Synoptic influences on snow accumulation on glaciers east and west of a topographic divide: Southern Alps, New Zealand

Understanding relationships between snow accumulation and synoptic climatology is important for assessing the way in which future climate variability will impact on glacier mass balance. However, few studies have as yet examined these relationships. Variability in snow accumulation on mid-latitude glaciers is strongly influenced by atmospheric circulation, orography, and redistribution of snow by wind. Very little is known about these processes in the New Zealand Southern Alps, where it is assumed that west-facing glaciers receive higher snow totals. However, few measurements are available to test this hypothesis. These processes were investigated over a 21-day period in winter 2008 on glaciers located west (Franz Josef Glacier) and east (Tasman Glacier) of the Main Divide of the Southern Alps. We directly measured snow accumulation and considered how it was affected by synoptic weather regime and location with respect to the Main Divide. Both glaciers received, ~ 75% of their snowfall during troughing regimes, which are characterized by strong westerly quadrant winds bringing humid air masses from the Tasman Sea over the Southern Alps. The Franz Josef Glacier site received, ~ 30% more snow than the Tasman Glacier site, but wind deflation meant that by the end of the study period, net snow accumulation was similar at both sites. Blocking synoptic regimes resulted in a reversal of prevailing westerly flow, generating strong downslope winds at Franz Josef Glacier and snow loss.