What Happens When Climate Predictions Get It Wrong?

How atmospheric rivers are rewriting the rules of seasonal forecasting.

Things are changing. For decades, scientists have relied on the El Niño–Southern Oscillation (ENSO) to forecast seasonal precipitation in the Western United States. In a nutshell, a strong El Niño typically means wet winters, while La Niña leans toward dry conditions. 

I still remember learning about El Niño-La Niña events in middle and high school or, later on, teaching about it to my undergrad students. The thing si that it’s a simple enough framework — except when it isn’t. 

The winter of 2023 was a prime example of how nature sometimes refuses to play by the rules. Despite La Niña conditions, which should have brought dryness to California and the Southwest, the region was hit with record-breaking rain and snow. 

Quite unexpected. 

But what happened? What caused this anomaly?

A new study from scientists at UC San Diego’s Scripps Institution of Oceanography finds that the real disruptor was a series of atmospheric rivers — massive bands of moisture that transport water vapor across the sky like aerial conveyor belts. 

As it turns out, these “rivers in the sky” overwhelmed the usual ENSO influence and turned what should have been a dry season into a deluge. Their research, published in Climate Dynamics, suggests that atmospheric rivers may be the missing piece in our understanding of why some years break the ENSO mold.

Their Methods

How did the scientists involved in the study figure this out?

The researchers used a simple yet well-thought-through approach. They examined over 70 years of climate data, looking at past El Niño and La Niña years to see when precipitation patterns matched expectations and when they didn’t. 

To pinpoint the role of atmospheric rivers, they separated rainfall into two categories: precipitation caused by these moisture-laden jets and precipitation from other sources.

They found that about 32% of ENSO years had unexpected precipitation patterns, what they called “heretical” years. Among those, nearly 70% could be explained by unusually strong or weak atmospheric river activity.

When ENSO Gets Overruled

Let’s look at their results a bit closer here.

Take 2023, for example. It was supposed to be dry, but nine powerful atmospheric rivers made landfall in California, making it one of the wettest years on record. 

This wasn’t an isolated case. Similar reversals happened in 1967, 2011, and 2017, all La Niña years that turned out wet. On the flip side, some El Niño years — like 1964, 1977, 1987, 2007, 2013, and 2015 — were drier than expected, and the data pointed to weaker atmospheric river activity as the culprit.

“One or two atmospheric rivers can turn it into a wet year, but a weak atmospheric river season can turn it into a dry year,” said Dr. Rosa Luna-Niño, lead author of the study. “This means we can’t trust El Niño and La Niña completely to make accurate water year predictions.”

This is a big deal for climate science and water management. Seasonal forecasts, used to guide everything from reservoir planning to drought preparedness, have long been based primarily on ENSO signals. But if atmospheric rivers can completely override those signals, then a new approach is needed — one that accounts for these unpredictable moisture surges.

At least we now know what else we should be on the lookout for.

A Climate Wildcard That’s Getting Stronger

The unpredictability of atmospheric rivers is part of what makes them such a challenge. Scientists can forecast their arrival about three weeks in advance, but anticipating their frequency over an entire season? That’s still out of reach.

Complicating matters further, research suggests these airborne water highways are likely to become even more dominant in shaping precipitation patterns as the climate warms. That means the traditional reliance on ENSO for long-term forecasting may become even less useful.

“Atmospheric rivers are the precipitation wildcards in the Western U.S.,” said Dr. Luna-Niño. “They don’t dance to the tune of ENSO.”

That’s a powerful reminder that climate systems aren’t as straightforward as we’d like them to be. It also highlights the need for improved forecasting methods that merge ENSO-based seasonal predictions with shorter-term atmospheric river outlooks. 

Luckily, the authors of the study are already working on refining these hybrid forecasts, which could help water managers make better decisions in an era where certainty is increasingly hard to come by.

Rethinking Seasonal Forecasts

For those of us who have worked in conservation, environmental policy, or the public sector, the stakes here are clear. Communities, farmers, and water agencies rely on these seasonal predictions to plan for the year ahead. But if forecasts fail, the consequences could ripple through everything from crop yields to wildfire risk to urban water supplies.

However, this study makes one thing clear: the old models we have relied on for so long need an update. ENSO still matters, but it’s only part of the picture. To truly understand and predict precipitation in the Western U.S., we need to track the erratic and powerful influence of atmospheric rivers.

The next step? 

Finding ways to better anticipate these moisture surges. Rest assured that scientists are already working to improve forecasting models, but in the meantime, the lesson is simple: expect the unexpected. 

When it comes to the weather, sometimes the biggest influences aren’t the ones we’ve been watching the longest.

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Best,

Sílvia P-M, PhD Climate Ages

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