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As I look out the window and once again view the downpours that have impacted California for months, it seems appropriate to describe one of the more interesting discoveries of the last decade, rivers in the atmosphere. Think of it as the "Pineapple Express", only in spades.
Completely unknown a decade ago, atmospheric rivers are turning out to be a key factor not only in Western flooding and water supply, but also in the Earth's water cycle. At any given time, somewhere between three and five atmospheric rivers are ferrying water in each hemisphere. More than 1,000 kilometers long, they are often no wider than 400 kilometers; they carry the equivalent, in water vapor, of the flow at the Mississippi River's mouth. This is not small potatoes. Some estimates are that they transport 90+% of Earth's water vapor.
"Ordinary" clouds do not carry water vapor long distances. They "rain out" as soon as water droplets coalesce and become heavy enough to fall as precipitation. In contrast, atmospheric rivers can drop very significant quantities of water indeed after conveying the water vapor long distances. Some examples. In January 2005 more than 20" of rain soaked Southern California, a huge amount for a desert region. In December 2010 atmospheric rivers dumped more than 10' of snow in parts of the Sierra Nevada mountains.
Atmospheric rivers happen because of the temperature differential between Earth's tropics and poles. The temperature differential causes low-pressure storms to spin off in the mid-latitudes. It is not just the Pineapple Express (an atmospheric river which delivers moisture from Hawaii to the West Coast). Atmospheric rivers can also come up through the Gulf of Mexico or along the East Coast of the U.S.
To investigate the subject in more detail, California is establishing four "observatories" to track the rivers as they arrive. Each river can have different effects depending on the angle and speed with which it approaches mountain ranges and watersheds. Contrary to the buzz that has built around this phenomenon, most atmospheric rivers are weak and cause little or no devastation, but some cause extreme West Coast floods.
Atmospheric rivers may become more relevant as global temperatures rise. Researchers are uncertain how climate change will affect the rivers, but warmer air generally means that the atmosphere can hold more water vapor. On the other hand, winds may weaken in a globally warmed world, meaning the rivers might carry more water but be less effective at delivering it. As they say, stay tuned.
More information on atmospheric rivers can be found at http://www.esrl.noaa.gov/psd/atmrivers/, http://www.esrl.noaa.gov/psd/atmrivers/, http://journals.ametsoc.org/doi/abs/10.1175/MWR2896.1, and http://journals.ametsoc.org/doi/abs/10.1175/1520-0493(1998)126%3C0725:APAFMF%3E2.0.CO;2?prevSearch=&searchHistoryKey=. Also, in Google, use this search term: "doi:10.1029/2010GL044696".