Frontal Ablation of Glaciers on Livingston Island 3 Frontal Ablation of Glaciers on Livingston Island

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This discussion paper is/has been under review for the journal The Cryosphere (TC). Frontal ablation of Please refer to the corresponding ﬁnal paper in TC if available. glaciers on Livingston Island

Frontal ablation and temporal variations B. Osmanoglu et al. in surface velocity of Livingston Island ice cap, Antarctica Title Page Abstract Introduction B. Osmanoglu1, M. I. Corcuera2, F. J. Navarro2, M. Braun3, and R. Hock1 Conclusions References 1Geophysical Institute, University of Alaska Fairbanks, P.O. Box 757320 Fairbanks, Alaska 99775, USA Tables Figures 2Dept. Matemática Aplicada, ETSI de Telecomunicación, Universidad Politécnica de Madrid, Av. Complutense, 30, 28040 Madrid, Spain J I 3Institute of Geography, University of Erlangen, Kochstrasse 4/4, 91054 Erlangen, Germany J I Received: 30 June 2013 – Accepted: 15 July 2013 – Published: 26 August 2013 Back Close Correspondence to: B. Osmanoglu ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Full Screen / Esc

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4207 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Abstract TCD Frontal ablation from marine-terminating glaciers and ice caps covering the islands off the western coast of the Antarctic Peninsula is poorly known. Here we estimate 7, 4207–4240, 2013 the frontal ablation from the ice cap of Livingston Island, the second largest island in 5 the South Shetland Islands archipelago, using glacier surface velocities obtained from Frontal ablation of intensity offset tracking of PALSAR-1 imagery and glacier ice thickness inferred from glaciers on principles of glacier dynamics and calibrated against ground-penetrating radar (GPR) Livingston Island measurements of ice thickness. Using 21 SAR images acquired between October 2007 and January 2011, we obtain surface velocities of up to 250 m yr−1 and an average B. Osmanoglu et al. −1 10 frontal ablation rate of about 509 ± 381 Mt yr , equivalent to a specific mass change of −0.7 ± 0.5 m w.e. yr−1 over the area of the ice cap (697 km2). A rough estimate of the surface mass balance of the ice cap gives 0.1 ± 0.1 m w.e. yr−1, resulting in a total Title Page −1 mass balance for Livingston Island ice cap of −0.6±0.5 m w.e. yr . We find that frontal Abstract Introduction ablation and surface ablation contribute equal shares to total ablation. We also find −1 15 large changes in frontal ablation rate (of ∼ 237 Mt yr ) due to temporal variability in Conclusions References surface velocities. This highlights the importance of taking into account the seasonality Tables Figures in ice velocities when computing frontal ablation with a flux-gate approach.

J I 1 Introduction J I

More than 99 % of the glacierized area of the islands in the periphery of the Antarctic Back Close 20 Peninsula drains through marine termini or into ice shelves (Bliss et al., 2013), but little is known about the magnitude and relative importance of mass loss through frontal Full Screen / Esc ablation (i.e. the sum of iceberg calving and submarine melting) at these termini. A few studies on marine-terminating ice caps in the Arctic show that frontal ablation might ac- Printer-friendly Version count for roughly 30–40% of the total ablation (Dowdeswell et al., 2002, 2008). Shep- Interactive Discussion 25 herd et al. (2012) give an estimate of mass loss (1992–2011) for the entire Antarctic Peninsula of −20±14 Gtyr−1 excluding glaciers and ice caps of the Antarctic periphery. 4208 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Since assessing and modelling of calving (Benn et al., 2007b, a; Amundson and Truffer, 2010; Otero et al., 2010; Bassis, 2011; Vieli and Nick, 2011) and submarine TCD melt (Motyka et al., 2003; Enderlin and Howat, 2013) are inherently difficult, mass 7, 4207–4240, 2013 changes by frontal ablation are often neglected or under-represented in regional and 5 global-scale mass budget assessments and projections (Radić and Hock, 2011; Cog- ley, 2012; Marzeion et al., 2012; Giesen and Oerlemans, 2013; Radić et al., 2013) Frontal ablation of leading to systematic underestimation of mass loss. While mass loss through surface glaciers on melting is reasonably well understood, the processes involved in frontal ablation are Livingston Island largely non-linear and operate on time scales that are not necessarily linked to re- B. Osmanoglu et al. 10 gional climate variations (Truffer and Fahnestock, 2007). There is a need to better quantify the dynamic mass losses because they provide a mechanism for glaciers to lose mass much more rapidly than is possible through other means. Title Page For the glaciers and ice caps covering the islands off the western coast of the Antarc- tic Peninsula, some estimates of frontal ablation have appeared recently (Osmanoglu Abstract Introduction 15 et al., 2013a; Navarro et al., 2013). Such estimates are crucial to understand the evo- lution of the mass balance in a region that has shown considerable regional warming Conclusions References

(Steig and Orsi, 2013; Turner et al., 2013). Tables Figures Here we estimate the average frontal ablation rate of the ice cap on Livingston Island, the second largest island in the South Shetland Islands archipelago, located northwest J I 20 of the tip of the Antarctic Peninsula (Fig. 1), for the period October 2007–January 2011. By frontal ablation we mean the loss of mass from the near-vertical calving fronts of J I the marine-terminating glaciers, including losses by calving, subaqueous melting, and Back Close subaerial melting and sublimation (Cogley et al., 2011). We adopt a flux-gate method approximating frontal ablation by the ice discharge through defined flux-gates close to Full Screen / Esc 25 the marine termini. Hence, the approach does not distinguish between the individual components of frontal ablation. It requires the knowledge of both ice velocities and Printer-friendly Version ice thickness at given flux gates. Radar remote sensing data are used to derive ice velocities, which in turn are used to approximate ice thicknesses based on principles Interactive Discussion of glacier dynamics and calibrated against the available GPR-retrieved ice thickness.

4209 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion We also investigate the temporal variations of ice velocity, and their seasonality, at the deﬁned ﬂux gates. For our analyses we compile a new 50 m × 50 m resolution DEM by TCD merging existing data sets with satellite-derived elevations. 7, 4207–4240, 2013

2 Study area Frontal ablation of ◦ 0 ◦ 0 ◦ 0 ◦ 0 glaciers on 5 Livingston Island ice cap (62 28 –62 45 S, 59 49 –60 59 W) is about 60 km long and 2 Livingston Island 30 km wide. The glacier-covered area was 734 km in 1956 and shrunk by 4.3 % during 2 the period 1956–1996, to a glacierized area of 703 km in 1996 (Calvet et al., 1999). B. Osmanoglu et al. Our latest estimate using the 2004 outlines (unpublished data from Jaume Calvet and David García-Sellés) is 697 km2. Glaciological ﬁeld campaigns have been conducted 10 in recent years on several glaciers of the ice cap. A 10 yr surface mass balance record Title Page is available for Hurd and Johnsons Glaciers, indicating a nearly balanced mass budget over the observation period 2002–2011 (Navarro et al., 2013) and a deceleration Abstract Introduction

of losses from these glaciers from 1957–2000 to 2002–2011. Jonsell et al. (2012) ap- Conclusions References plied a distributed temperature-radiation index melt model calibrated against automatic 15 weather station and in-situ surface mass balance data from Hurd Peninsula glaciers Tables Figures revealing a high sensitivity of the mass balance of the ice cap to climate change. They ◦ showed that a 0.5 C temperature increase results in 56% higher melt rates, which is J I mainly an eﬀect of the on-glacier summer average temperatures being close to zero. In- situ ice velocity measurements are available on Hurd Peninsula (Ximenis et al., 1999; J I 20 Otero, 2008; Otero et al., 2010), and ice thickness retrieved from GPR measurements Back Close are limited to certain locations on the island (see details in Sect. 3). A summary of other previous glaciological studies on the island and the Antarctic Peninsula region can be Full Screen / Esc found in Navarro et al. (2013). Printer-friendly Version

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4210 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3 Data TCD 3.1 Ice thickness 7, 4207–4240, 2013 Ice thickness data are only available for limited parts of the ice cap. These were retrieved from 20 MHz ground-based GPR measurements carried out in December 2000 Frontal ablation of 5 along the main ice divides of the western part of the island and, in December 2006, on glaciers on Bowles Plateau (BP in Fig. 2), which is the accumulation area of Perunika Glacier (21 Livingston Island in Fig. 2). The data are described in Macheret et al. (2009). Typical thickness under the western divides is ∼ 150 m, reaching maxima of ∼ 200 m, and the average thickness B. Osmanoglu et al. under Bowles Plateau is ∼ 265 m, with maximum thicknesses of 500 m. GPR measure- 10 ments on Hurd Peninsula glaciers carried out at diﬀerent radar frequencies and various dates are described in Navarro et al. (2009), and show an average thickness of ∼ 94 m Title Page and maximum values of ∼ 200 m. Abstract Introduction

