ICE SHEETS, OCEANS AND THE EARTH’S SHAPE: MODERN
PERSPECTIVES ON SEA LEVEL CHANGE (IAHS, IASPEI, IAG,
IAPSO, IAMAS, CMG)
Location: Law Building 115 LR2
Location of Posters: Old Gym
Thursday 22 July PM
Presiding Chair: C.R. Bentley (University of Wisconsin, USA)
JSH12/W/04-A4 Poster 1400-01
APPLICATION OF SAR INTERFEROMETRY TO DEM GENERATION OF ANTARCTICA
Taku Ozawa (The Graduate University for Advanced Studies, 1-9-10, Kaga, Itabashi-ku, Tokyo 173-8515, email: ozawa@nipr.ac.jp); Koichiro DOI, Kazuo Shibuya (both at National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku, Tokyo 173-8515, email: doi@nipr.ac.jp and shibuya@nipr.ac.jp)
SAR interferometry is an effective tool for estimating ice sheet motion as well as generating a digital elevation model (DEM). There are few precise DEMs in Antarctica except for some restricted regions, because of logistic difficulties to make dense ground surveys. It is true that satellite radar altimetry provides us data to generate DEM in Antarctica, but more precise DEM is required for monitoring ice sheet motion by SAR interferometry. As a first step to estimate the ice sheet motion, we tried to generate DEM by SAR interferometry technique. We employed the 3-pass method to separate the topographic fringe and the displacement fringe which are mixed in a SAR interferogram obtained on the Antarctic ice sheet. As a test site, we selected the ice sheet and outcropped area in the vicinity of Syowa Station (69.0degS, 39.5degE), which is the wintering base for the Japanese Antarctic Research Expedition, and analysed three serial JERS-1 SAR images acquired on June 16, July 30, and September 12 in 1996. We successfully extracted the topographic fringe on a large part of the ice sheet in the image.
JSH12/W/07-A4 Poster 1400-02
GEODETIC INDICATIONS ON RECENT ICE MASS CHANGES IN AN EAST ANTARCTIC REGION
Dietrich R. KORTH, W., Metzig, R., Perlt, J. (Institut fur Planetare Geoddasie Technische Universität Dresden, D-01062 Dresden, Germany)
During the last decade geodetic-glaciological investigations were carried out in Dronning Maud Land, Antarctica. The working area of about 300 by 300 square kilometres is located in the region of the Schirmacher Oasis (B=71° S, L=12° E). Different techniques were used to carry out observations, e.g. terrestrial geodetic measurements, static and kinematic GPS, tide gauge and SAR interferometry. As a result the flow of the inland and ice shelf glaciers has been completely studied. Detailed knowledge has been obtained on the ocean tide induced vertical motions of the ice shelf. Particular attention was paid to local rates of ice-sheet thickening or thinning. Such specific mass balances were derived from measurements along traverses crossing an ice stream. For one traverse running through an accumulation area the results are disturbed by short-term fluctuations in snowfall and snow densification. Measurements in ablation areas with blue ice at the surface are not affected by these influence. A thinning of the glacier of about 10 centimetres per year was determined for a large blue ice region. A significant lowering of the level of lake Untersee about 100 kilometres inland confirms this observed trend.
JSH12/W/09-A4 Poster 1400-03
COUPLED MARINE ICE-SHEET/EARTH DYNAMICS
Emmanuel LE MEUR, Richard Hindmarsh (both at British Antarctic Survey, Madingley Road, CB30ET Cambridge, U.K. Email: lemeur@nerc-bas.ac.uk; rcah@nerc-bas.ac.uk)
The West Antarctic ice sheet represents the only large marine icesheet on Earth. With most of the underlying bedrock below sea level, it strongly interacts with the sea which leads to specific ice dynamics and make these marine ice sheets potentially unstable. West Antarctica is believed to have substantially fluctuated during the last glacial/interglacial transition with a global sea level contribution of about 6 m. The question of its response to present-day conditions or to future environnemental changes is therefore of major concern. Previous studies have shown the key effect on the ice dynamics from the free water depth at the grounding line. Like sea level change, by modifying this water depth, bedrock displacements make the grounding line migrate and can significantly modify the ice sheet configuration. This presentation describes a simple 1-D axisymmetric marine ice sheet model which couples with a spherical visco-elastic earth model. By incorporating a kinematic grounding line migration formula, the ice model is able to generate the ice sheet span in response to the sea Level forcing, the ice flow and the bedrock characteristics. After testing the model response to some basic forcings we focus on the different effects resulting from including or excluding the bedrock response. The main effect of bedrock coupling consists of stabilizing the system by substantially reducing the grounding line migration. The overall time-dependent response of the system partly controlled by the delayed viscous mantle confirms the existence of specific ice/Earth dynamics and justifies the importance of realistic isostatics in ice sheet models.
