B. Legresy

University of Tasmania, Hobart Town, Tasmania, Australia

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Publications (89)151.5 Total impact

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    ABSTRACT: We investigated the evolution of two major rifts cutting across Mertz Glacier Tongue, East Antarctica, using a combination of satellite images and 60 day sets of GPS data from two stations deployed either side of the western rift in 2007. The eastern rift began to open in the early 1990s, and the western rift initiated in 2002 in conjunction with the collision of a large iceberg with the tongue. Velocity time series derived from the 2007 GPS data exhibited strong variations at tidal periods modulated by sea-surface height and sea-surface slope and reproduced here with a conceptually simple model. We found that opening of the western rift in 2002 leads to a dramatic change in behavior of the tongue as the large range in velocity (700–2400 m a–1) observed in 2000 was largely reduced (1075–1225 m a–1) in 2007. Opening of the western rift decoupled the glacier from the transverse loading on the tongue driven by east–west tidal circulation. This loading previously induced time-varying lateral drag, which caused the large velocity range. Our results suggest changes in the mechanical behavior of an ice tongue impact the dynamics of the outlet glacier system and should be considered in longer-term mass-balance evaluations.
    Full-text · Article · Dec 2015 · Journal of Glaciology

  • No preview · Conference Paper · Jul 2015
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    ABSTRACT: The rate of global mean sea-level (GMSL) rise has been suggested to be lower for the past decade compared with the preceding decade as a result of natural variability, with an average rate of rise since 1993 of +3.2 ± 0.4 mm yr â '1 (refs,). However, satellite-based GMSL estimates do not include an allowance for potential instrumental drifts (bias drift). Here, we report improved bias drift estimates for individual altimeter missions from a refined estimation approach that incorporates new Global Positioning System (GPS) estimates of vertical land movement (VLM). In contrast to previous results (for example, refs,), we identify significant non-zero systematic drifts that are satellite-specific, most notably affecting the first 6 years of the GMSL record. Applying the bias drift corrections has two implications. First, the GMSL rate (1993 to mid-2014) is systematically reduced to between +2.6 ± 0.4 mm yr â '1 and +2.9 ± 0.4 mm yr â '1, depending on the choice of VLM applied. These rates are in closer agreement with the rate derived from the sum of the observed contributions, GMSL estimated from a comprehensive network of tide gauges with GPS-based VLM applied (updated from ref.) and reprocessed ERS-2/Envisat altimetry. Second, in contrast to the previously reported slowing in the rate during the past two decades, our corrected GMSL data set indicates an acceleration in sea-level rise (independent of the VLM used), which is of opposite sign to previous estimates and comparable to the accelerated loss of ice from Greenland and to recent projections, and larger than the twentieth-century acceleration.
    No preview · Article · May 2015 · Nature Climate Change
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    ABSTRACT: Totten Glacier, the primary outlet of the Aurora Subglacial Basin, has the largest thinning rate in East Antarctica. Thinning may be driven by enhanced basal melting due to ocean processes, modulated by polynya activity. Warm modified Circumpolar Deep Water, which has been linked to glacier retreat in West Antarctica, has been observed in summer and winter on the nearby continental shelf beneath 400 to 500 m of cool Antarctic Surface Water. Here we derive the bathymetry of the sea floor in the region from gravity and magnetics data as well as ice-thickness measurements. We identify entrances to the ice-shelf cavity below depths of 400 to 500 m that could allow intrusions of warm water if the vertical structure of inflow is similar to nearby observations. Radar sounding reveals a previously unknown inland trough that connects the main ice-shelf cavity to the ocean. If thinning trends continue, a larger water body over the trough could potentially allow more warm water into the cavity, which may, eventually, lead to destabilization of the low-lying region between Totten Glacier and the similarly deep glacier flowing into the Reynolds Trough. We estimate that at least 3.5 m of eustatic sea level potential drains through Totten Glacier, so coastal processes in this area could have global consequences.
    Full-text · Article · Mar 2015 · Nature Geoscience
