Getting to the Bottom of Greenland’s Glaciers

story by Helen Hill

 

MIT postdoc Roberta Sciascia has been using MITgcm to explore the variations in submarine melt rate of Helheim Glacier induced by glacier and intermediary circulations.

Increasing evidence indicates that changes at the marine margin of Greenland’s tidewater glaciers may have triggered their recent acceleration and retreat and sizably increased Greenland’s contribution to sea level rise. One of the proposed mechanisms involves changes in submarine melting at the ice-ocean interface. What parameters and processes control the submarine melt rate are largely unclear however. In particular, the influence of surface melt water released at depth (sub glacial discharge) and ocean properties on the melt water plume dynamics have remained poorly understood.

Sciascia, working with Patrick Heimbach’s ice team at MIT in collaboration with researchers in Fiamma Straneo’s lab at WHOI, has been using a non-hydrostatic, high-resolution configuration of MITgcm to explore melting at the glacier’s base. The model, which includes an ice–shelf parameterization, initialized with data collected from Sermilik Fjord, where Helheim discharges, is forced by observed sub glacial discharge.

To date Sciascia’s glacier melting work has resulted in two papers:

Sciascia et al 2013 focused on understanding the seasonal variability of the submarine melting driven by a large tidewater glacier modeled on Sermilik Fjord conditions using a non hydrostatic, high-resolution configuration MITgcm with a melt rate parameterization at the vertical glacier front.

In Sermilik Fjord, near Greenland’s Helheim Glacier, the broadly two-layer stratification of the fjord’s ambient waters causes the melt water plume at the glacier front to drive a “double cell” circulation with two distinct outflows, one at the free surface and one at the layers’ interface.

The simulated “double cell” circulation reproduced in the model was demonstrated to be consistent, in both seasons, with Sermilik observations, and were observed to coincide with seasonal differences in the vertical structure of the melt rate, which were a maximum at the base of the glacier in summer and at the layers’ interface in winter.

Simulated submarine melt rates were noted to be strongly sensitive to the amount of sub glacial discharge, to changes in water temperature, and to the height of the layers. They were also shown to be consistent with those inferred from simplified one-dimensional models based on the theory of buoyant plumes.

In her new paper Sciascia et al 2014, the team have continued their investigation of the detail of submarine melting employing MITgcm, but now also comparing with idealized complementary laboratory experiments carried out by Claudia Cenedese at WHOI.

Here the advection mechanism in the numerical simulations is a density intrusion with a velocity an order of magnitude larger than the velocities associated with a glacier–driven circulation.

The authors observe that in summer, submarine melting is mostly influenced by the discharge of surface runoff at the base of the glacier and the intermediary circulation induces only small changes in submarine melting.

In winter, however, submarine melting depends only on the water properties and velocity distribution at the glacier front. Hence, the properties of the waters advected by the intermediary circulation to the glacier front are found to be the primary control of the submarine melting.

Finally, when the density of the intrusion is intermediate between those found in the fjord’s two layers a significant reduction in submarine melting is observed declining as the density approaches that of the bottom layer, when only a slight reduction in submarine melting is observed.

Getting to the Bottom of Greenland’s Glaciers – video from the laboratory together with three animations from the model (with audio)

To find out more about this work contact Roberta

This Month’s Featured Publications

Other New Publications this Month

L. Carone, R. Keppens and L. Decin (2014), Connecting the dots: a versatile model for the atmospheres of tidally locked Super-Earths, MNRAS (November 21, 2014) 445 (1): 930-945, doi: 10.1093/mnras/stu1793, published online October 8, 2014

Carsten Eden, Lars Czeschel, and Dirk Olbers (2014), Towards Energetically Consistent Ocean Models, Journal of Physical Oceanography 2014 ; e-View, doi: 10.1175/JPO-D-13-0260.1

Conrad, Patrick Raymond (2014), Accelerating Bayesian inference in computationally expensive computer models using local and global approximations, Thesis: Ph. D., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2014, uri: http://hdl.handle.net/1721.1/90599

Cooper, F.C, and L. Zanna (2014), Optimisation of an idealised ocean model, stochastic parameterisation of sub-grid eddies, arXiv:1410.5722 [physics.ao-ph]

David Halpern, Dimitris Menemenlis, and Xiaochun Wang (2014), Impact of Data Assimilation on ECCO2 Equatorial Undercurrent and North Equatorial Countercurrent in the Pacific Ocean, Journal of Atmospheric and Oceanic Technology 2014 ; e-View doi: 10.1175/JTECH-D-14-00025.1

Galassi, G., and G. Spada (2014), Sea–level rise in the Mediterranean Sea to 2050: Roles of terrestrial ice melt, steric effects and glacial isostatic adjustment, in press at Global and Planetary Change, doi: 10.1016/j.gloplacha.2014.10.007

Hill, Jenna C. and Alan Condron (2014), Subtropical iceberg scours and meltwater routing in the deglacial western North Atlantic, Nature Geoscience , published online October 12,  doi: 10.1038/ngeo2267

Tiffany Kataria, Adam P. Showman, Jonathan J. Fortney,Kevin B. Stevenson, Michael R. Line, Laura Kreidberg, Jacob L. Bean, and Jean-Michel De ́sert (2014), The Atmospheric Circulation of the Hot Jupiter WASP-43B: Comparing 3-Dimensional Models to Spectrophotometric Data http://arxiv.org/pdf/1410.2382.pdf

Samuel M. Kelly, Nicole L. Jones, Gregory N. Ivey, and Ryan J. Lowe (2014), Internal-tide spectroscopy and prediction in the Timor Sea, Journal of Physical Oceanography e-View, doi: 10.1175/JPO-D-14-0007.1

W. Koeve, H. Wagner, P. Kähler, and A. Oschlies (2014), 14C-age tracers in global ocean circulation models, Geosci. Model Dev. Discuss., 7, 7033–7074, doi:10.5194/gmdd-7-7033-2014

Gleb Panteleev, Max Yaremchuk, Jacob Stroh, Pamela Posey, David Hebert, and Dmitri A. Nechaev (2014), Optimization of the high-frequency radar sites in the Bering Strait region, Journal of Atmospheric and Oceanic Technology 2014 ; e-View, doi: 10.1175/JTECH-D-14-00071.1

Boris Sauterey, Ben A. Ward, Michael J. Follows, Chris Bowler and David Claessen (2014), When everything is not everywhere but species evolve: an alternative method to model adaptive properties of marine ecosystems, J. Plankton Res., published online: October 3, 2014,  doi: 10.1093/plankt/fbu078

Xiaoming Zhai and David R. Munday (2014), Sensitivity of Southern Ocean overturning to wind stress changes: Role of surface restoring time scales, Ocean Modelling, 84, pp. 12-25, doi: 10.1016/j.ocemod.2014.09.004

Do you have news about research using MITgcm? We are looking for contributions to these pages. If you have an interesting MITgcm project (ocean, atmosphere, sea-ice, physics, biology or otherwise) that you want to tell people about, get in touch. To make a post, contact Helen