JSH12/E/05-A4 Poster 1400-04
SPACEBORNE LASER ALTIMETRY OF THE POLAR ICE SHEETS
B. E. Schutz (University of Texas at Austin); C. R. Bentley (University of Wisconsin); R. Thomas (EG&G, NASA Wallops); J. Zwally (NASA Goddard)
In July 2001, NASA will launch a laser altimeter on ICES at into a near-circular, near-polar orbit to measure changes in polar ice-sheet topography. The ice sheet measurements will address fundamental questions about the growth or shrinkage of the polar ice sheets and their contribution to current and future global sea level change. The orbit inclination was chosen to provide coverage with the nominally nadir-looking altimeter, the Geoscience Laser Altimeter System, to a latitude of about 86 degrees. Exceptionally dense nadir tracks will occur at the high latitudes because of the orbit characteristics, thereby providing dense coverage of, for example, most West Antarctic features. The altimeter and related measurement systems have been designed to measure a secular height change with an accuracy of 15 mm/yr and over an area of 100 km x 100 km. This accuracy requirement was based on the analysis of van der Veen (1993) of the probability that a particular dh/dt is due to a real secular change in the ice sheet rather than arising from normal interannual variations in the snowfall. In the analysis of the results advantage will be taken of the synergy with the gravitational satellite GRACE to remove from the surface elevation changes any isostatic rebound signal that may exist. This paper will further describe the science requirements and the design considerations to meet the requirements, as well as the procedures planned for verification of the measurements and validation of the data products.
Friday 23 July AM
Presiding Chair: W.R. Peltier (Univ. of Toronto, Canada)
JSH12/W/03-A5 0830
SIX YEARS OF DUAL FREQUENCY TOPEX ALTIMETER DATA OVER SOUTH GREENLAND
Benoit LEGRESY+, Frederique Remy+, Patrick Vincent* (+: UMR5566/GRGS (CNES-CNRS-UPS) 18 Av. E. Belin, 31401 Toulouse cedex 4 FRANCE) (*: CNES DGA/T/ED/AL/MA 18 Av. E. Belin, 31401 Toulouse cedex 4 FRANCE E-mail: benoit.legresy@cnes.fr)
Since Topex/Poseidon was launched in 1991, it regularly releases altimetric data in C and Ku band every 10 days. As the satellite inclination only allows observations below 66 degree North latitude, only the southern part of the Greenland ice sheet is covered by a few tracks. The dual frequency altimetric information has been found useful for ice sheet surface properties investigation and for snow penetration induced error on the height measurement control. 6 years of data are now available. We have reprocessed the sensor level data of the time series for 8 tracks that cross Greenland. It allows us to analyse the seasonal cycle of the measurement and its relation to the ice sheet surface conditions in the accumulation area.
The C-Band radar waves penetrate deep into the snowpack, of the order of 10 to 30 m for dry snow, while Ku-Band radar waves penetrate 5 to 15 m. Ku-Band measurements are more sensitive to surface snow metamorphism. The difference of penetration between the two frequencies is linked to snow grain size and stratification. We also used radiometer brightness temperature data at 18, 21 and 37 Ghz which are simultaneously acquired on-board Topex/Poseidon. We find that brightness temperatures are complementary to the altimetric measurement in order to characterise snow surface metamorphism. Finally the 6 year long measurements provided by Topex/Poseidon are well controlled and can be accurately interpreted despite the sparse coverage of the satellite.
