Figure 1. More persistent cold conditions in boreal spring and early summer after a multiyear La Niña, compared to those before a strong El Niño (left, a, c), inhibit an eastward movement and enhancement of atmosphere deep convection (right, b, d), leading to a decoupled ocean and atmosphere that delay the growth of a subsequent El Niño.

In a paper published in Science Bulletin, an international team of scientists show that a transition from a multi-year La Niña to a strong El Niño rarely occurs. La Niña usually persists recharging the equatorial Pacific for consecutive years, as seen during 2020-2022. After the event, suggestions were ripe that the three-year heat recharge of the 2020-2022 La Niña could set the stage for an extreme El Niño. However, none of the extreme El Niño since 1950 have followed a multiyear La Niña.

To investigate whether multi-year La Niña events increase the likelihood of transitioning to an extreme El Niño, and how this transition might be influenced by greenhouse warming, the team analyzed reanalysis datasets and outputs from the latest models participating in Phase 6 of the Coupled Model Intercomparison Project (CMIP6).

Their findings reveal that a transition from a multi-year La Niña to a strong El Niño rarely occurs in observations or models, but is projected to increase under transient greenhouse warming. In boreal spring and early summer after a multiyear La Niña, despite the multiyear recharge, sea surface temperature remains anomalously cold in the equatorial central Pacific (Figure 1a, c), and the thermocline remains anomalously shallow in the central-eastern tropical Pacific. The cold conditions inhibit an ensuing eastward movement of atmosphere deep convection out of the warm western Pacific (Figure 1b, d), delaying onset of ocean-atmosphere coupling necessary for a strong El Niño.

The researchers found that under greenhouse warming scenarios, such transitions, while still rare, are projected to become more than twice as likely. The increase is attributed to a weakening in the climatological zonal sea surface temperature gradient, which facilitates the eastward movement of atmospheric convection, and enhanced upper-ocean stratification in the equatorial Pacific, which strengthens ocean-atmosphere coupling. These results provide predictive insights into the behavior of El Niño following multi-year La Niña events under greenhouse warming.

See the article:

Jia F., W. Cai, T. Geng, B. Gan, W. Zhong, L. Wu, and M. J. McPhaden, 2025: Transition from multi-year La Niña to strong El Niño rare but increased under global warming. Science Bulletin, 70, 756-764. https://doi.org/10.1016/j.scib.2024.12.034.

Summary submitted by the authors Fan Jia, Wenju Cai, Tao Geng, Bolan Gan, Wenxiu Zhong, Lixin Wu, and Michael J. McPhaden.