Abstract
Understanding the atmospheric controls on glacier mass balance is a key challenge in revealing the impacts of climate change on the global environment. Records of glacier behavior are biased towards the Northern Hemisphere, with high-quality observations from glaciers in the Southern Hemisphere rare and often of short duration. To advance knowledge of glacier-climate interactions in the Southern Hemisphere, this thesis uses high-quality meteorological observations and mass balance modelling, supported by glaciological and remote sensing observations, to resolve the climatic controls on mass balance of Brewster Glacier, a mountain glacier located on the Main Divide of the Southern Alps of Aotearoa New Zealand. A detailed mountain meteorology is resolved using a unique 10-year automatic weather station record obtained at the terminus of the glacier, enabling the diurnal and seasonal variability of cloudiness to be assessed for the first time. There is strong evidence that cloudiness is closely linked to variability in wind speed and humidity. Importantly, the length and quality of this unique observational record, which exceeds any other obtained in the Southern Alps, provides the basis for detailed distributed mass balance modelling of Brewster Glacier. Net radiation, driven primarily by net shortwave radiation, is the key contributor to melt energy on Brewster Glacier, with the turbulent sensible and latent heat fluxes, net longwave and the heat flux from precipitation governing melt variability. Atmospheric moisture reflected through precipitation, cloud cover and wind speed are linked to changes in snow accumulation and melt energy. The modelling also reveals that observations of glaciological mass balance are not capturing the full extent of the ablation season, resulting in a 35% underestimate of glacier-wide mass balance over the period 2010 to 2020. To extend the mass balance time series beyond the observational record, mass balance modelling is conducted using a bias-corrected long-term virtual climate station network data over the period 1982 to 2021, which provides confirmation there has been an almost three-fold increase in mass loss on Brewster Glacier over the last 14-years. A comparison of extreme positive and negative mass balance years reveals that 40% of the difference in mass balance is associated with snow accumulation changes, with surface (48%) and subsurface (10%) melt contributing to the majority of the remaining difference. The energy surplus in extreme melt years is due primarily to an increase in net shortwave radiation that is governed by an albedo feedback, as well as an increase in the turbulent heat fluxes. Collectively, the findings of this thesis demonstrate how atmospheric processes at a range of spatial and temporal scales control glacier mass balance and highlight the need for a multi-proxy approach to glacier monitoring in the mountainous regions of the Southern Alps. Future work should seek to homogenise the long-term glaciological mass balance of Brewster Glacier using a combination of remote sensing and distributed mass balance modelling.