JSH12/W/05-A5 0900
ELEVATION CHANGES OF THE GREENLAND ICE SHEET FROM REPEATED GPS AND SAR INTERFEROMETRY
K. KELLER, R. Forsberg, C. S. Nielsen (KMS, Rentemestervej 8, DK-2400 Copenhagen NV, Denmark, email: kk@kms.dk) N. Gundestrup, C. C. Tscherning (Niels Bohr Institute, Univ. of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen O, Denmark, email: ng@gfy.ku.dk) S. N. Madsen, J. Mohr, J. Dall (Institute of Electromagnetic Systems, DTU, DK-2800 Lyngby, email: snm@emi.dtu.dk)
Elevations of the Greenland ice sheet has been measured repeatedly over several years at two sites at the centre of the Ice Sheet (at the GRIP drilling sites), and at a marginal ice cap in central East Greenland (Geikie Plateau). Repeated GPS measurements at the GRIP site since 1992 shows the ice sheet in this region to be essentially stable, with measured elevation changes and strain rates in good accordance with accumulation and ice flow models. Satellite and airborne SAR interferometry, and airborne laser altimetry, has additionally been used to study the coastal ice cap, primarily in order to evaluate accuracy of methods, as well as detect possible large yearly elevation changes in a region of high and variable yearly snow accumulation. The paper will present the project activities, methods and some recent results.
JSH12/C/JSS31/E/05-A5 0930
ICE-3G RETREAT INCONSISTENT WITH GREENLAND OBSERVATIONS
R THOMAS and S Manizade(EG&G, NASA/WFF, Wallops Island, VA 23337, USA,
email: thomas@osb.wff.nasa.gov; manizade@osb.wff.nasa.gov); W Krabill (Lab for Hydrospheric Processes, NASA/WFF, Wallops Island, VA 23337, email: krabill@osb.wff.nasa.gov); J Wahr, K. Larson, and S Gross (U. Colorado, Boulder, CO 80309, USA, email: wahr@lemond.colorado.edu; kristine@lemond.colorado.edu; sjg@quake.colorado.edu); T van Dam (Obs. Royal de Belgique,
B-1180 Bruxelles, Belgium, email: tonie@oma.be); X Wu (JPL, Pasadena, CA 91109, USA,
email: xpw@cobra.jpl.nasa.gov)
ICE-3G representation of Holocene retreat implies present-day isostatic uplift of the east and west coasts of southern Greenland by about 3-4 mm/yr if the present-day ice sheet is in balance. Satellite radar and aircraft laser altimeter surveys indicate overall balance for higher-elevation regions since 1978, with significant recent thinning nearer the east coast and areas of both thickening and thinning near the west coast. We would expect the areas of thinning to cause increased uplift rates, but continuous GPS measurements at Kangerlussuaq on the west coast, since 1995, and at Kulusuk on the east coast, since 1996, show subsidence of 7.7 +- 2.6 mm/yr in the west and 6.1 +- 5.1 mm/yr in the east. Kangerlussuaq is near to areas of ice that are approximately in balance and Kulusuk is close to ice that is rapidly thinning.
These observations suggest that either the Holocene retreat of the Greenland ice sheet differed significantly from that described by ICE-3G, and/or there was substantial thickening sometime between about 5000 BP and a few decades ago. We have used our observations of ice-surface elevation change and coastal subsidence to infer a range of ice-sheet histories and mantle-viscosity profiles that are consistent with those data, and these suggest that the southern part of the ice sheet may have been substantially thinner than at present at some time during the past few thousand years.
JSH12/W/12-A5 1000
A RE-EVALUATION OF THE MASS BALANCE OF GREENLAND
Atsumu Ohmura and Pierluigi CALANCA (both at Deptartment of Geography, Swiss Federal Institute of Technology, Winterthurerstr. 190, CH-8057 Zurich, Switzerland, e-mail: calanca@geo.umnw.ethz.ch)
The present mass balance of the two major ice sheets, Antarctica and Greenland, is not known with sufficient accuracy. A re-evaluation of the mass balance of Greenland and its spatial distribution are presented. The accumulation is derived from direct observations. Compared to previous estimates, additional data for some 48 pits and 5 meteorological stations are included in the analysis. Moreover, care is taken to correct the precipitation measurements for the wind-induced defect, separately for the liquid and solid phases. The ablation is derived from the three-summer-months mean temperature. Finally, the calving rates are estimated from remote sensed data and the geometry of the outlet glaciers.
