Numerical investigations of the slow - down ofWhillans Ice Stream , West Antarctica : is it shutting down like Ice StreamC ?

We investigate whether the recent slow-down of Whillans Ice Stream (WIS),West Antarctica, may lead to its complete stoppage in the near future, using a numerical model. Basal resistance to ice-stream motion is represented by a continuous till layer whose strength changes in response to basal melting and freezing.We implement a basal drainage system, which acts to hinder ice-stream stoppage through supply of extra water to those parts of the bed that are experiencing freezing.The ice module is a standard flowline model with parameterized key out-of-plane effects (Raymond, 1996). The most important result of our modeling effort is that we never obtained a slow-down that would not be followed by a complete stoppage within5100years.WIS slow-down can be avoided in the model if large basal water-input rates are assumed, to satisfy basal freezing. In comparison, the tested perturbations inWISwidth andmass balance hada relatively small effect on the tendency of the simulated ice stream to slow down.These results underscore the need for more quantitative constraints on the efficiency of sub-ice-streamwater drainage.We conjecture that the present-day slow-down of WIS will evolve to shut-down in the next few decades, unless an addition of basal water prevents freeze-on-driven bed strengthening.