Physical Models for Moulins, Conduits to the Subglacial World

Glacier hydrology is a crucial factor in short-term (daily) and long-term (millennial) calculations of glacier mass loss into the ocean.  This is because meltwater influences the flow of glacier ice if it reaches the bed.   On the Greenland Ice Sheet, meltwater can run over bare ice, coalesce into streams, and pool into lakes on top of the ice. 

From there, the water is often -- but not always -- directed into moulins, near-vertical shafts that connect to the bed of the ice sheet hundreds of meters below.  Moulins drain large catchments of surface meltwater -- typically 10-30 km2, or ten Georgia Tech campuses -- and thus have a controlling influence on basal hydrology.

I am developing two models centered around moulins. (1) The Moulin Shape (MouSh) model predicts the radial expansion and contraction of a moulin connected to a subglacial channel and fed by hourly varying meltwater inputs.  We find that the moulin size and shape changes daily by some 20% as it accommodates changing meltwater fluxes. 

(2) The Fracture Sites  model (FraSM) predicts the likelihood of moulin initiation in space and time.  We find that moulin formation sites are strongly tied to drainages of neighboring supraglacial lakes, and that new moulins can form ~3-5 km upstream of a draining lake.  A new, upstream moulin could then drain an upstream lake, potentially inciting a chain reaction of lake drainage / moulin formation that steps its way upstream toward the vulnerable interior of the ice sheet, where changes to basal hydrology could significantly speed ice flow.

Ultimately, the moulin models connect climate, in the form of surface meltwater, to sea-level rise, via basal hydrology and ice flow.  As Greenland's climate changes, we need to be able to predict the sea-level rise that is coming as a consequence.