Atmospheric Rivers and Climate Change

animation by NOAA

An atmospheric river (AR) is a narrow corridor of concentrated moisture in the atmosphere. Atmospheric rivers consist of narrow bands of enhanced water vapor transport, typically along the boundaries between large areas of divergent surface air flow, including some frontal zones in association with extratropical cyclones that form over the oceans.

The term was originally coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s, to reflect the narrowness of the moisture plumes involved. Atmospheric rivers are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than the Earth's largest river, the Amazon River. There are typically 3-5 of these narrow plumes present within a hemisphere at any given time.

Atmospheric rivers have a central role in the global water cycle. On any given day, atmospheric rivers account for over 90% of the global meridional (north-south) water vapor transport, yet they cover less than 10% of the Earth's circumference. They also are the major cause of extreme precipitation events which cause severe flooding in many mid-latitude, westerly coastal regions of the world, including the West Coast of North America, western Europe, and the west coast of North Africa.

In California it has been recently recognized that much of the state's flood risks and its most extreme storms derive from atmospheric rivers. For instance, all 7 floods on the Russian River between 1997 and 2006 were due to an atmospheric river event. But it has also become evident that much of California's water resources derive from the same category of storms, precipitation in the state being highly episodic and falling in a small number of events each year (Dettinger et al. 2011).

Under climatic change, an analysis of a 7-model ensemble of climate change projections for California indicates that although the average intensity of atmospheric river events does not increase, there may occur more years with many AR events and occasional events that are much stronger than historical ones. Moreover, the length of the season over which AR events may occur is predicted to increase. These changes to the patterns of AR events may result in more frequent and more severe floods in California (Dettinger 2011).

The U.S. Geological Survey Multi-Hazard Demonstration Project developed a superstorm scenario for California involving a sequence of atmospheric river events which they termed the "ARkStorm". This hypothetical storm is portrayed as similar to the West Coast storms of 1861 and 1862 which flooded much of the Central Valley. This storm could potentially cause 1.5 million residents of the state to be evacuated and $725 billion of damage (Porter et al. 2010).

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