European dry spell regimes (1951–2000): Clustering process and time trends

Aiming to improve the knowledge of droughts in Europe, three indices related to dry spells, DS, regime have been analysed: the number of DS per year, N; the longest annual DS, Lmax; and the mean DS length per year, L, for different daily rainfall thresholds (0.1, 1.0, 5.0 and 10.0mm/day) at annual and seasonal scales. The database consists of daily records from 267 rain gauges, along the years 1951–2000. First, the mean values at annual and seasonal scales of these three indices are represented for the four thresholds. For 0.1 and 1.0mm/day, spatial patterns suggest a strong N–S gradient for latitudes south of 45°N, especially at annual scale and in summer season. For 5.0 and 10.0mm/day, the patterns are different, with a strong gradient in the Scandinavian Peninsula and the largest Lmax and L at NE Europe, except for summer. Second, a principal component analysis, PCA, is applied to the 60 variables (three indices at five time scales and four thresholds) characterising the DS regime of every gauge. A clustering process leads to a classification of the 267 rain gauges into 20 spatial clusters, on the basis of five selected principal components replacing the original variables. Most of clusters are spatially coherent, with greater spatial variability on DS regimes towards the south and west than to the north and east of Europe. And third, time trends on the three indices are quantified by the Kendall-tau algorithm, and statistical significances at 95% confidence level are assessed by the Mann–Kendall test. For all thresholds and seasons, there is a clear predominance of significant negative trends for N. Specifically, the highest number of rain gauges with significant negative trends corresponds to summer and winter, with average percentages from −2.7 to −8.1% per decade. In summer, significant negative trends are observed in Western Europe at 40°–60°N and between 10°W and 20°E. In annual and winter periods, negative trends are detected also at Western Europe for 0.1 and 1.0mm/day and at latitudes south of 45°N for the two highest thresholds. Spring, and especially autumn, are characterised by a low number of negative trends, particularly for 1.0, 5.0 and 10.0mm/day. In summer, Lmax depicts remarkable positive trends in Western Europe, at 40°N–60°N and between 0°E and 20°E, with high average values close to +10% per decade. Positive trends on L are dominant at annual scale and winter for 0.1mm/day, and in summer for 0.1 and 1.0mm/day, with average trends ranging from +4.8 to +8.1% per decade. Spring and autumn are characterised by numerous negative trends on L for all thresholds.