25 January 2022
Atmospheric rivers dust the Alps
Published online 7 December 2021
Huge streams of warm, humid air have deposited Saharan dust on the European Alps and may have led to a reduction in snow cover.
Atmospheric rivers are long stretches of cloud that can carry enormous quantities of water vapour from the tropics towards the poles. They were first described in 19941 and since then have come to be understood as the major mechanism transporting moisture from lower latitude oceans to higher latitude land masses, where they dump that moisture as snow and rain.
It now appears that atmospheric rivers could be transporting dust as well as water. In February 2021, satellite images showed two separate events in which vast dust plumes could be seen extending from the Sahara Desert in northwest Africa towards Europe, extending all the way up to Scandinavia. At the same time, the snow depth across the European Alps declined by around 50%.
With her longstanding interest in how dust is moved around by atmospheric phenomena, atmospheric scientist, Diana Francis, at Khalifa University in Abu Dhabi, wanted to investigate these dusty rivers, their cause and effects.
Francis and her team determined2 that, while these atmospheric rivers normally flowed from the tropical Atlantic over the Alps, in early 2021 they had instead been pushed south by high pressure in the north Atlantic so that they passed over North Africa and the Mediterranean Sea, collecting not only dust but also warmth and moisture.
When the atmospheric rivers encountered the Alps, the combination of dust and higher concentrations of water vapour had a warming effect. “They warmed the atmosphere and emitted this back to the surface,” Francis says. The dust deposited on the snow surface darkened it by 40% so it absorbed more solar radiation and melted at a higher rate. These two effects contributed to the extreme reduction in snow cover observed in early 2021, Francis says. Researchers also suggested the dust and nutrients it contained might have contributed to the growth of red algae on the snow surface, which was seen in the Alps during 2021.
“Incorporating the idea that the atmospheric rivers don’t just transport water vapour but also lots of dust – and lots of dust from the Sahara – seems to open up a lot of lines of new research that I think would be really exciting,” says Andrew Martin, a hydrometeorologist who studies atmospheric rivers at Portland State University, in the United States, who was not involved in the study.
- Zhu, Y. et al. Atmospheric rivers and bombs. Geophysical Research Letters 21, 1999-2002 (1994).
- Francis, D. et al. Atmospheric Rivers drive exceptional Saharan dust transport towards Europe. Atmospheric Research https://doi.org/10.1016/j.atmosres.2021.105959 (2021).