What Is An Atmospheric River? Explaining The Blessing And Curse

An atmospheric river is a weather feature that is both a necessity and, all too often, a destructive menace – particularly for those near the West Coast of the United States.

Put simply, an atmospheric river (AR) is a thin but long plume of moisture in the atmosphere that stretches from the tropics or subtropics into higher latitudes.

The term was first used in a 1994 research paper and is now widely used by meteorologists.

A more recent study published in August 2015 by the University of Reading and University of Iowa questions whether the term "atmospheric river" is misleading.

The study argues that the AR may not be so much a moisture pipeline from the tropics or subtropics, as previous conceptual models, but rather simply a "footprint" of moisture ahead of the cold front.

One type of AR you've probably heard of is the "Pineapple Express," a moisture band lined up from near Hawaii to the U.S. West Coast ahead of a strong Pacific cold front at least once or twice each fall, winter or spring.

They're Destructive ...

Regardless of how they're formed or maintained, these atmospheric rivers contain an incredible amount of moisture.

According to NOAA's Earth System Research Laboratory (ESRL), a strong AR can transport as much water vapor as up to 15 times the average flow of liquid water at the mouth of the Mississippi River.

A separate University of Reading study in 2013 found one AR in 2009 over the United Kingdom contained 4,500 times more water than the average flow in the River Thames in London.

Still not impressed? A 1998 MIT study estimated the moisture flux of an AR is roughly the same as that of the Amazon River, about 176,000 tons per second.

If an atmospheric river stalls over land, particularly when that moisture is lifted by mountainous terrain, significant flooding often is the result.

A 2006 study led by F. Martin Ralph of Scripps Institution of Oceanography found atmospheric rivers were responsible for all seven floods of northern California's Russian River in the seven-year period of study.

About 80% of California's major flood events can be traced to ARs, even though ARs accompany only 17% of West Coast storms, according to NASA.

But isn't all this moisture wrung out over California's Sierra a good thing, given the state's dependence on melting snowpack for water supply?

Only if it falls as snow.

A 2016 study by NASA, NOAA, UCLA, and Scripps Institution of Oceanography found that atmospheric river events are 2.5 times more likely to produce "rain over snow" events, or rain falling on existing Sierra snowpack.

These warm AR events diminish the snowpack available to slowly melt and recharge reservoirs for the summer dry season, instead, triggering more serious flooding. Oddly, it's a net loss for California's water supply.

NOAA/ESRL has a list of AR events, primarily along the U.S. West Coast, that have appeared in peer-reviewed studies since the mid-1980s.

... But They're Essential

You might say parts of the U.S. West Coast have a love-hate relationship with atmospheric rivers.

If the AR events aren't too warm to produce rain over snowpack, they're extremely beneficial for water supplies.

Atmospheric rivers can dump some of the heaviest multi-day snowfalls you'll find anywhere in North America over the Sierra and Cascades, sometimes amounting to over 100 inches of snow in a week or less.

This can turn worry into relief among local water authorities when a so-called "miracle March" AR event boosts Sierra snowpack after a disappointing winter.

Atmospheric rivers are, in fact, vital to the Earth's water cycle.

The 1998 MIT study mentioned above found that ARs are responsible for at least 90% of all atmospheric water vapor transport in the middle latitudes. 

A study published in February 2019 unveiled a new scale for ranking the severity and impacts of atmospheric rivers.

Not Just A U.S. West Coast Phenomenon

Atmospheric rivers are a flood concern in other parts of the world, as well.

In fact, there are three to seven such ARs present worldwide all the time.

These can be best seen on a global composite precipitable water (atmospheric moisture) loop from UW-CIMSS and Scripps Institution of Oceanography as thin fingers of moisture stretching poleward from the subtropics.

For instance, one impressive AR over 5,000 miles long accompanied Storm Desmond in December 2015, triggering massive flooding in parts of the U.K., Ireland, even Norway.

There's concern this type of winter AR event may become more common in the U.K., according to a 2013 University of Reading study.

The Nashville May 2010 flood was found to have had a contribution from an atmospheric river from the eastern tropical Pacific basin.

An AR even chipped its moisture into the infamous "Snowmageddon" mid-Atlantic snowstorm from Feb. 5-6, 2010, according to NOAA/ESRL.

The East Asian procession of heavy rain events from late spring into summer, known in China as the Mei-Yu season, often gets a boost from atmospheric rivers.

They're Predictable

Fortunately, thanks to advances in scientific research, numerical modeling and satellite technology, meteorologists can detect and forecast atmospheric river events sometimes up to a week in advance.

Given their impact, ARs remain an active area of research.

Field programs, headed by the Scripps Institution of Oceanography, often utilize hurricane hunter aircraft.

Dropping sensors into atmospheric rivers over the Pacific Ocean, the WC-130J aircraft collect data on moisture and winds, with the goal of improving forecasts, which are typically based heavily on satellite data only.

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