JSH12/E/02-A5 1100
ICE CAP VOLUME CHANGE ON FRANZ JOSEF LAND DURING LAST 40 YEARS
Yuri Macheret, Andrey GLAZOVSKY (bouth at Institute of Geography, Russian Academy of Sciences, Staromonetny per., 29, 109017 Moscow, Russia, email: andrey@glazov.msk.ru); Julian Dowdeswell (Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK, email: j.a.dowdeswell@bristol.ac.uk); Michael Gorman (Scott Polar Research Institute, University of Cambridge, Lensfield Road, CB2 1ER Cambridge, U.K)
Airborne radio-echo sounding of 26 ice caps of various morphology and area (26 to 1.892 km2) was carried out on Franz Josef Land archipelago in 1994. Ice thickness data were obtained on 12 ice caps as maps and on 14 other ice caps as profiles. These data allowed to find the close empirical relationships between ice cap area and volume as well as between average and maximum ice thickness. These yield exponents for the power-law relations between volume-area of 1.312 and 1.228 and between maximum ice thickness-area of 0.295 and 0.245 for ice caps with and without outlet glaciers, and coefficients for the linear average-maximum ice thickness relation of 0.504 and 0.510, respectively. Relations between volume and area parametrized by maximum ice thickness provide exponents equal to 1.295 and 1.245, respectively. Using these empirical relations the ice volume of all ice caps on archipelago is estimated.
Total ice cap volume is estimated as 2105.9 km3 in 1993 and as 2147.8 km3 in 1953 with volume decrease during this period by 41.9 km3 and area reduction by 209 km2. The volume decrease is equal to mean net specific mass balance -7.0 cm per year in water equivalent. Comparison of this value with mass-balance data of other authors suggests that the negative mass balance rate in the last 40 years has been approximately 3.3 times less than in 1930-1960 which in turn agrees well with data on decrease of mean summer air temperature in compared periods on Franz Josef Land.
JSH12/W/06-A5 1130
SEA LEVEL CHANGE AND SEA-ICE VARIATIONS – A LINK?
ANZENHOFER, M (GeoForschungsZentrum Potsdam (GFZ), c/0 DLR, Postfach 1116, D-82230 Oberpfaffenhofen, Germany. E-mail:anzenhof@gfz-potsdam.de)
Model runs simulating future climate conditions assuming a further increase of atmospheric greenhouse gases show various asymmetries. Especially for the sea-ice extension, a decrease is expected for the North pole and an increase for the South pole. Time series of sea- ice extensions of both poles exist from passive microwave satellite observations from 1978 till now.
Accurate sea level time series, however, only exist from 1992 on (the problem of linking Geosat and ERS or TOPEX data is still unsolved). Recent investigations have shown a surprisingly high correlation between sea level variations and sea-ice extension anomalies around Antarctica. The correlation time period was only 3 years long. The presentation will focus on an enhanced study of the correlation between vertical sea level and anomalies in spatial sea-ice distribution by the extension of the altimeter derived sea level data base, the analysis of North pole sea-ice, and the inclusion of sea surface temperature data.
JSH12/W/01-A5 1200
THE GLACIAL-INTERGLACIAL PARADOX
John A.T. BYE, (Flinders University, GPO Box 2100, Adelaide, Australia, 5001,
email: John.Bye@flinders.edu.au)
Sea level variability during the Quaternary is simulated using a stochastic climate model and a sensitivity relation for the change in net oceanic evaporation due to a change in sea surface temperature, on the assumption that the greater part of the change in net oceanic evaporation causes changes in the land ice volumes, rather than being directly returned to the ocean by rivers. The analysis suggests that the observed sea level changes can be interpreted as due to the transfer of heat to the deep ocean from the surface mixed layer, arising from random radiation perturbations of the same variance as would give rise to the interannual variability of the global temperature series.
