Spatial and temporal characteristics of atmospheric rivers in Scandinavia and their influence on the regional precipitation

EGU 2025

Erik Holmgren

Hans Chen

Chalmers University of Technology

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Atmospheric Rivers 101

Like rivers, but in the atmosphere

Long & Narrow
Temporary unusually high moisture transport
- Long distances
Local effects on precipitation
- North America (e.g. Rutz, Steenburgh, and Ralph 2014; Collow et al. 2022.)
- Flooding in the Yangtze basin (Pan and Lu 2019).
- Europe (e.g. Lavers and Villarini 2013)

Figure 1: Annual precipitation during ARs at the North American west coast. Adapted from Collow et al. (2022).

Atmospheric Rivers and their influence on precipitation over Scandinavia

When and where do ARs intersect Scandinavia?
What influence does ARs over Scandinavia have on precipitation?

Figure 2: Scandinavia as defined in the study: the combination Denmark, Norway, and Sweden.

Analysis

Data

ARTMIP Tier 2 (Collow et al. 2022)
- Four algorithms (ARDTs)
- Applied to ERA5
ERA5 precipitation
1980-2019

Method

Select ARs that intersect Scandinavia
Track ARs over time
Cluster AR tracks using K-means

Annual average AR frequency and precipitation

Frequencies

Maximum over Denmark
- Up to 5%
> 2% most of domain

Influence on precipitation

Up to 1300 mm during ARs
→ 45%

Figure 3: Annual average AR activity and precipitation.

AR clusters over Scandinavia

Southern coast of Norway (1)
- Up to 1% of the time, 9% of annual precipitation
- Southerly

The northern coast of Norway (2)
- 2.5% of the time, 15% precipitation

The central pattern (3)
- Relatively high frequencies, 16% of annual precipitation
- Westerly

The southern pattern (4)
- High frequencies
- 22.5% of annual precipitation

Figure 4: Pattern AR frequency and the corresponding annual precipitation.

But, as I’ve mentioned I we wanted to break this down further by using clustering the ARs. And through this we found four main AR patterns over Scandinavia which are shown in the current figure.
There are two southern patterns, cluster 1 and 4, panel a and g which are centered over Denmark. These are the most frequent ARs, with annual frequencies up to 10%.
And the ARs belonging to these patterns are responsible for 16 and 13 percent of the annual precipitation respectively.
But what is interesting about these is that they represent ARs with different orientations - as you can see pattern 1, in panel a, is more oriented from north to south, whereas pattern 4 is oriented in a more west-easterly direction.
The field correlation between the ARs and precipitation is also different, with pattern showing a relatively high correlation.
The second most frequent ARs are found in pattern 3 (looking at panel e), which is centred over the more northern west coast of norway. And they show relatively high frequencies and influence on precipitation, with a strong correlation between the AR and precipitation pattern.
ARs in pattern 2 are the least frequent, and are located over the more central parts of the domain, and also show less precipitation.
So this is great, we have an idea of where ARs intersect Scandinavia and if they influence precipitation in these locations, but if you remember, I also said that I want to know when. And this brings us to our next slide.

AR clusters: seasonal variation

Autumn (Sept-Oct-Nov) most frequent
- Least: Spring
Northern & Central cluster
- Summer & Autumn
Less variation in the southern cluster

Figure 5: Seasonal variation in AR patterns. Note the capped color scale.

Here we are breaking down the AR patterns from the previus slide, looking at how they vary throughout the year.
In this figure have the pattern again as rows, and the columns shows the frequency pattern during the four seasons.
So generally what we see here, is that the ARs are most frequent during autumn and least frequent during spring.
and this is quite consistent for all the patterns.
However, there are some interesting differences. In the two southern patterns, AR activity is high through most of the year, with the exception of spring.
While ARs in the northern pattern are mostly active during summer and autumn. Here we also see a southward shift of the pattern when going from summer to autumn.
But to better understand why we see these differences, we also need to look into how changes in the large scale circulation of the atmosphere affect these ARs.

AR clusters: variation with NAO

Strong positive phases (NAO++)
westerly flows →
Northern and central cluster (2 & 3)
- Large difference NAO– and NAO++
Southern cluster (4)
- High frequencies also during negative phases

Figure 6: Influence of NAO on AR frequency patterns.

Conclusions

ARs are a common feature in the Scandinavia climate system.
- AR activity peaking over Denmark, and the northern coast of Norway.
- Summer & Autumn
- NAO++
ARs have a strong influence on precipitation over Scandinavia.

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References

Collow, A. B. Marquardt, C. A. Shields, B. Guan, S. Kim, J. M. Lora, E. E. McClenny, K. Nardi, et al. 2022. “An Overview of ARTMIP’s Tier 2 Reanalysis Intercomparison: Uncertainty in the Detection of Atmospheric Rivers and Their Associated Precipitation.” Journal of Geophysical Research: Atmospheres 127 (8): e2021JD036155. https://doi.org/10.1029/2021JD036155.

Lavers, David A., and Gabriele Villarini. 2013. “The Nexus Between Atmospheric Rivers and Extreme Precipitation Across Europe.” Geophysical Research Letters 40 (12): 3259–64.

Pan, Mengxin, and Mengqian Lu. 2019. “A Novel Atmospheric River Identification Algorithm.” Water Resources Research 55 (7): 6069–87. https://doi.org/10.1029/2018WR024407.

Rutz, Jonathan J., W. James Steenburgh, and F. Martin Ralph. 2014. “Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States.” Monthly Weather Review 142 (2): 905–21. https://doi.org/10.1175/MWR-D-13-00168.1.