Abstract
Abstract. Harald Moltke Bræ, a marine-terminating glacier in north-western Greenland, shows episodic surges. A recent surge from 2013 to 2019 lasted significantly longer (6 years) than previously observed surges (2–4 years) and exhibits a pronounced seasonality with flow velocities varying by 1 order of magnitude (between about 0.5 and 10 m d−1) in the course of a year. During this 6-year period, the seasonal velocity always peaked in the early melt season and decreased abruptly when meltwater runoff was maximum. Our data suggest that the seasonality has been similar during previous surges. Furthermore, the analysis of satellite images and digital elevation models shows that the surge from 2013 to 2019 was preceded by a rapid frontal retreat and a pronounced thinning at the glacier front (30 m within 3 years). We discuss possible causal mechanisms of the seasonally modulated surge behaviour by examining various system-inherent factors (e.g. glacier geometry) and external factors (e.g. surface mass balance). The seasonality may be caused by a transition of an inefficient subglacial system to an efficient one, as known for many glaciers in Greenland. The patterns of flow velocity and ice thickness variations indicate that the surges are initiated at the terminus and develop through an up-glacier propagation of ice flow acceleration. Possibly, this is facilitated by a simultaneous up-glacier spreading of surface crevasses and weakening of subglacial till. Once a large part of the ablation zone has accelerated, conditions may favour substantial seasonal flow acceleration through seasonally changing meltwater availability. Thus, the seasonal amplitude remains high for 2 or more years until the fast ice flow has flattened the ice surface and the glacier stabilizes again.
Highlights
Surge-type glaciers are characterized by an alternation of long periods of low flow velocity and comparably short periods with velocities increased by at least 1 order of magnitude (1– 15 years, active phases or surge events) (Jiskoot, 2011; Benn and Evans, 1998; Bhambri et al, 2017)
By combining four different remotely sensed velocity data sets, we estimated a monthly velocity time series for Harald Moltke Bræ with high spatial and temporal resolution. Based on this time series we identified mainly two different signals of velocity variations close to the terminus of Harald Moltke Bræ: episodic surges and a pronounced seasonality
As we assume that there is a similar seasonality in most years of the observation period, we interpret the surges as phases with a strongly amplified seasonal amplitude
Summary
Surge-type glaciers are characterized by an alternation of long periods of low flow velocity (several to hundreds of years, quiescent phases) and comparably short periods with velocities increased by at least 1 order of magnitude (1– 15 years, active phases or surge events) (Jiskoot, 2011; Benn and Evans, 1998; Bhambri et al, 2017). These rapid changes in ice flow are triggered by internal instabilities and are mostly independent of external influences like weather or climate (Jiskoot, 2011; Benn et al, 2019). Surge-type glaciers normally cluster in certain regions (such as Alaska, Svalbard and the Karakoram) where the conditions are favourable for surge behaviour (e.g. a soft glacier bed and an at least partially temperate regime) (Sevestre and Benn, 2015; Jiskoot, 2011).
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