3.2 In situ surface velocities Conclusions References

In situ glacier surface velocity measurements in Livingston Island are only available Tables Figures 15 on Hurd Peninsula (Ximenis et al., 1999; Otero, 2008; Otero et al., 2010). Johnsons

Glacier, a tidewater glacier, has velocities increasing from zero at the ice divides to J I typical year-averaged values close to 40 myr−1 in the fastest part of its calving front. But, for most of its area, the velocities are below 10 myr−1. Hurd Glacier, which termi- J I

nates on land, has lower velocities, with observed year-averaged values always below Back Close −1 20 5 myr . The maximum velocities are observed in the upper ablation area, and strongly decrease near the glacier snout, which has been suggested to be frozen to bed based Full Screen / Esc on geomorphological analyses and GPR studies (Molina et al., 2007; Navarro et al., 2009). Printer-friendly Version

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4211 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3.3 SAR imagery TCD Synthetic Aperture Radar (SAR) data were used to derive surface ice velocities and to compile a new DEM for the ice cap. Data from two sources were included: (1) the 7, 4207–4240, 2013 PALSAR imaging system on board the Japanese Advanced Land Observing Satellite 5 (ALOS-1) satellite and (2) Bistatic TanDEM-X pair from the TerraSAR-X and TanDEM-X Frontal ablation of satellites. glaciers on PALSAR-1 provides L-band (1270 MHz) signals and was operational during 2006– Livingston Island 2011. We used two parallel tracks (124 and 125) covering the entire ice cap, which provided a total of 21 images between October 2007 and January 2011. All images B. Osmanoglu et al. 10 were collected in Fine Beam Single Polarization mode, which gives a ground resolution of about 9 m×5 m. The images have a swath width of ∼ 70 km in the range direction. The bistatic TanDEM-X pair was acquired by TerraSAR-X and TanDEM-X satellites Title Page

simultaneously, generating high quality interferometric data by removing the eﬀects Abstract Introduction of temporal decorrelation. These images have a ground resolution of about 3 m × 3 m 15 and cover an area of about 30 km × 30 km. The X-band (9.65 GHz) signal penetrates Conclusions References very little into the snow and ice, the penetration depth depending on surface condi- Tables Figures tions. It hence enables the generation of accurate surface elevation models. Details on the exact penetration depth of the SAR signal are unknown. However, X-band penetration is generally considered to have maximum penetration depths of ∼ 10 m in dry J I

20 snow, and less during wet snow conditions. The TanDEM-X acquisition occurred on 18 J I March 2012, at the transition from late summer to cooler winter conditions. Back Close 3.4 Digital elevation model Full Screen / Esc The only topographic maps available covering the entire Livingston Island are the 1 : 200 000 map by DOS (1968), based on aerial photos taken in 1957, and the Printer-friendly Version 25 1 : 100 000 map by SGE (1997), based on SPOT images of 1991 and 1996. Interactive Discussion

4212 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Since elevation is a crucial variable for our approach of calculating ice velocities, and an accurate high-resolution DEM is not available, we have compiled a new digital TCD elevation model for Livingston Island with 50 m × 50 m grid cells (Fig. 3) based on: 7, 4207–4240, 2013 1. the Radarsat Antarctic Mapping Project (RAMP) DEM (Liu, 2001),

5 2. radargrammetry using PALSAR-1 data, Frontal ablation of glaciers on 3. Tandem-X bistatic interferometry, Livingston Island

4. the Advanced Spaceborne Thermal Emission and Reﬂection Radiometer B. Osmanoglu et al. (ASTER) Global DEM v.2, 5. the Ice, Cloud and Land Elevation Satellite (ICESat) elevation proﬁles, level 1B Title Page 10 Global Elevation Data (GLA06) obtained from the National Snow and Ice Data Center (NSIDC). Abstract Introduction

The RAMP DEM covers the entire island with 200 m × 200 m grid cell resolution, Conclusions References which we resample to 50 m × 50 m (Fig. 3a). First, the RAMP DEM was sharpened using a SAR intensity image. The intensities of a SAR interferogram generated from Tables Figures 15 PALSAR-1 images were used to estimate local slopes (Eineder, 2003). These were then scaled to the range between 0.75 and 1.25, and multiplied by the RAMP DEM J I to superimpose the obtained structure from the intensity image to the RAMP DEM, without altering the histogram of original elevation values (Fig. 3b). Even though the J I sharpened RAMP DEM has smaller scale variability, statistically its misﬁt to ICESat el- Back Close 20 evations did not change after this operation. For comparison, the ICESat laser footprint is ∼ 60 m, separated by ∼ 170 m along the ground track (Fig. 3c). Full Screen / Esc Second, additional higher-quality partial DEMs for the ice cap were generated. Radargrammetry was used to derive a ∼ 160 m resolution DEM for the eastern half Printer-friendly Version

of the ice cap using PALSAR-1 data from two parallel tracks (Fig. 3d). PALSAR-1 data Interactive Discussion 25 provided the two different look angles necessary for radargrammetry, while the relatively short 16 day baseline limited the amount of surface change due to glacier motion 4213 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion and snow cover between the two images (Balz et al., 2009). In addition, a higher resolution (10 m) DEM was generated from bistatic TanDEM-X interferometry (Fig. 3f). The TCD unwrapping was done using a modified version of the SNAPHU unwrapping software, 7, 4207–4240, 2013 capable of multigrid unwrapping (Chen and Zebker, 2001). The entire InSAR process- 5 ing was done at full resolution, though the final DEM was put on a 10 m × 10 m grid to increase redundancy and reduce gaps due to radar shadows. The DEM is restricted Frontal ablation of to the parts in the north-east of the ice cap that are covered by the satellite scene. We glaciers on further used the 30 m resolution ASTER GDEM v.2 data, but the data are heavily af- Livingston Island fected by cloud cover. To ensure sufficient quality we only included pixels with three or B. Osmanoglu et al. 10 more available observations, which reduced the coverage mostly to the south-eastern part of the ice cap (Fig. 3e). A first order polynomial plane was removed from all digital elevation models and Title Page all were best-fitted to the ICESat data, finally getting resampled to 50 m pixel spacing before merging. The final elevation z for each pixel was obtained by taking weighted Abstract Introduction 15 averages of the available data. The weights were selected adaptively as a function of the expected error and number of neighboring points: Conclusions References P5 i i i Tables Figures wσwnz z = i=1 (1) P5 w i w i i=1 σ n J I where i denotes five different data sets (sharpened RAMP, ICESat, radargrammetry, J I ASTER and TanDEM-X), wσ denotes weighting based on expected error, and wn is 20 weighting based on the distance to the nearest neighbour. The expected RMSE for Back Close ASTER GDEM changes based on topography and number of observations available, Full Screen / Esc ranging from 3 to 50 m (Reuter et al., 2009; Hirt et al., 2010; Hengl and Reuter, 2011). The TanDEM-X DEM is expected to provide 10 m absolute and 2–4 m relative vertical Printer-friendly Version accuracy (Gonzalez et al., 2010). ICESat altimeter data is accurate to ∼ 0.3 m in ver- 25 tical (Magruder et al., 2007). Accuracy of radargrammetry changes with topography, Interactive Discussion accuracy of correlation and orbit accuracy, and is expected to be on the order of 10– 50 m (Balik et al., 2004; Balz et al., 2009, 2013). The RAMP DEM has spatially varying 4214 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion error, which increases with surface slope and is expected to be accurate to 30 m in vertical (Bamber and Gomez-Dans, 2005). The RMSE for the different DEMs compared TCD with ICESat were 125, 146, 200, and 368 m for TanDEM-X, sharpened RAMP, radar- 7, 4207–4240, 2013 grammetry and ASTER GDEM, respectively. The calculated deviations not only reflect 5 differences in elevation between data sets but also the correlation between number of samples available for each data set. There were 259, 792, 200 and 51 points available Frontal ablation of for comparisons between ICESat and the other four DEMs (TanDEM-X, sharpened glaciers on RAMP, radargrammetry and ASTER GDEM, respectively). For this analysis we gave Livingston Island equal weighting to ICESat and TanDEM-X DEM (std ∼ 5 m), as well as ASTER GDEM B. Osmanoglu et al. 10 and RAMP (std ∼ 25 m). The radargrammetry had the lowest weight (std ∼ 50 m). The combined DEM is shown in Fig. 3g and the standard deviation of errors relative to 792 ICESat measurements was 121 m. Title Page

Abstract Introduction 4 Methods Conclusions References 4.1 Flux gates Tables Figures 15 In this study frontal ablation is approximated by the ice ﬂux perpendicular to a theo-

retical surface across the glacier terminus called “flux gate”. For robust estimation of J I the ice discharge across each flux gate, ten parallel flux gates at intervals of ∼ 50 m were defined with the lowest gate as close as possible to the glacier termini, between J I

roughly 100 and 600 m upglacier from the calving front. Ice discharges for all ten flux Back Close 20 gates were calculated individually and averaged to obtain a mean value. Deviation of each flux gate from the mean was calculated and flux gates with deviations higher than Full Screen / Esc 20 % of the mean were discarded. On average 7.5 flux gates were used for the analysis. Flux gates are only defined for marine terminating glaciers where the ice velocities Printer-friendly Version at the flux gates exceed 20 myr−1. For the remaining tidewater and all land terminating Interactive Discussion 25 glaciers ice discharge into the ocean is assumed negligible.

