Satellite altimetry over ice shelves: tides and grounding lines
Helen Amanda Fricker(1) and Laurie Padman(2)
Scripps Institution of Oceanography,
9500 Gilman Drive,
La Jolla CA 92037,
(2) Earth & Space Research, 3350 SW Cascade Ave., Corvallis, OR 97333-1536, United States
The floating ice shelves are the areas of the Antarctic ice sheet that are most susceptible to the effects of climate change, justifying recent intense efforts to determine trends in ice-shelf mass balance terms including thickness and velocity. However, ice shelves move vertically with the underlying ocean tide, and any errors in predicted tide displacement can be aliased by the satellite orbits into apparent trends, seasonal cycles, and other periods. There are two factors making tide removal more challenging for Antarctic ice shelves than for the bulk of the global ocean. First, tide models for Antarctica are not as accurate as they are in mid-latitude oceans. This is because basic model inputs such as coastline/grounding-line locations and sub-ice-shelf bathymetry are poorly known, and the tide-resolving TOPEX/Poseidon and Jason radar altimeter missions only extend to 66 degrees S. Second, shelf ice within the grounding zone (GZ), a band a few kilometers wide adjacent to the grounding line, is not in hydrostatic equilibrium with the underlying ocean tide, but shows a reduced amplitude response.
Here, we review work we have done to help resolve this significant problem, including methods for improving the accuracy of Antarctic tide models through assimilation of multi-mission satellite altimeter data (ERS and ICESat) and in situ data from ice she
lf GPS and ocean moorings. We also present early results of a technique which uses repeat-track ICESat data across the GZ to determine the location and flexure characteristics of the GZ. Since ICESat covers all of the ice shelves, by analyzing ICESat data around the perimeters of the RIS and FRIS, we can significantly improve the GZ definition relative to existing estimates. By defining the flexure characteristics of the GZ, including width and curvature, it will be possible for the first time to apply tidal corrections within the GZ, which is frequently a region of intenseb asal melt and thus a significant focus of shelf-ice mass balance studies. Our studies will also contribute to tide removal for GRACE by improving not only the prediction of ocean tide but also the associated ocean load tide, whose influence extends over the entire Antarctic continent.