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    ABSTRACT: The Mertz Glacier tongue (MGT) in East Antarctica lost ~55% of its floating length in February 2010, when it calved large tabular iceberg C28 (78 x 35 km). We analyse the behavior of the MGT over the preceding 12 years using a variety of satellite data (SAR and Landsat imagery, and ICESat laser altimetry). Contact of its northwestern tip with the eastern flank of shoals from 2002/3 caused eastward deflection of the ice flow by up to ~47°. This change contributed to opening of a major rift system ~80 km to the south, along which iceberg C28 eventually calved. Paradoxically, the seabed contact may have also held the glacier tongue in place to delay calving by ~8 years. Our study also reveals the effects of other, more localised external influences on the MGT prior to calving. These include an abrupt sideways displacement of the glacier-tongue front by at least ~145 m following an apparent collision with iceberg C08 in early 2002; and calving of numerous small icebergs from the advancing northwestern front due to the “chiselling” action of small grounded icebergs and seabed contact, resulting in the loss of ~36 km2 of ice from 2001–2006. The example of the MGT confirms the need for accurate bathymetry in the vicinity of ice shelves and glacier tongues, and suggests that the cumulative effect of external factors might be critical to understanding and modelling calving events and ice shelf stability, necessarily on a case-specific basis.
    No preview · Article · Mar 2015 · Journal of Geophysical Research: Earth Surface
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    ABSTRACT: Remote sensing of the sea-ice thickness is one of the main objectives of the Ku-band radar altimeter SIRAL-CRYOSAT II mission. On the one hand, sea-ice thickness is derived from the measurement of the height of the freeboard of the floes, and based on isostasy, assuming that the density of the water, the ice, as well as the snow, are known. On the other hand even if the snow load is known, the penetration of the electromagnetic waves into the snow strongly depends on the electrical and geophysical characteristics of the snow layer (density, temperature, permittivity, roughness). The remote sensing of the snow layer thickness (SLT) remains a real challenge and will be useful to correct the snow load for converting freeboard measurements from satellite altimetry into sea-ice thickness. If the dual frequency radar altimetry data show a good potential for remote sensing of snow and more generally of penetrating media [1], providing the SLT from Ku band data alone is highly motivated by the orbit of CRYOSAT designed to cover the entire Arctic. In this framework, a theoretical study, based on a 3D modelling of the scattering of electromagnetic waves by a stratified medium at normal incidence has been carried out in order to investigate and quantify the capacity of snow and ice penetration of Ku-band waves. The stratified medium is modelled as a snow layer considered as a stack of 2 sub-layers and the boundary layer at the bottom represented by a semi-infinite layer of ice-sea as shown on the figure 1. The roughness of each interface is taken into account and the small slope approximation (SSA) is used to determine the coherent and incoherent components of the scattered intensity [2-4]. It is demonstrated that the coherent intensity is the the specular direction but it depends on the rms-roughness heights and does not depends on the shape of the correlation function. The incoherent intensity depends even on the rms-roughness heights, but also on the shape of the correlati- n function. Several simulations have been conducted by varying the permittivity, thickness and roughness of each interface [5-6]. The 3 interfaces are random processes with Gaussian autocorrelation functions with zero mean values. The main conclusion is that the backscattered signal from the stratified medium is strongly related to the dielectric characteristics. It can vary significantly even if the variations of the stratified medium are small. This is an important result to be kept in mind when attempting the signal inversion. In addition, another similar study based on a 2D modelling of the scattering of electromagnetic waves by the same stratified medium at normal incidence and simulations in the same conditions have been previously conducted [7]. The roughness of each interface was also taken into account and the first-order small perturbation method (SPM) was used to determine the coherent and incoherent components of the scattered intensity. Results from those two studies are also compared at the end.
    No preview · Conference Paper · Aug 2014

