Brief Communication: Further summer speedup of Jakobshavn Isbræ

Joughin, Ian; Smith, Ben; Shean, David; Floricioiu, Dana

doi:10.5194/tc-8-209-2014

Cited by 130 publications

(168 citation statements)

References 31 publications

(54 reference statements)

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“…For the force balance, we use the standard vertically integrated shallow shelf approximation (SSA) equations (MacAyeal, 1989). This approximation neglects the effects of vertical shearing and is, hence, more adapted to model the flow in areas where the friction is low, resulting in an ice motion dominated by sliding.…”

Section: Direct Modelmentioning

confidence: 99%

“…Jakobshavn Isbrae is one of Greenland's three largest outlet glaciers and has one of the largest drainage basin on the ice sheet's western margin (Bindschadler, 1984). It is also the fastest Greenland glacier with a terminus velocity greater than 13 km a −1 (Joughin et al, 2008(Joughin et al, , 2014. The flowline is 550 km long and runs from the ice divide to the ice front.…”

Section: Reference Experimentsmentioning

confidence: 99%

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Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

2016

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Abstract. Ice flow models are now routinely used to forecast the ice sheets' contribution to 21st century sea-level rise. For such short term simulations, the model response is greatly affected by the initial conditions. Data assimilation algorithms have been developed to invert for the friction of the ice on its bedrock using observed surface velocities. A drawback of these methods is that remaining uncertainties, especially in the bedrock elevation, lead to non-physical ice flux divergence anomalies resulting in undesirable transient effects. In this study, we compare two different assimilation algorithms based on adjoints and nudging to constrain both bedrock friction and elevation. Using synthetic twin experiments with realistic observation errors, we show that the two algorithms lead to similar performances in reconstructing both variables and allow the flux divergence anomalies to be significantly reduced.

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Section: Direct Modelmentioning

confidence: 99%

Section: Reference Experimentsmentioning

confidence: 99%

Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

2016

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“…IPCC Assessment Report (AR5), new modeling initiatives tend to show that both Pine Island and Thwaites glaciers may have initiated MISI (Favier et al, 2014;Joughin et al, 2014). Significant progress in the ability of marine ice sheet models to reproduce observed dynamical changes in coastal regions has led to these novel results .…”

Section: G Durand and F Pattyn: Antarctic Projections Uncertainitiesmentioning

confidence: 99%

“…Pine Island Glacier (Joughin et al, 2010;Favier et al, 2014;Seroussi et al, 2014) and Thwaites Glacier (Joughin et al, 2014) in particular have shown considerable contemporary grounding line retreat and thinning . Joughin et al (2010) present a first comprehensive modeling of Pine Island Glacier (PIG) based on a SSA model and using assimilation methods for initialization.…”

Section: Pig Ensemblementioning

confidence: 99%

“…Finally, Seroussi et al (2014) use a higher-order model over PIG to simulate its dynamical response to marine forcing over the next 50 years using a higher-order model, with spatial resolutions down to 500 m at the grounding line. This particular model did not perform the MISMIP experiments, but as with the Joughin et al (2014) model, physics and numerical implementation are conform to MISMIP and MISMIP3d recommendations.…”

Section: Pig Ensemblementioning

confidence: 99%

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Reducing uncertainties in projections of Antarctic ice mass loss

Durand

Pattyn

2015

The Cryosphere

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Abstract.Climate model projections are often aggregated into multi-model averages of all models participating in an intercomparison project, such as the Coupled Model Intercomparison Project (CMIP). The "multi-model" approach provides a sensitivity test to the models' structural choices and implicitly assumes that multiple models provide additional and more reliable information than a single model, with higher confidence being placed on results that are common to an ensemble. A first initiative of the ice sheet modeling community, SeaRISE, provided such multi-model average projections of polar ice sheets' contribution to sealevel rise. The SeaRISE Antarctic numerical experiments aggregated results from all models devoid of a priori selection, based on the capacity of such models to represent key ice-dynamical processes. Here, using the experimental setup proposed in SeaRISE, we demonstrate that correctly representing grounding line dynamics is essential to infer future Antarctic mass change. We further illustrate the significant impact on the ensemble mean and deviation of adding one model with a known bias in its ability of modeling grounding line dynamics. We show that this biased model can hardly be identified from the ensemble only based on its estimation of volume change, as ad hoc and untrustworthy parametrizations can force any modeled grounding line to retreat. However, tools are available to test parts of the response of marine ice sheet models to perturbations of climatic and/or oceanic origin (MISMIP, MISMIP3d). Based on recent projections of Pine Island Glacier mass loss, we further show that excluding ice sheet models that do not pass the MISMIP benchmarks decreases the mean contribution and standard deviation of the multi-model ensemble projection by an order of magnitude for that particular drainage basin.

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Distinct patterns of seasonal Greenland glacier velocity

Moon

Joughin

Smith

et al. 2014

Geophysical Research Letters

227

370

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Predicting Greenland Ice Sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms. We present a 5 year record of seasonal velocity measurements for 55 marine-terminating glaciers distributed around the ice sheet margin, along with ice-front position and runoff data sets for each glacier. Among glaciers with substantial speed variations, we find three distinct seasonal velocity patterns. One pattern indicates relatively high glacier sensitivity to ice-front position. The other two patterns are more prevalent and appear to be meltwater controlled. These patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. The difference may be determined by meltwater availability, which in some regions may be influenced by perennial firn aquifers. Our results highlight the need to understand subglacial meltwater availability on an ice sheet-wide scale to predict future dynamic changes.Key PointsFirst multi-region seasonal velocity measurements show regional differencesSeasonal velocity fluctuations on most glaciers appear meltwater controlledSeasonal development of efficient subglacial drainage geographically divided

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Brief Communication: Further summer speedup of Jakobshavn Isbræ

Cited by 130 publications

References 31 publications

Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

Reducing uncertainties in projections of Antarctic ice mass loss

Distinct patterns of seasonal Greenland glacier velocity

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