Winter Extremes of Beryllium-7 Surface Concentrations in Northern Europe

Specific activity of cosmogenic 7Be in surface air generally shows a spring-summer maximum. However, extremely high 7Be concentrations in surface air also occur during winter. The aim of our analysis is to characterise temporal and spatial prevalence of winter extreme events, and to investigate the associated synoptic meteorological conditions in northern Europe. Four measurement sites, with an approximate weekly sampling rate over the 2001–2010 period, are selected from the online Radioactivity Environmental Monitoring (REM) Database. The extremes in the 7Be surface concentration are defined as measurements above the 90th percentile in each location.The results indicate that at each measurement site, 10–20 % of the extremes occur during winter (November, December, January and February). Two types of 7Be extremes are distinguished: 1) approximately half of these occurrences are isolated events detected in one or two stations, and 2) the other half are events grouped within four months, when at least three 7Be extremes per month are observed. The monthly Scandinavia (SCAND) teleconnection index for isolated extreme events (type-1) is positive and, with only one exception, larger than 0.4, while in the case of type-2 events, the monthly SCAND is very high (larger than 1).This finding implies that in northern Europe during winter, the atmospheric conditions associated with a high SCAND index facilitate an occurrence of extreme 7Be surface concentration.