SUMMER MOISTURE VARIABILITY ACROSS EUROPE, 1892-1991 : AN ANALYSIS BASED ON THE PALMER DROUGHT SEVERITY INDEX

Monthly Palmer Drought Severity Indices (PDSI) have been calculated for the period 1892–1991 over a 5° latitude by 5° longitude grid covering Europe and south-west Asia from 35 °N to 70 °N and 10 °W to 60 °E. These indices are used to describe the spatial and temporal details of relative moisture variability during summer (June-August). The full series of mean summer PDSI maps are presented. Over Europe as a whole, there has been a consistent (but statistically insignificant) increase in moisture supply throughout the period of record. The 1930s and 40s were exceptionally dry. The summer of 1947 was the driest, both in terms of average PDSI but also in terms of the spatial extent of moderate drought. However, the summer of 1921 was by far the most extreme when considering the spatial extent of severe drought conditions. Widespread drought conditions also prevailed over many regions of Europe during the 1890s. The wettest summers occurred in 1987 and 1916. The total area of Europe experiencing either severe dry or severe wet conditions has increased noteably during recent years. Nine regions of coherent summer moisture variability are objectively defined by orthogonal rotation of the principal components of the summer PDSI. The nine rotated components, which explain 60 per cent of total variability of this data set, represent moisture variability in north-western Europe (NWEUR), the north-west Mediterranean (NWMED), the lower Volga (LVOLG), central European Russia (CERUS), the north-east Mediterranean (NEMED), the southern Caspian Sea (SCASP), the Pechora Basin (PECHB), the Danube Basin (DANUB), and northern Fennoscandia (NFENN). The last 3 years have been very dry in NWEUR and NWMED. The most recent summer in our analysis, 1991, was the wettest on record in both CERUS and NEMED. Preliminary spectral analyses provide little evidence for a solar-moisture relationship in these European data over this period.

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