Elmer/Ice News

ISMIP6 studies on Antarctic Ice Sheet


SeroussiEtAl2020AvatarThis study presents results from ice flow model simulations from 13 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015–2100 as part of the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). Elmer/Ice is one of these models, which are forced with outputs from a subset of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climate model results. Simulations of the Antarctic ice sheet contribution to sea level rise in response to increased warming during this period varies between −7.8 and 30.0 cm of sea level equivalent (SLE) under Representative Concentration Pathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment with constant climate conditions and should therefore be added to the mass loss contribution under climate conditions similar to present-day conditions over the same period. The simulated evolution of the West Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica mass change varies between −6.1 and 8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional simulated mass loss of 28 mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared to simulations done under present-day conditions for the two CMIP5 forcings used and display limited mass gain in East Antarctica.

 

Seroussi, H., et al. , 2020. ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century, The Cryosphere, 14, 3033–3070, doi:10.5194/tc-14-3033-2020

How seasonal cycle in basal melting contribute to observed changes in ice-shelf speed over the year?

klein2020Ice shelves play a critical role in modulating dynamic loss of ice from the grounded portion of the Antarctic Ice Sheet and its contribution to sea-level rise. Recent GPS observations of the Ross Ice Shelf (RIS), one of the largest ice shelves in Antarctica, reveal an annual cycle of ice velocity, with a maximal velocity anomaly reaching several meters per year at most stations. There are a lot of possible reasons for such a cycle. Recent measurements and modelling have shown that basal met rates along the ice front and those near Ross Island and Minna Bluff can change substantially at seasonal timescales, with high melt rates occurring during summer when the upper ocean along the western RIS ice front is warmed by insolation after the sea ice has been removed by melting and advection.

In this study, resulting from a collaboration between the Scripps Institution of Oceanography (University of California San Diego) and the Earth & Space Research Institute (Seattle), we use the Shallow-Shelf Approximation implemented in Elmer/Ice, forced with monthly basal melt rates from an ocean model, to explore the contribution of the strong seasonal cycle in basal melting toward changes in ice velocity over the year.

Our modelling shows that melt-rate response to changes in summer upper-ocean heating near the ice front will affect the future flow of RIS and its tributary glaciers. However, modelled seasonal flow variations from increased summer basal melting near the ice front are much smaller than observed, suggesting that other as-yet-unidentified seasonal processes are currently dominant.

Read more: Klein E., C. Mosbeux, P.D. Bromirski, L. Padman, Y. Bock, S.R. Springer and H.A. Fricker, 2020. Annual cycle in flow of Ross Ice Shelf, Antarctica: contribution of variable basal melting. Journal of Glaciology, doi:10.1017/jog.2020.61

 

The impact of temperature and crystal orientation fabric on the dynamics of mountain glaciers and ice streams

hruby2020In the first Elmer/Ice contribution from the University of Maine, M.S. student Kate Hruby explored how the strength, orientation, and distribution of temperature and crystallographic fabric in streaming ice affects the bulk volumetric flux. This work derives from a larger project, run by Chris Gerbi, Seth Campbell, Karl Kreutz, Peter Koons, and Bob Hawley, to measure fabric, temperature, and strain in the lateral margin of a glacier and relate the observed and model systems. Although shear margins have long been recognized as important mechanical factors in streaming ice flux, they have been the focus of very few rheological observations and model investigation.

This study uses the AIFlow solver plus new solvers written by Carlos Martín that permit node-scale control of temperature and fabric distribution. The main findings of the study are that flux is moderately to highly sensitive to both temperature and pressure and that the distribution of these parameters is as significant as their magnitude. Thus, calculating or predicting the fluxes to the precision we expect is needed in most studies will require that both temperature and fabric be incorporated into models. That, in turn, will require more extensive observations of natural systems.

The paper abstract is: Streaming ice accounts for a major fraction of global ice flux, yet we cannot yet fully explain the dominant controls on its kinematics. In this contribution, we use an anisotropic full-Stokes thermomechanical flow solver to characterize how mechanical anisotropy and temperature distribution affect ice flux. For the ice stream and glacier geometries we explored, we found that the ice flux increases 1–3% per °C temperature increase in the margin. Glaciers and ice streams with crystallographic fabric oriented approximately normal to the shear plane increase by comparable amounts: an otherwise isotropic ice stream containing a concentrated transverse single maximum fabric in the margin flows 15% faster than the reference case. Fabric and temperature variations independently impact ice flux, with slightly nonlinear interactions. We find that realistic variations in temperature and crystallographic fabric both affect ice flux to similar degrees, with the exact effect a function of the local fabric and temperature distributions. Given this sensitivity, direct field-based measurements and models incorporating additional factors, such as water content and temporal evolution, are essential for explaining and predicting streaming ice dynamics.

Read more: Hruby, K., C. Gerbi, P. Koons, S. Campbell, C. Martín and R. Hawley, 2020. The impact of temperature and crystal orientation fabric on the dynamics of mountain glaciers and ice streams, Journal of Glaciology, doi:10.1017/jog.2020.44

 

Elmer/Ice project © 2018 -- Conception : iGrafic