What El Nio could mean for California this winter

The specter of a developing El Niño invokes distressing memories for many Californians. Rainstorms tied to a super-strength El Niño episode overwhelmed southern parts of the state and killed 17 people in the winter of 1997-1998.

With forecasts that the infamous climate pattern could reach a similar intensity this winter, the Golden State is bracing for more floods, not even a year removed from one of its wettest stretches on record.

This fall, California officials are lining up $52 million for levee repairs and, compared to a year ago, longer stretches of temporary flood walls and millions more sandbags to prepare for what the winter might bring.

But whether California winds up making use of those preparations depends on an unpredictable mix of climate and weather indicators, none of which guarantee that California and the Southwest are in line for more slugs of precipitation. El Niño tilts the odds in favor of another wet year, but it doesn’t assure one.

“There are always ways in which Mother Nature can throw us a curveball,” Karla Nemeth, director of the state’s Department of Water Resources, told reporters at a briefing last week.

Though it’s clear El Niño is building — and could become one of the strongest ever observed — climate scientists and meteorologists said it’s too soon to know how it will influence weather in California and across the United States. While the 1997-1998 iteration of El Niño brought disaster, a more recent super El Niño in winter 2015-2016, for example, created unexpectedly drier conditions.

“They don’t always deliver in the way we expect them to,” said Andrew Hoell, a research meteorologist with NOAA’s Physical Science Laboratory in Boulder, Colo.

Why El Niño encourages wet California winters

A strong El Niño significantly raises the odds of heavy rainfall and a wet winter for the Southwestern U.S. and California, based on historic events and modeling studies. That is because of the ways El Niño shifts moisture around the globe.

The climate pattern stems from abnormally warm Pacific Ocean waters along the equator. That warmth leads to increased evaporation and more water vapor to fuel storm clouds in the central and eastern Pacific.

Meanwhile, changes to larger weather patterns mean that moisture is sent flowing across the southern U.S. During El Niño, the jet stream — a torrent of strong wind in the upper atmosphere that steers weather systems — tends to dip southward, sending storms into southern California.

Michael Anderson, the California state climatologist, said that during El Niño, storms also tend to move faster, zipping across the Pacific straight into California and the Southwest.

In years with especially strong El Niño patterns, such as 1982-1983 and 1997-1998, the wet trend can extend into Central California.

“Strong El Niño events increase the likelihood of above-average precipitation in the Southwest, and they do so to the tune of about 40, 50 or 60 percent,” Hoell said.

Scientists will be closely monitoring how the tropical Pacific evolves over the coming months to gauge El Niño’s effects. The West tends to avoid prolonged storminess, as it did in 2015-2016, when waters are most unusually warm over the central equatorial Pacific, they said.

But California tends to see more streams of moisture from the Pacific, known as atmospheric rivers, when the warmest waters set up farther east, near the coast of South America, as they have this year.

El Niño impacts shrouded in extra uncertainty

This time, Hoell pegs the chances of a wetter than normal winter in the Southwest at about 60 percent. That leaves at least a 40 percent chance of not seeing a wet winter, a point that is often lost in discussions of long-range El Niño forecasts.

While El Niño is a relatively reliable climate signal suggesting what weather many regions can expect, there are other phenomena that can outweigh its influence.

That includes a climate indicator known as the Madden-Julian Oscillation, a pattern that affects rainfall patterns across the Pacific basin and can dampen or enhance storminess in California.

Incidents known as sudden stratospheric warming — when polar regions dramatically warm and frigid air plunges southward — can also alter jet stream patterns and have dramatic effects on weather.

“There are so many different behaviors that can happen at weekly and monthly time scales that we can’t foresee three months in advance,” Hoell said.

Meanwhile, it’s not just the Pacific exhibiting unusual warmth — above-normal temperatures have dominated much of the world’s oceans in recent months. And it’s not clear how that could also influence weather patterns and potentially counter typical symptoms of El Niño.

Warmth in the Indian Ocean, for example, is thought to limit how much El Niño can strengthen and is even associated with the termination of an El Niño pattern, said Yuko Okumura, a research scientist at the University of Texas. A 2021 study found Indian Ocean warmth and rainfall patterns likely contributed to drier-than-expected conditions in California during the 2015-2016 El Niño, sending storms farther north into the Pacific Northwest instead.

Global climate data show slightly above-average sea surface temperatures across the Indian Ocean.

“I’m very curious how that’s going to affect the evolution of the current El Niño,” Okumura said.

The latest U.S. Climate Prediction Center forecast for December through February only shows a slight shift in the odds toward wetter conditions for Southern California and the Southwest, with a stronger likelihood of increased precipitation in the Southeast.

A monthly El Niño advisory the center issued Thursday found up to an 85 percent chance of a “strong” El Niño from November through January, and a 30 percent chance of a “historically strong” El Niño episode on par with the 2015-2016 and 1997-1998 events.

“Stronger El Niño events increase the likelihood of El Niño-related climate anomalies, but do not necessarily equate to strong impacts locally,” the center cautioned.

California preparing for the worst

Last winter’s parade of storms on California came at the tail end of La Niña — the counterpart to El Niño, marked by cooler-than-normal waters in the eastern and central Pacific. So California water officials said they are preparing for another wet year regardless of the El Niño forecast.

Along with the investments in levees, flood walls and sandbags, state officials said they plan to continue diverting as much surface water as possible into groundwater aquifers, which are still in deficit after years of drought.

Reservoir operators are set to release water to clear space for incoming flows ahead of any winter storms, the officials said.

Nemeth, of the state’s Department of Water Resources, called this past water year a “miracle year” given the drought turnaround, replenished reservoirs and the cooler weather that prevented serious snowmelt flooding over the spring and summer.

“We shouldn’t expect that necessarily again,” she said, “so there’s a lot of work to continue to be done particularly when it comes to flooding and protecting our communities.”

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Fernande Dalal

Update: 2024-08-29

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