Processed-Based Models in the Wild: A Forward Model Approach to Constraining the Processes Governing Basal Strength at Thwaites Glacier

Abstract:

Thwaites Glacier is a major contributor to uncertainty in near-term sea level estimates world wide. Current projections of the rate of retreat, and possible collapse, of Thwaites Glacier depends sensitively on the strength of the underlying bed and the physical processes controlling bed strength. While some of these processes, like subglacial hydrology, evolve rapidly and hence entail a highly dynamic bed, other processes such as the bed composition in terms of rock and subglacial sediments are evolving slowly, suggesting relatively static basal strength.

The goal of this study is to evaluate whether the basal strength at Thwaites Glacier is dynamic or static by identifying the dominant physical processes contributing to basal strength through a forward-modeling approach. We construct three independent, process-based forward models of basal strength to assess the relative importance of the following mechanisms: overburden stress, bed composition, and subglacial hydrology. These mechanisms are motivated by field observations and can be linked directly to a theoretical model of critical strength in our plastic bed assumption. By focusing on these physical processes, we are able to comment on the stability of each mechanism of basal strength and the time scales over which we expect these basal strength distributions to change.

An important contribution of this study is to apply process-based models, which are usually formulated only in highly idealized settings, to an actual field site. We focus our study on Thwaites Glacier by choosing our domain geometry and boundary conditions based on field observations. We additionally use these field observations to constrain each model of basal strength. To test each of our basal strength models we solve for ice speed using a 2D, depth integrated, thermomechanical free boundary model. We find that subglacial hydrology is a major control on basal strength at Thwaites glacier, largely in part to the ability of this mechanism to control the location of shear margins. This conclusion is particularly important given the rapid timescales over which subglacial hydrology can rearrange at the regional scale.