Enhanced Central European summer precipitation in the late 19th century: a link to the Tropics

In Central Europe, an unusually large number of floods were recorded in the summers and autumns of the late 19th century, causing considerable damage. Different factors contributing to the recurrence of these floods have been discussed in the literature, including the unusually high precipitation anomalies recorded at the time. Based on the frequency and spatial pattern of these floods, previous studies suggest that changes in the large-scale circulation may have played a relevant role. Here, we use an atmospheric global climate model forced with observed sea surface temperatures (SSTs) and atmospheric forcings (greenhouse gas concentrations and aerosol emissions) to test this hypothesis and identify the causes for the associated atmospheric circulation pattern. We find that our model is able to reproduce the high summer precipitation anomalies observed in Central Europe in the late 19th century, and we show that between 1875 and 1890, transient SSTs, as opposed to climatological ones, are the primary driver for this precipitation increase. Using a series of numerical experiments, we find that the SST variability in the central and eastern tropical Pacific Ocean (associated with the El Nino–Southern Oscillation region) was the primary driver for these anomalies. In addition, SST variability in the Indian Ocean as well as increasing anthropogenic aerosol emissions substantially enhanced European precipitation. The atmospheric circulation anomalies associated with this precipitation increase show a Pacific North American (PNA)-like pattern over North America and a substantially weakened mid-troposphere westerly flow over Europe and the North Atlantic, as well as increased pressure over Greenland and reduced pressure over Europe, resembling the negative phase of the summer North Atlantic Oscillation (NAO).