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CC Attribution 3.0 License. 7, 4207–4240, 2013 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 Printer-friendly Version Interactive Discussion 4207 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 bud- get 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 Interactive Discussion 4210 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 Data TCD 3.1 Ice thickness 7, 4207–4240, 2013 Ice thickness data are only available for limited parts of the ice cap.