Friday 21 July PM
Presiding Chair: A. Ohmura (Swiss Fed. Inst. of Tech., Zurich, Switzerland)
JSH12/E/04-A5 1400
SCANDINAVIAN ISOSTATIC REBOUND: CONSTRAINTS ON THE LATE WEICHSELIAN ICE MODEL
Kurt LAMBECK (Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia, email: Kurt.Lambeck@anu.edu.au)
Sea-level information from three different data sources have been combined to develop a comprehensive model of the isostatic rebound for Scandinavia in which it has been possible to separate effectively those parameters describing the earth response and those describing the ice sheet. The data sources used are: geological estimates of shoreline age-height relationships, the alternating marine freshwater stages of the Baltic Sea, and the recent tide-gauge and lake-tilt measurements. The essential glaciological result is an ice sheet that is distinctly asymetrical over Scandinavia, with the ice over the southeastern and southern areas being relatively thin when compared with the ice over the north and west. Maximum ice thickness at about 18,000 years ago is unlikely to have exceeded 2000 m with the maximum ice cover occurring over northern Sweden rather than Finland. Earth model results include a well defined stratification in the mantle viscosity, consistent with analyses from other parts of the world.
The paradox is that glacial conditions (increase in ice volume) are favoured by positive (temperate) sea surface temperature anomalies, and interglacial conditions (decrease in ice volume) by negative (temperate) sea surface temperature anomalies. The evolution of both these regimes, which are inherently unstable, appears to be controlled by the deep water formation process, while albedo feedback is of minor importance.
JSH12/W/13-A5 1430
GRAVITATIONAL-VISCOELASTIC PERTURBATIONS OF A SPHERICAL EARTH WITH COMPRESSIONAL DENSITY STRATIFICATION
G. Li, D. WOLF (both at GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany, email: dasca@gfz-potsdam.de)
We derive the analytic solution for the deformation and gravity change caused by the loading of a self-gravitating spherical earth model consisting of a viscoelastic mantle with compressional density stratification and a homogeneous inviscid core. The fundamental characteristics of the solution are evident in the Legendre-transform domain, where we show relaxation-time and amplitude spectra for the mantle and core modes that constitute the time-dependent part of the response. Following this, we predict in the space-time domain the radial displacement, the free-air gravity anomaly and the geoid height for an axisymmetric load model with parabolic cross-section and dimensions similar to those of the Laurentide ice sheet. A comparison with the corresponding predictions for an earth with homogeneous density in the mantle underlines the importance of allowing for the compressional density stratification when modelling load-induced gravitational-viscoelastic perturbations of the earth.
JSH12/E/03-A5 1500
THERMALLY INDUCED LATERAL VISCOSITY VARIATIONS AND POSTGLACIAL REBOUND: IMPLICATIONS FOR RELATIVE SEA LEVELS IN LAURENTIA
Patrick WU (Dept. of Geology & Geophysics, University of Calgary, Calgary, Alberta T2N-1N4, Canada , Email: ppwu@ucalgary.ca ), John Wahr (Department of Physics, University of Colorado, Boulder, Colorado 80309 USA, Email: wahr@longo.colorado.edu); W.R. Peltier ( Dept. of Physics, University of Toronto,Toronto, Ontario M5S-1A7, Canada, Email: peltier@atmosp.physics.utoronto.ca)
Quantifying lateral viscosity variations in the mantle is useful in understanding mantle dynamics. From seismic tomography, which maps the three dimensional structures in the mantle, one can convert lateral velocity variations to viscosity variations by simple scaling since both are affected by the same thermal structure. The assumption in doing so is that the contribution of lateral variations in chemical composition can be neglected. In reality, the viscosity variations obtained this way are only upper bound estimates. Let the ratio of the actual viscosity variation over this upper bound estimate be given by the factor Beta, the aim of this work is to estimate the value of Beta from the postglacial sea level data in Laurentia.
The finite element method is used to calculate the deformation and relative sea levels in a laterally and vertically heterogeneous flat Earth. The loading history is given by the realistic ICE4G model and its eustatic sea level loads. Several cycles of loading before the last glacial maximum are also included. The radial viscosity profile is assumed to be given by the VM1 or VM2 models of Peltier. The lateral viscosity variations are obtained by applying the simple scaling relation of Ivins & Sammis (1995) to Beta multiplied by the shear-wave velocity fluctuations in the S20A model of Ekstrom & Dziewonski (1998). A range of laterally heterogeneous viscosity models are constructed - each characterized by a different value of Beta. These viscosity models are tested to see which is consistent with the sea level data in North America. The maximum value of Beta that can explain the sealevel data simultaneously will give the maximum contribution of thermal effects on lateral viscosity variation.