  • No preview · Conference Paper · Dec 2013
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    Full-text · Dataset · Nov 2013
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    ABSTRACT: The deployment of a seismic network along the Adélie and George V coasts in East Antarctica during the period 2009–2012 provides the opportunity to monitor cryoseismic activity and to obtain new insights on the relationship between tidal cycles and coastal glacier dynamics. Here we focus on records from a seismometer located on a rocky outcrop in the vicinity of the grounding line of the 35 km broad Mertz glacier, a major outflow of this region. We detect numerous icequakes (50,000 events within 10 months and up to 100 events/h) and demonstrate their clear tidal modulation. We suggest that they result from ice friction and fracturing around the rocky peak and from the glacier flexure in response to the falling and rising tides at its grounding area. We propose that such icequake monitoring could be used as a climate proxy since grounding lines are subject to migrate with sea level changes.
    Full-text · Article · Oct 2013 · Geophysical Research Letters
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    ABSTRACT: In February 2010, the Mertz Glacier Tongue (MGT) calved, releasing an 80 × 40 km iceberg. We have developed a high-resolution barotropic ocean model of the region to simulate the local circulation in response to tides and atmospheric forcing. We improved the coastline, grounding line position and built a new bathymetry using satellite imagery and older bathymetry data to derive the best available tidal model for the region. We compared this and other available models to seven different sea level observations available in the area and significantly improved the tidal solutions reaching a root sum square of 2.3 cm. This model was then run in different bathymetric configurations, considering the ice draft of the major icebergs B9B and C28, to simulate the circulation before, during, and after the calving event. The currents changed substantially in the neighborhood of the MGT and icebergs. The barotropic model with tidal and atmospheric forcing and the atmospheric wind fields allow us to evaluate the forces acting on the MGT. The sea surface slope force dominates the budget. Calving occurred when high tide and strong nontidal currents (due to atmospheric forcing) combined to lead to the monthly maximum forces exerted on the MGT (i.e., between 10 and 13 February 2010). While the forces are not unusually large at the calving time, the currents are largely enhanced in the rifting area. Therefore, processes related to these currents, like melting the ice mélange inside the rifts, should be investigated to fully explain the final stage of the calving.
    Full-text · Article · Oct 2013 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: Ocean observations around the Australian‐Antarctic basin show the importance of coastal latent heat polynyas near the Mertz Glacier Tongue (MGT) to the formation of Dense Shelf Water (DSW) and associated Antarctic Bottom Water (AABW). Here, we use a regional ocean/ice shelf model to investigate the interannual variability of the export of DSW from the Adélie (west of the MGT) and the Mertz (east of the MGT) depressions from 1992 to 2007. The variability in the model is driven by changes in observed surface heat and salt fluxes. The model simulates an annual mean export of DSW through the Adélie sill of about 0.07 ± 0.06 Sv. From 1992 to 1998, the export of DSW through the Adélie (Mertz) sills peaked at 0.14 Sv (0.29 Sv) during July to November. During periods of mean to strong polynya activity (defined by the surface ocean heat loss), DSW formed in the Adélie depression can spread into the Mertz depression via the cavity under the MGT. An additional simulation, where ocean/ice shelf thermodynamics have been disabled, highlights the fact that models without ocean/ice shelf interaction processes will significantly overestimate rates of DSW export. The melt rates of the MGT are 1.2 ± 0.4 m yr−1 during periods of average to strong polynya activity and can increase to 3.8 ± 1.5 m/yr during periods of sustained weak polynya activity, due to the increased presence of relatively warmer water interacting with the base of the ice shelf. The increased melting of the MGT during a weak polynya state can cause further freshening of the DSW and ultimately limits the production of AABW.
    Preview · Article · Oct 2013

