Spatial patterns of basal drag inferred using control methods from a full‐Stokes and simpler models for Pine Island Glacier, West Antarctica

Abstract

[1] Basal drag is a fundamental control on ice stream dynamics that remains poorly understood or constrained by observations. Here, we apply control methods on ice surface velocities of Pine Island Glacier, West Antarctica to infer the spatial pattern of basal drag using a full-Stokes (FS) model of ice flow and compare the results obtained with two commonly-used simplified solutions: the MacAyeal shelfy stream model and the Blatter-Pattyn model. Over most of the model domain, the three models yield similar patterns of basal drag, yet near the glacier grounding-line, the simplified models yield high basal drag while FS yields almost no basal drag. The simplified models overestimate basal drag because they neglect bridging effects in an ice stream region of rapidly varying ice thickness. This result reinforces theoretical studies that a FS treatment of ice flow is essential near glacier grounding lines. Citation: Morlighem, M., E.

Dynamically and Observationally Constrained Estimates of Water-Mass Distributions and Ages in the Global Ocean

“…[ Khazendar et al [2007, 2009], Joughin et al [2010, and Morlighem et al [2010]. For BP and FS, ISSM extends the inversion to 3D models [Morlighem et al, 2010].…”

“…To minimize this effect, we use a cost function that measures the logarithm of the misfit instead [Morlighem et al, 2010]: where ɛ is a minimum velocity used to avoid zero velocity, and log is the natural logarithm. This cost function enables a robust estimation of the basal drag coefficient or depthaveraged ice hardness after only a few iterations over the entire model domain.…”

“…[49] This data assimilation technique has been successfully extended to BP and FS [Morlighem et al, 2010]. The major difference with SSA is that only the surface horizontal velocities are taken into account in the cost function evaluation, while the gradient with respect to the basal drag coefficient, a, or the depth-averaged ice hardness, B, are computed at the base only.…”

“…For each step of the optimization, one theoretically needs to re-calculate a thermomechanical equilibrium solution and update ice hardness, B, accordingly on grounded ice, to ensure consistency between ice flow velocity and ice viscosity. This can be done at the regional scale [Morlighem et al, 2010], on models for which computation time is small enough to allow for simultaneous inversion of the basal drag coefficient and update of the thermal regime. Practically, for large scale models such as Antarctica or Greenland, computation times are such that the inversion cannot include updates to the thermal regime.…”

“…These models are especially relevant for areas where both regimes of ice flow are of equal importance, as shown by Pattyn [1996] for the Shirase Glacier, East Antarctica. These models are usable over most of the ice sheet and ice shelf areas, but their assumptions are not strictly valid near grounding lines where a full-Stokes model (FS) is significantly more accurate [Lestringant, 1994;Nowicki and Wingham, 2008;Durand et al, 2009aDurand et al, , 2009bGagliardini et al, 2010;Morlighem et al, 2010]. FS is the most physically relevant model, but it is computationally expensive, therefore challenging to use at the continental scale at a high spatial resolution.…”