4215 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.2 Frontal ablation TCD For each marine-terminating glacier we compute the ice flux q across the corresponding flux gate. Following Rignot (1996) and Osmanoglu et al. (2013a), we derive the ice 7, 4207–4240, 2013 flux from surface velocities by Frontal ablation of 5 q = Hγu (2) sfc glaciers on where H is ice thickness and γ is the ratio between thickness-averaged and surface Livingston Island usfc horizontal velocities for each grid cell. We assume γ = 0.9 (Cuffey and Paterson, B. Osmanoglu et al. 2010). usfc and H are a function of position along the flux-gate. The ice discharge D is then defined as the integral of ice flux perpendicular to the flux-gate over the length L

10 of the ﬂux gate: Title Page

L Z Abstract Introduction D = qρicedl (3) Conclusions References 0 Tables Figures −3 where ρice is the density of ice (900 kgm ). Flow directions are computed from oﬀset tracking. Frontal ablation is given in units of Mtyr−1 throughout the paper. J I

4.3 Surface velocities J I

15 Intensity offset tracking method was used to obtain glacier surface velocities from Back Close PALSAR-1 intensity images (Gray et al., 1998; Strozzi et al., 2002; Werner et al., 2005; Full Screen / Esc Strozzi et al., 2008). We preferred intensity offset tracking, rather than coherence tracking, because of the large extent of incoherent areas in the available imagery (Strozzi Printer-friendly Version et al., 2002). In this study, inconsistent velocity measurements were masked out using 20 a spatial variance filter, such that surface velocity measurements that have Fisher’s Interactive Discussion Distance of 80 myr−1 (with a constant expected error of 4 myr−1) or above compared to their neighbors are discarded (Osmanoglu et al., 2011). 4216 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion The 21 ALOS PALSAR-1 scenes acquired between October 2007 and January 2011 from tracks 124 and 125 were grouped to form short paired temporal baselines to TCD reduce measurement errors (Osmanoglu et al., 2013b). However, due to acquisition 7, 4207–4240, 2013 gaps, especially over austral winter months, there are some pairs with longer base- 5 lines. Surface velocity time series can be constructed with an inversion similar to small baselines analysis (Berardino et al., 2002; Lanari et al., 2007). However, the poor ve- Frontal ablation of locity estimates for pairs with long temporal baselines do not allow for construction of glaciers on a redundant network, where each scene is connected with more than one pair. There- Livingston Island fore, the velocity time series were constructed based on the measured displacements B. Osmanoglu et al. 10 for each pair. In addition to averaging data from multiple pairs to increase the coverage and statistical significance of the annual surface velocity field, we also investigate the temporal Title Page variations of the surface velocities at the flux gates, and analyse their seasonality and their impact on resulting frontal ablation estimates for all calculated flux-gates. Abstract Introduction 15 To analyse the temporal variations of surface velocities at the flux gates, we computed average detrended velocities at the given flux gates. We did not attempt to esti- Conclusions References

mate trends in surface velocity, because our velocity measurement period is too short Tables Figures to estimate linear trends in velocity and, if any, these would likely be associated with the increase in velocity experienced as the glacier ice approaches the calving front, J I 20 and thus not representing a change in velocity with time at a given spatial location (Eulerian velocity) but a change in velocity of a given particle with time (Lagrangian J I velocity). Velocities were computed for all available periods spanning 46 to 368 days. Back Close The magnitude of the temporal variations of surface velocities was approximated as the standard deviation of the computed velocities. The flux gate length, average ice thick- Full Screen / Esc 25 ness and standard deviation of detrended surface velocities were used to calculate 1σ contribution of temporal variations of velocity to the estimated ice flux. Printer-friendly Version Seasonal variations were modelled by fitting a periodic signal (cosine) to the weighted observations of detrended velocities. The inverse of temporal baselines were Interactive Discussion selected as weights such that the shortest possible temporal baseline (of 46 days) has

4217 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion a weight of 1, while longer baselines have proportionally lower weights. The periodic signal does not account for interannual variations, yet it provides a measure of the TCD seasonal amplitude and timing over the study period. 7, 4207–4240, 2013 4.4 Ice thickness Frontal ablation of 5 Ice thickness observations are not available for any of the flux-gates, except for John- glaciers on sons Glacier (19 in Fig. 2). Therefore, we estimate the ice thickness at the flux-gates Livingston Island from surface velocity field following the method proposed by Rignot (1996) for ocean- terminating glaciers and also applied on King George Island by Osmanoglu et al. B. Osmanoglu et al. (2013a): τ n τ m d d Title Page 10 u = (1 − f ) EHb + f (4) sfc B R Abstract Introduction τd = ρicegHb sinα (5) Conclusions References where usfc is the surface velocity obtained from intensity feature tracking, f is an adjustable parameter between 0 and 1 setting the amount of sliding (f = 0, no sliding; Tables Figures f = 1, free sliding), n is the Glen’s flow law parameter, τd is the gravitational driving 15 stress, B is the column-averaged stiffness parameter in Glen’s flow law, E is the flow J I law enhancement factor, Hb is the estimated ice thickness, R is a constant including the effects of bed roughness, and m is the Weertman’s sliding law parameter. In Eq. (5), g J I is gravity, and α is the surface slope. The deformation component of Eq. (4) assumes Back Close deformation by simple shear, i.e. it does not include the effect of longitudinal stress 20 gradients. In contrast to Rignot (1996), we treat E as an adjustable parameter rather Full Screen / Esc than a constant. Typical values for E are in the range 0.5 − 10, however values outside this range have also been reported (Greve and Blatter, 2009). For this analysis Printer-friendly Version we calculate B based on ice temperature defined by an Arrhenius relationship (−3 ◦C, Interactive Discussion B = 231 866 kPa−1/3), while we set R as 4 kPam−1/2 a−1/2 (Rignot, 1996; Greve and 25 Blatter, 2009; Cuffey and Paterson, 2010). The m and n parameters are set to 2 and 4218 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3 respectively, while a truncated-Newton iterative optimization routine is used to find f and E using the available thickness data (Fig. 2). TCD In order to improve the fit we separated the surface velocity fields into slow (0– 7, 4207–4240, 2013 50 myr−1), medium (50–100 myr−1) and fast (>100 myr−1) moving glacier regions, and 5 fitted Eq. (4) for each region separately. Frontal ablation of 4.5 Error analysis glaciers on Livingston Island The analysis carried out in this study introduces errors at various steps: (1) derived surface velocities from intensity offset tracking of PALSAR-1 images, (2) conversion B. Osmanoglu et al. of surface velocity to thickness-averaged velocity, (3) inference of ice thickness from 10 thickness-averaged velocity and surface slope, including assumptions of the physical model and of the model parameter values, and (4) selection of flux gates. All error Title Page sources, except the latter, can be quantified by comparing the estimated ice thickness Abstract Introduction with available data. Errors in this paper are calculated from the difference between the observed and estimated ice thickness (Fig. 4), where 95 % of the data is covered inside Conclusions References ◦ 15 the dashed lines defined with the minimum β angle (30 ). There is a root mean square (rms) misfit of 103.44 m between the estimated and observed thickness data, indicating Tables Figures a poor fit. J I

5 Results J I

5.1 Surface velocities Back Close Full Screen / Esc 20 Average surface ice velocities obtained from SAR intensity offset tracking are shown in Fig. 5a. Spatially incoherent velocity measurements are masked out, and appear Printer-friendly Version as white. According to our results, Huron Glacier (7) has the fastest flowing ice, with −1 velocities up to 250 myr . Kaliakra (6), Perunika (21) and Charity (17) glaciers also Interactive Discussion show large surface velocities. Unfortunately, for none of these areas are there in-situ 25 ice surface measurements against which to compare our remotely-sensed velocities. 4219 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion The temporal variations in ice velocities for each flux-gate are shown in Fig. 6. The data show large temporal variability. The seasonality of these variations is approxi- TCD mated by fits to the periodic curves. Although the scatter is large and the data density 7, 4207–4240, 2013 limited, velocities generally tend to be higher during summer than winter as also indi- 5 cated in some cases by a relatively high correlation coefficient. However, in other cases the fits are rather poor or even meaningless, indicating that the velocity variations do Frontal ablation of not follow a simple seasonal pattern, or the data density insufficient, or there are un- glaciers on certainties too large to infer seasonal patterns. To explore a possible correlation of the Livingston Island velocity variations with air temperature, the figure also marks the periods of continuous ◦ B. Osmanoglu et al. 10 daily mean temperatures above 0 C.

