Official URL: http://dx.doi.org/10.1080/00291950701374328

The cumulative net mass balances of maritime glaciers in Norway displayed a net surplus over the 1963-2000 period, in contrast to the more continentally located glaciers and that of the global glacier trend, which was one of marked retreat. This period also corresponds to an increase in westerly circulation associated with an intensification of North Atlantic Oscillation (NAO). However, since 2000, all Norwegian glaciers have been decreasing in volume. This paper seeks to establish the causal mechanisms that resulted in the positive net balances occurring on Norwegian maritime glaciers. To achieve this, a Temporal Synoptic Index (TSI) was derived for a 30-year period, commencing in 1968, for a number of synoptic meteorological stations in Norway. This period coincides with the beginning of wide spread glacier mass balance measurements in Norway. The TSI is derived using Principal Components Analysis (PCA) and subsequent clustering of component scores to classify days for both winter and summer seasons. Findings indicate that the occurrence of ‘warm’ type air masses during the summer months have increased in frequency, particularly since the late 1980s. A general reduction in the frequency of ‘cold’ cluster types during the winter months is also evident after this period. These reductions in ‘cold’ cluster frequencies in winter appear to have been largely replaced with increases in the frequency of ‘warm’ types, with an increased moisture carrying capacity, particularly since the late 1970s. The frequency occurrence of these key air mass types is shown to be significantly related to glacier mass balance during both the accumulation and ablation season. Winter air mass types from maritime source regions act to enhance accumulation and suppress ablation, while summer continental source types suppress accumulation and enhance ablation.