JSH12/W/10-A5 1600
GLOBAL GLACIAL ISOSTASY, SEA LEVEL AND GLACIOLOGY
Richard PELTIER (Department of Physics, Universityof Toronto, Toronto,Ontario, Canada M5S-1A7, email: peltier@atmosp.physics.utoronto.ca)
The global viscoelastic and gravitationally self consistent theory of the glacial isostatic adjustment process has recently been refined considerably so as to incorporate the influence of both time dependent coastlines and the feedback onto sea level of the earth's changing rotational state. Although the primary focus of the analyses that have been performed with this theory has been on the inverse problem for mantle viscosity, it has other, equally interesting, applications. For example it may be invoked to provide important constraints on both the origins of the global rise in sea level that is apparent on modern tide gauge recordings and on fundamental questions in glaciology. In the former area the strength of the rotational feedback on sea level turns out to be interesting, especially from the perspective of space geodesy(eg GRACE), whereas in the latter area its application has led to important constraints upon ice rheology.
JSH12/W/14-A5 1630
ANTARCTIC REBOUND AND THE TIME-DEPENDENCE OF THE EARTH'S SHAPE
Erik R. IVINS (JPL, Caltech, Pasadena, CA 91109-8099 USA, email: eri@scn1.jpl.nasa.gov); Thomas S. James (Geol. Survey of Canada, Sidney, BC, Canada, email: james@pgc.nrcan.gc.ca)
Great strides have been made during the past 30 years in refining models of the last global glaciation. The refinements draw upon a vastly expanded relative sea level and sedimentary core record. Furthermore, we now possess a sharpened understanding of the mechanisms that drive climate changes associated with deglaciation. Some 15 years ago, using only 5.5 years of ranging data, analyses of the acceleration in LAGEOS I nodal drift was used to infer that postglacial rebound was responsible for a secular change in the Earth's ellipsoidal shape [Yoder et al., 1983]. Today there exists a wealth of geodynamics satellite orbit data that constrain the secular time-dependence of the Earth's shape and low order gravity field, which includes mass redistribution from present-day glacier and great ice sheet imbalance and from postglacial rebound. We have shown that an unambiguous determination of the secular variation in the Earth's pear shaped harmonic (l = 3, m = 0) might provide information that bears on the present-day mass balance of Antarctica. This issue is revisited in light of new constraints on glacial loading during the late-Pleistocene and Holocene. An especially critical issue for the interpretation of secular odd degree zonal harmonics, l = 3 to 7, is the timing and magnitude of the deglaciation of Antarctica from Last Glacial Maximum. We explore ways in which the recovery of secular variation In both zonal and non-zonal harmonics for l = 2 through 7 can improve constraints on both rebound and present-day ice sheet balance.
JSH12/W/11-A5 1700
SURFACE TEXTURE AND ICE-STREAM BASAL DRAGRICHARD
C.A. HINDMARSH, (British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET.
E-mail:rcah@bas.ac.uk)
Ice streams are restrained by a combination of lateral drag and basal drag. Borehole observations from Ice-Stream B indicate little basal coupling, but the problem of whether ice-stream surface texture originates from internal rheological variations, basal topography or coupling of ice, till and water flows remains unresolved. If it is from such coupling, we conclude (i) that significant basal drag is present and (ii) this is dynamic and can change, implying that the basal resistance can change very fast, changing the rate of ice export to the sea.
This paper addresses the coupling of flows of ice, till and water, and the issue of whether such coupling provides mechanisms for meso-scale (kilometres to tens of kilometres) variability in ice-sheet flow and texture. The question of whether effective pressures at the ice-bed interface are statically or hydraulically controlled is examined in this paper. The answer is scale dependent, and has a significant effect on the relationship between ice surface and basal topography.
Linear stability analyses indicate that there appears to be sufficient variability in the ice-till-water system to potentially explain texture in ice stream surfaces, variations in ice stream thickness of tens of metres not directly relatable to topography, and waves moving upstream or downstream. Most importantly, the ice-stream/bed system is shown to exhibit meso-scale variability simply by coupling ice-flow according to the shallow ice approximation, till flow according to the hydrostatic thin-till approximation and water flow according to an effective pressure-dependent hydraulics.