5.2 Ice thickness Title Page The ice thickness estimated from the average surface velocities (Fig. 5a) and the com- Abstract Introduction bined DEM (Fig. 3) using Eqs. (4) and (5) is shown in Fig. 5b. The computed ice thickness values are in the same range as the GPR measurements, however the fit Conclusions References 15 between the estimated and measured data sets indicates large errors (Fig. 4). Tables Figures 5.3 Frontal ablation J I Frontal ablation rates for all investigated tidewater glaciers are given in Table 1. The largest rate is found for Huron Glacier (7, Fig. 2), followed by glacier basin 3. Ice dis- J I −1 charge for all glaciers totals 509 ± 381 Mtyr . This is equivalent to a specific mass Back Close −1 2 20 change of −0.8 ± 0.6 mw.e.yr over the total area of the analysed basins (599 km ) and −0.7 ± 0.5 mw.e.yr−1 over the area of the whole ice cap (697 km2). Full Screen / Esc The changes in frontal ablation ∆Dseas for each basin, associated with the seasonal variations in surface velocity described earlier and characterised by their standard de- Printer-friendly Version viation σvel, are given in Table 1. The largest variation in frontal ablation occurs at basin −1 Interactive Discussion 25 3 and is 42.9 Mtyr . In terms of specific units, the discharge variations attain their highest value of 1.0 mw.e.yr−1 at basin 10. The total variation of frontal ablation from 4220 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion all basins reaches 237 Mtyr−1, which is less than the total uncertainty estimated for the frontal ablation (381 Mtyr−1) but is still a matter of concern, as it is 46 % of the best TCD −1 estimate for the frontal ablation (509 Mtyr ). 7, 4207–4240, 2013

6 Discussion Frontal ablation of glaciers on 5 6.1 Frontal ablation Livingston Island

The only available detailed estimate of calving losses on Livingston ice cap is that of B. Osmanoglu et al. Johnsons Glacier (19 in Fig. 2). Using a full-stokes dynamical model constrained by measured velocities near the calving front, Navarro et al. (2013) calculated calving −1 losses of 0.74 ± 0.17 Mtyr averaged over the period May 2004–August 2007. This Title Page −1 10 estimate compares reasonably well with our estimated value of 0.4 ± 0.3 Mtyr for the period 2008–2011 (Table 1). Abstract Introduction Our ice cap-wide frontal ablation estimate for Livingston Island is consistent with Conclusions References the results from a similar study on neighboring King George Island. Osmanoglu et al. (2013a) estimated that King George Island (1127 km2) lost 720 ± 428 Mtyr−1 Tables Figures 15 during the period October 2007–January 2011. The corresponding specific rate of −1 0 −1 0.6 ± 0.4 mw.e.yr is similar to Livingston s rate of 0.7 ± 0.5 mw.e.yr . The lower rel- J I ative error of the estimate for King George Island is mostly due to wider coverage of GPR ice thickness observations. On Livingston Island many of the available ice thick- J I ness measurements, mostly close to the ice divides in the western part of the ice cap, Back Close 20 could not be used for the tuning of model parameters because ice velocities could not be derived from SAR data in these areas. Full Screen / Esc Our ice cap-wide frontal ablation estimate is in the range of the ice discharge estimates for individual glaciers after collapse of the Larsen-B and respective dy- Printer-friendly Version namic adjustments, Evans Glacier (459 Mtyr−1, 2008) or Jorum Glacier main branch −1 Interactive Discussion 25 (534 Mtyr , 2008) (Rott et al., 2011), though the specific rates for these glaciers (2.19 and 1.68 mw.e.yr−1, respectively) are 2–3 times larger than that of Livingston Island. 4221 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Except for Columbia Glacier in Alaska (O’Neel et al., 2005) estimates in specific units are considerably higher for King George Island and Livingston Island ice caps than TCD those reported from glaciers in the Arctic (AMAP, 2011). 7, 4207–4240, 2013 6.2 Uncertainties Frontal ablation of 5 The large discrepancies between the calculated and observed thicknesses shown in glaciers on Fig. 4 (rms misfit of 103.4 m) indicate that our estimates of frontal ablation for Livingston Livingston Island Island should be considered only as a rough first-order approximation. The large errors result from a combination of those inherent to the estimation of surface velocities from B. Osmanoglu et al. PALSAR-1 images, those intervening in the conversion of surface velocity to thickness- 10 averaged velocity using Eq. (2), and those involved in the retrieval of ice thickness from thickness-averaged velocity and surface slope using Eqs. (4) and (5). In our case, Title Page the latter are expected to be dominant. This error component encompasses both the Abstract Introduction limitations of the physical model and the choice of values for the model parameters. The physical model represented by Eqs. (4) and (5) assumes deformation by simple Conclusions References 15 shear, neglecting longitudinal stress gradients which are known to be important near the calving fronts because of the large values of the along-flow gradient of the surface Tables Figures velocity. Consequently, the ice thickness inferred near the calving fronts, where the ice fluxes are computed, are expected to be poor, implying large errors in the ice discharge J I calculation. Using a single fit of the parameters E and f all over the Livingston ice cap, J I 20 as done in Osmanoglu et al. (2013a) for the neighboring King George Island ice cap, resulted in large errors (rms misfits >200 m). Using separate fits for the regions of slow, Back Close medium and fast flow, as described in Sect. 5.2, allowed us to significantly reduce the Full Screen / Esc error, though the current rms misfit (103.4 m) is still very large. Another limitation is the assumption of steady-state in Eqs. (4) and (5). The assump- Printer-friendly Version 25 tion is necessary to infer an ice-thickness distribution from velocity and surface slope data alone, without available thinning rate data. On Livingston Island, glacier thinning Interactive Discussion has only been studied on the Hurd Peninsula (Ximenis et al., 1999; Molina et al., 2007) (Fig. 2). Combined with observed front retreat on most of the ice cap (Calvet et al., 4222 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 1999) this suggests that the current geometry of the ice cap is not stationary. Even if the surface mass balance data for the last decade on Hurd Peninsula is rather close TCD to zero, indicating a deceleration of the mass losses as compared to previous decades 7, 4207–4240, 2013 (Navarro et al., 2013), the surface geometry needs some time to accommodate to the 5 changing mass budget. This, however, occurs faster in tidewater glaciers as compared to land-terminating glaciers, because the former have larger velocities. Frontal ablation of glaciers on 6.3 Temporal variations in surface velocities and associated changes in frontal Livingston Island ablation B. Osmanoglu et al. Noticeable temporal variations in surface velocities at the given flux gates are apparent 10 from both Fig. 6 and the σvel values in Table 1. However, these variations do not always exhibit a clear seasonality, as shown by the large scatter of values of the coefficients Title Page of determination r 2 given in Fig. 6. Clear seasonal variations in surface velocity are Abstract Introduction observed for several basins on Livingston Island. In particular, basins 6 (Kaliakra), 9 2 and 10 show large amplitudes and r values above 0.55. Since these measurements Conclusions References 15 are averaged along ten parallel flux gates for each basin and for every image-pair used in this analysis, it is very unlikely that these variations could arise from an error in our Tables Figures analysis. Fig. 6 also illustrates that the velocities tend to be higher during summer than − − winter (29.7 ± 17.3 myr 1 for summer vs. 19.7 ± 12.2 myr 1 for winter), which suggests J I that high velocities in Livingston Island glaciers could often correspond to periods of J I 20 strong melting at the glacier surface and associated changes in the water supply to the glacier bed, with corresponding changes in basal water pressure (e.g. Sugiyama Back Close et al., 2011). However, high velocities are sometimes observed during winter time. Full Screen / Esc Occasional periods of surface melting and liquid precipitation events during the winter are not unusual in this region, which could imply basal water pressure changes and Printer-friendly Version 25 associated speed-up events during the winter time. Regardless of the underlying mechanism for the temporal variations in surface veloc- Interactive Discussion ity, these can exert a relatively strong influence on the frontal ablation rates, as shown in Table 1. The group of basins to the northern and north-eastern parts of the island 4223 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion (1–6), most of them having large frontal ablation rates, shows a consistently large value for the frontal ablation change associated with seasonal variations of velocity as com- TCD pared to the frontal ablation estimated for each basin. The large value of the sum of 7, 4207–4240, 2013 frontal ablation changes for all the analysed basins, which is equivalent to 46 % of the 5 frontal ablation for the whole Livingston Island, stresses the importance of the strong effect that the period of measurement can have on the estimates of frontal ablation. Frontal ablation of glaciers on 6.4 Relative contribution of frontal ablation to total ablation Livingston Island

