Analysis of beryllium-7 variability in northern Europe

This work presents an overview of the results obtained in an analysis of the 7Be activity concentrations recorded in surface air in Helsinki, Finland, over a 25-year period (1987-2011), and stored in the online Radioactivity Environmental Monitoring (REM) database. First, lagged linear correlations of the 7Be specific activity with several meteorological variables: tropopause height (TPH); mean, minimum and maximum temperature; precipitation (Prec); atmospheric pressure (Press); potential vorticity (PV) at 300 hPa and 200 hPa; solar zenith angle (SZA); and sunspot number are analysed (Tab. 1). The time lag in the correlation calculations is allowed to vary between 0 and 7 days for TPH, meteorological parameters and PV, and between 0 and 31 days for SZA and sunspot number. The correlations are calculated for the total set of measurements as well as for different seasons. correlations for 7Be with TPH, temperatures, and atmospheric pressure. The strongest relationship is observed in summer, when it is accompanied by a short time lag (within two days). The absence of significant correlations with PV is probably due to the fact that stratospheric intrusions are not located directly above Helsinki. In the second part of our analysis, we looked into the 7Be extremes, defined as events with the 7Be surface concentrations above the 95th percentile. Even though the 7Be annual cycle is characterised by a maximum during the warm season and minimum during the cold period, 10% of the extreme events occur during cold months, between October and March. These “cold extremes” are analysed in more detail, and depending on their persistence, they are classified as “bursts” or “episodes”. Three representative episodes and one burst are analysed looking for common features. Our results imply that, in general, these events are characterised by anomalies in PV, sea level pressure, temperature and precipitation patterns over Europe and the North Atlantic. We further notice that the Scandinavia (SCAND) teleconnection index is above the 75th percentile during all the months in which the cold episodes are observed (Fig.1). This finding suggests a potential use of SCAND as a predictor of these events in the Scandinavian region.