  • No preview · Conference Paper · Jun 2013
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    ABSTRACT: Knowing the interannual variations in the Antarctic ice sheet net snow accumulation, or surface mass balance (SMB), is essential for analyzing and interpreting present-day observations. For example, accumulation events like the one in East Antarctica in 2009 (Shepherd et al. 2012, Science, doi: 10.1126/science.1228102) challenge our ability to interpret observed decadal-scale trends in terms of long-term changes versus natural fluctuations. SMB variations cause changes in the firn density structure, which need to be accounted for when converting volume trends from satellite altimetry into mass trends. Recent assessments of SMB and firn volume variations mainly rely on atmospheric modeling and firn densification modeling (FDM). The modeling results need observational validation, which has been limited by now. Geodetic observations by satellite altimetry and satellite gravimetry reflect interannual firn volume and mass changes, among other signals like changes in ice flow dynamics. Therefore, these observations provide a means of validating modeling results over the observational period. We present comprehensive comparisons between interannual volume variations from ENVISAT radar altimetry (RA) and firn densification modeling (FDM), and between interannual mass variations from SMB modeling by the regional atmospheric climate model RACMO2 and GRACE satellite gravimetry. The comparisons are performed based on time series with approximately monthly sampling and with the overlapping period from 2002 to 2010. The RA-FDM comparison spans the spatial scales from 27 km to the continental scale. The mass comparison refers to the regional (drainage basin) and continental scale. Overall, we find good agreement between the interannual variations described by the models and by the geodetic observations. This agreement proves our ability to track and understand SMB-related ice sheet variations from year to year. The assessment of differences between modeling and observations allows us to quantify upper bounds on the uncertainties of the results from either technique. Moreover, the joint analysis of the four techniques aids in attributing the remaining discrepancies to deficits in individual techniques. These analyses may guide further improvements in observing and modeling SMB-related ice sheet variations.
    No preview · Article · Apr 2013
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    ABSTRACT: The deployment of a seismic network along the Adélie and George V coasts in East Antarctica during the period 2009–2012 provides the opportunity to monitor cryoseismic activity and to obtain new insights on the relationship between tidal cycles and coastal glacier dynamics. Here we focus on records from a seismometer located on a rocky outcrop in the vicinity of the grounding line of the 35 km broad Mertz glacier, a major outflow of this region. We detect numerous icequakes (50,000 events within 10 months and up to 100 events/h) and demonstrate their clear tidal modulation. We suggest that they result from ice friction and fracturing around the rocky peak and from the glacier flexure in response to the falling and rising tides at its grounding area. We propose that such icequake monitoring could be used as a climate proxy since grounding lines are subject to migrate with sea level changes.
    No preview · Article · Jan 2013 · Geophysical Research Letters
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    ABSTRACT: Remote monitoring of the sea-ice thickness is one of the main objectives of the Cryosat mission. On the one hand, sea-ice thickness is derived from the measure of the freeboard of the ice, based on isostasy and assuming that the density of water, ice, as well as snow, are known. On the other hand, even if the snow load is known, the penetration of the electromagnetic waves into the snow strongly depends on the electrical and geophysical characteristics of the snow layer (density, temperature, permittivity, roughness). The remote sensing of the snow layer thickness (SLT) remains a real challenge