The last week of September, Western Norway saw the heaviest rainfall in 13 years. Such downpours will become more common in the future as global warming leads to more water in the atmosphere, climate scientists warn.
Over the course of just six hours in the early hours of 26 September, 91mm of rain fell from the sky over Jørpeland, near Stavanger, the heaviest downpour since 2005, according to Norwegian public service broadcaster NRK.
Bergen, Norway’s second largest city, saw 62mm, leading to severe flooding in several places, the broadcaster added.
Rivers in the sky
The rainfall was caused by a so-called atmospheric river, say researchers at CICERO Center for International Climate Research.
Atmospheric rivers can be thought of as “rivers in the sky”. They are long, narrow corridors in the atmosphere that transport most of the water vapour outside of the tropics. When an atmospheric river reaches mountainous regions on land, these very moist air masses are lifted, resulting in very heavy rain- or snowfall.
An atmospheric river will today, on 9 October, make landfall in Western Norway. You can see how it looks like in this tweet from the Norwegian Meteorological Institute (also known as MET Norway).
To become more intense
CICERO researchers are currently, together with colleagues from MET Norway and The Royal Netherlands Meteorological Institute (KNMI), the Norwegian energy company Statkraft, the University of Bergen and the Netherlands eScience Center, studying the link between heavy rainfall and atmospheric rivers.
In the research project “Translating Weather Extremes into the Future – a case for Norway”, also known as the TWEX project, they are trying to find out how atmospheric rivers will evolve in the future as the world warms due to climate change.
“A warmer climate means that atmospheric rivers will become somewhat more frequent and much more intense,” says KNMI researcher Kirien Whan.
“If the atmospheric rivers become more intense, there will potentially be more severe flooding, which could have an impact on the occurrence of landslides,” adds Jana Sillmann, research director at CICERO.
More rain, less snow
“Over the coming decades, the weather on the western coast of Norway will become increasingly wet and winters will be less snowy,” says Nathalie Schaller, senior researcher at CICERO.
This is because the average temperature here could increase by around 2°C by the end of the century, meaning that what once fell as snow could fall predominantly as rain over the coming decades, she explains.
“Global warming thus also increases the risk of atmospheric rivers causing flooding in western Norway during the colder parts of the year,” Schaller says.
“The increased risk of flooding, particularly during autumn and winter, should be taken into consideration both in future emergency response preparations and in the planning of new roads, office blocks and residential areas,” adds Helene Amundsen, senior researcher at CICERO.