Multi-year La Niña events (sometimes called “double-dip” or lingering La Niñas) occur when cold conditions in the equatorial Pacific persist for two or more years, leading to prolonged climate impacts like droughts, floods, and temperature extremes in various regions.
A recent study from researchers at Nanjing University of Information Science and Technology and the University of Hawaii identifies two distinct mechanisms or pathways that allow these events to develop and linger, challenging the idea that they are mostly triggered only by a preceding extreme (super) El Niño.
Multi-year La Niñas can amplify and prolong regional climate extremes (e.g., droughts in some areas, wetter conditions in others). Understanding these dual pathways improves ENSO prediction, especially for events not directly following a massive El Niño. Related research also points to roles for rainfall-salinity feedbacks and long-term changes in ocean stratification that may make multi-year events more frequent.
A recent study from researchers at Nanjing University of Information Science and Technology and the University of Hawaii identifies two distinct mechanisms or pathways that allow these events to develop and linger, challenging the idea that they are mostly triggered only by a preceding extreme (super) El Niño.
“Distinctive Routes for Multi-Year La Niña Formation” (Chen et al., 2026, Advances in Atmospheric Sciences) provides a nuanced framework for why some La Niña events persist for two or more years while others decay rapidly.
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Distinctive Routes for Multi-Year La Niña Formation
“Distinctive Routes for Multi-Year La Niña Formation” is the title of a research paper published on May 15, 2026, in Advances in Atmospheric Sciences by lead author B. R. Chen and co-authors including T. Li, X. Pan, and J. Liu (affiliations include Nanjing University of Information Science and Technology and the University of Hawaii).
The traditional explanation emphasizes the ocean heat content (OHC) discharge from a preceding extreme (super) El Niño. This leads to persistent negative thermocline anomalies that sustain La Niña via the Bjerknes feedback loop: stronger trade winds → enhanced equatorial upwelling → cooler SSTs → even stronger winds.
This mechanism explains only ~30% of historical multi-year La Niña (myLN) events over the past century. The study identifies a second, dominant pathway for the majority (~70%).
The study identifies two distinctive pathways (or “routes”) for multi-year La Niña (myLN) events, where cold equatorial Pacific conditions persist for two or more years, as opposed to single-year events.
Classic Heat Discharge (Super El Niño Precursor)
- Strong prior El Niño discharges upper-ocean heat.
- Creates deep negative thermocline depth anomalies.
- These anomalies resist warming, prolonging the cold phase through coupled ocean-atmosphere feedbacks.
- Aligns with events like those following 1982–83 or 1997–98.
South Pacific Meridional Mode (SPMM) — The Overlooked Dominant Pathway
During the decaying spring of the first-year La Niña:
- Meridionally extended (north-south) cold sea surface temperature (SST) anomalies develop, linked to a negative South Pacific Meridional Mode (SPMM).
- These off-equatorial anomalies (stronger in the South Pacific) generate or strengthen equatorial easterly wind anomalies.
- Enhanced easterlies drive:
- Stronger equatorial upwelling of cold water.
- Increased meridional divergence at the equator.
- This slows the natural damping of the cold SST anomaly.
- The cold conditions persist through the decaying summer.
- They re-intensify in the following autumn through season-dependent coupled instability (stronger Bjerknes feedback in certain seasons).
Evidence:
- Observational composites of myLN vs. single-year LN events.
- Idealized atmospheric general circulation model (AGCM) experiments that isolate the SPMM’s role in producing the critical equatorial easterlies.
Mechanistic Details and Seasonality
The SPMM pathway highlights extratropical-tropical interactions. The broader meridional structure of cold anomalies (unlike narrowly equatorial ones in single-year events) sustains the feedback.
Seasonality is crucial: spring SPMM anomalies set the stage, summer persistence occurs via wind-driven oceanic processes, and autumn re-development leverages peak coupled instability periods.
This route operates more independently of a massive prior El Niño, explaining many “non-canonical” multi-year events.
Broader Context and Implications
- Complementary Mechanisms: Other recent work highlights rainfall-salinity feedbacks (reduced precipitation increases salinity, affecting density and mixing to sustain cooling) and changes in upper-ocean stratification that may increase myLN frequency. communities.springernature.com
- Climate Change Links: Multi-year events have become more common recently and may increase further due to altered background states.
- Prediction Value: Distinguishing routes could improve seasonal-to-interannual forecasts, as myLNs amplify and extend impacts (e.g., prolonged droughts in the Maritime Continent/Australia, altered precipitation in the Americas, hurricane modulation).
- Distinctions from Related Work: This complements studies on North Pacific Meridional Mode (NPMM/PMM) influences or South Pacific Oscillation roles, emphasizing the South Pacific arm here.
The paper (DOI: 10.1007/s00376-026-5771-8, published May 15, 2026) uses reanalysis datasets (e.g., SST from ERSST, winds from ERA5) and model experiments for robustness. Full text is available via Springer or institutional access; supplementary materials include additional composites and experiment details.
This dual-route perspective refines ENSO theory by showing multiple pathways to persistence, moving beyond a one-size-fits-all recharge-discharge paradigm.
Published: Advances in Atmospheric Sciences
DOI: 10.1007/s00376-026-5771-8
Provided: Chinese Academy of Sciences
Authors: Borui Chen,
Tim Li,
Xiao Pan &
Jia Liu
Abstract
A prevailing hypothesis for generating a multi-year La Niña (LN) compared to a single-year LN is the strong discharge of ocean heat content (OHC) associated with preceding extreme El Niño, but this mechanism explains only a small portion (∼30%) of the multi-year LN events during the past century. For the majority of multi-year LN events, it is found that the meridional extent of the cold SST anomaly in the tropical Pacific, particularly the South Pacific Meridional Mode (SPMM), during the LN decaying spring plays a critical role in prolonging the LN. Idealized atmospheric general circulation model experiments confirm the role of the SPMM in generating equatorial easterlies, which may help enhance oceanic upwelling and meridional divergence at the equator, slowing down the LN damping. As a result, the cold anomaly associated with LN persists throughout the decaying summer and re-develops in the following autumn via a season-dependent coupled instability. Thus, this study emphasizes the role of the SPMM in causing multi-year LN events.
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