What Is an Atmospheric River? Overview and Impact on the Climate

Atmospheric rivers are similar to ordinary rivers in that they are responsible for transporting water over thousands of miles. There's one difference, though: The moisture they carry is water vapor, not liquid water. They carry lots of it, too.

According to NOAA, a typical atmospheric river carries the liquid water equivalent of the average flow of water at the mouth of the Mississippi River. And if an atmospheric river event is particularly strong, it can transport as much water as 7 to 15 Mississippi Rivers.

The intense moisture associated with these systems is a blessing to many regions worldwide, including the western US, which has had droughts squelched by it. But for all their benefits, atmospheric rivers can also be bad news, since their copious moisture can easily overwhelm regions, triggering downpours, mudslides, and floods. According to a recent study in Nature, by increasing atmospheric moisture (higher temperatures increase air's capacity to hold water vapor), climate change will undoubtedly enhance the intensity of these rivers as well as the precipitation delivered by them.

The Science of Atmospheric Rivers

Atmospheric rivers originate over the tropical Pacific Ocean. They are associated with a climate pattern called the Madden-Julian Oscillation (MJO)—eastward-moving disturbances of clouds, heavy rainfall, and winds that traverse the tropics every 30 to 60 days. As these disturbances drop heavy rainfall, they "wet" the environment out ahead of their path, thereby creating the 250- to 375-mile-wide plumes of moisture that we call atmospheric rivers.

If the MJO is in a convective (stormy and wet) phase and is located over the far western Pacific, and certain weather features, such as a blocking high pressure in the Gulf of Alaska, are also in place, this moisture plume can be steered, by the jet stream, to the northeast, taking aim at the U.S. West Coast.

Once an atmospheric river moves inland and sweeps over mountainous locales, its water vapor rises, cools, and condenses, creating heavy precipitation.

How Are Atmospheric Rivers Studied?

The Calwater2015 study provided scientists with one of the largest opportunities to study atmospheric rivers to date. During the multiyear research program, ships equipped with suites of weather instruments, including NOAA's Ronald H. Brown vessel, intercepted several landfalling atmospheric river events off the California coast, observing them directly as they passed overhead. Observations were also made by air; several aircraft flew directly into the sky rivers, releasing dropsondes.

Back on land, forecasters detect and study the presence and strength of atmospheric rivers, which are largely composed of water vapor, winds, and aerosols, by monitoring what's called integrated water vapor, or the concentration of water vapor in a column of air. Integrated water vapor transport, or how that moisture is transported over horizontal distances, is equally as important.

Weather satellite imagery in the infrared, visible, and microwave bands are also used to detect atmospheric rivers, as are numerical weather models, and upper air maps that chart relative humidity (700 millibars) and winds (300 millibars). They're typically recognized as conveyor belts of clouds and moisture stretching across the Pacific and onto the U.S. West Coast.

Where Do Atmospheric Rivers Occur?

Atmospheric rivers regularly impact the west coasts of the world's landmasses, most notably western North America, but they also occur in Europe, East Asia, and South Africa. (Atmospheric rivers also occur in western Greenland and Antarctica, but these events are less studied.)

According to NOAA, they are responsible for up to 50% of precipitation events in California and along the adjoining Canadian and Alaskan coastline.

One of the most well-known atmospheric river configurations is the Pineapple Express—the persistent flow of moisture that originates from the waters adjacent to the Hawaiian Islands. In November 2006, a strong Pineapple Express event dropped nearly 18 inches of rain over a period of 36 hours in Washington State's Mount Rainier National Park, triggering floods and a six-month closure. Years later in December 2010, a string of Pineapple Express events dumped 11 to 25 inches of rain from western Washington to southern California and blanketed the Sierras with 75% of its annual snowpack.