The flow of a polythermal glacier: McCall Glacier, Alaska, USA

We have analyzed the flow of polythermal McCall Glacier in Arctic Alaska. Using measurements of surface Velocity from the 1970s and 1990s, together with measurements of ice thickness and surface slope, we have investigated both the present flow, and seasonal and long-term flow variations. Our analysis of the present flow reveals that (i) longitudinal stress coupling is important along the entire length of the glacier; and (ii) there is significant basal sliding beneath a 2 km long section of the lower glacier, This sliding exists year-round and it accounts for more than 70% of the total motion there, We have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness. Accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and art increase in slope up-glacier from it. Both effects are reflected in the observed surface profile of McCall Glacier. The longitudinal stress-coupling length of McCall Glacier is three times the ice thickness, almost twice that typical of temperate glaciers. This is a direct effect of lower strain rates, which themselves are associated with the smaller mass-balance gradients of Arctic and continental glaciers. Long-term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution. This means that long-term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small, Seasonally, velocities reach their annual minimum in spring and increase during the short summer melt season by UP to 75% above mean winter values. However, the extra motion associated with the period of elevated velocities is only about 5% of the total annual motion. The speed-up, is due to an increase in basal sliding. This implies that most of the glacier bed is at the melting point. The zone affected by the melt-season speed-up extends well up-glacier of any moulins or other obvious sources for meltwater at the bed.