[1] The rapid drainage of supraglacial lakes around the ablation zone of the Greenland Ice Sheet forms an important link between water at the surface and the ice sheet base, allowing surface meltwater to reach the bed and hence increase glacial velocity. The conduits formed by lake drainages may remain open during the remainder of the melt season providing a pathway for further meltwater to reach the base. We investigated the drainage behavior of lakes from all regions of the Greenland Ice Sheet for the period [2005][2006][2007][2008][2009]. We mapped the evolution of 2600 lakes from 3704 MODIS images detecting a mean of 263 drainage events per year, of which 61% occurred in the south-west region. Only 1% of lake drainages occurred in the rapidly thinning south-east region. Our results show marked differences between the hydrology of the different regions of the ice sheet, with few lake drainages occurring in the regions where the highest dynamic mass loss is occurring. In the south-west and north-east, lake drainages are common and could impact glacier dynamics; in the south-east they are rare and are thus unlikely to do so.

[1] Synchronous acceleration and thinning of southeast (SE) Greenland glaciers during the early 2000s was the main contributor that resulted in the doubling of annual discharge from the ice sheet. We show that this acceleration was followed by a synchronized and widespread slowdown of the same glaciers, in many cases associated with a decrease in thinning rates, and we propose that ice sheet-ocean interactions are the first-order regional control on these recent mass changes. Sea surface temperature and mooring data show that the preceding dynamic thinning coincides with a brief decline in the cold East Greenland Coastal Current (EGCC) and East Greenland Current. We suggest this decline was partly induced by a reduction in ice sheet runoff, which allowed warm water from the Irminger Current to reach the SE Greenland coast. A restrengthening of the cold waters coincides with the glaciers' subsequent slowdown. We argue that this warming and subsequent cooling of the coastal waters was the cause of the glaciers' dynamic changes. We further suggest that the restrengthening of the EGCC resulted in part from cold water input by increased glacier calving during the speedup and increased ice sheet runoff. We hypothesize that the main mechanism for ice sheet mass loss in SE Greenland is highly sensitive to ocean conditions and is likely subject to negative feedback mechanisms.Citation: Murray, T., et al. (2010), Ocean regulation hypothesis for glacier dynamics in southeast Greenland and implications for ice sheet mass changes,

Geodetic measurements indicate that a number of glaciers in western Svalbard ranging in size from 5–1000 km2 are losing mass at an accelerating rate. The average thinning rate for Midtre Lovénbreen, the glacier with the best data coverage, has increased steadily since 1936. Thinning rates for 2003–2005 are more than 4 times the average for the first measurement period 1936–1962 and are significantly greater than presented previously. On Slakbreen, thinning rates for the latest measurement period 1990–2003 are more than 4 times that of the period 1961–1977. Thinning of several glaciers along a previously measured airborne lidar profile in Wedel Jarls Land has also increased, doubling between the period 1990–1996 and 1996–2002. Our results imply an increased sea level contribution from Svalbard. In addition, the mass loss is an important influence on measured rates of rebound on western Svalbard and should be factored into analysis of GRACE results.

No abstract

This paper is concerned with the application of automated digital photogrammetry, using 1:3000 scale photography, to complex, natural landform surfaces, of typical interest to geomorphologists. It assesses the quality of the results obtained using a relatively cheap and readily available area based stereomatching package, in terms of precision, accuracy and external reliability. Precision is investigated with reference to the confidence that can be placed in individual matches. Accuracy is evaluated using specially collected, independent datasets obtained from an area of complex topography in Glen Affric, Scotland. Data collection was stratified to areas of different surface roughness. External reliability is judged with respect to estimates of slope, a key parameter in geomorphological investigations. The results show that, whilst the effects of grid density and vegetation correction are the most important controls upon the accuracy and the external reliability of the photogrammetric results, collection parameters associated with the stereomatching process can also exert some control, particularly in areas of complex topography. It is impossible to generalize rules for choice of optimal collection parameters without careful consideration of the surface under investigation. Given that maximum grid densities are defined by the object space pixel resolution, the paper concludes that surface quality is largely governed by traditional controls upon photogrammetric data quality (camera calibration, base:distance ratio, ground control), combined with either scanning density or digital image resolution. However, over some surfaces, careful consideration has to be given to the effect of matching parameters.

Abstract.Calving is an important mass-loss process for many glaciers worldwide, and has been assumed to respond to a variety of environmental influences. We present a grounded, flowline tidewater glacier model using a physically-based calving mechanism, applied to Helheim Glacier, eastern Greenland. By qualitatively examining both modelled size and frequency of calving events, and the subsequent dynamic response, the model is found to realistically reproduce key aspects of observed calving behaviour. Experiments explore four environmental variables which have been suggested to affect calving rates: water depth in crevasses, basal water pressure, undercutting of the calving face by submarine melt and backstress from ice mélange. Of the four variables, only crevasse water depth and basal water pressure were found to have a significant effect on terminus behaviour when applied at a realistic magnitude. These results are in contrast to previous modelling studies, which have suggested that ocean temperatures could strongly influence the calving front. The results raise the possibility that Greenland outlet glaciers could respond to the recent trend of increased surface melt observed in Greenland more strongly than previously thought, as surface ablation can strongly affect water depth in crevasses and water pressure at the glacier bed.

During summer 2013 we installed a network of 19 GPS nodes at the ungrounded margin of Helheim Glacier in southeast Greenland together with three cameras to study iceberg calving mechanisms. The network collected data at rates up to every 7 s and was designed to be robust to loss of nodes as the glacier calved. Data collection covered 55 days, and many nodes survived in locations right at the glacier front to the time of iceberg calving. The observations included a number of significant calving events, and as a consequence the glacier retreated ~1.5 km. The data provide real‐time, high‐frequency observations in unprecedented proximity to the calving front. The glacier calved by a process of buoyancy‐force‐induced crevassing in which the ice downglacier of flexion zones rotates upward because it is out of buoyant equilibrium. Calving then occurs back to the flexion zone. This calving process provides a compelling and complete explanation for the data. Tracking of oblique camera images allows identification and characterisation of the flexion zones and their propagation downglacier. Interpretation of the GPS data and camera data in combination allows us to place constraints on the height of the basal cavity that forms beneath the rotating ice downglacier of the flexion zone before calving. The flexion zones are probably formed by the exploitation of basal crevasses, and theoretical considerations suggest that their propagation is strongly enhanced when the glacier base is deeper than buoyant equilibrium. Thus, this calving mechanism is likely to dominate whenever such geometry occurs and is of increasing importance in Greenland.

No abstract