Influence of solar forcing, climate variability and modes of low-frequency atmospheric variability on summer floods in Switzerland

摘要

The higher frequency of severe flood events in Switzerland in recent decadeshas given fresh impetus to the study of flood patterns and their possibleforcing mechanisms, particularly in mountain environments. This paperpresents a new index of summer flood damage that considers severe andcatastrophic summer floods in Switzerland between 1800 and 2009, andexplores the influence of external forcings on flood frequencies. Inaddition, links between floods and low-frequency atmospheric variabilitypatterns are examined. The flood damage index provides evidence that the1817–1851, 1881–1927, 1977–1990 and 2005–present flood clusters occur mostlyin phase with palaeoclimate proxies. The cross-spectral analysis documentsthat the periodicities detected in the coherency and phase spectra of 11(Schwabe cycle) and 104 years (Gleissberg cycle) are related to a highfrequency of flooding and solar activity minima, whereas the 22-yearcyclicity detected (Hale cycle) is associated with solar activity maxima anda decrease in flood frequency. The analysis of low-frequency atmosphericvariability modes shows that Switzerland lies close to the border of theprincipal summer mode. The Swiss river catchments situated on the centre andsouthern flank of the Alps are affected by atmospherically unstable areasdefined by the positive phase of the pattern, while those basins located inthe northern slope of the Alps are predominantly associated with thenegative phase of the pattern. Furthermore, a change in the low-frequencyatmospheric variability pattern related to the major floods occurred overthe period from 1800 to 2009; the summer principal mode persists in the negativephase during the last cool pulses of the Little Ice Age (1817–1851 and1881–1927 flood clusters), whereas the positive phases of the mode prevailduring the warmer climate of the last 4 decades (flood clusters from 1977 topresent).