How does El Nino work, and why might it bring rain and snow to California this winter? Here’s a Q&A:
Q: How might El Nino affect California?
A: There’s a favorable chance that this winter will be wetter than average in much of California — from San Diego to San Francisco. The greatest chance for wet conditions is in Southern California, said Mike Halpert of the National Oceanic and Atmospheric Administration’s Climate Prediction Center.
But there’s only an equal chance of a wetter-than-average rainy season north of San Francisco, where much of the state’s water supply is collected and stored in giant reservoirs. California needs both rain and snow up there. Snow slowly melting from the mountains is essential to recharging our reservoirs when the skies turn dry later in the spring.
Q: How can scientists make these forecasts about the winter half a year ahead of time?
A: We’ve had experience with El Nino before, a weather phenomenon characterized by the warming of Pacific Ocean waters west of Peru that causes changes in the atmosphere and can dramatically alter weather worldwide.
In the two strongest El Ninos on record, 1982-83 and 1997-98, that has meant relentless storms pelting California.
Q: What’s the latest on how El Nino is doing?
A: The ocean is getting hotter. On July 15, a key benchmark location in the Pacific Ocean was 3 degrees Fahrenheit above average. It’s very similar to the temperature reading on July 16, 1997, which was 3.2 degrees Fahrenheit above average.
Q: So we’re matching the pace we saw in the summer of 1997?
A: Yes, at least for ocean temperatures along the equator. The summer of 1997 was the precursor to the strongest El Nino in the modern record.
Q: What are other reasons scientists are so interested this year?
A: Winds along the Pacific Ocean at the equator typically move east to west. That’s why ocean water enjoyed by tourists on Indonesian beaches is so warm. Winds move warm water west, and the eastern Pacific Ocean’s surface along the equator is chilled as deep ocean water wells up.
In big El Nino years, so-called trade winds weaken, allowing the eastern Pacific Ocean to warm up more — making El Nino even stronger.
Q: Why do very strong El Ninos bring more rain to California?
A: El Nino can bring something to Southern California called the subtropical jet stream.
This current of air usually runs over the jungles of southern Mexico and Nicaragua, a reason Central America has rain forests, said Bill Patzert, climatologist with NASA’s Jet Propulsion Laboratory in La Canada Flintridge, Calif.
This subtropical jet stream has already shifted to the north, and is somewhat responsible for the devastating storms that pelted Texas and Oklahoma this spring and pushed those states out of drought, Patzert said.
“If it continues to warm up in the eastern Pacific, as we get into fall and winter, the subtropical jet stream could move across the southern tier of the United States,” Patzert said.
Q: Only the south? What about Northern California?
A: El Nino needs to be particularly powerful to affect Northern California.
“This is not strong enough yet,” Patzert said. “The really big El Ninos — we’re not there yet — can soak the whole state. But right now, it’s possible to get a lot of flooding and mudslides in the south. In Northern California, you could get below-normal rainfall and snowpack.
“So that’s why I’m not calling this a drought-buster yet,” Patzert said.
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SOURCE: LA Times, Rong-Gong Lin II, Thomas Suh Lauder and Paul Duginski