Causes for increased flood frequency in central Europe in the 19 th century 1 2

11 Historians and historical climatologists have long pointed to an increased flood frequency in Central Europe in 12 the mid and late 19 century. However, the causes have remained unclear. Here, we investigate the changes in 13 flood frequency in Switzerland based on long time series of discharge and lake levels, of precipitation and 14 weather types, and based on climate model simulations, focusing on the warm season. Annual series of peak 15 discharge or maximum lake level, in agreement with previous studies, display increased frequency of floods in 16 the mid 19th century and decreased frequency after the Second World War. Annual series of warm-season mean 17 precipitation and high percentiles of 3-day precipitation totals (partly) reflect these changes. A daily weather 18 type classification since 1763 is used to construct flood probability indices for the catchments of the Rhine in 19 Basel and the outflow of Lake Lugano, Ponte Tresa. The indices indicate an increased frequency of flood-prone 20 weather types in the mid 19 century and a decreased frequency in the post-war period, consistent with a climate 21 reconstruction that shows increased (decreased) cyclonic flow over Western Europe in the former (latter) period. 22 To assess the driving factors of the detected circulation changes, we analyse weather types and precipitation in a 23 large ensemble of atmospheric model simulations driven with observed sea-surface temperatures. In the 24 simulations, we do not find an increase in flood-prone weather types in the Rhine catchment in the 19th century, 25 but a decrease in the post-war period that could have been related to sea-surface temperature anomalies. 26