Ian D. Thomas

Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations

Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over?estimated, par t icularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root?meansquares of observation?model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.

Regional biases in absolute sea-level estimates from tide gauge data due to residual unmodeled vertical land movement

The only vertical land movement signal routinely corrected for when estimating absolute sea-level change from tide gauge data is that due to glacial isostatic adjustment (GIA). We compare modeled GIA uplift (ICE-5G + VM2) with vertical land movement at ?300 GPS stations located near to a global set of tide gauges, and find regionally coherent differences of commonly ±0.5–2 mm/yr. Reference frame differences and signal due to present-day mass trends cannot reconcile these differences. We examine sensitivity to the GIA Earth model by fitting to a subset of the GPS velocities and find substantial regional sensitivity, but no single Earth model is able to reduce the disagreement in all regions. We suggest errors in ice history and neglected lateral Earth structure dominate model-data differences, and urge caution in the use of modeled GIA uplift alone when interpreting regional- and global- scale absolute (geocentric) sea level from tide gauge data.

A Validation of Ocean Tide Models Around Antarctica Using GPS Measurements

Ocean tide models around the coastline of Antarctica are often poorly constrained, due to sparse data input and poorly known bathymetry in the ice-shelf regions. Land-based measurements of Ocean Tide Loading Displacements (OTLD), such as those made by GPS, provide a means of assessment of ocean tide models in such regions. Up to 11 years of daily GPS data from 18 stations on the Antarctic continent were processed using an up-to date estimation strategy based upon a precise point positioning analysis. Carrier-phase ambiguities were fixed, and parameters representing harmonic ground displacements at 4 diurnal frequencies (M2, S2, N2 and K2) and 4 semi-diurnal frequencies (K1, O1, P1 and Q1) were estimated on a daily basis, and then combined to form the GPS estimates of OTLD. These were compared with estimates of OTLD computed by means of a convolution process with a Green’s function from seven global ocean tide models: CSR4, FES99, FES2004, GOT00.2, NAO.99b, TPXO6.2, TPXO7.0, and four regional ocean tide models: CATS02.01, CADA00.10, MTOs.05, AntPen04.01. Fixing of carrier phase ambiguities was, unexpectedly, found to result in a poorer agreement between GPS estimates and models. For Antarctica as a whole, the TPXO6.2 and TPXO7.0 global models offer very good agreement with the GPS estimates of OTLD in all regions, with CADA00.10, MToS.05, CATS02.01 also generally being in good agreement. In East Antarctica, where the models are well constrained and in good agreement, the GPS estimates offer good agreement with the models – often to a sub-millimetre level – particularly for the lunar N2 and Q1 constituents. In West Antarctica, there is greater divergence amongst the modelled estimates of OTLD due to the complex coastline and less well modelled ice sheet regions. Here, the TPXO6.2, TPXO7.0 and CADA00.10 models offer equally good agreement. In summary, GPS measurements of OTLD are of sufficient accuracy to distinguish between the models in certain regions of Antarctica, although some systematic biases remain at solar frequencies.