Abstract
Extreme precipitation events in Norway in all seasons are often linked to atmospheric rivers (AR). We show that during the period 1979–2018 78.5% of the daily extreme precipitation events in Southwestern Norway are linked to ARs, this percentage decreasing to 59% in the more northern coastal regions and ~40% in the inland regions. The association of extreme precipitation with AR occurs most often in fall for the coastal areas and in summer inland. All Norwegian regions experience stronger winds and 1–2°C increase of the temperature at 850 hPa during AR events compared to the climatology, the extreme precipitation largely contributing to the wet climatology (only considering rainy days) in Norway but also in Denmark and Sweden when the rest of Europe is dry. A cyclone is found nearby the AR landfall point in 70% of the cases. When the cyclone is located over the British Isles, as it is typically the case when ARs reach Southeastern Norway, it is associated with cyclonic Rossby wave breaking whereas when the ARs reach more northern regions, anticyclonic wave breaking occurs over Northern Europe. Cyclone-centered composites show that the mean sea level pressure is not significantly different between the eight Norwegian regions, that baroclinic interaction can still take place although the cyclone is close to its decay phase and that the maximum precipitation occurs ahead of the AR. Lagrangian air parcel tracking shows that moisture uptake mainly occurs over the North Atlantic for the coastal regions with an additional source over Europe for the more eastern and inland regions.
Highlights
We focus on the extreme precipitation events occurring in Norway during all seasons of the last 40 years (1979–2018) using ob servations and an atmospheric reanalysis to emphasize some regional differences when other studies focused on specific regions, one season or even a few months (Sodemann and Stohl, 2013; Azad and Sorteberg, 2017; Benedict et al, 2019)
More than 50% of the extreme precipitation events occur in summer in the inland regions of Norway and only ~38% of all extremes are associated with Atmospheric rivers (AR)
Extreme precipitation associated with ARs contributes to the wet climatology and is associated with an increase in temperature of 1–2◦C at 850 hPa due to the passage of the warm front and sector
Summary
Atmospheric rivers (AR) are transient elongated bands of large moisture fluxes that usually extend from the tropics to the higher lati tudes (Newell et al, 1992) and can produce large amounts of precipi tation along with strong winds when reaching land (Zhu and Newell, 1998; Ralph et al, 2004, 2006; Leung and Dian, 2009; Gimeno et al, 2016; Waliser and Guan, 2017) leading to floods, landslides, and wind storms (Stohl et al, 2008; Liberato et al, 2012; Trigo et al, 2014, Table 1 in Ralph et al, 2019). It illustrates the features of ARs reaching Norway addressed in this study in a statistical way, such as the large-scale atmospheric circulation, the association with synoptic cyclones, anticyclones, and wave breaking when such an event occurs. A more detailed description of the large-scale flow configuration promoting ARs towards Europe is given by Pasquier et al (2018) using the decomposition in weather regimes considering all seasons together They showed that southwestern Norway is located at the end of the climatological jet regularly hit by ARs especially during the Zonal (often associated with NAO+) and Atlantic Trough weather regimes.
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