A time-series analysis of the changing seasonality of precipitation in the British Isles and neighbouring areas

Complex demodulation of monthly precipitation data is used to determine temporal changes in the decadal averages of the annual precipitation cycle. Good spatial coherence is found between sites from as far apart as Scotland, western Ireland and southern Sweden. The annual precipitation cycle is found to have been unusually late in the 1870s and early 1900s and to have been noticeably early in the 1860s and 1890s. Fluctuations of up to 13 weeks are found in the timing of the annual precipitation cycle, as recorded at Kew, during the last 300 years. Complex demodulation provides a technique to quantify annual climate changes and hence to provide a quantitative context in which detection of anthropogenic-induced changes to the annual climate signal can be assessed. Ten percent changes in the decadal averages of the amplitude of the annual precipitation cycle and in precipitation amounts are found. Although precipitation in the British Isles is often cited as rising, no persistent increases are observed in this study. (C) 1999 Elsevier Science B.V. All rights reserved.