Solar activity cycles recorded in long-term data on tritium activity concentration in precipitation at Zagreb, Croatia

It has been expected that the solar cycle may have an influence on the production rate of cosmogenic tritium. However, prevailing anthropogenic bomb tritium in precipitation in the second half of the 20th century prevented such studies. We analyzed long-term data (1976-2020) of tritium activity concentration in precipitation at Zagreb, Croatia, by various statistical methods looking for the solar cycle periodicity. Frequency and Wavelet analyses were performed on the whole data set, while sinusoidal fitting was performed on data since 1996, when bomb tritium is negligible. All methods resulted in two distinct periods. The shorter 1-year period corresponds to the annual variation of tritium in precipitation typical for the Northern Hemisphere stations. The longer period of 10.21 yr, approximate to 11 yr and 12.17 yr was obtained by frequency analysis, wavelet analysis and sinusoidal fitting, respectively. These values are close to the solar cycle with 11 yr period on the average (ranging from 9 yr to 13.6 yr). Additionally, we compared the maxima in the smoothed monthly sunspot number in the 23rd and 24th solar cycles and the minima in tritium activity concentration in precipitation. A delay of about 30 months between them was observed and plausibly explained by the residence time of cosmogenic tritium in the stratosphere before entering the troposphere.