As California experiences ongoing drought, scientists say the state accumulated a "rain debt" between 2012 and this year of around 20 inches—a full year's worth of average rainfall for the Golden State.

That debt was driven upward by a lack of atmospheric air currents that normally move inland off the Pacific Ocean and deliver significant amounts of water vapor, researchers at NASA's Goddard Space Flight Center in Greenbelt, Md., say.

These sporadic but extreme weather events, known as atmospheric rivers, bring the state anywhere from 20 percent to 50 percent of the state's rainfall in an average years, they say.

"When they say that an atmospheric river makes landfall, it's almost like a hurricane, without the winds. They cause extreme precipitation," says the study's lead author Andrey Savtchenko. "They cause extreme precipitation."

California normally gets around 20 inches of rain annually on average, with wet years and dry years usually balancing themselves out over the short term, the researchers say.

However, from 2012 to 2014 the state built up a deficit of almost 13 inches and the wet season of 2014-2015 failed to deliver any significant precipitation, increasing the debt by another 7 inches, the researchers report in the Journal of Geophysical Research: Atmospheres.

An atmospheric high-pressure system over the eastern Pacific Ocean since 2011 has inhibited the formation of usual atmospheric rivers, they say.

Atmospheric rivers, found all around the globe, are concentrated bands of water vapor blowing across the atmosphere like the winds seen in jet streams.

Typically moving from west to east, the rivers that arrive in California begin life in the tropical Pacific.

Like jet stream winds, they typically move from west to east. The ones that arrive in California originate over tropical seas where large amounts of moisture evaporate into the air off warm ocean waters.

Informally dubbed the Pineapple Express, the moisture-laden rivers then move northward in the direction of North America.

As they come onshore, the state mountain ranges force them upward, causing them to cool and condense and release their precipitation.

The current situation is not the first time the state has faced a rain debt caused by extended drought, Savtchenko says.

Analysis of climate records shows California racked up a rain debt of more than 27.5 inches between 1986 and 1994, he explains. There is concern that may become much more common in the future, which raises questions about meeting future water demands, he says.

"Drought has happened here before. It will happen again, and some research groups have presented evidence it will happen more frequently as the planet warms," Savtchenko says. "But, even if the climate doesn't change, are our demands for fresh water sustainable?"