For Johnsons Glacier, using Navarro et al. (2013) data it can be estimated that, over the B. Osmanoglu et al. period April 2006–March 2008, the losses by frontal ablation represent only 16 % of the 10 total annual ablation, the remaining originating from surface ablation (assuming basal melting and internal accumulation were negligible). However, this glacier has a very Title Page particular setting, with a very shallow pro-glacial bay (just a few meters depth), a nearly Abstract Introduction flat bed in the area close to the calving front and moderate frontal velocities (maximum −1 values of the order of 40 myr ), implying a small flux of ice into the ocean. Aside from Conclusions References 15 some observations on Rotch Dome (westernmost part of the ice cap) during 1971– 1974 (Orheim and Govorukha., 1982), the surface mass balance of the ice cap has Tables Figures only been studied on Hurd Peninsula (Ximenis et al., 1999; Navarro et al., 2013). The latter study includes mass balance profiles (summer, winter and annual) averaged over J I the period 2002–2011 for Johnsons (tidewater, 5.36 km2) and Hurd (land-terminating, 2 J I 20 4.03 km ) glaciers. To compare the frontal ablation rates to mass change due to surface processes we Back Close perform a simple back-of-the-envelope calculation and approximate ice cap-wide an- Full Screen / Esc nual surface mass balance and total ablation rates. Based on surface mass balance observations on land-terminating Hurd and marine-terminating Johnsons glaciers we Printer-friendly Version 25 determine linear summer balance gradients by regressing summer surface balances averaged over 20 m altitude bands vs. altitude for the mass-balance years 2008–2011 Interactive Discussion (approximately overlapping with the time span of our velocity measurements; unpublished data from Francisco Navarro) and apply them to the hypsometry of our new 4224 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion DEM. Hence, we assume the gradient to be valid for the entire ice cap (Fig. 7). We apply Johnsons Glacier’s gradient to all tidewater basins (96.8 % of total area), and TCD Hurd Glacier’s gradient to all land-terminating basins (3.2 %). For elevations where the 7, 4207–4240, 2013 gradient yields positive summer balances we assume 0 mw.e.yr−1. 5 We use the same approach for computing glacier-wide winter balances but we assume that above 600 ma.s.l. the winter balance remains constant. Altitudes above Frontal ablation of 600 m correspond to mountain areas (mostly to the Friesland Massif, reaching 1700 m) glaciers on which occupy a limited planar area of ∼ 6.3 % of the ice cap. As a sensitivity test we Livingston Island also computed the winter balance where the gradient of winter surface mass balance B. Osmanoglu et al. 10 vs. elevation was applied for the entire elevation range. We found no significant difference in results between both methods. The gradient method yielded mean glacier-wide summer balance and winter balance Title Page of −0.7 ± 0.0 mw.e.yr−1 and 0.8 ± 0.1 mw.e.yr−1, respectively. The resulting mean an- −1 nual surface mass balance for the entire Livingston Island is 0.1±0.1 mw.e.yr , which, Abstract Introduction 15 added to the contribution to mass balance by frontal ablation (−0.7 ± 0.5) gives a total mass balance for Livingston Island of −0.6 ± 0.5 mw.e.yr−1. Conclusions References

To quantify the partitioning of total annual ablation we equate summer balance with Tables Figures total annual surface ablation, thus assuming summer snow accumulation to be negligible. We ﬁnd that frontal ablation and surface ablation contribute equal shares to the J I 20 total annual ablation of Livingston Island. This contribution of frontal ablation to the total ablation is even larger than that of Arctic ice caps such as the Academy of Sciences J I Ice Cap, in Severnaya Zemlya (Dowdeswell et al., 2002), and Austonna, in Svalbard Back Close (Dowdeswell et al., 2008), which show contributions of frontal ablation to the total ablation of 30–40 %. Full Screen / Esc

Printer-friendly Version 25 7 Conclusions Interactive Discussion Major outlet glaciers on Livingston Island are discharging a total of 509 ± 381 Mtyr−1, which is equivalent to a specific mass change of −0.7 ± 0.5 mw.e.yr−1 calculated over 4225 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion the entire ice-covered area of 697 km2, and is coincident with a rough estimate of the surface ablation during the period analysed. Therefore, frontal ablation and surface TCD ablation contribute equal shares to total ablation of Livingston Island. Uncertainties in 7, 4207–4240, 2013 ice thickness estimation hinder the accuracy of the obtained results, so future work 5 should focus on improving the ice thickness estimates across flux-gates. Considerable temporal variations in ice velocity at the analysed flux gates are ob- Frontal ablation of served, with associated changes in frontal ablation for the entire Livingston Island of glaciers on ∼ 237 Mtyr−1, which is 46 % of the estimated frontal ablation. This stresses the im- Livingston Island portance of taking into account the temporal variations in ice velocity when computing B. Osmanoglu et al. 10 frontal ablation with a flux-gate approach. The velocity variations do not always show a clear seasonality. Velocities tend to be higher during the summer, though occasion-

ally high values are also observed during the winter. This suggests that changes in Title Page basal water pressure, associated with either strong surface melting or rainfall events (which sometimes occur during the winter), are likely the main drivers of the temporal Abstract Introduction 15 variations in surface velocity, but further studies are needed to explore the causes of Conclusions References the observed temporal variations in ice velocities. Tables Figures Acknowledgements. Funding was provided by NSF project #ANT-1043649, NASA project #NNX11A023G, DFG #BR2105/9-1 and the National Plan of R&D (Spain) project CTM2011- 28980. We thank the ESF ERANET Europolar IMCOAST project (BMBF award AZ 03F0617B), J I 20 EU FP7-PEOPLE-2012-IRSES IMCONet grant 318718, and Alaska Satellite Facility for data provision. J I Back Close

References Full Screen / Esc

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4230 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion SGE: Spanish Antarctic Cartography, Livingston and Deception Islands, scale 1 : 100 000, 1st edn. Pub. Servicio Geográfico del Ejército, 1997. 4212 TCD Shepherd, A. Ivins, E. R., A, G., Barletta, V. R., Bentley, M. J., Bettadpur, S., Briggs, K. H., Bromwich, D. H., Forsberg, R., Galin, N., Horwath, M., Jacobs, S., Joughin, I., King, M. A. 7, 4207–4240, 2013 5 Lenaerts, J. T. M., Li, J., Ligtenberg, S. R. M., Luckman, A., Luthcke, S. B., McMillan, M., Meister, R., Milne, G., Mouginot, J., Muir, A., Nicolas, J. P., Paden, J., Payne, A. J., Pritchard, H., Rignot, E., Rott, H., Sandberg Sørensen, L., Scambos, T. A., Scheuchl, B., Schrama, E. Frontal ablation of J. O., Smith, B., Sundal, A. V., van Angelen, J. H., van de Berg, W. J., van den Broeke, M. glaciers on R., Vaughan, D. G., Velicogna, I., Wahr, J., Whitehouse, P. L., Wingham, D. J., Yi, D., Young, Livingston Island 10 D., and Zwally, H. J.: A reconciled estimate of ice-sheet mass balance, Science, 338, 1183– 1189, 2012. 4208 B. Osmanoglu et al. Steig, E. J. and Orsi, A. J.: Climate science: the heat is on in Antarctica, Nat. Geosci., 6, 87–88, 2013. 4209 Strozzi, T., Luckman, A., Murray, T., Wegmuller, U., and Werner, C.: Glacier motion estimation Title Page 15 using SAR offset-tracking procedures, IEEE Geosci. Remote S., 40, 2384–2391, 2002. 4216 Strozzi, T., Kouraev, A., Wiesmann, A., Wegmüller, U., Sharov, A., and Werner, C.: Estimation Abstract Introduction of Arctic glacier motion with satellite L-band SAR data, Remote Sens. Environ., 112, 636– 645, 2008. 4216 Conclusions References Sugiyama, S., Skvarca, P., Naito, N., Enomoto, H., Tsutaki, S., Tone, K., Marinsek, S., and Tables Figures 20 Aniya, M.: Ice speed of a calving glacier modulated by small fluctuations in basal water pressure, Nat. Geosci., 4, 597–600, 2011. 4223 Truffer, M. and Fahnestock, M.: Rethinking ice sheet time scales, Science, 315, 1508–1510, J I 2007. 4209 Turner, J., Barrand, N. E., Bracegirdle, T. J., Convey, P., Hodgson, D. A., Jarvis, M., Jenk- J I 25 ins, A., Marshall, G., Meredith, M. P., Roscoe, H., Shanklin, J., French, J., Goosse, H., Back Close Guglielmin, M., Gutt, J., Jacobs, S., Kennicutt, M. C. I., Masson-Delmotte, V., Mayewski, P., Navarro, F., Robinson, S., Scambos, T., Sparrow, M., Summerhayes, C., Speer, K., and Full Screen / Esc Klepikov, A.: Antarctic climate change and the environment: an update, Polar Rec., 1– 23, doi:10.1017/S0032247413000296, available at: http://journals.cambridge.org/article_ Printer-friendly Version 30 S0032247413000296, 2013. 4209 Vieli, A. and Nick, F. M.: Understanding and modelling rapid dynamic changes of tidewater Interactive Discussion outlet glaciers: issues and implications, Surv. Geophys., 32, 437–458, 2011. 4209

