Dynamic patterns of ice stream flow in a 3-D higher-order ice sheet model with plastic bed and simplified hydrology

Abstract

[1] Predicting ice sheet mass balance is challenging because of the complex flow of ice streams. To address this issue, we have coupled a three-dimensional higher-order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress. The model was tested for its sensitivity to regional water availability. First, with an assumed undrained bed, the ice stream oscillates between active and stagnant phases, solely as a result of thermodynamic feedbacks occurring at the ice-till interface. However, the velocity amplitude decreases over time, as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing "ice sheet mode." Second, we assume that the till is able to assimilate water from a hypothetical regional hydrological system. This leads to significantly different long-term behavior, as a continuously oscillating "ice stream mode" is maintained. The extra water incorporated in the till leads to higher velocities, triggering stronger thermodynamic feedbacks between the ice and till layer. Results also suggest that fast-flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase. Similarly, till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase. Our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system, underscoring the need to accurately describe the coupling between ice dynamics, basal conditions and regional subglacial hydrology in ice sheet models.

Response of the large-scale subglacial drainage system of Northeast Greenland to surface elevation changes

“…It is also known that glaciers in other parts of Greenland (e.g. Bjørk et al, 2012) advanced during the Little Ice Age. These changes must have influenced ice thicknesses in the drainage basin, but the extent of the impact is unknown.…”

“…Hence, recent years have seen an increasing interest in the response of different regions of the GrIS to climatic forcings (e.g. van den Bolch et al, 2013). While the outlet glaciers in southern Greenland have been speeding up during the past decade (Bevan et al, 2012), the glaciers in the northeast have until recently been considered relatively stable, partly due to the presence of sea ice and/or small ice shelves (Joughin et al, 2010).…”

“…Anandakrishnan and Alley, 1997;Fahnestock et al, 2001), and recent modelling efforts have begun to address this (e.g. Bougamont et al, 2011;de Fleurian et al, 2014). The coupling between ice flow and subglacial processes is, however, complicated and not easily incorporated into ice-sheet models.…”

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

“…Ice flow response to surface meltwater drainage events was investigated by coupling the higher-order thermodynamic model CISM to a model of subglacial sediment 40 , as well as a model of subglacial hydrology 46 . The ice thickness evolves according to the continuity equation; conservation of energy is expressed through the advective-diffusive heat equation.…”

“…In addition, by focusing explicitly on the character of the hydrological system, previous work has inherently assumed that the ice-bed interface consists of hard bedrock. However, thick subglacial sediments have been observed 34,35 and furthermore are known to exert firstorder control on flow in other glaciated regions [36][37][38][39][40][41] . To date, theoretical considerations on the implications of a soft sedimentary bed on GrIS dynamics are only starting to emerge 42 , but have never been implemented and tested in modelling studies.…”