Research articleSimulated and reconstructed atmospheric variability and their relation with large Pre-industrial summer floods in the Hasli-Aare catchment (Swiss Alps) since 1300 CE
Introduction
The Swiss Alps are extremely sensitive to changes in atmospheric circulation and to environmental perturbations that impact the hydrological and flood regime of the region. Indeed, the climate and flood variability of the Swiss Alps have been reconstructed using both instrumental data and documentary sources (Hächler-Tanner, 1991; Röthlisberger, 1991; Gees, 1997; Pfister, 1999; Weingartner and Reist, 2004; Burger, 2008; Schmocker-Fackel and Naef, 2010a, Schmocker-Fackel and Naef, 2010b; Wetter et al., 2011) and, in the case of longer time series, using natural proxies such as lake records, dendrochronology, and delta floodplain deposits (Tinner et al., 2003; Casty et al., 2005; Holzhauser et al., 2005; Wirth et al., 2013a, Wirth et al., 2013b; Schulte et al., 2015, Schulte et al., 2019b, this issue). These centuries-long flood records can be compared with other time series reconstructed from solar and climate proxies.
The influence of atmospheric variability modes on the climate and hydrology of the Swiss Alps has been highlighted over the past few decades, mostly in relation to the winter climate (Beniston and Jungo, 2002; Efthymiadis et al., 2007; Bartolini et al., 2009; López-Moreno et al., 2011; Marzeion and Nesje, 2012; Haslinger et al., 2017). However, flooding in the Swiss Alps presents a marked seasonal distribution, with the highest frequencies being recorded in the summer months (Röthlisberger, 1991; Pfister, 1999) in relation with summer atmospheric processes (Peña and Schulte, 2014; Peña et al., 2015; Schulte et al., 2015, Schulte et al., 2019b, this issue).
For instance, a number of studies have examined the variability between climate and flood proxies in Switzerland (e.g. Röthlisberger, 1991; Schmocker-Fackel and Naef, 2010b; Wirth et al., 2013a, Wirth et al., 2013b; Peña et al., 2015) and, in particular, in the Bernese Alps, including the Hasli-Aare catchment (Schulte et al., 2009, Schulte et al., 2015, Schulte et al., 2019b, this issue). However, the integrated multi-archive study of Alpine paleofloods in the Bernese Alps (Schulte et al., 2019a, Schulte et al., 2019b, this issue) has shown that the correlations between the different proxies call for a better understanding of the processes involved so as to identify flood uncertainties (i.e. out-of-phase flood pulses, and/or opposite flood trends) and thresholds in the archives.
Based on the advances made by this integrated multi-archive approach, our objective here is to analyse the summer paleoflood variability in the Alpine Hasli-Aare catchment (46°41′N, 6°04′E; 596 km2; Fig. 1). We do so by simulating the atmospheric variability of the flood periods, reconstructed by Schulte et al. (2015) from floodplain sediment and historical sources, between 1300 and 2005 CE. We also seek to determine whether the paleoflood and atmospheric variability of recent centuries was similar to that of the Industrial period. We use the Community Earth System Model-Last Millennium Ensemble (CESM-LME, Otto-Bliesner et al., 2016) to investigate the extent of atmospheric variability and specifically address the questions of i) how the simulated atmospheric variability changed over the Pre-industrial era (1300–1849 CE) and ii) how the results of this model can help to explain the flood variability.
In short, this paper seeks to show that proxy derived paleoclimate series, including paleoflood evidences, are essential data to improve our understanding/knowledge of past climate dynamics and, in this way, that they can contribute to improving future climate simulations and our knowledge of future flood drivers. On the understanding that paleoclimate modeling has improved over the last two decades (Cane et al., 2006; Ludwig et al., 2019), this paper focuses on the analysis of the atmospheric patterns of past periods of extreme hydrological events in Alpine catchments, which have been rarely studied to date using paleoclimate simulations.
Section snippets
Environmental proxies from delta plain sediments
Alluvial plain sediment sections and cores provide excellent geoarchives for reconstructing Late Holocene floodplain aggradation, flood and climate history in the Hasli-Aare catchment (Fig. 1).
