Spatial and temporal variability of Be-7 surface concentrations

Be-7 (t(1/2) = 53 days) is generated by cosmic rays in the upper troposphere and stratosphere, and is a tracer of downward aerosol transport. We apply a singular value decomposition (SVD)based technique, which simultaneously determines patterns of significant spatial and temporal variability, to Be-7 surface concentration data records that are 22 years long, from 19 locations distributed globally. Significant variability occurs on time scales corresponding to the solar cycle (approximate to 11 years), El Nino/Southern Oscillation (approximate to 2-3 years), an annual cycle, and a semiannual cycle. The phase of the solar cycle signal varies little globally, except at the south pole, where low magnetic shielding may permit direct penetration of solar particles during times of high solar activity. The spatial variability of the phase of the ENSO signal corresponds with the effects of observed rainfall anomalies during ENSO. The phases of the annual and semiannual signals depend upon the spatial distribution of vertical transport and precipitation. The temporal variabilities, or modes, corresponding to these signals sum together to form a variety of annual Be-7 cyclicities at different locations. We compare reconstructions of the time series based on the SVD analysis with the observed time series at 6 locations. The 4 modes explain between 19% and 47% of the monthly variance at these 6 sites. We show how various locations are affected by differing relative amounts and phases of the 4 signals. We also calculated the spatial variability of the annual cycle phase from a larger data set, by fitting a sinusoid to the average annual cycles at the sites. The agreement between the SVD and the sine-fitting results serves to validate both approaches.