and will be useful to correct for the snow load for converting freeboard measurements from satellite altimetry into sea-ice thickness. If dual frequency radar altimetric data show a good potential for remote sensing of snow and more generally of penetrating media, (Legrésy et al., 2005), providing the SLT from Ku band data alone is highly motivated by the orbit of Cryosat designed to cover the entire Arctic. A theoretical study, based on the 2D modelling of the scattering of electromagnetic waves by rough layered interfaces at normal incidence, has been carried out in order to investigate the capacity of snow penetration of Ku-band waves. The multi-layered model used in this study is based on the first-order small perturbation method (Afifi et al. 2010, 2012). Within its domain of validity, this approximate model allows a fast analysis of the multi-layered structures by means of analytical equations giving the scattered field and intensities. The total backscattered intensity IT is written as a sum of a coherent IC and a fluctuating IF contribution: IT = IC + IF IC is the coherent contribution to the total intensity coming from the scattering of the stack of layers, and IF is the fluctuating contribution which takes into account the first order roughness effects. The medium is considered as a stack of three layers, with two interfaces, air/snow and snow/ice. Several simulations have been conducted by varying the temperature, permittivity, roughness and thickness of each layer and the results are presented. More specifically, the influence of the snow thickness on the backscattering is analysed.
    No preview · Conference Paper · Sep 2012
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    ABSTRACT: Interannual variations of the Antarctic ice sheet due to surface mass balance (SMB) fluctuations are important for estimates and interpretations of ice sheet mass balance. Recent assessments mainly rely on atmospheric modeling, but SMB variations on an ice-sheet scale are also reflected in satellite altimetry and GRACE satellite gravimetry data. All three approaches have their limitations. On the one hand, modeled interannual SMB variations need observational validation, which has been limited by now. On the other hand, the space-geodetic data do not exclusively reflect SMB phenomena, but also ice dynamics, glacial isostatic adjustment, and errors. Moreover, quantifying ice mass changes from altimetry requires assumptions on firn density and compaction. Finally, the spatial sensitivity of GRACE is limited to a few hundred kilometers. Nonetheless, previous work has shown promising qualitative agreement between interannual change patterns in ENVISAT radar altimetry (RA) and GRACE and between regional-scale interannual signals from GRACE and atmospheric modeling. Here we present comprehensive comparisons between all three techniques for the Antarctic ice sheet. We use the RACMO2/ANT high-resolution regional atmospheric model, ENVISAT RA estimates from an advanced along-track repeat analysis, and GRACE estimates from regional analyses based on different series of time-variable gravity field solutions. The overlapping period of the three techniques is August 2002 to October 2010. The high-resolution (27 km) spatial representations of interannual phenomena in ENVISAT RA and RACMO2/ANT match well for many regions, and show good agreement with the larger-scale patterns of mass varations determined with GRACE. We quantify this agreement by statistical parameters. Next, we combine all three techniques to characterize and quantify selected temporal SMB anomalies, such as an excess accumulation in West Antarctica and Wilkes Land in September/October 2005. The presented overall agreement indicates that, to a large extent, interannual signals in ENVISAT and GRACE are well understood as temporal SMB variations represented by RACMO2/ANT. Remaining differences between the three techniques help us to assess their limitations, and may guide further improvement in observing and modeling SMB-related ice sheet variations.
    No preview · Article · Apr 2012
  • Lydie Lescarmontier · Benoît Legrésy · Felix Perosanz · R. Coleman

