Simulated and reconstructed atmospheric variability and their relation with large Pre-industrial summer floods in the Hasli-Aare catchment (Swiss Alps) since 1300 CE

Abstract Previous studies have suggested a relation between the variability of the climate and flood events, a conclusion supported by recent findings. Indeed, evidence has emerged that the atmospheric variability modulated by solar activity could play a role in driving the flood dynamic. The aim of this paper is to analyse summer paleoflood variability in the Alpine Hasli-Aare catchment (Switzerland) by simulating the atmospheric variability of flood periods identified from both floodplain sediment and historical sources dating from between 1300 and 2005 CE. We use the Community Earth System Model-Last Millennium Ensemble (CESM-LME) to investigate the extent of this atmospheric variability and address the questions of i) how the simulated atmospheric summer variability changed over the Pre-industrial era (1300–1849 CE) and ii) how this model is related to sedimentary floodplain evidence of flood variability. The results for the observed, reconstructed and simulated SLP grids showed a year-to-year summer atmospheric variability (Summer North Atlantic Oscillation; SNAO) related to a northwest-southeast pressure gradient with positive anomalies extending over the Scandinavian Peninsula and the British Isles, while the Mediterranean area was under slight low-pressure anomalies. The phase changes of the simulated SNAO in the Industrial era (1850–2005 CE) are consistent with changes in the reconstructed and observed SNAO. The comparison of flood reconstruction using geochemical proxies from floodplain sediments and paleoclimate simulations provides evidence that the SNAO in negative/positive phase modulated by solar variability (negative/positive anomalies) can play a substantial role in driving flood frequencies in the Hasli-Aare catchment and even in influencing their spatial distribution.