SPATIAL PATTERNS OF SUMMER SPEEDUP ON SOUTH-CENTRAL ALASKA GLACIERS FROM REPEAT SATELLITE IMAGERY

Seasonal changes in glacier basal motion are detectable from variations in glacier surface speed. They are attributable to variations in subglacial hydrology, and cause variations in both ice discharge and glacier erosion. We develop a novel workflow based upon Landsat 8 feature tracking, to document differences between spatial patterns of summer and winter glacier surface speed, which reflect changes in the distribution of basal motion. Of 64 glacier profiles from ice divide to terminus across the Wrangell-St Elias Ranges of Alaska, 19 have sufficiently high quality data to allow diagnosis of seasonal patterns. Of these, we identify summer speedup on 13 glaciers. The speedups are relatively uniform over much of the ablation zones, and their magnitudes vary by only a factor of ~2 between glaciers whose velocities span an order of magnitude. Summer speedups extends up to ~30 km up-glacier from termini, and often end at the bases of icefalls. These data provide systematic observation of the spatial pattern of enhanced summer glacier basal motion and suggest the possibility of its simple parameterization in glacier models.