Simulated ice extent and velocity in April (left) and November (right) of 1995.
|The Autumn of break-ups: When Jakobshavn Isbrae lost its floating tongue|
|investigators:||A. Aschwanden, M. Fahnestock, M. Truffer, and R. Motyka|
|venue:||2015 AGU Fall Meeting|
Jakobshavn Isbrae, Greenland's fastest-flowing outlet glacier, lost its floating tongue in 1995, an event which is often attributed to changes in ocean temperature. This poster and movie show the results of PISM simulations of this event, based on a step increase from 180 m/yr to 225 m/yr in sub-shelf melt rate during 1995 (Motyka et al. 2011). The simulations are started from reasonably-detailed observations of the 1985 state of the outlet glacier. A high-resolution HIRHAM5 reanalysis (Langen et al. 2015) is used for the atmospheric 1989–2011 climate. The results show that general patterns are simulated correctly, with ice speeds which almost double after break-up of the floating tongue. The timing of the break-up is too early and too fast, but these simulations do not include the “ice rumple” (Echelmeyer et al. 1991), which may add stability to the floating tongue.
See the official announcement for complete details.
The Antarctic Research Centre, Victoria University of Wellington, New Zealand, is offering a FULLY FUNDED scholarship for an enthusiastic and talented Ph.D student to undertake numerical ice-sheet modelling research. Experiments will focus on better understanding and simulating the processes involved in ice-sheet – ocean interactions. Such processes determine the basal mass balance of marine-based ice-sheets such as the West Antarctic Ice Sheet, and as such, control the pattern and timing of grounding-line migrations.
Collaborating with scientists at a variety of New Zealand (VUW, GNS, NIWA) and Australian (UNSW, UTAS, AAD) institutions, the researcher will use present-day glaciological and oceanographic observations as primary constraints to a suite of model simulations that will explore the sensitivity of Antarctic ice-sheets to changes in ocean circulation. A key aspect of the project lies in trying to identify and quantify thresholds and feedback mechanisms that may either accelerate or inhibit ice-shelf melt. The ultimate aim of the project is to build on recent work to provide more robust simulations of ice-shelf and ice-sheet changes under future scenarios of perturbed atmospheric and oceanographic conditions.
The research project will span a range of temporal and spatial scales, but will primarily use the Parallel Ice Sheet Model and will focus initially on the Ross Ice Shelf. The successful applicant may also have the opportunity to spend time in Antarctica acquiring new data.
Skills: Applicants must have a strong background in geophysics, maths or other numerical Earth Sciences. Experience working in a UNIX / Linux environment, including shell scripting, is essential. Programming abilities in any of the usual languages and experience with high-performance computing facilities would also be extremely useful.
Applications: We wish to have the successful applicant starting no later than July 2016, and therefore request completed applications by 18th December 2015.
For details of the application process or to lodge an expression of interest, contact Dr. Nick Golledge (firstname.lastname@example.org) as soon as possible.
A new paper in the Proceedings of the National Academy of Sciences by J. Feldmann and A. Levermann, of the Potsdam Institute for Climate Impact Research, uses PISM simulations to show that nearly-complete WAIS collapse is triggered by present-day melt rates in the Amundsen Sea. Modeled WAIS deglaciation follows after relatively-short (60–200a) periods in which the present-day sub-shelf (i.e. ocean-caused) melt rates are sustained.
The simulations use conservative assumptions about, and (necessarily) modeling of, the interaction of the ice sheet with the ocean and atmosphere. In particular, subshelf melt rates for the present ice shelf geometry are taken from Finite Element Sea Ice-Ocean Model (FESOM) results. These are then extended to the evolving cavity geometry by a diffusive algorithm into regions below sea level, but with a pressure adjustment using the ice shelf base elevation. This leads to melt rates further inland that are similar to corresponding present-day-cavity-geometry-induced melt rates.
In most other ways this application of PISM is as expected, though at high (5km) resolution and using a full suite of marine ice sheet submodels: 50ka spinup, SIA+SSA model with plastic till, subgrid motion of the calving front, ocean-water stress boundary condition at the calving front, the "eigen-calving" calving law, and an interpolated grounding line.
The results of the simulations are most easily understood by seeing what happens:
This work appeared today, 2 November 2015. It is already featured in commentaries at the Washington Post, The Guardian, and Bloomberg Business News. It is also featured in Nature journal's "News:Explainer", and in Science magazine's "Latest News".
The last of these includes this high-level view from two well-known students of the behavior of Amundsen Sea-sector glaciers:
“This paper does confirm what we hypothesized, that knocking out the Pine Island Glacier and Thwaites takes down the rest of the West Antarctic Ice Sheet,” says Ian Joughin, a glaciologist at the University of Washington, Seattle, who co-authored last year’s Science paper. He adds, however, that the model’s weakness is its [temporal] resolution; it shows the destabilization of the glaciers occurring roughly 60 years from now, whereas present observations suggest that collapse is already underway. As a result, Joughin says, the model’s time scale for the collapse is probably too long. “It’s more likely measured in centuries rather than millennia.”
Indeed, “the jury is still out” on whether Feldmann and Levermann’s study got the time scale right, [Eric] Rignot [of the University of California, Irvine] says. The long-term evolution of an ice sheet “is a very complex modeling problem. Some of the variables controlling the models are not all that well known,” he adds, including forces such as winds, ocean circulation, and how icebergs calve. “There is not a model out there that is getting it right, because they all have caveats. I think the discussion is ongoing, and is only going to be more interesting with time.”
PISM is jointly developed at the University of Alaska, Fairbanks (UAF) and the Potsdam Institute for Climate Impact Research (PIK). For more about the team see the UAF Developers and PIK Developers pages.
UAF developers, who are in the Glaciers Group at the GI, are supported by NASA's Modeling, Analysis, and Prediction and Cryospheric Sciences Programs (grants NAG5-11371, NNX09AJ38C, NNX13AM16G, NNX13AK27G) and by the Arctic Region Supercomputing Center.