SUMMER MOISTURE AVAILABILITY OVER EUROPE IN THE HADLEY CENTRE GENERAL CIRCULATION MODEL BASED ON THE PALMER DROUGHT SEVERITY INDEX

Abstract

Palmer Drought Severity Index (PDSI) values have been calculated for Europe (35–70°N, 10°W–60°E) for the control and perturbed integrations of the Hadley Centre high-resolution coupled ocean–atmosphere general circulation model (GCM). The control integration is 75 years in length. The perturbed experiment, with CO2 increasing at a rate of 1 per cent compound per year from a starting value of 323 ppm, is also 75 years in length. The control integration was analysed by unrotated andVARIMAX rotated principal components and these were projected on to the perturbed integration results. The control integration shows similar features to the real world, but is different particularly with respect to low-frequency aspects. Variability on time-scales beyond 20 years is absent in the various model area-average, principal component (PC) and rotated principal component (RPC) time series. Both sets (PC and RPC) of patterns, however, show many of the features of the real world. The first three PCs of the control integration are clearly present in the real world, although in higher order components. The perturbed integration shows a dramatic change to drought conditions affecting most of Europe by year 75. In some regions the PDSI quickly approaches (within 30–40 years) severe drought values of \,−3 to \,−4 before stabilizing at this level until year 75, but in a few areas little change in drought severity is seen. The pattern of change in PDSI during the last 10 years of the perturbed integration is somewhat different from that shown by soil moisture calculated directly by the GCM. Some reasons for the differences are discussed.