Influence of solar activity changes on European rainfall

European hydroclimate shows a high degree of variability on every time scale. The variability is controlled by natural processes such as Atlantic ocean cycles, changes in solar activity, volcanic eruptions and anthropogenic factors. This contribution concentrates on the solar influence on European precipitation, a relationship which has been documented by a large body of published case studies. Here we are concentrating on the period 1901-2015 for which we compare sunspot data with monthly precipitation series of 39 European countries by calculating Pearson correlation coefficients for a multi-year cross-correlation window. The coefficients have been mapped out across Europe with the aim to identify areas in which solar activity may have influenced precipitation. Results show that February precipitation in Central and Western Europe yields the strongest solar response with coefficients reaching up to +0.61. Rainfall in June-July is equally co-driven by solar activity changes, whereby the solar-influenced zone of rainfall shifts from the British Isles towards Eastern Europe during the course of summer. Other months with noteworthy solar responses are April, May and December. On a decadal scale, the correlation between precipitation and solar activity in central Europe appears to be mostly positive, both statistically and by visual curve comparison. Yet, best positive correlations coefficients of February, June, July and December are typically reached when the solar signal lags rainfall by 1.5-2 years. Taking into account cause and effect, it is suspected that increases in Central European rainfall are actually triggered by the solar minimum some 3-4 years before the rainfall month, rather than the lagging solar maximum. Similar lags of a few years occur between solar activity and the solar-synchronized North Atlantic Oscillation (NAO) due to memory effects in the Atlantic. The literature review demonstrates that most multidecadal studies from Central Europe encountered a negative correlation between solar activity and rainfall, probably because short time lags of a few years are negligible on timescales beyond the 11 year solar Schwabe cycle. Flood frequency typically increases during times of low solar activity associated with NAO- conditions and more frequent blocking.