Reconstruction of mass balance of glaciers in southern Norway back to 1948

A model using upper-air meteorological variables in the US National Centers for Environmental Prediction and US National Center for Atmospheric Research (NCEP/NCAR) re-analysis database is used to extend net balance b n back to 1948 for seven glaciers in southern Norway. The observational record of another glacier, Storbreen, began in 1948. Over the observational record of each of the seven glaciers, correlation with Storbreen estimates b n more accurately than the upper-air model does for four of them and less accurately for three. In all seven cases, however, an average of the model and the Storbreen correlation is more accurate than either alone, so the average is used to reconstruct b n for years when it was not observed. For the seven glaciers other than Storbreen, a combined series is formed from observations during their period of record and from reconstructed values prior to then back to 1948. There are three distinct sections in all eight b n series: prior to 1989; 1989–1995, when the North Atlantic Oscillation index was strongly positive; and after 1995. The 1989–95 mean b n was anomalously positive because of both decreased ablation and especially increased accumulation. The mean b n since 1995 has been anomalously negative because of increased ablation, whilst accumulation has been nearly the same as over 1948–88. The first principal component of the eight 1949–2005 b n series explains 78% of the total variance, and the second explains 12%. Over 1949–88 there were no substantial shifts in b n or in either winter balance b w or summer balance b s at any of the seven glaciers where observations began after 1948, nor were there in the Storbreen record. There is a distinction between the three glaciers that gained mass over 1948–2005 and the five that lost mass. Each of the three that gained had accumulation-area ratio AAR ≥ 0.64 and < 0.7% of its area δS in the lowest tenth of its altitude range, while the five that lost had AAR ≤ 0.46 and 1.9 ≤ δS ≤ 4.4%. Because of these hypsometries, the five glaciers that lost mass now have an especially large ablation area.