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Padman, L, Fricker HA.  2005.  Tides on the Ross Ice Shelf observed with ICESat. Geophysical Research Letters. 32   10.1029/2005gl023214   AbstractWebsite

The Ice, Cloud and land Elevation Satellite (ICESat) provides the first opportunity for measurement of surface elevation h(i) over the portions of the Antarctic ice shelves that are south of the European Remote Sensing (ERS) satellite maximum latitude (81.5 degrees S). The dominant source of short-period variability in h(i) is ocean tides. We use crossover elevation difference (Delta h(i)) data from the Ross Ice Shelf (RIS) to demonstrate ICESat's ability to detect the tidal signal, and to compare the accuracy of several tide models. The root-mean-square (rms) value of all RIS measurements of Delta h(i) is approximate to 0.74 m; after removing the tide using the most accurate model, the rms of the residual signal in regions of optimal model performance is approximate to 0.16 +/- 0.03 m. This value corresponds to an uncertainty in h(i) of 0.11 +/- 0.02 m. We postulate that the primary sources of the residual signal are tide model errors and the inverse barometer effect.

Padman, L, Fricker HA, Coleman R, Howard S, Erofeeva L.  2002.  A new tide model for the Antarctic ice shelves and seas. Annals of Glaciology, Vol 34, 2002. 34( Winther JG, Solberg R, Eds.).:247-254., Cambridge: Int Glaciological Soc   10.3189/172756402781817752   Abstract

We describe a new tide model for the seas surrounding Antarctica, including the ocean cavities nucler the floating ice shelves. The model uses data assimilation to improve its fit to available data. Typical peak-to-peak tide ranges on ice shelves are 1-2 m but can exceed 3 m for the Filchner-Ronne and Larsen Ice Shelves in the Weddell Sea. Spring tidal ranges are about twice these values. Model performance is judged relative to the similar to5-10 cut accuracy that is needed to fully utilize ice-shelf height data that will be collected with the Geoscience Laser Altimeter System, scheduled to be launched on the Ice, Cloud and land Elevation Satellite in late 2002. The model does not yet achieve this level of accuracy except very near the few high-quality tidal records that have been assimilated into the model. Some improvement in predictive skill is expected from increased sophistication of model physics, but we also require better definition of ice-shelf grounding lines and more accurate water-column thickness data in shelf seas and under the ice shelves. Long-duration tide measurements (bottom pressure gauge or global positioning system) in critical data-sparse areas, particularly near and on the Filchner-Ronne and Ross Ice Shelves and Pine Island Bay, are required to improve the performance of the data-assimilation model.

Padman, L, Costa DP, Dinniman MS, Fricker HA, Goebel ME, Huckstadt LA, Humbert A, Joughin I, Lenaerts JTM, Ligtenberg SRM, Scambos T, van den Broeke MR.  2012.  Oceanic controls on the mass balance of Wilkins Ice Shelf, Antarctica. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007301   AbstractWebsite

Several Antarctic Peninsula (AP) ice shelves have lost significant fractions of their volume over the past decades, coincident with rapid regional climate change. Wilkins Ice Shelf (WIS), on the western side of the AP, is the most recent, experiencing a sequence of large calving events in 2008 and 2009. We analyze the mass balance for WIS for the period 1992-2008 and find that the averaged rate of ice-shelf thinning was similar to 0.8 m a(-1), driven by a mean basal melt rate of < w(b)> = 1.3 +/- 0.4 m a(-1). Interannual variability was large, associated with changes in both surface mass accumulation and < w(b)>. Basal melt rate declined significantly around 2000 from 1.8 +/- 0.4 m a(-1) for 1992-2000 to similar to 0.75 +/- 0.55 m a(-1) for 2001-2008; the latter value corresponding to approximately steady-state ice-shelf mass. Observations of ocean temperature T obtained during 2007-2009 by instrumented seals reveal a cold, deep halo of Winter Water (WW; T approximate to - 1.6 degrees C) surrounding WIS. The base of the WW in the halo is similar to 170 m, approximately the mean ice draft for WIS. We hypothesize that the transition in < w(b)> in 2000 was caused by a small perturbation (similar to 10-20 m) in the relative depths of the ice base and the bottom of the WW layer in the halo. We conclude that basal melting of thin ice shelves like WIS is very sensitive to upper-ocean and coastal processes that act on shorter time and space scales than those affecting basal melting of thicker West Antarctic ice shelves such as George VI and Pine Island Glacier.