4231 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Werner, C., Wegmuller, U., Strozzi, T., and Wiesmann, A.: Precision estimation of local oﬀsets between pairs of SAR SLCs and detected SAR images, in: Geoscience and Remote Sensing TCD Symposium, 2005, IGARSS’05, Proceedings, 2005 IEEE International, vol. 7, 4803–4805, 2005. 4216 7, 4207–4240, 2013 5 Ximenis, L., Calvet, J., Enrique, J., Corbera, J., Fernández de Gamboa, C., and Furdada i Bellavista, G.: The measurement of ice velocity, mass balance and thinning-rate on Johnsons Glacier, Livingston Island, South Shetland Islands, Antarctica, Acta geológica hispánica, 34, Frontal ablation of 406–409, 1999. 4210, 4211, 4222, 4224 glaciers on Livingston Island

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4232 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD Table 1. Estimated frontal ablation rates for the period between October 2007 and Jan- 7, 4207–4240, 2013 uary 2011, basin area, and average thickness and length of the flux-gates of all investigated tidewater glaciers on Livingston Island. σ are the standard deviations of the computed tempo- vel Frontal ablation of ral variations in velocities averaged over the flux gates and ∆Dseas are their associated changes in frontal ablation. Frontal ablation rates are given in Mtyr−1 and in specific units (mw.e.yr−1). glaciers on Livingston Island

Basin Frontal ablation Area Avg. thick. Length σvel ∆Dseas Mtyr−1 mw.e.yr−1 % km−2 % m km myr−1 Mtyr−1 mw.e.yr−1 B. Osmanoglu et al. 1 42.7 ± 31.3 0.61 ± 0.45 8.4 69.6 11.6 180.3 16.8 10.2 25 0.4 2 5.3 ± 3.9 0.80 ± 0.59 1 6.7 1.1 126.6 3.7 7.8 3 0.4 3 69.8 ± 51.2 0.85 ± 0.62 13.7 82.1 13.7 172.2 22.7 13.5 42.9 0.5 4 58.8 ± 43.2 0.86 ± 0.63 11.6 68.3 11.4 152.5 18.1 11 24.6 0.4 Title Page 5 18.8 ± 13.8 0.93 ± 0.68 3.7 20.3 3.4 153.8 7.7 16.5 15.9 0.8 6 (Kaliakra) 53.1 ± 39 0.83 ± 0.61 10.4 64.3 10.7 166.7 10.6 20.8 29.7 0.5 Abstract Introduction 7 (Huron) 145.4 ± 114.1 2.69 ± 2.11 28.6 54.1 9 100.4 7.2 19.2 11.2 0.2 8 0.7 ± 0.5 0.15 ± 0.11 0.1 4.4 0.7 64.8 2.1 15.4 1.7 0.4 9 1.3 ± 0.9 0.74 ± 0.54 0.3 1.7 0.3 64.5 1.2 17.1 1.1 0.6 Conclusions References 10 4.8 ± 3.5 0.90 ± 0.66 0.9 5.3 0.9 120.7 3.8 14.9 5.5 1.0 11 (Strandzha) 1.8 ± 1.3 0.82 ± 0.60 0.3 2.2 0.4 54.1 2.1 14.3 1.3 0.6 Tables Figures 12 (Dobrudzha) 4.1 ± 3 0.48 ± 0.35 0.8 8.7 1.5 84.8 2.8 19.6 3.8 0.4 13 (Magura) 0.4 ± 0.3 0.37 ± 0.28 0.1 1.1 0.2 52.4 1 14 0.6 0.6 14 (Srebarna) 4.8 ± 3.5 1.07 ± 0.78 0.9 4.4 0.7 74.2 2.3 22.7 3.1 0.7 15 (Macy) 2.4 ± 1.8 0.08 ± 0.06 0.5 30.1 5 70.6 3.4 17.2 3.4 0.1 J I 16 (Prespa) 8.7 ± 6.4 0.68 ± 0.50 1.7 12.7 2.1 81.9 3.5 23.3 5.4 0.4 17 (Charity) 1.0 ± 0.7 0.15 ± 0.11 0.2 6.6 1.1 95.2 3.1 14.5 3.4 0.5 J I 18 (Huntress) 15.2 ± 11.2 0.37 ± 0.27 3 40.8 6.8 108.1 4.3 15 5.7 0.1 19 (Johnsons) 0.4 ± 0.3 0.07 ± 0.05 0.1 5.3 0.9 120.5 2.1 8.7 1.8 0.3 Back Close 20 2.6 ± 1.9 0.20 ± 0.15 0.5 13.2 2.2 141.4 3 10.9 3.8 0.3 21 (Perunika) 23.8 ± 17.5 0.71 ± 0.52 4.7 33.7 5.6 168.9 6.2 18 15.3 0.5 22 10.0 ± 7.3 1.21 ± 0.89 2 8.3 1.4 131.9 5 9.3 4.9 0.6 Full Screen / Esc 23 6.9 ± 5.1 0.58 ± 0.43 1.4 11.8 2 120.1 4.8 10.5 4.9 0.4 24 26.1 ± 19.2 0.60 ± 0.44 5.1 43.7 7.3 152.2 13.8 11 18.8 0.4 Total 508.9 ± 381.0 0.85 ± 0.64 100 599.4 100 236.8 0.4 Printer-friendly Version Entire ice cap 0.73 ± 0.55 697.3 0.3 Interactive Discussion

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J I Fig. 1. Location of Livingston Island, to the northwest of the tip of the Antarctic Peninsula. Green colour denotes ice-free areas, while grey is used for glaciated areas. Note that in subse- J I quent ﬁgures only the glaciated area of the island is shown. Map base: SCAR Antarctic Digital Back Close Fig. 1. LocationDatabase, of Livingston Vers. Island, 6.0; toMOA the northwest coastline of of the Antarctica, tip of the Antarctic NSIDC. Peninsula. Green colour denotes ice-free areas, while grey is used for glaciated areas. Note that in subsequent ﬁgures only the glaciated area of the island is shown. Map base: SCAR Antarctic Digital Database, Vers. 6.0; MOA coastline of Antarctica, NSIDC. Full Screen / Esc

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Fig. 2. Livingston Island glacier basins, according to Bliss et al. (2013). Numbers indicate the basins analysed in this study. Grey shading marks the area for which ice velocities could be derived from SAR data. Thick black lines denote ﬂux gates used for computing ice discharge. Ice thicknesses obtained from GPR measurements are shown in color. HP denotes Hurd Peninsula, and BP Bowles Plateau. of about 3 m×3 m and cover an area of about 30 km×30 2012,at the transition from late summer to cooler winter con- km. The X-band (9.65 GHz) signal penetrates very little into ditions. 165 the snow and ice, the penetration depth depending on surface conditions. It hence enables the generation of accurate surface elevation models. Details on the exact penetration depth 3.4 Digital Elevation Model of the SAR signal are unknown. However, X-band penetration is generally considered to have maximum penetration 175 The only topographic maps available covering the entire Liv- 170 depths of ∼10 m in dry snow, and less during wet snow con- ingston Island are the 1:200,000 map by DOS (1968), based ditions. The TanDEM-X acquisition occurred on 18 March on aerial photos taken in 1957, and the 1:100,000 map by SGE (1997), based on Spot images of 1991 and 1996. B. Osmanoglu et al.: Frontal ablation of glaciers on Livingston Island 3 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Fig. 1. Location of Livingston Island, to the northwest of the tip of the Antarctic Peninsula. Green colour denotes ice-free areas, while grey TCD is used for glaciated areas. Note that in subsequent ﬁgures only the glaciated area of the island is shown. Map base: SCAR Antarctic Digital Database, Vers. 6.0; MOA coastline of Antarctica, NSIDC. 7, 4207–4240, 2013

