Hydraulic run-away: a tnechanistn for therll1ally regulated surges of ice sheets

By using a simple paramet erized model of thermomech anically coupled flow in cold ice sheets, toge ther with a physical ly based sliding law which includes a description of basal drainage, we show that relationships between ice flux and ice thickness can realistically be m ul t i -valued, and hence t h a t hydraulically ind uced s urges can occur. 'vVe term t his m echanism h)draulic TllIl-aWa), as i t relies on the positive feed-back bet ween sliding veloc i t y and basal melt production. For t his feed­ back to operate, it is esse n tial that water p ressure increases with water storage. This is consiste n t with various recent ideas concerning drainage under ice sheets, be it t hrough a system of canals, a distri buted film or a subglacial aquifer. For confined flows, such as valley glaciers (e.g. Tra p r i dge Glacier) or topographically cons trained ice s tream s (e.g. Hudson S trait in the Laurentide ice sheet) , which are u n de rlain by sufficie n t ly deformable sediment, we c a n expect thermally regulated surges to occur, while in a laterally unconfined drainage basin (such as that which flows into t he Ross Ice Shel!), we might expect ice stream s to develop.