The state of California has been dealing with an unprecedented amount of moisture this year, thanks to a slew of long, narrow currents of exceptionally wet air called atmospheric rivers. Much like a river is water that moves over land, an atmospheric river is water (vapor) moving through the air. The mountains of California and the Pacific Northwest usually wring out the moisture from the air, but there’s just so much of it that some is also reaching further inland than it otherwise would.


Atmospheric rivers are fairly common in the winter and are an important source of fresh water in the Western United States. Just a few events each year cause up to half of the annual precipitation on the West Coast. However, this heavy precipitation — which can park over a region for several days, as we have been seeing in California — can lead to flooding, mudslides and other disasters. El Niño and La Niña, which are large climate patterns in the Pacific Ocean, can be used as proxies for atmospheric river forecasts. The simple narrative is that El Niño is wet and La Niña is dry. This is the third year of La Niña and expectations were set up by the first two years, when winters were not very wet. But there is a transition going on from La Niña to El Niño at the moment. Climate change is playing a role here as well, given the oceans are warming and warm water is more energetic. This means the water can evaporate more readily, adding more water vapor into the air, which eventually precipitates out as rain or snow.


While most of the attention has been on impacts to California, there’s a sensitive region of the planet that is also being affected – the Arctic. The Arctic is already warming at four times the rate as the rest of the planet. Satellite observations and climate models are finding that atmospheric rivers are reaching the Arctic more frequently during the winter, especially over the Barents-Kara Seas off the north coast of Norway, affecting the sea ice. In winter, temperatures in the Arctic are below freezing and the ice should be recovering. Instead these powerful storms are slowing down seasonal ice recovery. The storms can be blamed for a third of the Arctic’s wintertime sea ice loss. Atmospheric rivers are inherently warmer than the surrounding Arctic environment and can act like a blanket, trapping heat near the Earth’s surface and preventing it from escaping out to space. The heavy rainfall from atmospheric rivers also has a melting effect, especially on fragile, re-growing ice.