    No preview · Article · Jan 2012
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    ABSTRACT: In February 2010, the tongue of Mertz Glacier calved, releasing an 80 km � 30 km iceberg. We had anticipated this calving event and during austral summer 2008 we started observing its development as well as gathering data to monitor the dynamics of the glacier upstream. In addition to the regular collection of satellite images and altimetry data, a number of observations have been made within the CRACICE-Mertz (Cooperative Research into Antarctic Calving and Iceberg Evolution) project and more generally under the framework of the IPY. We investigated the glacier dynamics and rift development using the following data types: ERS SAR interferometry, RADARSAT and ENVISAT SAR images, LANDSAT and SPOT images, SPOT stereo imagery, airborne ice-thickness radar profiles and in situ GPS measurements. Using numerical model studies to integrate and compare the various derived information, we compare the basal melt/freeze rates derived from ocean modeling using ROMS with that from mass-balance studies of the glacier tongue derived from the surface velocity fields and airborne ice-thickness measurements. Considering the last calving event of the glacier, we investigated several possible mechanisms leading to the opening of the rift and the calving of the ice tongue. First, we used the ocean circulation in the Mertz region derived from a barotropic model (TUGO) together with continuous GPS measurements to study the movement and flexure of the ice tongue. Then using a PPP (Precise Point Positioning) GPS processing technique, we increased the accuracy of our GPS measurements and observed the sub-daily scale movement of the glacier induced by its natural vibration modes. Finally, we conclude on the importance of the ocean forcing which is found to be the main driver of the rifting and calving and to describe the sequence of events in the latest calving process.
    No preview · Conference Paper · Jun 2011
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    ABSTRACT: We present our investigations on the accuracy of the GPS processing for ice studies. To this end, we used data from geodetic receivers both on rock sites and on a fast moving glacier in Antarctica, the Mertz glacier. From raw data to accurate position, we tried several GPS processing strategies and software in order to evaluate the accuracy level. We used the CSRS-PPP online processing tool from NRCAN, the GINS geodetic software from CNES-GRGS and the TRACK-GAMIT MIT tool. The processing strategies were based on the double difference (DD) or Precise Point Positioning (PPP) mode depending on the capabilities of these software. In any case, PPP kinematic series were computed at a 30 seconds sampling using IGS and GRGS-IGS precise orbits and (30s) clocks products. The GINS-PPP processing showed the best accuracy level with a floating PPP but with various spurious signals (random, periodic, jumps. . . ) that appeared on our series. We investigated the impact of PPP ambiguities fixed to integer values using the GRGS IGS products on the “non-geophysical” signals and the sub-daily scale movements of our glacier. The Mertz glacier ice tongue is floating on the ocean, so the two main signals were the glacier flow (approximately 3 meters a day) and the tidal signal. Thanks to this accurate processing and the flexibility offered by the PPP processing, we succeed in looking at sub hourly signals with few centimeters amplitude.
    No preview · Conference Paper · Jan 2011
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    ABSTRACT: The Mertz Glacier tongue (MGT), East Antarctica, has a large area of multi-year fast sea ice (MYFI) attached to its eastern edge. We use various satellite data sets to study the extent, age, and thickness of the MYFI and how it interacts with the MGT. We estimate its age to be at least 25 years and its thickness to be 10-55 m; this is an order of magnitude thicker than the average regional sea-ice thickness and too thick to be formed through sea-ice growth alone. We speculate that the most plausible process for its growth after initial formation is marine (frazil) ice accretion. The satellite data provide two types of evidence for strong mechanical coupling between the two types of ice: The MYFI moves with the MGT, and persistent rifts that originate in the MGT continue to propagate for large distances into the MYFI. The area of MYFI decreased by 50% following the departure of two large tabular icebergs that acted as pinning points and protective barriers. Future MYFI extent will be affected by subsequent icebergs from the Ninnis Glacier and the imminent calving of the MGT. Fast ice is vulnerable to changing atmospheric and oceanic conditions, and its disappearance may have an influence on ice tongue/ice shelf stability. Understanding the influence of thick MYFI on floating ice tongues/ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change, and thus to predicting the future of the Antarctic Ice Sheet.
    Full-text · Article · Dec 2010

Publication Stats

792 Citations
151.50 Total Impact Points

Institutions

  • 2003-2015
    • University of Tasmania
      • Institute for Marine and Antarctic Studies (IMAS)
      Hobart Town, Tasmania, Australia
  • 2013
    • Telecommunications for Space and Aeronautics
      Tolosa de Llenguadoc, Midi-Pyrénées, France
  • 2006-2013
    • Laboratoire d'Etudes en Géophysique et Óceanographie Spatiales
      Tolosa de Llenguadoc, Midi-Pyrénées, France
  • 2003-2010
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2008-2009
    • University of Toulouse
      Tolosa de Llenguadoc, Midi-Pyrénées, France
  • 2002
    • Centre D'Etudes Spatiales De La Biosphere
      Tolosa de Llenguadoc, Midi-Pyrénées, France