Causes of increased flood frequency in central Europe in the 19th century

Abstract

Abstract. Historians and historical climatologists have long pointed to an increased\nflood frequency in central Europe in the mid- and late 19th century.\nHowever, the causes have remained unclear. Here, we investigate the changes\nin flood frequency in Switzerland based on long time series of discharge and\nlake levels, precipitation, and weather types and based on climate model\nsimulations, focusing on the warm season. Annual series of peak discharge or\nmaximum lake level, in agreement with previous studies, display increased\nfrequency of floods in the mid-19th century and decreased frequency after\nthe Second World War. Annual series of warm-season mean precipitation and\nhigh percentiles of 3\u2009d precipitation totals (partly) reflect these\nchanges. A daily weather type classification since 1763 is used to construct\nflood probability indices for the catchments of the Rhine in Basel and the\noutflow of Lake Lugano, Ponte Tresa. The indices indicate an increased\nfrequency of flood-prone weather types in the mid-19th century and a\ndecreased frequency in the post-war period, consistent with a climate\nreconstruction that shows increased (decreased) cyclonic flow over western\nEurope in the former (latter) period. To assess the driving factors of the\ndetected circulation changes, we analyze weather types and precipitation in\na large ensemble of atmospheric model simulations driven with observed\nsea-surface temperatures. In the simulations, we do not find an increase in\nflood-prone weather types in the Rhine catchment in the 19th century but a\ndecrease in the post-war period that could have been related to sea-surface\ntemperature anomalies.