Abstract
The Greenland Ice Sheet is currently in a period of negative mass balance in response to rising global temperatures. Accumulation in the interior is exceeded by increased mass loss at the margins, most notably on marine-terminating outlet glaciers. The Petermann Glacier, located in north western Greenland is one of only seven outlet glaciers that form floating ice tongues, and is thought to drain between 4 – 6% of the ice sheet. As with other outlet glaciers such as Jakobshavn Isbrae, the Petermann has experienced large-scale calving events in recent years, which have caused concern for the future stability of the ice tongue. While the calving and basal melt rates are reasonably well understood for the Petermann, there is no published data describing the surface climatology and ablation regimes operating at the surface. This thesis took a desktop-based approach to filling these data gaps, utilising data from the Greenland Climate Network to assess the near-surface climate and its links to surface melt for the 2002 – 2005 melt seasons.
A combination of surface lowering observations from an AWS-mounted sonic ranging instrument, a degree day model and an energy balance model were employed. The Petermann is characterised by low accumulation during winter months, with no surface snow cover observed in 2002, and a maximum of 0.24 m was present in 2003. The average mass loss through surface ablation was 1.09 m we, with a range of 0.89 (2005) to 1.33 m we (2002). Melt season length as defined by the period of observed surface lowering ranged from 80 days in 2002 to 103 days in 2005. Snow melt was found to not play a major role in total surface mass loss. Consequently, a surface degree day factor was found to be applicable, rather than requiring a separate value for snow and ice. Degree day factors (DDFs) ranged from 0.005 m d-1 ˚C-1 in 2004 to 0.009 m d-1 ˚C-1 in 2009. Air temperature was found to correlate well to the total energy balance (Pearson correlation coefficient of 0.82. Net radiation (Q*) was found to dominate the energy balance, accounting for over 90% of the available energy in both 2003 and 2005; these two seasons also saw the best agreement between modelled and observed melt.
In addition to characterising the ice surface from 2002 – 2006, long-term air temperature trends were assessed by creating a proxy temperature record using longer term climate data from Alert Station and the NCEP/NCAR reanalysis. A statistically significant warming trend (p < 0.01) was observed at an annual resolution. The most significant outcome of this analysis is the finding that strongest warming occurred during autumn and winter months. While this does not directly cause melt, an increase in the background temperature of the ice mass means that less energy is required to raise the near-surface ice to melting point. The implications of this may be seen in earlier onset of melting and greater total mass loss over individual seasons.
Average surface lowering of 1.09 m per year means that surface melt equates to approximately ten percent of the mass loss attributed to basal melting at the ice-ocean interface. While the total mass loss contribution is relatively small, significance of any increase in melt lies in the potential contribution to downward crevasse propagation combining with basal crevasses resulting in full-thickness fracture and calving events. In light of previous work, it was thought that increased surface melting would promote enhanced basal sliding; however, recent studies find no definitive evidence for such effects. Therefore, basal melt currently plays a dominant role in mass loss, and is likely to remain of greater significance for stability of the floating section in the future.