Climate: El Nino weakens, La Nina threatens

May 24th, 2010 By Pat Brennan in Earth / Earth Sciences

The Pacific weather pattern known as El Nino is all but gone, climate scientists say, while its alter ego, La Nina, might soon appear on the horizon.

If it does, it could tilt the scale in favor of wildfires when Santa Ana winds rage in the fall, though it's too soon to tell.

"I always love to call La Nina the diva of drought -- El Nino's dry sibling," said Bill Patzert, an ocean research scientist at the Jet Propulsion Laboratory in Pasadena, Calif.

In Southern California, , a periodic warming of equatorial Pacific waters, can mean increased rainfall, and the region did in fact have a bit more rain than normal -- 14.6 inches, normally 12.76 -- though it might have felt like a lot more after recent dry years.

But for those expecting an El Nino deluge, it didn't quite live up to its early billing.

La Nina, meanwhile, can mean just the opposite for Southern California: drier conditions that could persist through next winter.

"I think it's definitely hasta la vista, El Nino," Patzert said.

And La Nina?

"At this point, it's up in the air, but if you look at the data the dice is definitely loaded for La Nina," he said.

During La Nina episodes, ocean waters cool in the east-central Pacific. And ocean temperatures in the area are cooling already, leading to a bit of a spread in the predictions offered by climate scientists and the computer models they rely on.

Patzert said he's betting on La Nina developing within about two months. Once in place, it could last a year or more, although no one is making such predictions yet for the episode that might be developing.

Mike Halpert, the deputy director for the National Oceanic and Atmospheric Administration's Climate Prediction Center, said he, too, thinks El Nino is on its way out, though an official pronouncement from his agency won't come till next month.

He put the chances of a La Nina developing at about 50-50. We're now moving into a "neutral" period, and Halpert said computer models are split on what will happen next, with some forecasting a weaker or stronger La Nina and some no La Nina at all.

"La Nina doesn't last a month or two," he said. "We look for basically six to nine months of colder () temperatures having some impacts in circulation."

La Nina is known for its drying effects in Southern California, which could be bad news in the fall. The rains fueled growth of grasses and shrubs in the hills, which dry out to create fuel for wildfire just as Santa Ana winds begin to blow.

Most wildfires are caused by people, either by accident or arson, a far bigger factor than La Nina, Patzert said.

Still, La Nina can tip the scales a bit, he said.

"As we get into fall, there is always a race between early rainfall -- heavy rainfall -- and the Santa Anas," he said. "If we have a La Nina, what it does is put a damper on early rainfall, so it gives the Santa Anas more of a foot up on it. So advantage Santa Ana during a La Nina."

(c) 2010, The Orange County Register (Santa Ana, Calif.).
Distributed by McClatchy-Tribune Information Services.

"Climate: El Nino weakens, La Nina threatens." May 24th, 2010. http://phys.org/news193944559.html