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Fig. 2. Livingston Island glacier basins, according to Bliss et al. (2013). Numbers indicate the J I Fig. 2. Livingston Island glacier basins, according to Bliss et al. (2013). Numbers indicate the basins analysed in this study. Grey shading basins analysed in this study. Grey shading marks the area for which ice velocities could be de- marks the area for which ice velocities could be derived from SAR data. Thick black lines denote ﬂux gates used for computing ice discharge. Back Close Ice thicknessesrived obtained from from SAR GPR data. measurements Thick black are lines shown denote in color. ﬂux HP gates denotes used Hurd Peninsula,for computing and BP ice Bowles discharge. Plateau. Ice thicknesses obtained from GPR measurements are shown in color. HP denotes Hurd Penin- Full Screen / Esc sula, and BP Bowles Plateau. × × of about 3 m 3 m and cover an area of about 30 km 30 2012,at the transition from late summer to cooler winter con- Printer-friendly Version km. The X-band (9.65 GHz) signal penetrates very little into ditions. 165 the snow and ice, the penetration depth depending on surface Interactive Discussion conditions. It hence enables the generation of accurate surface elevation models. Details on the exact penetration depth 3.4 Digital Elevation Model of the SAR signal are unknown. However, X-band penetra-4235 tion is generally considered to have maximum penetration 175 The only topographic maps available covering the entire Liv- 170 depths of ∼10 m in dry snow, and less during wet snow con- ingston Island are the 1:200,000 map by DOS (1968), based ditions. The TanDEM-X acquisition occurred on 18 March on aerial photos taken in 1957, and the 1:100,000 map by SGE (1997), based on Spot images of 1991 and 1996. icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 7, 4207–4240, 2013

Frontal ablation of B. Osmanoglu et al.: Frontal ablation of glaciers on Livingston Island 5 glaciers on Livingston Island

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Fig. 3. Digital elevation data used in this analysis. There are 792 ICESat data points distributed among five different satellite tracks. Radar- Fig. 3. Digitalgrammetry elevation and ASTER data data provide used elevations in outside this the analysis. TanDEM-X coverage. There Hurd are Glacier 792 and ICESatJohnsons Glacier, data where points surface mass distributed J I among fivebalance di observationsfferent aresatellite available, are tracks. labelled by Radargrammetry H and J, respectively. and ASTER data provide elevations outside the TanDEM-X coverage. Hurd Glacier and Johnsons Glacier, where surface mass balance J I −3 observations600 m upglacier are available, from the calving are front. labelled Ice discharges by H for andwhere J, respectively.ρice is thedensityof ice(900kgm ). Flow directions 280 all ten flux gates were calculated individually and averaged are computed from offset tracking. Frontal ablation is given −1 Back Close to obtain a mean value. Deviation of each flux gate from the 305 in units of Mt a throughout the paper. mean was calculated and flux gates with deviations higher than 20% of the mean were discarded. On average 7.5 flux Full Screen / Esc 4.3 Surface velocities gates were used for the analysis. Flux gates are only defined 285 for marine terminating glaciers where the ice velocities at the −1 flux gates exceed 20 m a . For the remaining tidewater and Intensity offset tracking method was used to obtain glacier Printer-friendly Version all land terminating glaciers ice discharge into the ocean is surface velocities from PALSAR-1 intensity images (Gray assumed negligible. et al., 1998; Strozzi et al., 2002; Werner et al., 2005; Strozzi 310 et al., 2008). We preferred intensity offset tracking, rather Interactive Discussion 4.2 Frontal ablation than coherence tracking, because of the large extent of incoherent areas in the available imagery (Strozzi et al., 2002).In 290 For each marine-terminating glacier we compute the ice this study, inconsistent velocity measurements were masked flux q across the corresponding flux gate. Following Rignot out using a spatial variance filter, such that surface velocity −1 (1996) and Osmanoglu et al. (2013a), we derive the ice flux4236315 measurements that have Fisher’s Distance of 80 m a (with −1 from surface velocities by a constant expected error of 4 m a ) or above compared to their neighbors are discarded (Osmanoglu et al., 2011). The 21 ALOS PALSAR-1 scenes acquired between Oc- q = Hγu (2) sfc tober 2007 and January 2011 from tracks 124 and 125 320 were grouped to form short paired temporal baselines to re- 295 where H is ice thickness and γ is the ratio between surface duce measurement errors (Osmanoglu et al., 2013b). How- usfc and thickness-averaged horizontal velocities for each ever, due to acquisition gaps, especially over austral winter grid cell. We assume γ =0.9 (Cuffey and Paterson, 2010). months, there are some pairs with longer baselines. Surface usfc and H are a function of position along the flux-gate. velocity time series can be constructed with an inversion sim- The ice discharge D is then defined as the integral of ice flux 325 ilar to small baselines analysis (Berardino et al., 2002; Lanari 300 perpendicular to the flux-gate over the length L of the flux et al., 2007). However, the poor velocity estimates for pairs gate: with long temporal baselines do not allow for construction of a redundant network, where each scene is connected with L more than one pair. Therefore, the velocity time series were D = qρ dl (3) 330 constructed based on the measured displacements for each Z ice 0 pair. B. Osmanoglu et al.: Frontal ablation of glaciers on | Paper Discussion Livings | Paper Discussion | ton Paper Discussion | Island Paper Discussion 7 TCD − of 1.0 m w.e.7, 4207–4240, a 1 at basin 2013 10. The total variation of frontal ablation from all basins reaches 237 Mt a−1, which is less than the totalFrontal uncertainty ablation of estimated for the frontal ablation (381 Mt a−1)glaciers but is still on a matter of concern, as it is 46% of Livingston Island −1 465 the best estimate for the frontal ablation (509 Mt a ). B. Osmanoglu et al.

6 DISCUSSION Title Page 6.1 FrontalAbstract ablationIntroduction Conclusions References The only available detailed estimate of calving losses on Liv- Tables Figures ingston ice cap is that of Johnsons Glacier (19 in Fig.2). Us- 470 ing a full-stokesJ dynamicalI model constrained by measured velocities near the calving front, Navarro et al. (2013) cal- J I culated calving losses of 0.74 ± 0.17 Mt a−1 averaged over the period 2004/05-2007/08.ThisBack Close estimate compares reason- −1 Fig. 4. Measured vs. estimated ice thickness according to Eq. (4). The dashed line indicates Full Screen / Esc ± ◦ ably well with our estimated value of 0.4 0.3 Mt a for the 1-to-1Fig. line. 4. ContinuousMeasured lines versus show estimated the 95 % error ice thickness boundary corresponding according to to Equa-β = 30 . Red, green and blue colours are used to distinguish points from the slow, medium and fast475 movingthe period 2008-2011 (Table 1). glacier regions,tion 4. respectively. The dashed line indicates the 1-to-1 line. Continuous lines Our ice cap-widePrinter-friendly frontal Version ablation estimate for Livingston show the 95% error boundary corresponding to β = 30◦. Red, green Island is consistentInteractive Discussion with the results from a similar study on and blue colours are used to distinguish points from the slow, neighboring King George Island. Osmanoglu et al. (2013a) medium and fast moving glacier regions, respectively. 2 4237 estimated that King George Island (1127 km ) lost 720±428 −1 480 Mt a during the period October 2007-January 2011. The ± −1 the fits are rather poor or even meaningless, indicating that corresponding specific rate of 0.6 0.4 m w.e. a is similar ± −1 the velocity variations do not follow a simple seasonal pat- to Livingston’s rate of 0.7 0.5 m w.e. a . The lower rel- 435 tern, or the data density insufficient, or there are uncertainties ative error of the estimate for King George Island is mostly too large to infer seasonal patterns. To explore a possible cor- due to wider coverage of GPR ice thickness observations. On relation of the velocity variations with air temperature, the 485 Livingston Island many of the available ice thickness mea- figure also marks the periods of continuous daily mean tem- surements, mostly close to the ice divides in the western part peratures above 0 ◦C. of the ice cap, could not be used for the tuning of model parameters because ice velocities could not be derived from 440 5.2 Ice thickness SAR data in these areas. 490 Our ice cap-wide frontal ablation estimate is in the range The ice thickness estimated from the average surface veloci- of the ice discharge estimates for individual glaciers after col- ties (Fig.5a) and the combined DEM (Fig.3) using equations lapse of the Larsen-B and respective dynamic adjustments, 4 and 5 is shown in Fig.5b. The computed ice thickness val- Evans Glacier (459 Mt a−1, 2008) or Jorum Glacier main ues are in the same range as the GPR measurements, however branch (534 Mt a−1, 2008) (Rott et al., 2011), though the −1 445 the fit between the estimated and measured data sets indicates 495 specific rates for these glaciers (2.19 and 1.68 m w.e. a , large errors (Fig.4). respectively) are 2-3 times larger than that of Livingston Is- land. Except for Columbia Glacier in Alaska (O’Neel et al., 5.3 Frontal ablation 2005) estimates in specific units are considerably higher for King George Island and Livingston Island ice caps than those Frontal ablation rates for all investigated tidewater glaciers 500 reported from glaciers in the Arctic (AMAP, 2011). are given in Table 1. The largest rate is found for Huron 450 Glacier (7, Fig.2), followed by glacier basin 3. Ice discharge 6.2 Uncertainties for all glaciers totals 509 ± 381 Mt a−1. This is equivalent to a specific mass change of −0.8 ± 0.6 m w.e. a−1 over the The large discrepancies between the calculated and observed total area of the analysed basins (599 km2) and −0.7±0.5 m thicknesses shown in Fig.4 (rms misfit of 103.4 m) indi- w.e. a−1 over the area of the whole ice cap (697 km2). cate that our estimates of frontal ablation for Livingston Is- 455 The changes in frontal ablation △Dseas for each basin, as- 505 land should be considered only as a rough first-order ap- sociated with the seasonal variations in surface velocity de- proximation. The large errors result from a combination of scribed earlier and characterised by their standard deviation those inherent to the estimation of surface velocities from σvel, are given in Table 1. The largest variation in frontal ab- PALSAR-1 images, those intervening in the conversion of lation occurs at basin 3 and is 42.9 Mt a−1. In terms of spe- surface velocity to thickness-averaged velocity using Equa- 460 cific units, the discharge variations attain their highest value 510 tion 2, and those involved in the retrieval of ice thickness 8 B. Osmanoglu et al.: Frontal ablation of glaciers on Livingston Island icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 7, 4207–4240, 2013

