Seasonal changes in ice sheet motion due to melt water lubrication

• Coupled model of subglacial drainage system and ice dynamics.
• Accounts for seasonal evolution of drainage system and ice velocity.
• Model used to determine annual mean ice velocity.
• Predicts generally larger velocities in years with more melting.

A numerical model is used to calculate how the motion of an idealized ice-sheet margin is affected by the subglacial drainage of melt water from its surface. The model describes the evolution of the drainage system and its coupling with ice flow through a sliding law that depends on the effective pressure. The results predict ice acceleration during early summer when the inefficient drainage system is temporarily overwhelmed. The growth of a more efficient drainage system leads to a subsequent slowdown of the ice very close to the margin, but high water pressure and ice velocity are maintained through much of the summer further inland. Annual mean ice velocity increases with the total quantity of melt water except close to the margin, where it is almost insensitive to the amount of melting. Short-term variability of melt water input leads to rapid changes in ice velocity that result in a slight increase in the mean velocity relative to a smoother input. Linked-cavity and poroelastic models for the distributed drainage system are compared, and their relative merits discussed. Two different sliding laws are considered, and the need for a holistic description of hydraulically controlled sliding is highlighted.