Interannual to millennial scale variability of the River Ammer floods and its relationship with extreme climate and solar forcing

The number and strength of weather and climate extremes, such as severe floods and droughts, have increased significantly during recent decades. Part of such increase is attributed to human-induced climate change. To better assess the role of natural and anthropogenic forcing we should put the recent climate extreme variability into a long-term perspective. We analyze the variability of extreme precipitation and temperature over Europe in connection with observed daily discharge data of the River Ammer (southern Germany) and the 5500-year flood layer record from varved sediments of the downstream Ammersee. We show that daily River Ammer floods are related to upper level  Rossby waves breaking over Europe. This process, which is related to European scale extreme precipitation and temperature anomalies, is consistent with extreme precipitation and temperature patterns associated with River Ammer floods. From a synoptic scale perspective, the observed out-of-phase relationship between solar irradiance forcing and river Ammer floods, as presented in previous studies, is related to enhanced blocking activity over Eastern Europe-western Russia during low solar forcing which favors upper level positive potential vorticity anomalies over western Europe, a more unstable atmosphere and more floods. A singular spectrum analysis of a flood layer record from lake Ammer and a total solar irradiance reconstruction, going back in time to the mid-Holocene, reveals coherent variability at ~900 years and ~2300 years. We argue that similar cycles should dominate the millennial scale variations of blocking activity in the Eastern Europe-western Russia as well as the frequency of extreme temperatures, precipitation and floods over Europe.