Hagen Bræ: A Surging Glacier in North Greenland—35 Years of Observations

Abstract We use remotely sensed ice velocities in combination with observations of surface elevation and glacier area change to investigate the dynamics of Hagen Bræ, North Greenland in high detail over the last 35 years. From our data, we can establish for the first time that Hagen Bræ is a surge‐type glacier with characteristics of both Alaskan‐ and Svalbard‐type surging glaciers. We argue that the observed surge was preconditioned by the glacier geometry and triggered by englacially stored meltwater. At present, the glacier is in a transitional state between active and quiescence phases and is not building up to its pre‐surge geometry. We suggest that the glacier is adjusting to the loss of its floating section, general thinning, and changes in fjord conditions that occurred over the study period which are unrelated to the surge behavior. The high temporal resolution of the ice velocity data gives insight to the sub‐annual glacier flow.


Glacier Area Change
The glacier area change was measured by manually digitising the annual glacier front using optical imagery and arcGIS. The origin and resolution of the used imagery is summarised in Table 2. The annual area change was measured from 1985 to 2018 using imagery from Landsat and Sentinel-2. In order to measure the glacier area change consistently the satellite images best representing the annual maximum seasonal retreat were chosen among the available images. Furthermore, only ice free areas (such as large crevasses) with connection to the front line were excluded from the area measurement.
In 2003 the Scan Line Corrector (SLC) on Landsat-7 Enhanced Thematic Mapper Plus (ETM+) failed resulting in bands of missing data in all images Jensen et al. [2016].
If the image representing the maximum retreat had missing-data bands crossing the glacier front, the front was digitised as a straight line across the band.
When finding the images, the time resolution of the glacier front was close to daily in the period covered by Landsat 7, Landsat 8 and Sentinel-2, making cloudy weather con- ditions and personal judgment the only possible uncertainty sources when finding the image showing the glacier front position at the end of melt season. Landsat 5 had the same return period as Landsat 7 and Landsat 8 but the number of images available online is much smaller. Thereby, introducing a higher uncertainty as to whether the digitised glacier front represents the front position at the end of melt season correctly. Due to the poor temporal resolution during the first 14 years, the annual glacier area change from this period is associated with a higher uncertainty than the glacier area change later in the period.
The annual glacier area change is also associated with an uncertainty related to human precision and ability to correctly asses the glacier front position, which is often difficult due to shadows and unattached ice floating close to the glacier front. This uncertainty is higher in the measurements from the period before the glacier tongue broke up in the summer of 2008 because the ice bergs had a tendency to stay in the fjord. Thus making it difficult to see which parts of ice were attached to the glacier tongue and which were not.
After the tongue broke up in 2008 the tendency for the calved-off ice to stay in the fjord was smaller.

Surface Mass balance Modelling
Using output of surface mass balance from the regional climate model HIRHAM5, we tested whether the increase in surface slope was indeed due to the dynamical behaviour of the glacier or to an increased surface mass balance over the period. Figure S9 was calculated using winter velocities (weighted average of available surface velocity maps between October 1st and March 31st) since 1992. The total surface mass balance of the area between the gates was added to the flux through the upper gate and compared the flux through the lower. There is a slight decreasing trend in the surface mass balance over the period already indicating that it was not the controlling factor in the surface elevation changes. In Figure 9 the difference between the amount of ice going in and out of