Changes in the flood hazard in Germany through changing frequency and persistence of circulation patterns

The link between trends in circulation patterns and trends in the flood magnitude is studied for 122 meso-scale catchments in Germany for a period of 52 years (1951-2002). Flood trends, significant at the 10% level, are detected for a large number of catchments. The catchments are pooled into three regions, based on flood seasonality and flood trends. Field-significant increasing trends are found for winter in Regions West and East. For summer, increasing and decreasing flood trends are detected for Regions South and East, respectively. The temporal behaviour of three flood indicators of each region is compared to atmospheric indicators derived from circulation patterns. Significantly increasing frequency and persistence of flood-prone circulation patterns intensify the flood hazard during the winter season throughout Germany. Moreover, a trend towards a reduced diversity of circulation patterns is found causing fewer patterns with longer persistence to dominate the weather over Europe. This indicates changes in the dynamics of atmospheric circulations which directly influence the flood hazard. Longer persistence of circulation patterns which in general do not favour large precipitation amounts may lead to large runoff coefficients due to soil-moistening and hence cause floods.