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Interactive Discussion Fig. 5.Fig.a) Surface 5. (a) velocitiesSurface obtained velocities from SAR obtained feature from tracking. SAR Huron feature Glacier tracking. (7) shows Huronthe fastest Glacier ﬂow. b) (7) Estimated shows ice the thickness computedfastest from surface ﬂow. (b) velocityEstimated using Equation ice thickness 4. computed from surface velocity using Eq. (4).

4238 from thickness-averaged velocity and surface slope using flow, as described in subsection 5.2, allowed us to signifi- equations 4 and 5. In our case, the latter are expected to be cantly reduce the error, though the current rms misfit (103.4 dominant. This error component encompasses both the limi- 530 m) is still very large. tations of the physical model and the choice of values for the Another limitation is the assumption of steady-state in 515 model parameters. equations 4 and 5. The assumption is necessary to infer an The physical model represented by equations 4 and 5 as- ice-thickness distribution from velocity and surface slope sumes deformation by simple shear, neglecting longitudinal data alone, without available thinning rate data. On Liv- stress gradients which are known to be important near the 535 ingston Island, glacier thinning has only been studied on the calving fronts because of the large values of the along-flow Hurd Peninsula (Ximenis et al., 1999; Molina et al., 2007) 520 gradient of the surface velocity. Consequently, the ice thick- (Fig.2). Combined with observed front retreat on most of ness inferred near the calving fronts, where the ice fluxes are the ice cap (Calvet et al., 1999) this suggests that the cur- computed, are expected to be poor, implying large errors in rent geometry of the ice cap is not stationary. Even if the the ice discharge calculation. Using a single fit of the pa- 540 surface mass balance data for the last decade on Hurd Penin- rameters E and f all over the Livingston ice cap, as done in sula is rather close to zero, indicating a deceleration of the 525 Osmanoglu et al. (2013a) for the neighboring King George mass losses as compared to previous decades (Navarro et al., Island ice cap, resulted in large errors (rms misfits >200 m). 2013), the surface geometry needs some time to accommo- Using separate fits for the regions of slow, medium and fast date to the changing mass budget. This, however, occurs icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 10 B. Osmanoglu et al.: Frontal ablation of glaciers on Livingston Island TCD

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Fig. 6. Detrended velocity time series for all analysed tidewater glaciers. Velocities are averaged over each glacier’s flux gate and shown as J I Fig. 6. Detrendeddeviations of each velocityglacier’s mean timeover the periodseries October for 2007-January all analysed 2011. Fits to tidewater a periodic signal glaciers.and their amplitu Velocitiesde A, phase φ and are aver- 2 aged overr value each are shown glacier’s to illustrate fluxthe seasonality. gate Larger and ampli showntudes indicate as stronger deviations seasonal effects, of and each phase values glacier’sare reported mean in partial over the Back Close years. Continuous periods with daily air temperature exceeding 0 ◦C are shaded in blue, using the temperature records from Juan Carlos I period Octoberweather station 2007–January (12 m a.s.l.). Horizontal bars 2011. indicate Fits the time to interval a periodic of each measurement signal (temporal and baseline). their amplitude A, phase φ and r 2 value are shown to illustrate the seasonality. Larger amplitudes indicate stronger sea- Full Screen / Esc sonal effimportanceects, and of the phase strong effect values that the areperiod reported of measure- in(westernmost partialyears. part of the Continuous ice cap) during 1971-74 periods (Orheim with daily 585 ment can have on the estimates of◦ frontal ablation. 600 and Govorukha., 1982), the surface mass balance of the ice air temperature exceeding 0 C are shaded in blue,cap has using only been the studied temperature on Hurd Peninsularecords (Ximenis et al., from Juan Printer-friendly Version Carlos I6.4 weather Relative station contribution (12 of frontalma.s.l.). ablation Horizontal to total 1999; bars Navarro indicate et al., 2013). the The time latter interval study includes of mass each mea- mass loss balance profiles (summer, winter and annual) averaged over surement (temporal baseline). the period 2002-2011 for Johnsons (tidewater, 5.36 km2) and Interactive Discussion For Johnsons Glacier, using Navarro et al. (2013) data it can 2 605 Hurd (land-terminating, 4.03 km ) glaciers. be estimated that, over the period April 2006-March 2008, To compare the frontal ablation rates to mass change 590 the losses by frontal ablation represent only 16% of the to- due to surface processes we perform a simple back-of-the- tal annual ablation, the remaining originating from surface envelope calculation and approximate ice cap-wide annual ablation (assuming basal melting and internal accumulatio4239n surface mass balance and total ablation rates. Based on sur- were negligible). However, this glacier has a very particu- 610 face mass balance observations on land-terminating Hurd lar setting, with a very shallow pro-glacial bay (just a few and marine-terminating Johnsons glaciers we determine lin- 595 meters depth), a nearly flat bed in the area close to the calv- ear summer balance gradients by regressing summer surface ing front and moderate frontal velocities (maximum values −1 balances averaged over 20 m altitude bands versus altitude of the order of 40 m a ), implying a small flux of ice into for the mass-balance years 2008-2011 (approximately over- the ocean. Aside from some observations on Rotch Dome icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 7, 4207–4240, 2013

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Title Page 200 200 Marine Terminating Elevation (m) Terminating Marine Land Terminating Elevation (m) Terminating Land Abstract Introduction

0 2 4 6 8 -2 0 2 -2 0 2 -2 0 2 -2 0 2 0 0.6 1.2 -2 0 2 -2 0 2 -2 0 2 -2 0 2 % Area m w.e. m w.e. m w.e. m w.e. % Area m w.e. m w.e. m w.e. m w.e. Conclusions References

Fig. 7. Surface mass balance estimates extrapolated from Johnsons and Hurd Glaciers to the hypsometry of the whole ice cap for different Tables Figures Fig. 7. Surfaceyears. Cumulative mass distributions balance of total area estimates for each glacier extrapolated type are shown as the from black solid Johnsons line, with a second and x-axi Hurds shown on Glaciers the upper to the hypsometryborder. Theof greenthe and whole blue lines ice show cap extrapolated for di summerfferent and winter years. balances. Cumulative For winter balances distributions the dashed lines indi ofcate total constant area for each glaciergradient typeapproach, are while shown the solid line as indicates the constant blackfinal solid value approach. line, Summerwith a balances second use the constant x-axis gradi shownent approach on with the a upper cut-off at zero. J I border. The green and blue lines show extrapolated summer and winter balances. 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