Schulte et al. (2015) reconstructed the frequencies of paleofloods from geochemical records, primarily alluvial delta plain sediments since 1300 CE, and these series were dated by radiocarbon. Calibration of the natural archive records was provided by textual sources from the last 520 years (Swiss Federal
Methods
The first step involves correcting the simulation of the CESM-LME SLP grid by performing a quantile-quantile mapping transformation (Amengual et al., 2012). The procedure computes the changes, quantile by quantile, in the cumulative distribution frequency (CDF) of daily SLP of the LME outputs and the observed data. The statistical adjustment is based on the relationship between the ith ranked value of the corresponding CDFs for the past calibrated (1300–1849 CE), the control instrumental or
Coherence of the EOF analysis in the CESM-LME simulations. Climate variability during the Industrial era: 1850–2005
The EOF analysis of observed and reconstructed summer SLP anomalies for July–August (JA) (Fig. 2) shows a principal spatial pattern with positive anomalies extending over the Scandinavian Peninsula and the British Isles, and slight low-pressure anomalies over the Mediterranean area. This summer principal mode of atmospheric variability, the Summer North Atlantic Oscillation (SNAO), differs from the winter NAO pattern insofar as it presents a northwest-southeast pressure gradient. In contrast,
Discussion
The CESM simulations performed for the Last Millennium Ensemble Project (CESM-LME) were obtained using a state-of-the-art model, the results of which have been widely published (see, for example, Otto-Bliesner et al., 2016). However, they are not without their limitations given that the simulations performed do not follow the climate system's internal variability. This means that the simulated atmospheric conditions in a particular year may be quite different from those actually observed,
Conclusions
Using the CESM-LME climate model, we have shown that the phase changes of the simulated SNAO in the Industrial era (1850–2005 CE) are consistent with changes in the reconstructed and observed SNAO during flood periods in the alpine Hasli-Aare catchment. We found also that solar forcing (SOL) in the Industrial era was the most important forcing in the simulated SNAO variability. When we compared the flood pulses reconstructed from geochemical proxies of the Hasli floodplain with the SNAO, TSI,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The work was funded by the Catalan Institution for Research and Advanced Studies (ICREA Academia program) and the Spanish Agency of Science and FEDER/UE (project CGL2016-75475/R). The authors are members of the Past Climate Change (PAGES) Floods Working Group (2016-2018 and 2019-2021). The authors wish to thank the CESM1(CAM5), Last Millennium Ensemble Community Project, NSF/CISL/Yellowstone, DOE INCITE program, Office of Biological and Environmental Research and NOAA, which provided the
Links to the different data are as follows
CESM Paleoclimate Working Group at NCAR. (Last accessed: September 2019): htpp://www.cesm.ucar.edu/projects/community-projects/LME/.
Twentieth Century Reanalysis Project dataset (Last accessed: September 2019): http://www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2.html .
Reconstruction of Sea Level Pressure fields over the eastern North Atlantic and Europe back to 1500 AD (Luterbacher et al., 2002; last accessed: September 2019):
//www.ncdc.noaa.gov/paleo/pubs/luterbacher2002/luterbacher2002.html
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2021, Science of the Total EnvironmentCitation Excerpt :Climate variability may also have a significant indirect influence on positive metal anomalies, particularly when: i) the provenance of alluvial sediment of different textures is controlled by erosion and deposition during episodes of flooding and ii) organic soils are developed during periods of decreasing flood activity. This is a crucial point because Schulte et al. (2009a, 2009b, 2015) showed that floodplain deposition and their related floods in the Hasli-Aare, Lütschine and Lombach rivers are strongly influenced by climatic and solar activity (Steinhilber et al., 2009; Peña et al., 2015; Peña and Schulte, 2020). The development of organic-rich horizons during periods of lower flood activity also signifies a lower deposition of metal-laden alluvium, but in some cases our records show a high metal content (between 160–100 BCE, 105–140 CE, 250–350 CE, 490–590 CE and 940–1000 CE, Fig. 6).