California drenched by ‘atmospheric river’: What is it?

The American state of California has been pummeled by flash floods and heavy rains this past week as a result of an “atmospheric river,” officials said.

Governor Gavin Newsom has declared a state of emergency in several southern counties including Los Angeles and San Diego as a result of the torrential rains brought on by the atmospheric river. 

These atmospheric rivers have resulted in “the wettest storm systems to impact the greater Los Angeles area,” Ariel Cohen, chief National Weather Service (NWS) meteorologist told Reuters. 

California, a state on the west coast of the US, is known for extreme weather events, including torrential downpours, earthquakes, landslides, and torrid winds that can cause wildfires. 

What are atmospheric rivers?

Atmospheric rivers are a powerful weather phenomenon often nicknamed “sky rivers” for their ability to transport immense amounts of moisture. Imagine a river, but instead of water, it flows with concentrated water vapor, stretching thousands of miles across the sky. 

As they encounter landmasses, especially mountains, they are forced to rise higher into the atmosphere. This ascent cools the water vapor within the river, causing it to condense into raindrops or snow.

The “Pineapple Express” is a well-known atmospheric river that delivers moisture-laden air from the warm waters near Hawaii all the way to the US west coast.  

In the region, atmospheric rivers deliver 30-50% of annual precipitation, acting as a crucial source of water for the region. 

They replenish reservoirs, nurture forests, and sustain diverse ecosystems. However, these sky rivers can turn dangerous if too powerful, unleashing torrential downpours, and leading to devastating floods, mudslides, and even wildfires. 

Image: National Oceanic and Atmospheric Administration

These sky rivers, though narrow by extreme weather event standards, can be 300 miles wide and are formed by winds associated with cyclones. Some are gentle and beneficial to a drought-prone state like California, but the strong ones can be devastating. 

In fact, between 1950 and 2010, 33% to 74% of droughts on the west coast were ended by these very storms.

These bands of moisture can transport water vapor at 7 to 15 times the rate of the Mississippi river every day. While they might seem “narrow” compared to vast weather systems, they can stretch hundreds to thousands of miles long and be up to 300 miles wide, according to the National Oceanic and Atmospheric Administration (NOAA) and the US Geological Survey (USGS). 

Globally, about 4-5 atmospheric rivers hold a staggering 90% of the water vapor traveling towards the earth’s poles. These narrow bands of moisture might seem insignificant, covering less than 10% of the planet’s circumference, but their impact is colossal. 

These sky rivers can affect people worldwide, delivering over half the annual water runoff in the coastal areas of North and South America, western European countries like France, Spain, Portugal, and the United Kingdom, as well as Southeast Asia and New Zealand.