Controls on Spatial and Temporal Variation in Snow Accumulation on Glaciers in the Southern Alps, New Zealand
Mountain glaciers are already responding to climatic warming, and are expected to make a substantial contribution to sea-level rise in the coming decades. The aim of this investigation in the New Zealand Southern Alps was to improve our understanding of snow accumulation variability on mid-latitude maritime glaciers, in order to allow for better estimation of future glacier mass balance. The specific aim was to investigate snow accumulation processes at a range of spatial and temporal scales, focussing on synoptic-scale atmospheric circulation influences, moisture sources for snow accumulation and local-scale dependencies of snow accumulation in relation to topography. A range of methods were utilised including direct measurement, snow and ice core analysis, statistical analysis and modelling. Snow accumulation in the Southern Alps was found to be derived predominantly from the Tasman Sea, and deposited during low pressure troughs and fronts. Although precipitation increased with elevation, wind processes redistributed this mass. On a ~monthly timescale this redistribution caused an unexpected result, namely that wind deflation of snow on Franz Josef Glacier countered the effects of greater accumulation, and total accumulation was similar at both Franz Josef and Tasman Glaciers over this period. These processes make it challenging to simulate snow accumulation patterns by simply extrapolating snowfall over an orographic barrier from lowland climate station data. On an inter-annual basis, temperature, especially during the ablation season, had most influence on net accumulation, and warm summers served to homogenise winter variability. Consequently, atmospheric circulation patterns that affect summer temperature, for example the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) also influence inter-annual variability in net accumulation. Together, these results highlight the dependence of maritime glaciers in the New Zealand Southern Alps on the prevailing westerly circulation. Although some uncertainty surrounds how global warming will affect atmospheric circulation and synoptic weather patterns, the results of this research indicate that New Zealand glaciers can be expected to lose significant mass in the coming decades if the current positive trend in the SAM continues, and if La Niña events (positive ENSO) become more frequent.