Long term variations of extreme rainfall in Denmark and southern Sweden

Abstract

A high number of studies have detected changes in the observed heavy rainfall in Northern and Central Europe, all adding to the debate on anthropogenic climate change and its potential impact on rainfall extremes. However, it is equally relevant to understand natural variations on which the anthropogenic changes are imposed. This study identifies multi-decadal variations in daily rainfall extremes from Denmark and southern Sweden, with a recurrence level relevant for flood hazard analysis. Based on smoothed series it is concluded that the frequency of the extreme events shows both a general increase from 1874 to present and an oscillation with a cycle of 25–40 years. The magnitude of the extreme events also oscillates, but with a cycle of 15–30 years and a smaller amplitude. Regional analysis of a larger Danish dataset with a shorter observations period found a countrywide low period in 1970–1979. It is furthermore concluded that the oscillation signal along the west coast of Denmark is dominated by the changeable coastal weather of this region. The eastern part of Denmark shows a more consistent signal, which partly can be explained by an index derived from sea level pressure differences between Gibraltar and Haparanda. The identification of a cyclic pattern in the extreme rainfall is highly relevant for our understanding of the non-stationarities in flood hazard.