MULTISCALE DETECTION OF ABRUPT CLIMATE CHANGES - APPLICATION TO RIVERNILE FLOOD LEVELS

The historical flood-level time series of the River Nile (AD 622-1470) is chosen to identify abrupt climate changes by applying global and l ocal analysis techniques: the Mann-Kendall test and a non-hierarchical cluster analysis method to improve the Mann-Kendall test; a multiscal e moving t-test with correction to the degree of freedom and an antisy mmetric wavelet transform. The global estimates show three distinct ep ochs, AD 622-1078, 1079-1325 and 1326-1470, coinciding with larger sca le climate changes: a relatively cool age, the Little Climatic Optimum of the Middle Ages, and an interim period before the Little Ice Age. The local estimates reveal the following results. The reference time o f abrupt changes can be clearly identified, the associated time-scale coincides with the persistent anomaly period, and the maximum absolute t-value is statistically significant. There are about eight almost sy nchronous abrupt changes in the minimum and maximum River Nile flood l evels, many of them are associated with 35-45 year persistence time-sc ales. An association of these short time-scales with those of interdec adal variability reported for the mid-and high-latitude sea-surface te mperature of the North Atlantic is suggested, although information on phase coherence is not available. (C) 1997 by the Royal Meteorological Society. (No. of Figures: 9 No. of Tables: 1 No. of References: 40).