Changing Contribution of Peak Velocity Events to Annual Velocities Following a Multi-decadal Slowdown at White Glacier

As the focus of intensive glaciological studies in the 1960–70s, White Glacier on Axel Heiberg Island, Canada, has played an important role in understanding the dynamics of mostly-cold polythermal glaciers in the high Arctic. In this study, we examine the magnitude, duration and timing of peak velocity events in the summers of 2013–15 using continuous dual-frequency GPS observations, and compare them with similar measurements made in 1968. Summer speed-up events in 1968 and 2014, in which ice velocities reached 200% above winter values, were found to occur in conjunction with formation and drainage of an ice-marginal lake. Despite thinning of the glacier by >20 m and a decrease in annual surface velocities of 15–35% since the 1960s, the relative magnitude and duration of these peak events has increased, particularly at lower elevations, in comparison with the observations at the same locations many decades ago. Given the long-term slowdown of the glacier, the relative contribution of summer displacement to the net annual motion has therefore increased significantly, with summer motion over the span of <2 months now accounting for nearly half of the total annual displacement.