water sensed by GRACE, which are significantly larger than those predicted by some hydrology models. The results also demonstrate that GRACE can be an important tool for monitoring deformation of the Earth, and suggest that combined analysis of GRACE and GPS may be a useful approach for estimation of geocenter variations.
Figure 1. Maps of the (a) in-phase and (b) out-of-phase annual amplitude of radial deformation inferred from GRACE gravity harmonics (Eq. (1)).
Figure 2. In-phase (left) and out-of-phase (right) annual radial deformation amplitudes for South America. Also shown are the locations of the GPS sites used the analysis, with the site names given in the right figure.
Figure 3. Time series of estimated vertical position Δz (relative to average value) for GPS site BRAZ. The error bars represent ±1σ. The red line is the model for this site calculated using the best-fit parameters (including annual and semi- annual variations). The yellow line represents the annual variations predicted for the location of site BRAZ from the GRACE observations (after l=1 corrections).
Figure 4. Comparison of in-phase and out-of-phase annual vertical variations estimated using GPS and GRACE (after l=1 corrections). The dotted line has unit slope and passes through the origin.
This research was supported by NASA grants NNG04GF09G, NNG04GL69G, and NAG5-13748, and by both the Natural Sciences and Engineering Research Council of Canada and the Miller Institute for Basic Sciences (JXM). Comments from G. Blewitt and an anonymous referee were greatly appreciated. We thank the organizations responsible for operating and maintaining the GPS sites used in this study. The GPS data files were obtained from archives operated by SOPAC, UNAVCO, Inc., and NASA. We thank W. Bertiger and S. Kedar for providing GIPSY scripts and assistance. Some figures were generated using Generic Mapping Tools version 3 [Wessel and Smith, 1995].
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