d is characterized by multi-year sea ice, which undergoes periodic disintegration events. These are thought to allow the release of icebergs and to reduce back stress, thus promoting calving events and glacier retreat (Higgins, 1991; Reeh et al., 2001). Subsequent studies at Humboldt (Carr et al., 2015) and Petermann (Johannessen et al., 2013) have partly supported this theory, although the relationship appears to be more complex. At Petermann, the impact of sea ice buttressing appears to be less important than surface melt on seasonal velocity increases (Nick et al., 2012), whilst at Humboldt, icebergs were able to move away from the terminus, despite the formation of winter sea ice (Carr et al., 2015).
The outlets of the NEGIS are thought to be particularly susceptible to the effects of sea ice buttressing (Khan et al., 2014). At Nioghalvfjerdsfjorden, evidence suggests that sea ice holds icebergs in place at the calving front (Helk and Dunbar, 1953; Reeh et al., 2001), and that sea ice disintegration led to a large calving event in August 1997 (Reeh et al., 2001). Ten kilometers of frontal retreat occurred at Zachariae Isstrøm during 2002–2003, which led to the complete loss of its floating ice tongue (Mouginot et al., 2015). Khan et al. (2014) linked this to reduced sea ice concentration, due to high atmospheric temperatures, however, others have suggested it was primarily due to warmer subsurface temperatures (Mouginot et al., 2015). With Arctic sea ice predicted to decrease in the near-future (e.g., Collins et al., 2013), there is clear potential for reduced sea ice buttressing on glacier termini to allow for faster, enhanced ice discharge from the northern regions of the ice sheet. However, uncertainty remains as to the importance of sea ice buttressing on all outlet glaciers in northern Greenland. There is therefore a need for more detailed study of the impact of these processes on glacier retreat and inland ice velocities.
Across the GrIS, glacier-specific factors (basal topography and fjord geometry) have been identified as the cause of differing glacier responses to external climatic forcing (Howat and Eddy, 2011; Enderlin et al., 2013), and research suggests that this is also the case in northern Greenland (e.g., Porter et al., 2014; Carr et al., 2015). This section presents evidence for the effect of glacier geometry, the presence of floating ice tongues, and basal topography, on outlet glacier dynamics in northern Greenland.
Fjord width has been shown to have a strong influence on glacier dynamics (e.g., Jamieson et al., 2012; Enderlin et al., 2013; Carr et al., 2015). Fjord width variations can influence the stability of marine-terminating outlet glacier front positions by either promoting equilibrium or advance in a narrowing fjord, or rapid retreat in a widening fjord (e.g., Benn et al., 2007; Carr et al., 2014). At Petermann Glacier, the narrow fjord is thought to hinder the movement of icebergs away from the glacier front, which may facilitate ice mélange formation, which, also referred to as sikussak, is defined as a mixture of calved icebergs and sea ice. This mélange may “choke” the fjord with icebergs, which could exert resistive back-stress on the glacier tongue (Johnson et al., 2011). This ice mélange has been identified as a key control on iceberg calving rates elsewhere in Greenland (Amundson et al., 2010; Cassotto et al., 2015). Narrow fjords may also result in more ice contact with the fjord walls and, consequently, greater lateral drag exerted on the glacier sides (Raymond, 1996). These processes may apply to other glaciers in the region, several of which also terminate in narrow fjords (e.g., Ryder, Steensby and Hagen Bræ), and there is large variability in fjord geometries across northern Greenland, ranging from the wide Humboldt Glacier to the narrow sinuous fjord at Steensby Glacier. Thus, contrasting fjord widths between glaciers in northern Greenland could contribute to their varying response to external drivers.
Floating Ice Tongues
Changes in the floating ice tongues in front of seve