Cosmogenic 7Be and 22Na in Finland: Production, observed periodicities and the connection to climatic phenomena

Abstract

This study presents theoretical production calculations and time series analyses of two cosmogenic isotopes, 7Be and 22Na, from the four high-latitude stations of Kotka, Kajaani, Rovaniemi and Ivalo located in Finland. We used published results for 7Be but performed full simulations of the 22Na production in atmosphere. For the first time, lookup tables of 22Na production by cosmic rays in the atmosphere are presented. In conjunction with calculations using the new model of 22Na production, the 7Be/ 22Na ratios in the atmosphere were also calculated. The wavelet transform of the 7Be and 22Na time series revealed sets of periodicities in the 2.5–8 year range. The wavelet coherence method was used to study coherences between 7Be and 22Na and 7Be/ 22Na data and AO, NAO, AMO, QBO and SO teleconnection indices representing different climatic variations in Northern Europe. In the wavelet coherence analyses, the 7Be activities were found to be mainly modulated by NAO and AMO at an interannual scale, while 22Na activity was found to be less effected by climatic phenomena. The 7Be coherence with other indices was intermittent where the coherence with SO was limited to Ivalo data and in the case of QBO, to Kotka data. The 22Na data was not found to be in coherence with any of the studied indices. In the 7Be/ 22Na ratio a clear seasonal pattern was observed where low 7Be/ 22Na ratios were observed during summer and high ratios during winter. This was speculated to be caused by the height of atmospheric vertical mixing. During 2006–2011, the 7Be/ 22Na ratios from Kotka, Kajaani and Rovaniemi showed variance at synoptic time scales but were nearly identical at the seasonal scale. The 7Be/ 22Na ratio was proposed to be a radiochronometer to estimate residence times of aerosols carrying 7Be and 22Na.