Atmospheric rivers shift toward the poles

Atmospheric rivers, which are narrow bands of moisture in the atmosphere, have moved significantly over the past 40 years. In the Northern Hemisphere, they have shifted about 6 degrees of latitude toward the pole, and in the Southern Hemisphere, about 10 degrees. This shift translates to movements of over 600 kilometers in the north and more than 1,100 kilometers in the south.

Researchers study atmospheric river movements

Zhe Li and Qinghua Ding from the University of California, Santa Barbara, studied how atmospheric rivers transport moisture to high latitudes and contribute to rainfall in mid-latitude regions. Their findings, published in Science Advances on October 11, 2024, highlight the significant shift in these moisture corridors.

Climate models and observed patterns differ

Climate models that include past human influences predicted an increase in atmospheric river activity in mid-latitudes. However, reanalyses of past climate data show a more complex picture, with increases near 50°N and 50°S and decreases near 30°N and 30°S. This pattern, which starts in the tropics, is not well captured by average climate models.

Cooling in the eastern Pacific drives the shift

The main cause of the shift, according to the researchers, is the cooling of surface waters in the eastern tropical Pacific since 2000. This cooling is linked to the Walker circulation, a large-scale air movement in the tropics. La Niña conditions, which involve colder eastern Pacific waters, strengthen this circulation, pushing atmospheric rivers toward the poles.

Shift impacts different regions differently

Regions that receive fewer atmospheric rivers may face more droughts and heat, while those receiving more are at greater risk of flooding. Southern California could see fewer atmospheric river events, worsening water shortages, while areas like British Columbia and Alaska may experience more frequent and intense events, increasing the risk of flooding and landslides.

Shift not uniform across latitudes

The study does not provide specific data about France, so applying the findings to regions at similar latitudes requires caution. A shift of 6 to 10 degrees in latitude does not necessarily mean more or less rain in a specific area, as the study focuses on entire latitude bands rather than individual regions.

Natural and human factors both involved

While global warming influences future atmospheric river behavior, the shift observed so far cannot be attributed solely to warming. The study suggests a combination of natural variability and human-induced climate change. The Pacific Ocean's natural temperature cycles, such as La Niña and El Niño, also play a significant role in these patterns.