Low-frequency variability of beryllium-7 surface concentrations over the Eastern Mediterranean

7Be measurements, performed in Northern Greece (40°N) since 1988, were analyzed, in order to investigate the variability of the surface concentrations that can be attributed to processes with frequencies below the synoptic variability. Spectral analysis on the 7Be time series revealed three characteristic spectral regions, a 1-year periodicity corresponding to the well-established annual cycle, a periodicity between 20 and 30 months and a peak corresponding to a period of 11 years. The relationship with the 11-year cycle of solar activity was investigated through the correlation between the sunspot number and 7Be (−0.86) and between the heliocentric potential and 7Be (−0.80). 7Be was in coincidence with the heliocentric potential whereas it lagged the sunspot number by 5 months, expressing the time needed for the solar wind variations to be reflected on the production of 7Be during the specific solar cycle. The amplitude of the annual cycle was not constant throughout the whole period. An anti-correlation of −0.83 was revealed between the amplitude and the sunspot number with higher amplitudes during solar minimum and smaller amplitudes during solar maximum. With the use of a simplistic model, the amplitude modulation was attributed to the changing with the 11-year cycle contribution of the upper tropospheric and lower stratospheric reservoirs to the surface concentrations. More specifically, during solar maximum the contribution of upper troposphere down to the surface via winter-mixing is as much as 55% compared to the summer-mixing contribution, whereas during solar minimum winter-mixing is no more than 35%, thus inducing a higher amplitude. The superposed epoch method revealed that the 20–30 months periodicity is related to the quasi-biennial oscillation (QBO), with the 7Be minimum following the QBO maximum with ≈8 months time delay, whereas for the same time lag total ozone showed a reverse behavior. Cross spectrum analysis between 7Be and total ozone with the QBO shows a significant (90% confidence level) squared coherence, indicating that 68% and 86% of 7Be and total ozone variability of the 20–30 months periodicity, respectively, can be explained by the QBO.