# ENSO History — Oceanic Niño Index (ONI) Analysis **Data source:** NOAA Climate Prediction Center (CPC) - ONI series: (downloaded 2026-08-09) - CPC ENSO definitions: Local files: - `data/enso/raw/oni.ascii.txt` — raw CPC file - `data/enso/oni.json` — parsed monthly series, 918 points, Jan 1950 → Jun 2026 - `data/enso/events.json` — 42 derived events - `data/enso/build_enso.py` — parsing/event-derivation script (reproducible) ## How the index is defined The **Oceanic Niño Index (ONI)** is the 3-month running mean of sea surface temperature (SST) anomalies in the **Niño 3.4 region** (5°N–5°S, 120°–170°W), computed from the **ERSSTv5** dataset against centered 30-year base periods (updated every 5 years). Each value is labelled by its 3-month season (e.g. NDJ = Nov–Dec–Jan); in `oni.json` each value is keyed by the **center month** of that season. NOAA CPC definitions: - **El Niño:** ONI ≥ **+0.5 °C** for at least **5 consecutive overlapping 3-month seasons** - **La Niña:** ONI ≤ **−0.5 °C** for at least 5 consecutive overlapping 3-month seasons - Strength by peak |ONI|: 0.5–0.9 weak · 1.0–1.4 moderate · 1.5–1.9 strong · ≥ 2.0 very strong ## Very strong and strong events since 1950 (from the downloaded data) ### El Niño | Event | Peak month | Peak ONI | Class | |---|---|---|---| | 1957–58 | 1958-01 | +1.69 | strong | | 1965–66 | 1965-11 | +1.82 | strong | | 1972–73 | 1972-11 | +1.84 | strong | | **1982–83** | 1983-01 | **+2.14** | **very strong** | | 1991–92 | 1992-01 | +1.54 | strong | | **1997–98** | 1997-12 | **+2.37** | **very strong** | | 2009–10 | 2009-12 | +1.50 | strong | | **2015–16** | 2015-12 | **+2.59** | **very strong** | | 2023–24 | 2023-12 | +1.99 | strong (borderline; rounds to +2.0) | | 2026–27 (in progress) | — | — | developing; NOAA declared El Niño 2026-06-11 | ### La Niña | Event | Peak month | Peak ONI | Class | |---|---|---|---| | 1955–56 | 1955-11 | −1.59 | strong | | **1973–74** | 1973-12 | **−2.04** | **very strong** | | 1975–76 | 1975-12 | −1.58 | strong | | 1988–89 | 1988-12 | −1.85 | strong | | 1998–2000 | 1999-12 | −1.53 | strong | | 2007–08 | 2008-01 | −1.76 | strong | | 2010–11 | 2010-11 | −1.57 | strong | Peak values cross-check against CPC's published figures (1997–98 ≈ +2.4, 2015–16 ≈ +2.6, 1982–83 ≈ +2.1/2.2): all match. ## Notes on the benchmark events - **1997–98 (peak +2.37, Dec 1997):** The strongest event of the 20th century in Niño 3.4 terms. Developed rapidly from May 1997, peaked in OND/NDJ 1997, decayed by AMJ 1998 and flipped directly into a strong, long-lived La Niña (Jul 1998 – Jun 2000, peak −1.53). Global surface temperature spiked in 1998. - **2015–16 (peak +2.59, Dec 2015):** Largest ONI peak in the record. By the strict CPC 5-consecutive-season rule it is continuous with the warm conditions that began in Oct 2014 (ONI stayed ≥ +0.5 through May 2016), so this analysis treats 2014–2016 as **one episode** — as does CPC's own cold/warm-episodes table. Some agencies (WMO, BOM, Met Office) instead distinguish a weak 2014–15 event from the very strong 2015–16 event. - **2023–24 (peak +1.99, Dec 2023):** Developed mid-2023 after the 2020–23 triple-dip La Niña, peaked in NDJ 2023 at +1.99 — just 0.01 below the +2.0 "very strong" threshold in the unrounded data (CPC's 1-decimal table rounds it to +2.0, so it is often described as very strong). Decayed through spring 2024; followed by neutral conditions and a weak, short-lived cool spell in late 2024 (3 consecutive seasons ≈ −0.5 to −0.6, below the 5-season threshold → **not** an official La Niña in ONI terms). ## Current status (as of latest data, MJJ 2026) The 2025–26 winter was neutral-to-cool (SON–NDJ 2025 ≈ −0.6, only 3 consecutive seasons < −0.5 → no La Niña event). Since then the Pacific has warmed **rapidly**: | Season | ONI | |---|---| | DJF 2026 | −0.39 | | JFM 2026 | −0.21 | | FMA 2026 | +0.11 | | MAM 2026 | +0.46 | | AMJ 2026 | +0.95 | | MJJ 2026 | +1.39 | **NOAA CPC declared El Niño on 2026-06-11** under its new official index, the Relative Oceanic Niño Index (RONI, NWS PNS 26-05). RONI removes the background warming trend and ran ~0.5°C cooler than the raw ERSSTv5 anomaly this spring (AMJ 2026 RONI ≈ +0.5 vs raw ONI +0.95), which is why the declaration came at a time when the legacy ONI was already well above +1.0. With 2 consecutive seasons ≥ +0.5 in ONI terms, the event is still officially "developing" under the historical 5-season rule, but the NOAA Advisory is in effect and the weekly Niño 3.4 anomaly was +2.3°C in late July. A **very strong event peaking OND/NDJ 2026** is the dominant scenario in the IRI/CPC model plume. ## Caveats - The series starts at DJF 1950, so the 1949–51 La Niña is truncated at its start (listed as 1950-01 → 1951-01; it actually began in 1949). - ONI is revised when ERSSTv5 base periods are updated; values here are those published by CPC on the download date. - Event boundaries here are derived mechanically from the ±0.5 °C / 5-season rule; minor differences vs. CPC's narrative chronology (e.g. the 2014–16 grouping) are noted above.