Influence of solar forcing, climate variability and atmospheric circulation patterns on summer floods in Switzerland

Abstract

Abstract. The higher frequency of severe flood events in Switzerland in recent decades has given fresh impetus to the study of flood patterns and their possible forcing mechanisms, particularly in mountain environments. This paper presents an index of summer flood damage that considers severe and catastrophic summer floods in Switzerland between 1800 and 2009, and explores the influence of solar and climate forcings on flood frequencies. In addition, links between floods and low-frequency atmospheric circulation patterns are examined. The flood damage index provides evidence that the 1817–1851, 1881–1927, 1977–1990 and 2005–present flood clusters occur mostly in phase with palaeoclimate proxies. The cross-spectral analysis documents that the periodicities detected in the coherency and phase spectra of 11 (Schwabe cycle) and 104 years (Gleissberg cycle) are related to a high frequency of flooding and solar activity minima, whereas the 22 year cyclicity detected (Hale cycle) is associated with solar activity maxima and a decrease in flood frequency. The analysis of atmospheric circulation patterns shows that Switzerland lies close to the border of the summer principal mode: the Summer North Atlantic Oscillation. The Swiss river catchments situated on the centre and southern flank of the Alps are affected by atmospherically unstable areas defined by the positive phase of the Summer North Atlantic Oscillation pattern, while those basins located in the northern slope of the Alps are predominantly associated with the negative phase of the pattern. Furthermore, a change in the low-frequency atmospheric circulation pattern related to the major floods occurred over the period from 1800 to 2009: the Summer North Atlantic Oscillation persists in negative phase during the last cool pulses of the Little Ice Age (1817–1851 and 1881–1927 flood clusters), whereas the positive phases of SNAO prevail during warmer climate of the last four decades (flood clusters from 1977 to present).

Integration of multi-archive datasets for the development of a four-dimensional paleoflood model of alpine catchments

“…In the northern Bernese Alps, floods have been triggered mainly by precipitation anomalies, such as intensive summer thunderstorms and long-lasting advective rainfall events, combined with pronounced snowmelt as well as glacier lake or landslide-dammed lake outburst floods (Röthlisberger, 1991;Gees, 1997;Weingartner et al, 2003;Peña et al 2015). According to the 500-year historical flood record of the Aare River (Schulte et al, 2015), 95% of the floods occurred during the summer months of July and August.…”

“…Such field data are able to capture flood dynamics both under different climate conditions during cooler and warmer climate periods (e.g. Medieval Climate Anomaly, Little Ice Age, 20 th Century Global Warming) (Büntgen et al, 2011) and during transition periods when storm tracks shifted and atmospheric circulation patterns changed (Mudelsee et Schulte et al, 2008Schulte et al, , 2015Wilhelm et al, 2012;Peña et al, 2015;Brönnimann et al, 2018). Moreover, they can also undoubtedly mirror the natural forces that prevailed during pre-industrial times.…”

Quantitative historical hydrology in Europe

“…Assumptions concerning the hydraulic method and models applied to calculate discharge, type of flow (uniform versus non-uniform), the effective flow area and choice of energyloss coefficients cause uncertainty in discharge estimates. For instance, in the River Elbe in Dresden, the official peak discharge of the 1845 flood is 5700 m 3 s −1 , whereas the water profile calculations by means of one-and two-dimensional hydraulic models provide a value of 4335 m 3 s −1 , a discrepancy attributed to an inaccurate stage-discharge relationship at the gauge (Pohl, 2008).…”

“…during the Little Ice Age (Sheffer et al, 2008). In many mountain catchments historical floods are considerable larger than the instrumental data, which can be explained by different reasons (Peña et al, 2014;Schulte et al, 2015): (i) changes in atmospheric dynamics (e.g. from the 1930s to 1977 in Switzerland); (ii) possible inaccuracy of instrumental data during flood peak conditions (inundation or malfunction of gauge station); (iii) changes in discharge contribution from snowmelt and glacier melt during past cooler climate periods (e.g.…”

Quantitative historical hydrology in Europe

“…during the Little Ice Age (Sheffer et al, 2008). In many mountain catchments historical floods are considerable larger than the instrumental data, which can be explained by different reasons (Peña et al, 2014;Schulte et al, 2015): (i) changes in atmospheric dynamics (e.g. from the 1930s to 1977 in Switzerland); (ii) possible inaccuracy of instrumental data during flood peak conditions (inundation or malfunction of gauge station); (iii) changes in discharge contribution from snowmelt and glacier melt during past cooler climate periods (e.g.…”