Meltwater migration in temperate ice

Frictional heating in the fast deforming ice of an ice stream potentially warms the ice to the melting point and there is evidence that a substantial region of temperate ice may exist near the base of the ice-stream margins. The production of meltwater in such a region alters the heat balance and the usual temperature equation solved in ice-sheet models must be modified to take account of this. In some cases, simply capping the temperature at the melting point and assuming excess heating produces meltwater can give the correct dynamics. But in general this is too simplistic an approach. Meltwater drains under gravity and dynamic pressure gradients, and the resulting transport of heat must be accounted for in order to correctly locate the boundaries between temperate and subtemperate ice. In this study, we look in detail at the dynamics of the temperate ice, focussing on the movement of meltwater in a compacting, viscous matrix of ice and the way in which meltwater transport influences the subtemperate-temperate transition. Four canonical cases can be considered: when the ice flows into or out of the temperate region, and when the meltwater flows towards or away from the interface. We examine the possible formation of boundary layers near these regions, and discuss how these may be responsible for channeling meltwater to the ice bed. We also discuss the use of numerical enthalpy and enthalpy-gradient methods, and the conditions under which these can correctly describe the dynamics.