Ice-Shelf Backpressure: Form Drag Versus Dynamic Drag

Abstract

The traditional concept of ice-shelf backpressure, defined as the stress deficit reducing ice-shelf spreading rates below the unconfined-expansion limit, is inadequate to predict the stress regime at an ice-stream grounding line when the ice-shelf flow geometry differs from ideal, rectangular channel flow. This inadequacy results from the action of glaciostatic stresses distributed around the margins of an ice shelf, which lead to a reaction force, termed form drag, at the grounding line of an ice stream. Here, I examine the stress regime at the grounding line of the West Antarctic Ice Sheet in terms of form drag and dynamic drag, the latter of which arises purely due to ice-shelf motion and viscous coupling at the ice-shelf shear margins. Finite-element simulations of the Ross Ice Shelf discussed here show that form drag dominates dynamic drag at the grounding line of ice streams B and C. As a demonstration of the consequence of this dominance, the future evolution of the Ross Ice Shelf, and of the stress regime at the grounding line of ice streams B and C are simulated to assess the response to impulsive removal of the Crary Ice Rise. This simulation shows that the forces restraining Ice Stream B do not change by a significant amount even after 1000 years of simulated adjustment. The forces restraining Ice Stream C, however, reduce by 40% over the 1000 year period, with an initial 25% change occuring within the first 250 years. This contrast between ice streams B and C is attributed to the dominance of form drag, its dependence on the ice-shelf thickness distribution, and the effect Crary Ice Rise has on the ice-shelf thickness at the grounding lines of the two ice streams.