The friction coefficient and the base topography of a stationary and a dynamic ice sheet are perturbed in two models for the ice: the full Stokes equations and the shallow shelf approximation. The sensitivity to the perturbations of the velocity and the height at the surface is quantified by solving the adjoint equations of the stress and the height equations providing weights for the perturbed data. The adjoint equations are solved numerically and the sensitivity is computed in several examples in two dimensions. A transfer matrix couples the perturbations at the base with the perturbations at the top. Comparisons are made with analytical solutions to simplified problems. The sensitivity to perturbations depends on their wavelengths and the distance to the grounding line. A perturbation in the topography has a direct effect at the ice surface above it, while a change in the friction coefficient is less visible there.
Parts of ice sheets that flow into the oceans and affect sea level can flow unusually fast by slipping over their beds. We use Elmer/Ice to solve for the first time in three dimensions the equations that describe the flow of ice as it slips over a bumpy rock bed. We include the important tendency for glaciers to separate from rock and form water‐filled cavities down‐glacier from bumps. These calculations indicate that resistance to slip depends sensitively on the bump shape and spacing. Cavities can cause the bed to become more slippery the faster the ice slides, with this destabilizing effect being more severe for bumps that are laterally narrow and widely spaced. However, bumps with steeply sloping up‐glacier sides can reverse this effect and cause resistance to slip to increase over a wide range of increasing slip velocity. This diverse behavior highlights the need for estimates of glacier slip velocity to incorporate the actual topography of glacier beds.
Read more: Helanow C., N. R. Iverson, L. K. Zoet and O. Gagliardini, 2020. Sliding relations for glacier slip with cavities over three‐dimensional beds, Geophysical Research Letters, 47, doi:10.1029/2019GL084924.
This article highlights the influence of surface meltwater and tides on crevasse opening leading to major calving events at grounded tidewater glaciers, such as Bowdoin Glacier, Northwest Greenland, where most calving occurs by a few large events resulting from kilometre-scale fractures forming parallel to the calving front. High-resolution terrestrial radar interferometry data of such an event reveal that crevasse opening is fastest at low tide and accelerates during the final 36 h before calving. Using Elmer/Ice, the authors identify the crevasse water level as a key driver of modelled opening rates. Sea water-level variations in the range of local tidal amplitude (1 m) can reproduce observed opening rate fluctuations, provided crevasse water level is at least 4 m above the low-tide sea level. The accelerated opening rates within the final 36 h before calving can be modelled by additional meltwater input into the crevasse, enhanced ice cliff undercutting by submarine melt, ice damage increase due to tidal cyclic fatigue, crevasse deepening or a combination of all these processes.
Read more: van Dongen E.,G. Jouvet,A. Walter,J. Todd,T. Zwinger,I. Asaji,S. Sugiyama, F. Walter and M. Funk, 2019. Tides modulate crevasse opening prior to a major calving event at Bowdoin Glacier, Northwest Greenland. Journal of Glaciology 1–11, DOI: 10.1017/jog.2019.89.