Ice sheet flow with thermally activated sliding. Part 1: the role of advection

Contemporary ice sheet thinning drives subglacial groundwater exfiltration with potential feedbacks on glacier flow

Observations indicate that groundwater-laden sedimentary aquifers are extensive beneath large portions of the Greenland and Antarctic ice sheets. A reduction in the mechanical loading of aquifers is known to lead to groundwater exfiltration, a discharge of groundwater from the aquifer. Here, we provide a simple expression predicting exfiltration rates under a thinning ice sheet. Using contemporary satellite altimetry observations, we predict that exfiltration rates may reach tens to hundreds of millimeters per year under the fastest thinning parts of the Antarctic Ice Sheet. In parts of West Antarctica, predicted rates of exfiltration would cause the total subglacial water discharge rate to be nearly double what is currently predicted from subglacial basal melting alone. Continued Antarctic Ice Sheet thinning into the future guarantees that the rate and potential importance of exfiltration will only continue to grow. Such an increase in warm, nutrient-laden subglacial water discharge would cause changes in ice sliding, melt of basal ice and marine biological communities.

The role of sliding in ice stream formation

Ice streams are bands of fast-flowing ice in ice sheets. We investigate their formation as an example of spontaneous pattern formation, based on positive feedbacks between dissipation and basal sliding. Our focus is on temperature-dependent subtemperate sliding, where faster sliding leads to enhanced dissipation and hence warmer temperatures, weak- ening the bed further, and on a similar feedback driven by basal melt water production. Using a novel thermomechanical model, we show that formation of a steady pattern of fast and slow flow can occur through the downstream amplification of noise in basal conditions. This process can lead to the establishment of a clearly defined ice stream separated from slowly flowing, cold-based ice ridges by narrow shear margins. Our model is also able to predict the downstream widening of ice streams due to dissipation and heat transport in these margins. We also show that downward advection of cold ice induced by accelerated sliding is the primary stabilizing mechanism that can suppress ice steam formation altogether, and give an approximate, analytical criterion for pattern formation.

Ice sheet flow with thermally activated sliding. Part 2: the stability of subtemperate regions

The onset of sliding in ice sheets may not take the form of a sharp boundary between regions at the melting point, in which sliding is permitted, and regions below that temperature, in which there is no slip. Such a hard switch leads to the paradox of the bed naturally wanting to refreeze as soon as sliding has commenced. A potential alternative structure is a region of subtemperate sliding. Here temperatures are marginally below the melting point and sliding velocities slower than they would if the bed was fully temperate. Rather than being controlled by a standard sliding law, sliding velocities are then constrained by the need to maintain energy balance. This thermal structure arises in temperature-dependent sliding laws in the limit of strong sensitivity to temperature. Here, we analyse the stability of such subtemperate regions, showing that they are subject to a set of instabilities that occur at all length scales between ice thickness and ice sheet length. The fate of these instabilities is to cause the formation of patches of frozen bed, raising the possibility of highly complicated cold-to-temperate transitions with spatial structures at short length scales that cannot be resolved in large-scale ice sheet simulation codes.