Padman, L, Siegfried MR, Fricker HA.  2018.  Ocean tide influences on the Antarctic and Greenland ice sheets. Reviews of Geophysics. 56:142-184.   10.1002/2016rg000546   AbstractWebsite

Ocean tides are the main source of high-frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide-induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite-based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice-ocean models and analytical glacier models that quantify the effect of ocean tides on lower-frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice-ocean interface, and higher-resolution models with improved representation of tidal energy sinks.

Padman, L, Erofeeva SY, Fricker HA.  2008.  Improving Antarctic tide models by assimilation of ICESat laser altimetry over ice shelves. Geophysical Research Letters. 35   10.1029/2008gl035592   AbstractWebsite

Assimilation of laser altimeter data from the Ice, Cloud and land Elevation Satellite (ICESat) significantly improves the accuracy of ocean tide models for the Ross Ice Shelf (RIS). For the most energetic tidal harmonic, K(1), assimilation reduces the root-mean-square error (RMSE) between the model and a set of 16 independent tide records on and near the RIS from 6.0 to 2.8 cm, and the combined RMSE for the six most energetic tidal harmonics from 7.7 to 5.4 cm. When only the six most recent and highest-quality tide records are considered, the combined RMSE is 4.8 cm. This value is close to the uncertainty expected from tidal analyses of the short (similar to 1-2 month) validation records, indicating that assessing further improvements in tide model accuracy will require development of a higher quality validation data set. Citation: Padman, L., S. Y. Erofeeva, and H. A. Fricker (2008), Improving Antarctic tide models by assimilation of ICESat laser altimetry over ice shelves, Geophys. Res. Lett., 35, L22504, doi:10.1029/2008GL035592.

Paolo, FS, Fricker HA, Padman L.  2016.  Constructing improved decadal records of Antarctic ice shelf height change from multiple satellite radar altimeters. Remote Sensing of Environment. 177:192-205.   10.1016/j.rse.2016.01.026   Abstract

Antarctica's ice shelves, the floating extensions of the ice sheet, exert an important dynamic constraint on the flow of ice from the grounded ice sheet to the ocean and, therefore, on changes in global sea level. Thinning of an ice shelf reduces its ability to restrain the ice discharge from the grounded ice sheet. However, our understanding of how ice shelf processes couple ice-sheet changes to climate variability is still rudimentary. In part, this is due to the brevity and low temporal resolution of surveys of ice shelf thickness relative to the broad range of time scales on which ice-sheet mass fluctuates. Here, we present improved procedures to construct 18-year (1994–2012) time series of Antarctic ice shelf surface height at high spatial resolution (~30 km) by merging data from three overlapping satellite radar altimeter missions (ERS-1, ERS-2, and Envisat). We apply an averaging scheme to enhance the signal-to-noise ratio of height changes over the floating ice shelves, and extract low-order polynomial trends using a robust approach (regularized regression with cross-validation) that accounts for both bias and variance in the fit. We construct formal confidence intervals by bootstrap resampling of the residuals of the fit. The largest source of height error arises from the interaction of the radar signal with the snow and firn surface; on annual time scales, changes in surface and sub-surface scattering and radar penetration lead to apparent height changes that are larger than the true surface-height change arising from densification. Our 18-year time series of surface height provide an insight into how ice shelves respond to the changing atmospheric and oceanic conditions. Our methods could also be applied to grounded portions of the ice sheets, both in Antarctica and Greenland.

Paolo, FS, Padman L, Fricker HA, Adusumilli S, Howard S, Siegfried MR.  2018.  Response of Pacific-sector Antarctic ice shelves to the El Nino/Southern Oscillation. Nature Geoscience. 11:121-+.   10.1038/s41561-017-0033-0   AbstractWebsite

Satellite observations over the past two decades have revealed increasing loss of grounded ice in West Antarctica, associated with floating ice shelves that have been thinning. Thinning reduces an ice shelf's ability to restrain grounded-ice discharge, yet our understanding of the climate processes that drive mass changes is limited. Here, we use ice-shelf height data from four satellite altimeter missions (1994-2017) to show a direct link between ice-shelf height variability in the Antarctic Pacific sector and changes in regional atmospheric circulation driven by the El Nino/Southern Oscillation. This link is strongest from the Dotson to Ross ice shelves and weaker elsewhere. During intense El Nino years, height increase by accumulation exceeds the height decrease by basal melting, but net ice-shelf mass declines as basal ice loss exceeds ice gain by lower-density snow. Our results demonstrate a substantial response of Amundsen Sea ice shelves to global and regional climate variability, with rates of change in height and mass on interannual timescales that can be comparable to the longer-term trend, and with mass changes from surface accumulation offsetting a significant fraction of the changes in basal melting. This implies that ice-shelf height and mass variability will increase as interannual atmospheric variability increases in a warming climate.

