European Alpine moisture variability for 1800–2003

Moisture availability for the European Greater Alpine region (GAR) (43°N–49°N and 4°E–19°E) for the period 1800–2003 is analyzed on the basis of maps of monthly self‐calibrating Palmer Drought Severity Index (scPDSI) with a 10′ × 10′ spatial resolution. To represent the impact of seasonal snow cover on the water budget, a simple snow‐accumulation and snowmelt model is added to the water balance calculations on which the (self‐calibrating) Palmer Drought Severity Index is based. Over the region as a whole, the late 1850s into the 1870s and the 1940s to the early 1950s stand out as persistent and exceptionally dry periods, whereas the first two decades of the nineteenth century and the 1910s were exceptionally wet periods. Dividing the Greater Alpine Region into four subregions, with the subregions based on coherence of precipitation variability, we find a large degree of heterogeneity in the behavior of the drought index over the subregions. The driest summers on record, in terms of the amplitude of the index averaged over the Alpine region, are 1865 and 2003. In these years, 75.6% and 85.1% of the region was suffering from a moderate drought (or worse). The areas northwest of the high mountains were affected most severely in the 1865 drought, whereas the 2003 drought impacted all subregions more equally. By substituting climatological monthly mean temperatures, from the period 1961–1990, for the actual monthly means in the parameterization for potential evaporation, an estimate is made of the direct effect of temperature on drought. It is observed that a major cause for the vast areal extent of the area affected by the summer drought in the last decade is the high temperatures. Temperatures in the 12 months preceding and including the summer of 2003 explain an increase in the area percentage with moderate (or worse) drought of 31.2%. Copyright © 2006 Royal Meteorological Society