atmospheric river

atmospheric river, any long, narrow, and concentrated horizontal corridor of moisture in Earth’s troposphere. Such formations transport vast amounts of water vapor—at flow rates more than double that of the Amazon River—and heat from tropical regions near the Equator toward the middle and higher latitudes. They serve as the primary source of horizontal water transport in the midlatitudes, providing more than half of the precipitation to coastal areas in parts of Europe, North America, South America, New Zealand, and Southeast Asia and facilitating the movement of more than 90 percent of the world’s moisture from the tropics to the poles. Most atmospheric rivers can be found in the North Pacific, Atlantic, southeastern Pacific, and South Atlantic oceans away from the tropics, and they produce moderate amounts of rain and snow. However, some atmospheric rivers are responsible for extreme precipitation and flooding events that may last up to several days in some regions. An average of four to five atmospheric rivers are active in Earth’s atmosphere at any given time.

Characteristics

An atmospheric river varies in size, shape, and strength, appearing as a ribbon of moisture that may stretch more than 2,000 km (1,245 miles) long but somewhat less than 1,000 km (621 miles) wide across oceans. Such formations are components of extratropical cyclones that possess a low-pressure cell at their leading edges. They are associated with low-altitude jet streams—such as those that occur at heights of 2–3 km (1.2–1.9 miles)—that appear ahead of a cold front. Studies have shown that the intensity, location, and frequency of atmospheric rivers are affected by larger-scale climate phenomena, such as polar vortices, El Niño/Southern Oscillation (ENSO), and the Madden-Julian Oscillation (MJO).

As the leading edge of an atmospheric river passes over a coastal area, the river’s path is forced upward by topography and pockets of warm air. The water vapor entrained within the river lifts and cools, forming clouds and precipitation. Most atmospheric rivers are relatively benign phenomena that deliver much-needed rain and snow to dry areas, providing water to vegetation and helping to recharge snowpacks, reservoirs, and groundwater systems that contribute to the growth of all forms of life, the maintenance of biodiversity, domestic and industrial water supplies, agriculture, and hydroelectric power generation. However, some atmospheric rivers can produce extreme snowfall and rainfall events (including thunderstorms), which generate local high winds and flooding. Such events typically last about 20 hours, but some can continue over the course of several days, delivering steady, massive amounts of precipitation that accumulate so rapidly that the ability of the soil to absorb water and the capacity of local flood-control infrastructure are quickly overwhelmed. If such heavy precipitation events occur in mountainous coastal regions, such as those of California’s Pacific coast, they can also result in landslides and mudslides, which can lead to widespread property damage, injury, and loss of life (see also orographic precipitation).

The AR scale

The effect of an atmospheric river in a given area is ranked by its duration and intensity according to the AR scale, which was developed by American meteorologists F. Martin Ralph, Jonathan J. Rutz, Jason M. Cordeira, and others in 2019. Atmospheric rivers are evaluated using a unit of flow measurement called integrated vapor transport (IVT), which accounts for winds and moisture quantities to determine total water volume carried by an atmospheric river across a line perpendicular to its flow. Rankings on the AR scale range from 1 to 5: AR-1 events produce an IVT greater than 250 kg of water vapor per meter (about 168 pounds per foot) per second, and each subsequent rank represents an increase in IVT of 250 kg per meter per second (500 kg per meter per second is needed to reach AR-2, 750 kg per meter per second is needed to reach AR-3, and so on).