Control of seasonal ice dynamics by overdeepenings.

Ice loss from the Greenland Ice Sheet (GrIS) is currently the most significant single global contributor to barystatic sea level rise. The discharge of ice directly into the ocean from marine terminating glaciers is the cause of approximately 40% of this sea level rise. Understanding the processes that control how ice slides over the bed is fundamental to improving predictions of future GrIS mass loss.

Ice flow through major outlet glaciers dominates discharge of ice to the ocean, and this often involves flow through complexly overdeepened glacially eroded troughs. The adverse slopes of overdeepenings have the potential to modulate subglacial water pressure both by reducing hydraulic gradient, and via supercooling processes.

Here, we explore the control exerted on ice dynamics by the prominent overdeepening near the terminus of Upernavik Isstrøm II, an outlet glacier on the west coast of Greenland. We observe a ‘marine-isolating’ effect on the flow of inland ice, with ice dynamics dominated by marine processes downstream of the riegel and by melt processes inland of the riegel. Further, intriguing patterns of seasonal velocity variation were observed within the overdeepening under high melt conditions that support the possibility that adverse slopes of overdeepenings suppress the seasonal development of efficient channelised subglacial drainage, which is a key mediator of rates of sliding.