From steady streaming to oscillations: the role of subglacial drainage and temperate ice in ice-stream dynamics

The majority of Antarctic ice is discharged through fast-flowing ice streams. Some of these ice streams exhibit variations in velocities and ice stream discharge on decadal to centennial time scales, but the factors controlling these variations are still insufficiently understood.  Using computational models of ice flow and hydrology, we predict the existence of two dynamical regimes: stable ice streaming associated with high hydraulic permeability of the bed, and ‘binge-purge’ oscillations associated with low permeability.

Observations indicate that the fast-flow of ice streams is enabled by meltwater lubricating the ice stream bed, and models suggest that this lubrication is the result of a positive feedback between fast flow, heat dissipation at the ice stream bed and meltwater production within the ice. In particular, recent studies have highlighted that heat dissipation in temperate ice stream margins, which are regions of high lateral strain, can contribute significantly to the subglacial water balance. However, the role of this meltwater flux in ice stream dynamics remains unclear. Here, we investigate the roles of subglacial drainage and feedbacks between fast flow and heat dissipation in ice-stream evolution. 

The ice is modelled as a vertically uniform plug flow. Water flow at the bed is modelled as a Darcian system whose hydraulic transmissivity increases with decreasing effective pressure. Dynamical feedbacks in the energy balance include both frictional heating along the bed and lateral shear heating. Within our model, two distinct dynamic regimes can be identified: if the hydraulic permeability of the bed is sufficiently high to evacuate all meltwater produced at the ice stream bed and in its margins, a moderately-fast steady ice stream forms. Conversely,`binge-purge’ oscillations between fast and stagnant flow emerge when the hydraulic permeability of the bed is too low to evacuate the meltwater produced within the ice stream. Topographic controls can suppress this oscillatory behaviour, while the formation of temperate ice in ice stream margins amplifies it.