Analysis of Ice Plains of Filchner/Ronne Ice Shelf Using ICESat Data

We use repeat-track laser altimeter data from the Ice, Cloud, and land Elevation Satellite (ICESat) to map the grounding zone of Filchner/Ronne Ice Shelf (FRIS), Antarctica. Repeated passes of ICESat reveal ice flexure in the grounding zone occurs as the ice shelf responds to ocean height changes due primarily to tides. In the course of our mapping, we have confirmed or identified three major "ice plains", regions of low surface slope near the GZ where the ice is close to hydrostatic equilibrium: one on Institute Ice Stream, another to its east, and another west of Foundation Ice Stream. The vertical information from repeated ICESat tracks enables us to study the topography and flexure characteristics across these three ice plains, and we use this to develop a classification scheme for ice plains based on their surface topography and their state of flotation. We show that one of these ice plains indicates changes in lateral extent on short time-scales, depending on the state of the ocean tide. Understanding the location and nature of ice plains is important for ice sheet modeling, since they add uncertainty to the absolute boundary between floating and grounded ice

A high resolution, three-dimensional view of the D-28 calving event from Amery Ice Shelf with ICESat-2 and satellite imagery

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 48(3), (2021): e2020GL091200, doi:10.1029/2020GL091200.Tabular calving events occur from Antarctica's large ice shelves only every few decades, and are preceded by rift propagation. We used high-resolution imagery and ICESat-2 data to determine the propagation rates for the three active rifts on Amery Ice Shelf (AIS; T1, T2, and E3) and observe the calving of D-28 on September 25, 2019 along T1. AIS front advance accelerated downstream of T1 in the years before calving, possibly increasing stress at the rift tip. T1 experienced significant acceleration for 12 days before calving, coinciding with a jump in propagation of E3. ICESat-2's high resolution and repeat acquisitions every 91 days allowed for analysis of the ice front before and after calving, and rift detection where it was not visible in imagery as a ∼1 m surface depression, suggesting that it propagates as a basal fracture. Our results show that ICESat-2 can provide process-scale information about iceberg calving.We received funding from the following sources: NASA NNX15AC80G and NSF grant 1443677 (Fricker and Becker) and NASA 80NSSC20K0960 (Walker). We are grateful to Mike Cloutier, Polar Geospatial Center for assistance with WorldView imagery. Geospatial support was provided by Polar Geospatial Center under NSF-OPP awards 1043681 and 1559691.2021-07-1

Instantaneous Antarctic ice-sheet mass loss driven by thinning ice shelves

Abstract Recent observations show that the rate at which the Antarctic Ice Sheet (AIS) is contributing to sea level rise is increasing. Increasing ice-ocean heat exchange has the potential to induce substantial mass loss through the melting of its ice shelves. Lack of data and limitations in modelling, however, have made it challenging to quantify the importance of ocean-induced changes in ice-shelf thickness as a driver for ongoing mass loss. Here, we use a numerical ice-sheet model in combination with satellite observations of ice-shelf thinning from 1994 to 2017 to quantify instantaneous changes in ice flow across all AIS grounding lines, resulting from changes in ice-shelf buttressing alone. Our process-based predictions are in good agreement with observed spatial patterns of ice loss, providing support for the notion that a significant portion of the current ice loss of the AIS is ocean driven and caused by a reduction in ice-shelf buttressing

Seismicity within a propagating ice shelf rift: The relationship between icequake locations and ice shelf structure

Iceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important process involved in calving is the initiation and propagation of through‐penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the austral summers of 2004–2007. We apply a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h. Even during periods of quiescence, we find significant deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes ( M w  > −2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift‐transverse crevasses. Small‐scale variations in the mechanical structure of the ice shelf, especially rift‐transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts. Key Points Rift‐related seismicity controlled by mechanical heterogeneity Back projection reveals that seismic deformation is continuous in region Spacing of rift‐transverse crevasses controls the timing of seismic swarmsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106935/1/jgrf20203.pd

Impact of Tide-Topography Interactions on Basal Melting of Larsen C Ice Shelf, Antarctica

Basal melting of ice shelves around Antarctica contributes to formation of Antarctic Bottom Water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers. Tides influence ice shelf basal melt rate (w(b)) by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf. We use a three-dimensional ocean model, thermodynamically coupled to a nonevolving ice shelf, to investigate the relationship between topography, tides, and w(b) for Larsen C Ice Shelf (LCIS) in the northwestern Weddell Sea, Antarctica. Using our best estimates of ice shelf thickness and seabed topography, we find that the largest modeled LCIS melt rates occur in the northeast, where our model predicts strong diurnal tidal currents (similar to 0.4 m s(-1)). This distribution is significantly different from models with no tidal forcing, which predict largest melt rates along the deep grounding lines. We compare several model runs to explore melt rate sensitivity to geometry, initial ocean potential temperature (theta(0)), thermodynamic parameterizations of heat and freshwater ice-ocean exchange, and tidal forcing. The resulting range of LCIS-averaged w(b) is similar to 0.11-0.44 m a(-1). The spatial distribution of w(b) is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of w(b) is influenced by theta(0). These sensitivities in w(b) predictions reinforce a need for high-resolution maps of ice draft and sub-ice-shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