Paolo, FS, Fricker HA, Padman L.  2015.  Volume loss from Antarctic ice shelves is accelerating. Science.   10.1126/science.aaa0940   AbstractWebsite

The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using eighteen years of continuous satellite radar altimeter observations we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 km3 per year for 1994-2003 to rapid loss of 310 ± 74 km3 per year for 2003-2012. West Antarctic losses increased by 70% in the last decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades.

Phillips, HA, Laxon SW.  1995.  Tracking of Antarctic Tabular Icebergs Using Passive Microwave Radiometry. International Journal of Remote Sensing. 16:399-405. AbstractWebsite

Passive microwave images of Antarctica from the Special Sensor Microwave Imager (SSM/I) are used to track two giant tabular icebergs that originated from the Larsen ice shelf in 1986. Since microwave radiation is relatively insensitive to weather and unaffected by lighting conditions, the SSM/I instrument provides all-weather, year-round viewing. The icebergs are visible almost every day giving an ideal temporal resolution for tracking their motion. One of the icebergs was tracked until October 1988, six months after its last position noted in the Navy/NOAA Joint Ice Centre ice charts. The tracks of both icebergs reveal motion in sympathy with observed oceanographic currents and eddies.

Phillips, HA, Allison I, Coleman R, Hyland G, Morgan PJ, Young NW.  1998.  Comparison of ERS satellite radar altimeter heights with GPS-derived heights on the Amery Ice Shelf, East Antarctica. Annals of Glaciology, Vol 27, 1998. 27( Budd WF, Ed.).:19-24., Cambridge: Int Glaciological Soc Abstract

In the spring of 1995 an extensive global positioning system (GPS) survey was carried out on the Amery Ice Shelf, East Antarctica, providing ground-truth ellipsoidal height measurements for the European remote-sensing satellite (ERS) radar altimeters. GPS- and altimeter-derived surface heights have been compared at the intersecting points of the ERS ground tracks and the GPS survey. The mean and rms height difference for all ERS-1 geodetic-phase tracks across the survey region is 0.0 +/- 0.1m and 1.7 m, respectively. The spatial distribution of the height differences is highly correlated with surface topographic variations. Comparisons of GPS-derived surface-elevation profiles along ERS ground tracks show that the ERS altimeters can closely follow the GPS representation of the actual surface.

Phillips, HA.  1998.  Surface meltstreams on the Amery Ice Shelf, East Antarctica. Annals of Glaciology, Vol 27, 1998. 27( Budd WF, Ed.).:177-181., Cambridge: Int Glaciological Soc Abstract

A topographic map of a 120 km by 20 km section of the Amery Ice Shelf, East Antarctica, mapped with the global positioning system (GPS) in the spring of 1995, revealed two long, shallow troughs in the ice-shelf surface. Smooth features coinciding with these troughs appeared in a synthetic aperture radar image acquired 18 months earlier. ERS-1 altimeter waveform sequences and backscatter measurements along repeat satellite ground tracks across the same section of the Amery Ice Shelf, for the 1993-94 summer, exhibited a dramatic change over a 2 km sector between 30 January and 2 February 1994. The change is consistent with the presence of liquid water on the ice-shelf surface, located in the deeper of the two troughs. A time series of special sensor microwave/ imager brightness temperatures over the Lambert Glacier-Amery Ice Shelf region for the same period has sharp maxima on 5 January and 21 January 1994. These maxima are interpreted as the melting events leading to the meltstream observed in the altimeter data 25 days later.

Pritchard, HD, Ligtenberg SRM, Fricker HA, Vaughan DG, van den Broeke MR, Padman L.  2012.  Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature. 484:502-505.   10.1038/nature10968   AbstractWebsite

Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying(1,2) glacier acceleration along Antarctic ice-sheet coastal margins(3). Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers(4). Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula(5). But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted(3). Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow(2). The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen(6) and Bellingshausen(7) seas, and atmospheric warming on the Antarctic Peninsula(8). This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.