Environment & Science

Forget El Niño, the 'PDO' could be the real drought buster

An image showing the positive / warm phase of a PDO and the negative / cold phase. The terms warm and cold refer to the temperature of water off the west coast of America.
An image showing the positive / warm phase of a PDO and the negative / cold phase. The terms warm and cold refer to the temperature of water off the west coast of America.

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A new forecast out Thursday on the El Niño climate pattern shows it could be one of the strongest on record. And that could deliver much needed rain to Southern California and possibly northern parts of the state, too. But El Niños are usually fleeting, lasting only a year or two.

Now, evidence is building that a longer-term climate pattern — one that might bring years of rainy winters — could be forming in the Pacific well north of the equatorial waters that give rise to El Niño.

The PDO game change

For the past several months, researchers have been tracking warmer temperatures in this northerly patch of ocean. And they're beginning to question whether we're about to see a switch in something called the Pacific Decadal Oscillation or PDO.

Given the data, the PDO could be shifting from a cool phase to a warm one — a shift that could mean a wetter decade ahead for much of California. Still, the phenomenon could also turn out be a short-lived blip, not a years-long flip. 

Unlike El Niño, which focuses on sea surface temperatures in a stretch of the Pacific near the equator, the PDO looks at water in the northern part of the ocean, from Hawaii all the way to Alaska.

According to research scientist Nathan Mantua with the National Oceanic and Atmospheric Administration, the PDO has a warm phase and a cool phase, and each one can last anywhere from a few years to decades.

During the warm phase, waters along the coast of the western U.S. tend to heat up while the larger ocean about 200 miles off the coast cools down. During the cool phase these trends are reversed.

"When you have the warm pattern of the PDO, it tends to be wet in the southwest U.S. and northern Mexico," he explained.

During those same years you are more likely to see drought in the Northern Rockies, Idaho, Eastern Washington, Western Montana and Southern British Columbia.

Likewise, the cool phase is linked to wetter periods up north but dry conditions in Southern California and neighboring states.

Mantua says the PDO has been mostly in a cool phase since 1998, coinciding with some of California's driest years on record.

Climate scientist Bill Patzert with the Jet Propulsion Laboratory thinks it's this PDO pattern that is responsible in large part for the severe drought in the region.

Long-term drought buster on the horizon?

However, since January of 2014, the PDO has been shifting into a warm mode.

Patzert thinks this could be the drought-buster the state has been hoping for.

"Perhaps in the long term, rooting for a [warm] PDO... is probably the most important thing for California and the American West," he said.

Thursday and Friday at JPL's von Kármán Auditorium, Patzert will give a free public lecture on the PDO, El Niño and drought.

He said even a strong El Niño isn't likely to supply all the water needed to get California out of a drought this bad.

In fact, a recent NASA study found the state would need double the average rainfall in a single year to break the drought

"In the long run these decadal or multi-decade variations in the Pacific are really the key to sustaining California agriculture and California civilization," Patzert said.

That may be true for Southern California, but it is less clear how a warm PDO will affect Northern California, said NOAA's Nathan Mantua.

That's because the northern part of the state is between the two regions that switch from wet and dry as the PDO shifts.

"Northern California sort of sits between the ends of this sort of north-south see-saw," he said.

Still, he's optimistic that a warm PDO is coming, since the major index predicting this pattern has been positive for 19 months.

But Mantua cautions that even such a strong signal can result in a warm PDO that only lasts a year or so.

"Beyond that, it's going depend on what the winds do and the weather patterns," he said.

Sometimes those can change rapidly and dramatically, bringing drought conditions anew to California.

Another wild card, according to JPL's Patzert, is how climate change will affect the PDO and related weather patterns.

"As we move into the 21st Century, climate is shifting beneath our feet... nobody really understands what the impact will be," he said.

For now, climate watchers will keep their eyes on the ocean for signals of the weather to come.