Speed-ups and slowdowns: insights into velocity variations measured from 2009-2011 on Yahtse Glacier, Alaska

Abstract

Water at the bed of a glacier can alter the basal stress distribution that governs basal motion and is responsible for short-term velocity variations. Accelerated basal motion has lead to increased ice discharge on tidewater glaciers, influencing the glacial contribution to sea level rise. The development of a temporally high-resolution model requires understanding the dynamic elements of the subglacial drainage system that govern short-term velocity variations. Here, we use global positioning system (GPS) observations to document the relationship between basal motion and widely variable water inputs on Yahtse Glacier over three years. Yahtse Glacier is a temperate, grounded, tidewater glacier terminating in Icy Bay, southcentral Alaska. We simultaneously measured surface ice velocity along the glacier's centerline using high-resolution GPS stations, weather data and glaciohydraulic tremor amplitude from 11 June 2009 through 11 September 2011. Yahtse exhibits a repeated summer slowdown facilitated by speed-up events and winter speed-up that ends in mid-winter when ice velocities return to pre-melt-season values. We find that the extent of the summer slowdown is determined by the presence of late-summer heavy rainfall that triggers speed-up events coupled with extra slowdowns. The summer minimum speed impacts flow speeds until mid-winter, therefore controlling average annual ice velocities. Measurements of glaciohydraulic tremor and surface uplift indicate extra slowdowns are the result of the release of subglacially stored water and the dynamic response of the isolated cavity system. We apply the basal hydrology model developed by Bartholomaus [Journal of Glaciology, 57, 206 (2011)] and adapted by Brinkerhoff [Annals of Glaciology, 57, 72 (2016)] to the ablation area of Yahtse Glacier. The model reproduces a majority of the transient speed-up events observed, but not the associated extra slowdowns or the season trend of a summer slowdown and winter speed-up. For the model to reproduce the short lived extra slowdowns and subsequent seasonal velocity trends, we suggest the model may need to account for the isolated subglacial cavity system.