Jeremy Bassis, The Department of the Geophysical Sciences, University of Chicago
When: Friday, February 13, 2009, 10:30 a.m. to 11:30 a.m.
Where: Seminar Room 1.603, 10100 Burnet Road, Bldg 196-ROC, Austin, Texas 78758
Host: Charles Jackson, UTIG
Iceberg calving not only accounts for the majority of ice discharged from the Antarctic Ice Sheet and close to half of the ice lost from the Greenland Ice Sheet, it is also one of the most efficient mechanisms of transferring ice from the ice sheets to the surrounding oceans. Recent, observations now indicate that both the Greenland Ice Sheet and the West Antarctic Ice Sheet have transitioned into states of negative mass balance. With a few spectacular exceptions, most of this change has been attributed to increased melting near the seaward margins of the ice sheet (perhaps caused by the infiltration of warm ocean waters near the ice sheet edges). However, evidence from past ice sheets as well as a few regions in Greenland, West Antarctica and Alaska show that increased iceberg calving rates can lead to extremely rapid loss of ice mass – rates of mass lost two to three orders of magnitude larger than by melting alone. This raises the question: will iceberg calving play a major role in determining sea level in future climate warming scenarios or will it remain "quiet" on the seaward front? We do not know the answer to this question nor do we yet fully understand the triggers that occasionally lead to massive increases in calving rates. More worrisome, at present iceberg calving is almost entirely absent from large-scale ice sheet models and has so far eluded realistic parameterization. In this talk, I will present an approach towards elucidating physically consistent forms for a calving "law", i.e., a large-scale parameterization of the calving process. that relies on in part on scaling arguments. The parameterization that I will present differs significantly from previous efforts in two regards. First, the calving law is formulated as a probability distribution, thus explicitly includes the sporadic nature of iceberg calving and the random nature of some of the forcings (ocean swell, winds, etc.). Second, I use the same calving law to treat both grounded and floating ice. In this model, differences in calving style are due entirely to differences in the stress regime. Incorporating this calving parameterization into a flowline ice sheet-shelf model shows that the calving parameterization is able to qualitatively explain observations of (1) the pattern of advance and retreat of several ice shelves (the floating seaward extension of ice sheets) and marine terminating outlet glaciers; (2) the size-frequency distribution of icebergs calved from floating and grounded ice; (3) the effect of increased basal and surface melting on iceberg calving rates.