The Algorithm Theoretical Basis Document for Tidal Corrections

This Algorithm Theoretical Basis Document deals with the tidal corrections that need to be applied to range measurements made by the Geoscience Laser Altimeter System (GLAS). These corrections result from the action of ocean tides and Earth tides which lead to deviations from an equilibrium surface. Since the effect of tides is dependent of the time of measurement, it is necessary to remove the instantaneous tide components when processing altimeter data, so that all measurements are made to the equilibrium surface. The three main tide components to consider are the ocean tide, the solid-earth tide and the ocean loading tide. There are also long period ocean tides and the pole tide. The approximate magnitudes of these components are illustrated in Table 1, together with estimates of their uncertainties (i.e. the residual error after correction). All of these components are important for GLAS measurements over the ice sheets since centimeter-level accuracy for surface elevation change detection is required. The effect of each tidal component is to be removed by approximating their magnitude using tidal prediction models. Conversely, assimilation of GLAS measurements into tidal models will help to improve them, especially at high latitudes

Modeling 5 Years of Subglacial Lake Activity in the MacAyeal Ice Stream (Antarctica) Catchment Through Assimilation of ICESat Laser Altimetry

Subglacial lakes beneath Antarctica’s fast-moving ice streams are known to undergo ~1km3 volume changes on annual timescales. Focusing on the MacAyeal Ice Stream (MacIS) lake system, we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from Ice, Cloud and land Elevation Satellite (ICESat) laser altimetry. We construct a steady-state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the ICESat-derived filling or drainingrates. The modeled volume change rates of five large subglacial lakes in the downstream portion of MacIS are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered. However, the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream. Overall, we show that, for this fast-flowing ice stream, most subglacial lakes receive \u3e90% of their water from distant distributed sources throughout the catchment, and we confirm that water is transported from regions of net basal melt to regions of net basal freezing. Our study provides a geophysically based means of validating subglacial water models in Antarctica and is a potential way to parameterize subglacial lake discharge events in large-scale ice-sheet models where adequate data are available

High basal melting forming a channel at the grounding line of Ross Ice Shelf, Antarctica

Antarctica's ice shelves are thinning at an increasing rate, affecting their buttressing ability. Channels in the ice shelf base unevenly distribute melting, and their evolution provides insight into changing subglacial and oceanic conditions. Here we used phase-sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable Ross Ice Shelf. Melt rates of 22.2 ± 0.2 m a−1 (>2500% the overall background rate) were observed 1.7 km seaward of Mercer/Whillans Ice Stream grounding line, close to where subglacial water discharge is expected. Laser altimetry shows a corresponding, steadily deepening surface channel. Two relict channels to the north suggest recent subglacial drainage reorganization beneath Whillans Ice Stream approximately coincident with the shutdown of Kamb Ice Stream. This rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability

A new tide model for the Antarctic ice shelves and seas

We describe a new tide model for the seas surrounding Antarctica, including the ocean cavities under the floating ice shelves. The model uses data assimilation to improve its fit to available data. Typical peak-to-peak tide ranges on ice shelves are 1-2 m but can exceed 3 m for the Filchner-Ronne and Larsen Ice Shelves in the Weddell Sea. Spring tidal ranges are about twice these values. Model performance is judged relative to the ~5-10 cm accuracy that is needed to fully utilize ice-shelf height data that will be collected with the Geoscience Laser Altimeter System, scheduled to be launched on the Ice, Cloud and land Elevation Satellite in late 2002. The model does not yet achieve this level of accuracy except very near the few high-quality tidal records that have been assimilated into the model. Some improvement in predictive skill is expected from increased sophistication of model physics, but we also require better definition of ice-shelf grounding lines and more accurate water-column thickness data in shelf seas and under the ice shelves. Long-duration tide measurements (bottom pressure gauge or global positioning system) in critical datasparse areas, particularly near and on the Filchner-Ronne and Ross Ice Shelves and Pine Island Bay, are required to improve the performance of the data-assimilation model

ICESat GLAS Altimetry Measurements: Received Signal Dynamic Range and Saturation Correction

NASAs Ice, Cloud, and land Elevation Satellite (ICESat), which operated between 2003 and 2009, made the first satellite-based global lidar measurement of Earths ice sheet elevations, sea-ice thickness and vegetation canopy structure. The primary instrument on ICESat was the Geoscience Laser Altimeter System (GLAS), which measured the distance from the spacecraft to Earths surface via the roundtrip travel time of individual laser pulses. GLAS utilized pulsed lasers and a direct detection receiver consisting of a silicon avalanche photodiode (SiAPD) and a waveform digitizer. Early in the mission, the peak power of the received signal from snow and ice surfaces was found to span a wider dynamic range than planned, often exceeding the linear dynamic range of the GLAS 1064-nm detector assembly. The resulting saturation of the receiver distorted the recorded signal and resulted in range biases as large as 50 cm for ice and snow-covered surfaces. We developed a correction for this saturation range bias based on laboratory tests using a spare flight detector, and refined the correction by comparing GLAS elevation estimates to those derived from Global Positioning System (GPS) surveys over the calibration site at the salar de Uyuni, Bolivia. Applying the saturation correction largely eliminated the range bias due to receiver saturation for affected ICESat measurements over Uyuni and significantly reduced the discrepancies at orbit crossovers located on flat regions of the Antarctic ice sheet