PACIFIC OCEAN

After soggy El Niño, beware the 'Diva of Drought'

Californians hoping El Niño storms this winter will end the state's historic drought should start looking further ahead to its devilish -- and much drier -- sister, La Niña.

Often referred to as the "Diva of Drought," strong La Niña weather patterns often follow strong El Niños.

The two systems are defined by water temperatures in the Pacific Ocean and present opposite weather patterns, particularly in Southern California and the southwestern United States.

El Niños tend to funnel storms toward California, historically leading to above-average rainfall. La Niñas do the reverse, leading to drier-than-usual conditions.

Weather forecasts are always subject to change, but there are already signs that this year's El Niño pattern -- one of the strongest ever -- won't last beyond this winter.

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A sizable La Niña pattern may follow, and the change can happen rapidly, potentially setting the stage for more drought conditions.

Moreover, some recent research suggests the lingering effects of a La Niña pattern -- what one meteorologist called a La Niña "hangover" -- contributed to the severity of the current drought.

All of this leads some meteorologists to suggest that even if El Niño produces monster storms where California needs them most, the state will likely remain in a drought and will be facing challenging forecasts in the coming years.

"By winter 2016-2017, the El Niño will return to normal conditions, if not La Niña conditions," said Paul Ullrich, a climate modeler at the University of California, Davis.

"In that case, we predict below-average rainfall. So, unless this year is very dramatic -- meaning twice average -- it's going to be very hard to erase our rain dent."

The two patterns are part of the El Niño Southern Oscillation, or ENSO, which refers to the sea surface temperatures in the Pacific Ocean.

During El Niño cycles, the usual trade winds running from east to west in the Pacific die down, allowing water temperatures to rise. In some instances, including the current pattern, the rise in water temperatures is dramatic, up to 5.4 degrees Fahrenheit along the equator.

More ocean water consequently evaporates and storms heading toward the West Coast pick up steam, intensifying the storms that reach California.

La Niña events are rarer than El Niños, and there can be a "normal" phase between the two. But La Niñas are known for opposite characteristics. Water temperatures in the same swath of the Pacific Ocean are up to 7 F cooler than usual.

They are associated with drier weather in the southwestern United States. La Niñas can also raise temperatures in those regions in the winter, and, unlike El Niños, La Niñas can persist for up to three years.

An analysis this April from the National Oceanic and Atmospheric Administration's Climate Program Office analyzed potential La Niña effects on California's current drought, which began in 2011-12 -- the second year of a La Niña cycle.

The researchers found that even when that cycle ended, dry conditions continued for two years, intensifying the drought conditions.

They looked at data from other years following La Niña patterns and found they "suggest that the dryness can linger for multiple years after a La Niña -- until the Pacific shifts all the way over to its warm phase, El Niño."

Uncertainty in the forecast

Meteorologists caution against looking too far ahead, noting that there remains some uncertainty about this year's El Niño forecast -- let alone what will happen a year from now.

Recent readings show that water temperatures in the equatorial Pacific have hit a record high for this point in an El Niño cycle. That increases the odds of significant El Niño storms hitting California this winter, including in the north and central parts of the state where the majority of its reservoirs and water infrastructure are found (Greenwire, Nov. 20).

That temperature is similar to readings in 1997-98 and 1982-83, when El Niño storms pummeled Southern California, leading to flooding and property damage.

It remains unclear, however, whether the storms will produce snow in the Sierra Nevada in Northern California. Historically, mountain snowpack has provided up to three-quarters of the state's water supply, and it is now at record lows.

And conditions in the Pacific can shift rapidly, said Nick Bond, a meteorologist at the University of Washington. He noted that a La Niña pattern quickly followed the 1997-98 El Niño.

"Even some long-range models are showing there is a good chance for a Niña in the winter for 2016-2017," he said.

NOAA's Nate Mantua agreed that strong La Niña patterns can follow El Niño events like the one occurring this year.

But he noted that for California, La Niña doesn't necessarily pose a significant drought risk. There is "close to zero" correlation between La Niña cycles and rainfall in north and central California, he said, where the state collects most of its drinking water. Three-quarters of the state's water storage is in Northern California.

A La Niña, he said, "doesn't necessarily mean that California would be thrown into an amplified drought."

However, Mantua said the NOAA survey was intriguing because there is a significant possibility that the conditions it examined could align in the coming years. If a La Niña develops in the winter of 2016-17, there would be lingering effects the following years. Mantua called it a "La Niña hangover."

Pay attention to details

A lot of the uncertainty surrounding El Niño, La Niña and drought impacts stems from trying to understand high-pressure systems and how they interact with the Pacific Ocean's water temperatures.

There is general consensus that a "ridiculously resilient ridge" of high pressure off the West Coast contributed significantly to the current drought by deflecting storms from reaching the United States.

That ridge has dissipated, and researchers are studying how El Niño and La Niña cycles influence those high-pressure systems.

Mantua suggested that the La Niña hangover "could be one factor that favors the kind of persistent ridge on the West Coast."

Kevin Trenberth of the National Center for Atmospheric Research agreed with the NOAA study but explained La Niña's influence another way.

He said a better barometer for when drought conditions crop up in California is where major storms are occurring over the Pacific Ocean.

If they are east of the international date line -- closer to the United States -- rain typically gets funneled toward the West Coast. That's what happens in an El Niño cycle.

Usually, however, most storms occur west of the date line. And that is amplified during a La Niña cycle.

Those circumstances, he said, create the sort of high-pressure ridging that shields the West Coast from storms.

"As long as the action is in the western Pacific, west of the date line," he said, "that often tends to set up a ridge and create droughtlike conditions for California."

He added, "La Niña certainly sets the stage, but some of the details also matter."

Daniel Swain, a Stanford University climate scientist, said La Niña's influence on California drought varies.

"Historically, big La Niña events have followed big El Niño events," he said. "And La Niña events can influence the likelihood of drought in California but not in a super-reliable way."

But he added that La Niñas -- and the potential dry weather they can bring -- are worth keeping in mind because this year's El Niño will produce powerful storms but likely won't solve the state's water shortages.

"It's not going to last until next winter," he said. "And our drought right now is big enough that that's not going to cut it."

Twitter: @GreenwireJeremy Email: jjacobs@eenews.net

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