A climatology of atmospheric rivers over Scandinavia and related precipitation

Abstract. Atmospheric rivers (ARs) play an important role in the global climate system, facilitating both meridional moisture transport and regional weather patterns that are important for the local water supply. While previous research has mainly focused on the relationship between ARs and precipitation in North America and East Asia, the role of ARs in the regional climate of Scandinavia remains understudied. In this study, we used data from the Atmospheric River Tracking Method Intercomparison Project to characterize ARs making landfall in Scandinavia during 1980–2019. Combined with ERA5 reanalysis precipitation data, we quantified the AR-related precipitation over the region. We found that ARs are present during up to 5 % of the 6-hourly time steps in the most active areas. During these AR events, the region receives up to 40 % of the total annual precipitation. Additionally, the precipitation histograms show that the probability density is greater for the highest precipitation rates during AR events compared to non-AR events. By clustering the AR pathways using a k-means algorithm, we identified four typical AR pathways over Scandinavia (maximum annual AR frequencies and AR-related precipitation fraction in parentheses): over southern Denmark (4 %, 18 %), along the northern coast of Norway (2.5 %, 12 %), over the southern parts of Norway and the south-central parts of Sweden (1.8 %, 15 %), and along the southern coast of Norway (1 %, 7 %). Furthermore, we found that ARs over Scandinavia are typically most common during autumn and least frequent in spring, with some differences in seasonality between AR clusters. To investigate how large-scale atmospheric circulation affects Scandinavian ARs, we used the North Atlantic Oscillation (NAO) index to characterize circulation patterns during AR events. We found that AR activity over Scandinavia generally peaks during strong positive phases (>0.5) of the NAO. Our results indicate that ARs over Scandinavia, despite being relatively infrequent, are associated with a large fraction of the annual precipitation, which emphasizes their important role in the regional weather and climate.