Model simulations investigate Totten thinning
Enhanced oceanic heat flux and changing ocean dynamics are believed to be the key factors in making the Totten Glacier one of the fastest thinning glaciers in East Antarctica. To investigate this, a model of the ocean circulation beneath and around the Totten Glacier is currently being developed by scientists at the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) and the Australian Antarctic Division.
The Totten Glacier is located approximately 400 km east of Casey station, on the eastern side of Law Dome, and discharges up to 70 Gt/year of fresh glacial meltwater into the ocean. This is equivalent to 100 times the volume of Sydney Harbour every year. It has a maximum thickness of over 2.5 km at its grounding line – the region at which the glacier departs the continental ice sheet and begins to float – and is nearly 200 m thick at the calving front, 150 km to the north. Recent measurements show that the Totten Glacier is thinning at up to 1.9 m per year, a three-fold increase over the past 10 years. The direct cause of this alarming statistic isn't yet known, but is believed to be ocean driven.
The leading hypothesis is that relatively warm water derived from Circumpolar Deep Water (CDW), is mixed and modified and flows southwards onto the continental shelf, enhancing the melting of the glacier.
Once on the continental shelf, and with the appropriate bathymetric pathways to reach the glacier, the modified CDW, which is denser than the surrounding shelf water masses, is able to sink to the grounding line of the glacier and cause increased melting and rapid glacier acceleration. This is also suspected to be the key cause of the increased melting of other ice shelves showing rapid thinning, such as the Pine Island Glacier in the Amundsen Sea region of West Antarctica.
Since the ice shelf acts to slow glacier flow, ice shelf thinning by increased melting could lead to rapid acceleration of the Totten Glacier, similar to what was observed in the wake of the disintegration of the Larsen A and B ice shelves on the Antarctic Peninsula (Australian Antarctic Magazine 14: 22-23, 2008). Observations suggest a transport of modified CDW onto the continental shelf region near the Totten Glacier, but are too sparse to be definitive. Modelling is an obvious way to address the difficulty in obtaining high-resolution observations of the ocean near the Totten Glacier.
At the ACE CRC we are developing a numerical model to examine the thermodynamic interaction between floating ice shelves and the ocean on Antarctica's coastal margins (see Australian Antarctic Magazine 19: 6, 2010 for more details).
The output from the ice shelf-ocean model includes the time-evolution of ocean currents, and salinity and temperature of the water. From this, the melt rates of the ice shelves and the dynamics of massive water bodies can be determined.
The circulation and water temperature in the open ocean and under the Totten and Dalton ice shelves is illustrated in Figure 1. This shows the depth averaged ocean currents for March 2006, coloured for ocean temperature. Warm modified CDW can be seen to flow onto the shelf break and towards the eastern side of the front of the ice shelf. The fresh meltwater then flows out of western side and continues westwards around Law Dome.
The melt rate of the Totten Glacier ice shelf is calculated within the model. Figure 2 shows the melt rate (in metres per year) under the Totten ice shelf, with depth-averaged currents overlaid. Melt rates of more than 50 m per year are observed occurring at the deepest part of the ice shelf.
These simulations can assist the planning of future scientific expeditions. Interpreting the output gives a clear picture of the ideal positions for mooring oceanography instruments such as CTD (conductivity, temperature, depth) buoys, as well as for taking glaciology measurements (for example, bore-hole or GPS receiver locations). The model can also be extended to run into the future, and by driving it with various climate scenarios, we can assess the sensitivity and response of East Antarctica's ice shelves to the effects of climate change.
Ultimately, these results will aid in the planning of future observation programs in this region. They will also provide information to the Intergovernmental Panel on Climate Change, about the contribution of the region to global sea level rise.
Supplementary material is available, including references and 3D animations of CDW flow onto the continental shelf.
DAVID GWYTHER, BEN GALTON-FENZI and GUY WILLIAMS
ACE CRC and Australian Antarctic Division
Rignot, E. and S. S. Jacobs, 2002: Rapid bottom melting widespread near Antarctic ice sheet grounding lines. Science, 296, 2020–2023.
Pritchard, H. D., R. J. Arthern, D. G. Vaughan, and L. A. Edwards, 2009: Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature, 461, 971–975.
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