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[1] Discharge from marine-terminating outlet glaciers represents a key component of the Greenland Ice Sheet mass budget and observations suggest that mass loss from northwest Greenland has recently accelerated. Despite this, the factors controlling outlet glacier dynamics within this region have been comparatively poorly studied. Here we use remotely sensed data to investigate the influence of atmospheric, oceanic, and glacier-specific controls on the frontal position of Alison Glacier (AG), northwest Greenland, and nine surrounding outlet glaciers. AG retreated by 9.7 km between 2001 and 2005, following at least 25 years of minimal change. Results suggest that sea ice and air temperatures influence glacier frontal position at seasonal and interannual timescales. However, the response of individual outlet glaciers to forcing was strongly modified by factors specific to each glacier, specifically variations in fjord width and terminus type. Overall, our results underscore the need to consider these factors in order to interpret recent rapid changes and predict the dynamic response of marine-terminating outlet glaciers to atmospheric and oceanic forcing.

Discharge from marine-terminating outlet glaciers accounts for up to half the recent mass loss from the Greenland ice sheet, yet the causal factors are not fully understood. Here we assess the factors controlling the behaviour of Humboldt Glacier (HG), allowing us to evaluate the influence of basal topography on outlet glacier response to external forcing since part of HG’s terminus occupies a large overdeepening. HG’s retreat accelerated dramatically after 1999, coinciding with summer atmospheric warming of up to 0.19°C a–1 and sea-ice decline. Retreat was an order of magnitude greater in the northern section of the terminus, underlain by a major basal trough, than in the southern section, where the bedrock is comparatively shallow. Velocity change following retreat was spatially non-uniform, potentially due to a pinning point near HG’s northern lateral margin. Consistent with observations, numerical modelling demonstrates an order-of-magnitude greater sensitivity to sea-ice buttressing and crevasse depth (used as a proxy for atmospheric warming) in the northern section. The trough extends up to 72 km inland, so it is likely to facilitate sustained retreat and ice loss from HG during the 21st century.