El Niño conditions — the warming of ocean waters off South America that can alter weather across the globe, including California’s summer temperatures and the amount of rain it might receive next winter — are emerging in the Pacific Ocean for the first time in 4 years.
While El Niños do not automatically guarantee wet weather for California, historically, the stronger they are, the more likely it is that the state will have a rainy winter season. And after the dramatic series of storms this past winter that ended the drought and filled nearly empty reservoirs, another one back-to-back could increase flood risks.
“The climate models are in strong agreement that there will be an El Niño,” said Michelle L’Heureux, a climate scientist at the National Oceanic and Atmospheric Administration who led a new report out Thursday. “At this point it’s looking likely.”
The chances of any El Niño forming are now 82% by July and 94% by November, according to the NOAA report.
More significant for California, there’s a 46% chance of a strong El Niño by November, increasing to 54% by January, NOAA researchers concluded Thursday.
El Niño occurs when ocean temperatures warm up, as they are rapidly doing now, near the equator off the coast of Peru. Combined with changing trade winds, the pattern historically has meant increased chances of wet conditions in the southern half of the United States and drier conditions in the northern half. But the Bay Area, located in the middle, doesn’t have as clear a signal.
The opposite is La Niña, a cooling of ocean waters off Peru, which has been in effect for much of the past three years.
During El Niño years, droughts are more likely in Australia and India. Wet weather is more likely in eastern Africa, now gripped by drought. And the risk of hurricanes in the Gulf of Mexico decreases.
What does the emerging El Niño mean for California? The state just came out of a severe three-year drought. Will there be another rainy winter to keep reservoirs full? Or the beginning of the next drought?
It depends on how strong the El Niño becomes. That strength will be defined by how much warmer than normal ocean waters off South America end up later this year.
Since 1951, there have been 26 El Niño events. Of those, 11 have been weak, 7 moderate, 5 strong and 3 very strong. Overall, rainfall in Southern California averaged 126% of normal. In the Bay Area it was 109% of normal.
But two of the three very strong El Niños, during the winters of 1982-83 and 1997-98, were associated with seasons that were among the wettest in recorded California history, with massive Sierra Nevada snow packs and major flooding.
The third, in 2015-16, was something of a bust. Hyped as a “Godzilla El Niño,” that event did see big winter storms, but they ended up hitting Oregon and Washington, dashing hopes of breaking a California drought that didn’t end until big storms arrived in 2017.
“People hear El Niño and they think it’s a guarantee we are going to have a wet rainy year,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay, who has closely studied the historical trends. “The reality is that we have seen that’s not always the case.”
Generally speaking, in strong or very strong El Niño years, Southern California has a higher chance of a wet winter than Northern California.
There have been eight strong or very strong El Niño events back to 1951. Of those, Southern California saw wet winters in six, an average winter in one, and a dry winter in the other. In the Bay Area, however, there were wet winters in only four of those years, an average winter in two, and a dry winter in two.
Null said that there are many other factors at play, including the warming climate and other large weather patterns off Asia, the Arctic and elsewhere that scientists are still working to better understand.
“El Niño is the superstar of the team, the Stephen Curry,” he said, using an NBA analogy. “It is normally the dominant player. But there are still other supporting players. Sometimes Draymond Green is the dominating player.”
Even so, the chances of a strong El Niño are beginning to raise concerns for two reasons.
First, El Niño years tend to be hotter across the globe than other years. El Niño is a natural phenomenon. But the planet has been warming from the build-up of greenhouse gases that trap heat in the atmosphere from the burning of fossil fuels.
Each of the last four decades has been steadily warmer than the previous decade. And the 10 hottest years back to 1850 globally have all occurred since 2010. In fact, three of the five warmest years — 2019, 2015 and the hottest, 2016 — were El Niño years.
Many scientists expect a strong El Niño later this year to further push up the Earth’s temperature. That could increase the risk of droughts, heat waves, forest fires and coral reef bleaching in some parts of the world.
“2024 is very likely to be the warmest year on record, once again breaking all of our previous records,” said Daniel Swain, a climate scientist at UCLA. “There will be a lot of global heat that emerges out of the tropical Pacific Ocean as this subsurface warmth materializes and surfaces and exchanges a large amount of that heat with the atmosphere.”
Swain cautioned that this past wet winter already increases flood risk for next year, and another rainy season will only exacerbate the potential. More than a dozen atmospheric river storms filled reservoirs, ended the state’s three-year drought, and gave California the largest Sierra snowpack since 1982-83.
When those storms hit, there was plenty of room in reservoirs, which were low due to the drought. Next year, they will start the winter fuller.
Also, if a heat wave in the coming weeks causes much of the remaining Sierra snow — about 70% is still there — to melt rapidly, that could cause further flooding in places such as the Tulare Basin between Bakersfield and Fresno, an area that has taken more than a year to drain after prior deluges submerged farmland and communities.
“It looks like it’s full steam ahead for a significant El Niño event,” Swain said.
“It might have some influence on late summer coastal heat wave potential,” he added, “But really, all eyes will be on what might happen next winter in terms of precipitation.”
