Variations pluviométriques durant la dernière période séculaire en Europe Occidentale

Changes in rainfall conditions in Western Europe over the last century

CNRS Equipe de Recherche n° 30, Grenoble

Abstract

This paper presents an analysis of the rainfall fluctuations during the last century over the West part of Europe (Fig. 1). Our purpose is not to apply systematic random tests but to caracterize the principal changes common to large areas of annual and seasonal rainfall series. The main sources of information on monthly and annual rainfall totals are : Meteorologie Nationale (Garnier, 1974) in France - Instituto Geofisico de Coimbra (1942-1955) in Portugal - Service meteorologique national (1943) in Spain - Annual rainfall in England since 1725 (Craddock, 1976). Two techniques have been applied to caracterize rainfall variations : for each individual station the mass curve (accumulated departures from the average of consecutive rainfall totals expressed as percentage of the average) and principal components analysis for the whole network. Ireland and South Spain excepted, the oceanic coastal regions of Europe show common changes in their annual rainfall records (Fig. 2. Fig. 3) : a 10 % mean deficiency during the period 1887-1909 and 1042-1957 ; a 8 % mean excess from 1922 to 1941 and a low excess since 1958. If some other stations of the inland (Paris, Colmar, Toulouse, ...) exhibit the same evolution, on the other hand mediterrean stations show completely different variations. Further investigations thanks to seasonal analysis (Fig. 3) show that the deficiency of 1887-1909 is mainly due to a winter deficiency ; the excess of 1922-1941 and the successive deficiency (1942-1957) result from important contrast in their mean spring rainfalls (40 % of difference in some stations). For winter and spring the two seasons which are caracterized by noticeab1e changes in their pluviometric evolution, each monthly rainfall serie indicate the same similar evolution as that of the seasonal total (Fig. 4). These results are re1ated to changes in the general circulation of the atmosphere during the last century and espacially to variations in the strength of the zonal circulation.