Will La Niña return this fall? The tea leaves are unusually strong » Yale Climate Connections

Seasonal forecasting experts at NOAA and elsewhere are more confident than usual that La Niña conditions will develop by mid- to late 2024. If so, it would mark the fourth year out of five that La Niña has prevailed. It would also raise a number of red flags, including enhanced risks of widespread U.S. drought and a rip-roaring Atlantic hurricane season.

El Niño and La Niña are rooted in large-scale warmings and coolings, respectively, of the sea surface across the eastern tropical Pacific. Lasting from one to several years, each El Niño and La Niña event includes atmospheric responses that generate a variety of far-flung impacts and weather extremes. Although El Niño and La Niña themselves are natural processes, they are playing out in a warming world that’s likely affecting some of their impacts and perhaps how often they evolve.

What would the return of La Niña mean?

If a La Niña event does materialize later this year, it would raise the odds of a wide range of impacts. These include reduced wind shear over the tropical North Atlantic, especially across the Gulf of Mexico and Caribbean, that in turn would increase the chance of a busy Atlantic hurricane season. During the “three-peat” La Niña period from 2020 to 2022, the Atlantic spewed out 65 named storms, 28 hurricanes, 12 major hurricanes, and an accumulated cyclone energy of 420.6, according to Colorado State University. Three years of average behavior (1991-2020) would have yielded 43.2, 21.6, 9.6, and 369.

In addition, the U.S. Sun Belt is more likely to experience drought and intense heat during La Niña. Some of the worst wildfires on record across California and the Southwest have occurred during La Niña events, particularly in 2020 and 2021 (see photo at top).

An unusually confident forecast

In the latest monthly outlook issued on January 19 by NOAA and the International Research Institute for Climate and Society, the odds of La Niña prevailing by the July-to-September period are pegged at 58%, with a 36% chance of neutral conditions and only a 6% chance that the current strong El Niño conditions would persist or recur by then.

That’s the highest confidence for the July-to-September period from any outlook issued in January during the past 13 years that NOAA and the institute have jointly issued such outlooks (see Figure 1).

The high odds for La Niña reflect the strong agreement among longer-range weather-climate models that the eastern tropical Pacific will be cooling this year. It’s also a sign of forecaster confidence arising from what they’ve learned about strong El Niño events and what follows them, said Michelle L’Heureux, a scientist at NOAA’s Climate Prediction Center who manages the monthly outlooks.

“Chances are enhanced that we will have a follow-up La Niña,” L’Heureux said in an email.

Why this year’s El Niño is boosting forecast confidence

Predicting months in advance how the El Niño–Southern Oscillation, or ENSO, will evolve is a tricky endeavor. That’s especially true during northern winter, before the spring predictability barrier (which simply refers to the difficulty of predicting ENSO events prior to northern summer) has been breached.

Research increasingly shows that the most predictable sequence of events during ENSO is for a strong early-year El Niño event — which happens about once per decade, on average — to evolve into La Niña conditions by later that year. Such research is being turbo-charged by the recent growth in large ensembles, or parallel runs of a single climate model each carried out for the same time period. Small variations in the starting point of each ensemble member help reveal where there is greater or lesser confidence in a particular outcome, as deduced by looking at how closely the ensemble members agree or differ.

When the models are used to reproduce historical climate, scientists can better understand the links between one year’s ENSO event and the next.

What’s more, climate models have become more skillful in recent years in characterizing how El Niño and La Niña evolve. It’s a perennially tough challenge, especially since these oceanic warmings and coolings span thousands of miles yet extend only tens to hundreds of feet below the surface. (Climate models still tend to overestimate the strength of El Niño and La Niña events, for example.)

For the last few years, researchers based at the University of Texas at Austin and the National Center for Atmospheric Research, or NCAR, have been collaborating on ways to improve long-range outlooks of El Niño and La Niña. A 2021 paper led by UT’s Xian Wu reproduced 62 years of past climate, using model ensembles of 10 to 40 members. The study found that forecasts produced in November can provide up to 25 months’ notice of a multi-year La Niña event, especially when issued while a strong El Niño is in progress.

A paper now in review led by Nathan Lenssen, a teaching assistant professor at the Colorado School of Mines now visiting NCAR, takes that research a step further. It uses a set of analog “hindcasts” (forecasts of replicated past events) across multiple large ensembles covering more than 500 years each, with an 1,800-year model run and a 20th-century reanalysis serving as de facto ground truths. The result was more than 10,000 years of simulated ENSO forecasts, all analyzed on a laptop instead of a supercomputer.

The new work finds even more support for the enhanced predictability that occurs when a strong El Niño is in progress. Lenssen discussed the ongoing work on January 30 in Baltimore in a talk entitled “Year 2 ENSO Predictability is Highly State Dependent” at the American Meteorological Society’s annual meeting.

Lenssen and colleagues found that nearly all of the ability to predict ENSO states from one to two years in advance was rooted in the reliable sequence from a strong El Niño to La Niña, with little skill found for the forecasts that started with other ENSO states. “This is a story that’s been hidden in the [overall] skill across multiple states,” said Lenssen in his talk.

Simple real-world climatology also supports the idea of an El Niño-to-La Niña handoff this year, albeit with a much small number of examples to draw on. In NOAA data going back to 1950, there have been six El Niño events before the current one that ranked informally as “strong” (Oceanic Niño Index of at least +1.5) during the October-to-December period. They were 1957-58, 1965-66, 1972-73, 1982-83, 1997-98, and 2015-16. The most recent four of these six events were each followed by La Niña conditions — and not just for the subsequent year, but also for the year after that.

Why have we seen so many La Niñas lately?

Although climate scientists once expected that global warming would lead to semi-permanent El Niño conditions, the real world has been going the other way, which is a topic of active research. As we discussed in a 2023 post, large-scale cooling has predominated over the eastern tropical Pacific during the past 40 years in spite of warming over most other parts of the world’s oceans.

Read: A mystery in the Pacific is complicating climate projections

At the same time, multi-year La Niña events have grown increasingly common in recent decades, for reasons that are unknown, according to a 2023 paper, “Understanding the recent increase in multiyear La Niñas.” The researchers found that there were only were two multi-year La Niña events before 1970; these followed a neutral ENSO state. Since 1970, there have been eight multi-year La Niña events, and these always followed an El Niño (Figure 3).

It’s important to keep in mind the maxim that enhanced odds don’t mean any particular outcome is guaranteed, as every El Niño and La Niña event is different.

“Our study is only discussing the prediction of ENSO state, not any ENSO impacts or teleconnections,” Lenssen said in an email. “However, we are excited by our robust results showing that there is multi-year predictability of the ENSO system following large El Niño events and expect this predictability to extend to impacts.” He added: “Note that this doesn’t mean that every forecast out of a large El Niño will be correct!”

It’s also still beyond the ability of forecasters to predict with confidence how strong an upcoming event might be a year or more out. L’Heureux points to the example of the 2015-16 El Niño, the most recent strong event prior to 2023-24. She noted that by late 2016, “We had a La Niña, but it was very weak.” In fact, the 2016-17 La Niña barely cleared the minimal threshold for a weak event and lasted just five months. The subsequent 2017-18 La Niña persisted for seven months and only hit the minimal threshold for a moderate event.

L’Heureux summed it all up: “As these things go, the chances for an upcoming La Niña are not 100%, but they are elevated and it bears watching going forward.”

Jeff Masters contributed to this post. Website visitors can comment on “Eye on the Storm” posts (see comments policy below). Sign up to receive notices of new postings here.