18 research outputs found
Modelled glacier dynamics over the last quarter of a century at Jakobshavn Isbræ
Observations over the past 2 decades show substantial ice loss associated
with the speed-up of marine-terminating glaciers in Greenland. Here we use a
regional three-dimensional outlet glacier model to simulate the behaviour of Jakobshavn
Isbræ (JI) located in western Greenland. Our approach is to model and
understand the recent behaviour of JI with a physical process-based model.
Using atmospheric forcing and an ocean parametrization we tune our model to
reproduce observed frontal changes of JI during 1990–2014. In our
simulations, most of the JI retreat during 1990–2014 is driven by the ocean
parametrization used and the glacier's subsequent response, which is largely
governed by bed geometry. In general, the study shows significant progress in
modelling the temporal variability of the flow at JI. Our results suggest
that the overall variability in modelled horizontal velocities is a response
to variations in terminus position. The model simulates two major
accelerations that are consistent with observations of changes in glacier
terminus. The first event occurred in 1998 and was triggered by a retreat of
the front and moderate thinning of JI prior to 1998. The second event, which
started in 2003 and peaked in the summer 2004, was triggered by the final
break-up of the floating tongue. This break-up reduced the buttressing at the
JI terminus that resulted in further thinning. As the terminus retreated over
a reverse bed slope into deeper water, sustained high velocities over the
last decade have been observed at JI. Our model provides evidence that the
1998 and 2003 flow accelerations are most likely initiated by the ocean
parametrization used but JI's subsequent dynamic response was governed by its
own bed geometry. We are unable to reproduce the observed 2010–2012 terminus
retreat in our simulations. We attribute this limitation to either
inaccuracies in basal topography or to misrepresentations of the climatic
forcings that were applied. Nevertheless, the model is able to simulate the
previously observed increase in mass loss through 2014
Insights into Spatial Sensitivities of Ice Mass Response to Environmental Change from the SeaRISE Ice Sheet Modeling Project I: Antarctica
Atmospheric, oceanic, and subglacial forcing scenarios from the Sea-level Response to Ice Sheet Evolution (SeaRISE) project are applied to six three-dimensional thermomechanical ice-sheet models to assess Antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies. Results indicate (i) growth with warming, except within low-latitude basins (where inland thickening is outpaced by marginal thinning); (ii) mass loss with enhanced sliding (with basins dominated by high driving stresses affected more than basins with low-surface-slope streaming ice); and (iii) mass loss with enhanced ice shelf melting (with changes in West Antarctica dominating the signal due to its marine setting and extensive ice shelves; cf. minimal impact in the Terre Adelie, George V, Oates, and Victoria Land region of East Antarctica). Ice loss due to dynamic changes associated with enhanced sliding and/or sub-shelf melting exceeds the gain due to increased precipitation. Furthermore, differences in results between and within basins as well as the controlling impact of sub-shelf melting on ice dynamics highlight the need for improved understanding of basal conditions, grounding-zone processes, ocean-ice interactions, and the numerical representation of all three
The effect of climate forcing on numerical simulations of the Cordilleran ice sheet at the Last Glacial Maximum
We present an ensemble of numerical simulations of the Cordilleran ice sheet during the Last Glacial Maximum performed with the Parallel Ice Sheet Model (PISM), applying temperature offsets to the present-day climatologies from five different data sets. Monthly mean surface air temperature and precipitation from WorldClim, the NCEP/NCAR reanalysis, the ERA-Interim reanalysis, the Climate Forecast System Reanalysis and the North American Regional Reanalysis are used to compute surface mass balance in a positive degree-day model. Modelled ice sheet outlines and volumes appear highly sensitive to the choice of climate forcing. For three of the four reanalysis data sets used, differences in precipitation are the major source for discrepancies between model results. We assess model performance against a geomorphological reconstruction of the ice margin at the Last Glacial Maximum, and suggest that part of the mismatch is due to unresolved orographic precipitation effects caused by the coarse resolution of reanalysis data. The best match between model output and the reconstructed ice margin is obtained using the high-resolution North American Regional Reanalysis, which we retain for simulations of the Cordilleran ice sheet in the future.AuthorCount:5;</p