|dc.description.abstract||This thesis aims to better understand the relationships between basal water pressure, friction,
and sliding mechanisms at ice sheet scales. In particular, it develops a new subglacial
hydrology model (Hydro) to explicitly predict water pressures in response to basal water
production and water injection from the surface.
Recent research suggests that the Greenland ice sheet (gis) is losing a substantial volume
of ice through dynamic thinning. This process must be modelled to accurately assess the
contribution of the gis to sea-level rise in future warming scenarios. A key control on dynamic
thinning is the presence of water at the ice-bed interface; Zwally et al. (2002) highlight the
importance of supraglacial lakes' impact on basal ice dynamics, a process now con rmed by
Das et al. (2008) and Shepherd et al. (2009).
Many studies focus on the effects of surface meltwater reaching the bed of the gis but
the underlying processes are often ignored. Geothermal, strain, and frictional melting, which
evolves with basal hydrology, provide the background basal pressure profile that surface
meltwater perturbates. Without understanding how these heat terms affect the background
profile it is difficult to define basal boundary conditions in models and therefore difficult to
model the dynamic response of the gis to surface melting.
Hydro tracks subglacial water pressures and the evolution of efficient drainage networks.
Coupled with the existing 3D thermomechanical ice sheet model Glimmer, model outputs
include effective pressure N and the efficient hydraulic area. Defining frictional heat flux
and basal traction as functions of N allow the modelling of seasonal dynamic response to
randomly draining supraglacial lakes.
Key results are that frictional heat flux, as a function of N, caps potential runaway
feedback mechanisms and that water converges in topographic troughs under Greenland's
outlet glaciers. This leads to a background profile with low N under outlet glaciers. Therefore,
outlet glaciers show a muted dynamic speedup to the seasonal surface signal reaching the bed.
Land-terminating ice does not tend to have subglacial troughs and so has higher background
N and consequently a larger seasonal response. This, coupled with effects of ice rheology,
can explain the hitherto puzzling lack of observed seasonal velocity change on Jakobshavn
Isbræ and other outlet glaciers.||en