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El Niño Explained - Matt Weiser -


El Niño Explained - Matt Weiser -

By Matt Weisler

The celebrated weather phenomenon is forecast to be in play this coming winter, and it’s often thought to be a drought-buster. Think again.

El Niño is the weather phenomenon in which equatorial waters of the Pacific Ocean become warmer than average. It’s a natural cycle in the ocean and unrelated to climate change.

This shift in Earth’s largest ocean also shifts the jet stream, that highway of high-altitude air currents that drives storms around the globe. As a result, El Niño can change where storms strike around the world, and particularly in California.

The Spanish name, by the way, means “little boy” or “Christ child”, a tribute to the fact that El Niño often reveals its influence on the weather around December.

El Niño conditions are expected to be in place again this coming winter. Many people are hoping this will break the severe drought in California. They feel this way because El Niño is often portrayed by the media as a drought-busting weather phenomenon. But the reality is quite different.

Before you get too excited about El Niño, here are five important things to understand. We interviewed Jan Null, a consulting meteorologist based in Saratoga and a leading El Niño myth buster.

1. Please excuse the media. We get excited, too.

An El Niño was in effect last winter, and the media made sure we all knew about it, with heady predictions by some that a wet winter was ahead. Well, it proved to be the driest winter in state history.

Rain AnomaliesJan Null, who owns Golden Gate Weather Services , said the hype arises from just two memorable winters: 1982-83 and 1997-98. Both were strong El Niño winters, and both produced equally strong images in the media of severe flooding, wave erosion and wind damage. People remember those images, he said, and they’re often dragged out of the archives and reprinted every time another El Niño is forecast.

In reality, some of the state’s wettest winters and worst flooding have occurred when no El Niño was present, or during the opposite condition, La Niña, in which the Pacific Ocean is cooler than normal.

The fact is, out of 23 El Niño events over the past 65 years, only nine resulted in wetter-than-average winters in California.

“Trying to overcome the hype, once it gets out there, is really tough,” said Null, who was on a panel that briefed Congress about El Niño in January. “Part of the obsession comes from the front-page stories you see.”

2. It’s a knob, not a switch.

El Niño is a nuanced thing. It can be strong or weak, depending on various factors that are hard to measure and even harder to predict. It doesn’t flick on and off like a light switch. It’s more like a volume knob.

So it isn’t enough to simply say, “It’s an El Niño year.” You have to understand how strong that year’s El Niño really is.

Current predictions for the winter ahead suggest El Niño will be a “borderline-strong” event, Null said.

It’s true, a strong El Niño, as measured by temperature change in the ocean, does seem more likely to produce wet winters. But it is not enough reason to start building that new swimming pool.

There have been only four strong El Niños in the past 65 years. Two of those led to a wet winter. The other half were drier than average.

A composite of El Niño effects over the past 60 years shows only a modest beneficial effect on precipitation in California. But in the important Northern Sierra region, noted by the red arrow, there is no significant effect at all.

3. Location, location, location.

A crucial factor with any El Niño is where the jet stream decides to position itself. When the Pacific Ocean warms up, it usually pushes the jet stream south. This is why dry Southwest states are often wetter than normal during an El Niño. The same can be true in Southern California.

But Northern California is where droughts are made and broken. The state as a whole, thanks to massive water diversion systems, gets 50 percent of its freshwater from snowmelt in the Northern Sierra Nevada. If that region experiences drought, it will hurt the whole state.

El Niño often creates an atmospheric Mason-Dixon line in California:

Wet in the south, not so wet in the north. Where that line falls is not constant, and it makes all the difference in water supply.

For reasons that aren’t clear, the dividing line often sets up around the Interstate 80 corridor in the Sacramento region.

“If it sits at Interstate 80, and we get most of our water from north of Interstate 80, then it obviously is problematical,” Null said.

4. The LeBron James effect.

LeBron James is one of the most influential professional basketball players currently engaged in the sport. Like El Niño, he generates a lot of media attention. And like the weather phenomenon, he had two standout years: LeBron led the Miami Heat to two straight championships in 2012 and 2013.

But those victories wouldn’t have been possible unless the other players on his team were also scoring points.

The same goes for El Niño. It doesn’t act alone to determine the weather. It works in concert with many other phenomena. And if they’re not getting along, California might not get a wet winter.

Null names just some of the other weather patterns that often influence El Niño, and vice versa: Pacific Decadal Oscillation, Madden-Julian Oscillation, Arctic Oscillation, North Atlantic Oscillation.

We won’t attempt to explain all those here. Just know that, like El Niño, they are poorly understood and difficult to predict. How they all work together is even more mysterious.

“There’s a whole alphabet soup of other things going on out there,” Null said. “Sometimes all these things line up right to make El Niño stronger. Sometimes they line up to have a negative feedback. But you can’t win if you don’t have the other players contributing.”

5. Don’t pick cherries.

A basic problem with El Niño is there simply aren’t a lot of examples from which to seek inspiration. Weather experts have been studying El Niño only since the 1950s, and there have been just 29 events to analyze.

They’ve learned a lot about El Niño in that time. But comparing present conditions to a sample size that small is bound to produce error and uncertainty.

After 65 years of studying and research, only those two El Niño events in 1982-83 and 1997-98 were considered “very strong.” They both produced wet winters. But letting just two examples shape our expectations about El Niño is sure to create disappointment.

“That’s cherry-picking data, which is bad science,” Null said. “I would love to be optimistic and think it’s going to be a very wet year. But there’s not enough information to say that.”

Matt Weiser is managing editor of He can be reached at