Atmospheric rivers in northern high latitudes and their impacts on the regional climate

Licentiate thesis, 2026

Atmospheric rivers (ARs) are long and narrow temporary pathways of unusually high and organized moisture transport in the lower troposphere. ARs have an important role in the global climate system as they facilitate a majority of the meridional moisture transport. This AR related moisture then stimulates regional weather patterns and is in many ways crucial for the local water supply in many regions. In the early days of the field, AR research primarily focused on local case studies over the west coast of North America. Today, while much of AR research remains focused on North America, studies on ARs and their interactions with the climate system now cover most regions of the world. For these studies, the level of detail typically decreases as the spatial scope increases. Hence, in recent years ARs over Europe have become relatively well studied. However, ARs over Scandinavia, and their impacts on the regional climate, have received limited scientific attention.

In this thesis I introduce a detailed climatology of ARs over Scandinavia. The study builds on the current state of global and regional AR research and leverages publicly available data from multiple AR detection algorithms to identify ARs that make landfall over Scandinavia. Using a clustering algorithm we could distinguish four different AR pathways through which ARs reach Scandinavia: a spatial level of detail not common for AR studies. We found that AR activity peaks over southern Denmark and the northern coast of Norway, with annual average frequencies reaching up to 4.2 % and 2.6 % respectively. Simultaneously, these regions receive up to 19.7 % and 13.1 % of their annual average precipitation during AR events. Furthermore, our use of multiple detection algorithms enables us to estimate the uncertainties in Scandinavian AR characteristics associated with algorithm choice.

This research serves as a robust foundation for future work focused on ARs over Scandinavia during the 21st century. Additionally, the novel clustering method can be applied to other regions, further uncovering how AR impact regional climates.